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Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions Eiji Yashima,*,† Naoki Ousaka,† Daisuke Taura,† Kouhei Shimomura,† Tomoyuki Ikai,‡ and Katsuhiro Maeda‡ †

Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan ‡ Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan ABSTRACT: In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

CONTENTS 1. Introduction 2. Helical Assemblies of Small Molecules 2.1. Introduction to Helical Assemblies of Small Molecules 2.2. Hydrogen-Bond-Assisted Helical Assemblies 2.3. Metal-Coordination-Assisted Helical Assemblies 2.4. Solvophobic and π−π Interaction-Assisted Helical Assemblies 2.5. Multiple Helical Assemblies 2.6. Chiral Amplification in Helical Assemblies of Small Molecules 2.6.1. Sergeants and Soldiers Effect and Majority Rule 2.6.2. Chiral Self-Sorting 2.6.3. Helix-Sense Induction 2.6.4. Memory of Helical Chirality 2.7. Helix Inversion 2.8. Helical Template-Assisted Helical Assemblies 2.8.1. Inorganic Helices 2.8.2. Helical Arrangement of Inorganic and Metal Nanoparticles 2.9. Function of Helical Assemblies of Small Molecules 2.9.1. Chiral Recognition and Separation 2.9.2. Asymmetric Catalysis 2.9.3. Circularly Polarized Luminescence © 2016 American Chemical Society

2.9.4. Miscellaneous Applications 3. Helical Assemblies of Foldamers 3.1. Helical Cavity Formation and Inclusion Complexation 3.1.1. π-Conjugated Aromatic Foldamers 3.1.2. Nonheterocyclic and Heterocyclic Aromatic Oligoamides 3.1.3. Memory of Helical Chirality 3.2. Foldamer-Based Helical Assemblies 3.2.1. Metal- and Solvent-Induced Helical Assemblies 3.2.2. Guests-Induced Helical Assemblies 3.3. Foldamer-Based Supramolecular Helical Assemblies 3.4. Functions of Foldamers and Helically-Assembled Foldamers 3.4.1. Chiral Recognition and Asymmetric Catalysis 3.4.2. Artificial Ion Channels 4. Multistranded Helical Assemblies 4.1. Double Helices 4.1.1. Homo-Double Helices 4.1.2. Complementary (Hetero-) Double Helices

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Received: June 6, 2016 Published: October 18, 2016 13752

DOI: 10.1021/acs.chemrev.6b00354 Chem. Rev. 2016, 116, 13752−13990

Chemical Reviews 4.1.3. Sequence-Specific and Templated-Directed Complementary (Hetero-) Double-Helix Formations 4.1.4. Homo- and Hetero-Double Helices Based on Helicene Oligomers 4.2. Triple and Quadruple Helices 4.3. Inclusion Complexation 4.4. Chiral Amplification during Double-Helix Formation 4.5. Memory of Double-Helical Chirality 4.6. Helix Inversion 4.7. Functions of Multi-Stranded Helical Assemblies 4.7.1. Chiral Recognition 4.7.2. Chiral Self-Sorting through Double-Helix Formation 4.7.3. Asymmetric Catalysis 4.7.4. Miscellaneous Applications (Bioactivity) 4.8. Miscellaneous 5. Helical Polymers and Their Assemblies 5.1. Helical Polymers 5.1.1. Progress in Helical Polymer Synthesis 5.1.2. Progress in Static Helical Polymers: Helix-Sense-Selective Polymerization 5.1.3. Progress in Dynamic Helical Polymers: Chiral Amplification and Helix Inversion 5.1.4. Other Types of New Helical Polymers 5.1.5. Helical Polymers with Helicity Memory 5.1.6. Photoresponsive Helical Polymers and Helix-Sense Control with Light 5.1.7. Stabilization of Helical Structures 5.2. Helical Assemblies of Polymers 5.2.1. Helical Assemblies of Helical Polymers 5.2.2. Helical Assemblies of Achiral Polymers 5.3. Helically-Assembled Polymers 5.3.1. Helically-Assembled π-Conjugated Polymers 5.3.2. Helically-Assembled Block Copolymers 5.4. Multi-Stranded Helical Polymers 5.4.1. Double-Stranded Helical Polymers 5.4.2. Stereocomplex Formation 5.5. Helical Cavity of Helical Polymers 5.5.1. Inclusion Complexation of Small Molecules 5.5.2. Inclusion Complexation of Polymers 5.5.3. Inclusion Complexation of CNT 5.5.4. Polymerization in Helical Cavity 5.6. Determination of Helical Handedness of Helical Polymers 5.6.1. Microscopic Observations of Helical Polymers 5.6.2. Microscopic Observations of Synthetic Helical Polymers 5.7. Functions of Helical Polymers and Their Assemblies 5.7.1. Chiral Recognition 5.7.2. Asymmetric Catalysis 5.7.3. Circularly Polarized Luminescence 5.7.4. Miscellaneous Applications 6. Template-Assisted Helical Assemblies 6.1. Helical Arrangements along Biological Helical Polymer and Oligomer Templates

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6.2. Super-Structured Helices and Helical Assemblies 6.3. Applications 7. Molecular Spring 7.1. Supramolecular Polymers, Foldamers, and Duplexes 7.2. Helical Polymers 7.3. Super-Structured Helices 8. Conclusions and Outlook Associated Content Special Issue Paper Author Information Corresponding Author Notes Biographies Acknowledgments References

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1. INTRODUCTION The helix is one of the topological structures and is inherently chiral; therefore, molecules, supramolecules, oligomers, polymers, and their assemblies can be optically active as a result of being only due to the helicity, despite lacking asymmetric carbons and stereogenic centers in their components when they take a preferred-handed helical conformation. Mother nature applies the one-handed helical structure in biological systems at the macromolecular and supramolecular levels, as seen in the right-handed α-helix for proteins1 and the right-handed double helix for DNA2 that mostly relies on the homochirality of each component, L-amino acids, and D-sugars, respectively. These biological helical polymers further assemble to form supramolecular helical structures, such as the coiled-coil helix bundle proteins, superhelices of DNA, and protein−DNA hybrid superstructures, which are responsible for their vital functions that involve recognition, catalysis, ion transport, genetic information storage, replication, etc.3 With these findings, chemists, for a long time, have made a noble effort to develop a large variety of artificial helical molecules, oligomers, and polymers, and helical aggregates from small molecular components through self-assembly. The purpose of the initial studies appeared just to mimic the structures of biological helices and some functions related to the biological ones. However, remarkable progress has been made during the past couple of decades in developing synthetic helical systems that demonstrate unique chiral phenomena, properties, and specific functionalities, some of which have not been observed in or achieved by natural systems. In 2009, we published a review article entitled “Helical Polymers: Synthesis, Structures, and Functions” in this journal4 which encompassed the complete progress in the synthesis of helical polymers, their unique properties, helical structure determinations, and functions toward applications of chiral materials, mostly since 2001, thus demonstrating a clue or guideline for the rational design and synthesis of singlestranded helical polymers with optical activity, the helical structures of which could be unambiguously determined by high-resolution atomic force microscopy (AFM) observations for certain helical polymers when combined with their X-ray diffraction (XRD) analysis. A brief overview of the synthesis and structures of foldamers,5−8 an interesting class of helically folding oligomers in specific solvents, and those of doublestranded helical polymers and oligomers9,10 was also described

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Figure 1. Scanning electron microscopy (SEM) images of self-assembled helices (A and B) and flat ribbons (C) of lithium 12-hydroxystearate (1): (A) D-form, right-handed twist; (B) L-form, left-handed twist; (C) racemic form, no twist. (Reproduced with permission from ref 62. Copyright 1965 American Chemical Society.)

molecules and the progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described. The ortho-condensed polycyclic aromatic compounds, helicenes, are some of the long-known and wellestablished, helically shaped π-conjugated molecules prepared for the first time in the 1950s.33−35 The chemistry of helicenes and their derivatives with a stable or static helical conformation showing an optical activity has attracted continuous interest because of their unique structural and chiroptical properties,36−39 but is beyond the scope of this study and not included in this review except for their helical assemblies and related helically shaped π-conjugated dynamic foldamers.

in the review. However, the vast areas of supramolecular helical assemblies of small molecules and oligomers, thus producing an important class of supramolecular polymers, one of the emerging research areas in supramolecular chemistry and polymer chemistry, were not included.4 Given the extreme importance of self-assembled helical structures rather than monomeric helices in biological systems for their sophisticated functions, in the present review article, we will comprehensively describe supramolecular helical assemblies formed from artificial small molecules, foldamers, and polymers from the viewpoints of their formations, structures, and functions. Noncovalent bonding interactions, such as hydrogen bonding, π−π aromatic stacking, ion-dipolar interactions, charged electrostatic interactions, and their combinations, are of key importance for constructing supramolecular helical architectures. A number of related topics on supramolecular aggregates, including gelators,11−21 will not be covered in this review due to our present focus on helical assemblies with amplification of chirality during the dynamic helically assembled processes. Amplification of chirality is a unique and quite intriguing phenomenon being linked to the origin of homochirality in biological systems22−27 together with the development of practically important synthetic methods to obtain optically active products by asymmetric synthesis.28−31 The chiral amplification was for the first time observed in synthetic helical polymer systems by Green et al. in 1988.22,24,32 They discovered the outstanding features of the dynamic macromolecular helicity of polyisocyanates, typically stiff rodlike polymers, of which the right- and left-handed helical conformations are interconvertible from each other and separated by rarely occurring helical reversals because of very small helix inversion barriers. Therefore, the chirality is significantly amplified in dynamic helical polyisocyanates by a small chiral bias as a result of the strong cooperative interactions between the monomer units, thus producing polymers with a large helix-sense excess.22,24 This discovery cannot be overemphasized and remains as a landmark in recognition of the fact that the underlying principle discovered in dynamic helical polyisocyanates has been proved to be universal and applicable to other varieties of polymeric and supramolecular helical systems that will be described in this review. In addition to supramolecular helical assemblies formed from a broad range of molecular and macromolecular components, a brief historical overview of the helical assemblies of small

Figure 2. TEM image of a multilayered vesicle formed from didodecyldimethylammonium bromide. (Reproduced with permission from ref 63. Copyright 1977 American Chemical Society.)

2. HELICAL ASSEMBLIES OF SMALL MOLECULES One of the attractive advantages of the self-assembly of small molecules into supramolecular helical structures is their high tunability and controllability with respect to size, shape, and handedness that rely on multiple weak noncovalent interactions, such as hydrogen bonding, electrostatic interactions, solvophobic interactions, metal-ion complexation, and combinations thereof; all of these weak interactions can be intelligently regulated by external stimuli (solvent, pH, 13754


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Figure 3. TEM images of helical ribbons formed from 2 (A) and 3 (B). (Reproduced with permission from refs 64 and 66. Copyright 1984 The Chemical Society of Japan.)

Figure 5. Polarized light micrograph of representative right-handed fibers formed from glyco-bolaamphiphile 6 (n = 12). (Reproduced with permission from ref 72. Copyright 1997 American Chemical Society.)

processes as well as related helical systems except for selfassembled liquid crystalline systems50−59 that are out of the scope of this section, although this section will describe a brief historical overview of helical assemblies formed from small amphiphilic molecules. In 1945, the first example of helical assemblies of small molecules was reported using amphiphilic carboxylic acids with a long alkyl chain terminated by an (S)-sec-butyl group that formed a one-handed twisted assembly from its dispersed solution.60 Hotten and Birdsall first pointed out that singlehanded helical fibers could be formed from aqueous dispersions of lithium 12-hydroxystearate (1), but they did not mention the helical handedness or direction.61 Later, Tachibana and Kambara clearly demonstrated the effect of chirality on the helical assemblies of 1.62 They used the enantiomers and racemate of 1 and found that the D- and L-forms of 1 helically assembled into right- and left-handed twisted fibers, respectively (Figure 1A and B, respectively), while no helical aggregates were formed from the racemate (Figure 1C).

Figure 4. (A) TEM image of helical aggregates of N-octyl-Dgluconamide (5) and (B) Computer generated model of a quadruple helix formed from 5. (Reproduced with permission from ref 71. Copyright 1993 American Chemical Society.)

temperature, additive, etc.). In contrast to such an attractive noncovalent supramolecular approach, it seems to be almost impossible to construct identical helical structures based on a typical approach using covalent bonds. 2.1. Introduction to Helical Assemblies of Small Molecules

Since a number of comprehensive reviews on chiral supramolecular assemblies formed from amphiphilic chiral molecules have been published,40−49 this section will mainly focus on supramolecular helical assemblies of artificial small molecules showing a unique chiral amplification during the assembly 13755


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Figure 6. TEM images of the self-assembled ribbons formed from 7 with different degrees of ee of tertrate: (A) 0% (racemate); (B) 50% (L-rich); (C) 100% (pure L). (Reproduced with permission from ref 74. Copyright 1999 Nature Publishing Group.) (D) Schematic illustration of the effect of the ee of tartrate anion on the helical pitch of the multibilayer twisted ribbons. (Reproduced with permission from ref 75. Copyright 2002 American Chemical Society.)

an ammonium headgroup through an L-glutamic acid residue, was found to self-assemble into highly developed vesicles, which were slowly transformed into right-handed helices upon aging at 20 °C (Figure 3A).64 A related observation of helical assemblies was reported by Yamada and co-workers, who used amphiphile 3, bearing an oligo(L-gultamic acid)-headgroup instead of the ammonium one (Figure 3B).66 In 1984, Yager and Schoen also found a similar helical assembly derived from an artificial phospholipid L-4 bearing fatty acyl chains with polymerizable diacetylenic units (Figure 3).65 This helical morphology is fragile and sensitive to temperature changes but was locked after polymerization of the diacetylenic groups.65,67 In sharp contrast to the earlier example of lithium 12hydroxystearate (1) as already described, the racemic form of 4 produced both right- and left-handed helices, probably due to phase separation based on chirality.68 This kind of “chiral selfsorting” behavior will be discussed in section 2.6.2. Just after these pioneering studies, Pfannemüller and Welte synthesized an optically active N-alkyl gluconamide 5 (Figure 4), comprised of an open-chain glucose linked to an alkyl chain through an amide linkage, and observed highly ordered helical ropes with a right-handed twist-sense in its hydrogel formed at the concentration as low as 1%.69 Based on the computerassisted high resolution electron micrographs of 5, Fuhrhop et al. proposed a unique structure of two intertwined micellar fibers termed as “bulgy double helices”.70 A further study by the same group concluded the formation of a quadruple helix consisting of four micellar fibers of 5 in water (Figure 4).71 These kinds of multiple helices will be discussed in section 2.5. In contrast to the common amphiphiles described above, bolaamphiphiles consisting of two water-soluble headgroups linked through a hydrophobic spacer prefer to form stable monolayered membranes rather than the bilayer membranes due to their hydrophilic−hydrophobic−hydrophilic structural feature. Various kinds of synthetic bolaamphiphiles have been reported, and the relationship between the molecular structures and their morphologies has been summarized in review papers by the groups of Fuhrhop43 and Shimizu.44 One of the earliest examples of supramolecular helices assembled from chiral

Chart 1

Figure 7. Schematic illustration of the helical columnar stack of C3symmetrical discotic 9. (Reproduced with permission from ref 78. Copyright 2008 American Chemical Society.)

Since the first example of a totally synthetic bilayer membrane made of a simple double-chain ammonium salt, didodecyldimethylammonium bromide, discovered by Kunitake and Okahata in 1977 (Figure 2),63 various kinds of artificial amphiphilic molecules that self-assemble into vesicles, lamellae, rods, and other related morphologies have been synthesized, leading to an in-depth understanding of the relationship between amphiphilic molecular structures and their selfassembled morphologies. However, studies regarding the helical assemblies of synthetic amphiphiles had not been significantly advanced until three groups independently reported work on the self-assembled helices consisting of molecular bilayers of synthetic chiral amphiphiles in 1984.64−66 Amphiphile 2, composed of double-alkyl chains connected to 13756


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Figure 8. (A) Supramolecular polymerization of 10a with 10b driven by complementary hydrogen bonding between diaminopyridine and uracil moieties. (B) SEM image (platinum−carbon shadowing) of right-handed superhelical structures formed from an equimolar mixture of L-10a and L10b. (Reproduced with permission from ref 79. Copyright 1993 The National Academy of Sciences.)

bolaamphiphiles was reported by Shimizu and Masuda,72 who synthesized glycol-bolaamphiphiles 6 comprised of two 1-Dglucosamide-headgroups connected by an alkyl chain (Figure 5). Because of the relative orientation of the two carbonyl dipoles in 6, a remarkable effect of the odd−even number of methylene groups of the connecting linkers on the supramolecular morphologies has been found for the first time. At the end of the 1990s, Oda, Huc, and co-workers demonstrated a novel method to modulate supramolecular helical morphologies using the chiral source that comes from the counteranion to a cationic achiral surfactant.73,74 This electrostatic chiral interaction allows control of the helical pitch by adjusting the enantiomeric excess (ee) of the counteranion. Gemini surfactants are composed of two identical cationic surfactants connected by a hydrocarbon spacer. The achiral cationic gemini surfactant 7, having L-tartrate as the chiral counteranion, was found to self-assemble into multilamellar twisted ribbons in both water and some organic solvents, as shown in Figure 6C.73,74 The handedness of the twisted ribbons formed from 7 and L-tartrate was right-handed as a result of the chiral conformation of 7 induced by L-tartrate,75 while the opposite-handed ribbons were formed from the Dtartrate. Interestingly, mixing the L- and D-enantiomers in various molar ratios resulted in the formation of helices with continuously varying pitches and widths (Figure 6B and D). In addition, an infinite twist pitch (flat ribbons) was formed from the racemic mixture (Figure 6A),74 thus demonstrating the first example of supramolecular chirality control through noncovalent chiral interactions between small achiral and chiral

molecules with opposite charges to each other during the selfassembly. The related systems will be discussed in section 2.6.3.1. 2.2. Hydrogen-Bond-Assisted Helical Assemblies

In supramolecular chemistry, hydrogen bonding is one of the most useful driving forces to construct higher-order structures such as helices.13 A simple alkyl-diamide 8 (Chart 1) derived from enantiopure trans-1,2-diaminocyclohexane has been found to form helical fiber-like aggregates in some organic solvents as a result of intermolecular hydrogen bond formation between the amide groups of adjacent molecules along with a hydrophobic interaction between the long alkyl chains.76 Meijer and co-workers prepared an enantiomeric pair of C3symmetrical benzene-1,3,5-tricarboxamide (BTA) derivatives bearing chiral alkyl chains (9a and 9b in Chart 1) which selfassemble into a long and stable helical columnar stack with a controlled helicity in a diluted heptane solution due to unidirectional hydrogen bonding and cooperative interactions between the chiral side chains (Figure 7).77,78 Based on temperature-dependent UV−vis spectroscopy measurements, achiral 9c also self-assembles into a helical stack in a highly cooperative manner, but it lacks a preferred-handed helicity.78 Taking advantage of specific and directional multihydrogen bonding formations, Lehn and co-workers succeeded in the supramolecular polymerization of two chiral monomers bearing complementary hydrogen bonding units, thus producing supramolecular polymers with an optical activity (Figure 8A).79 The two hydrogen bonding units contain an array of donor−acceptor−donor (DAD) and its complementary ADA 13757


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right-handed supramolecular helical fibers when the concentration of an equimolar solution of the (L-10a + L-10b) mixture exceeded 50 μg/mL (Figure 8B). In addition, diastereomeric mixtures of L-10a and D-10b and of D-10a and L-10b also formed left- and right-handed supramolecular helical fibers, respectively. Thus, the handedness of these supramolecular helices is imposed by the stereochemistry of the 10b component. No helicity was observed for an equimolar mixture of the meso compounds. The self-complementary ureidotriazine-based monomer 11a bearing chiral alkyl chains at the periphery forms a dimer through the formation of quadruple hydrogen bonds (ADAD array) between the units, and further solvophobically driven stacking of the dimers produced helical columnar architectures with a controlled helicity in apolar solvents such as n-dodecane (Figure 9). A similar assembly was also observed for 12a, consisting of the bis-ureidotriazine units connected by an alkyl spacer and carrying long chiral solubilizing chains at the periphery (Figure 9).80 Its water-soluble analogue 12b also selfassembles into a columnar supramolecular polymer even in water, while the monofunctionalized derivative 11b does not stack via hydrogen-bonded pairs.81 The hydrophobic parts of the stacked dimeric moieties of 12b, which appear to shield the quadruple hydrogen bonding from water, likely contribute to this unusual stability in water. Meijer and co-workers further applied this self-complementary hydrogen bonding unit to functionalize chiral oligo(pphenylenevinylene) (OPV) (13), which also dimerizes with the dimerization constant (Kdim) of (2.1 ± 0.3) × 104 L/mol in chloroform. When dissolved in n-dodecane, the hydrogenbonded dimers further assemble into a helical stack that shows a thermochromic reversibility, as revealed by UV−vis, fluorescence, and circular dichroism (CD) spectroscopic measurements.82 A detailed spectroscopic monitoring of a nucleation process during the hierarchical self-assembly of 13 into supramolecular helical structures supports a nucleation− growth pathway with a remarkable cooperativity (Figure 10).83 Upon cooling from the molecularly dissolved state, 13 initially dimerizes via quadruple hydrogen bonding. Until aggregates consisting of 10−15 stacked dimers are formed, the dimers associate into short disordered stacks with the disordered stacking direction via a noncooperative isodesmic pathway. Upon further cooling, the relative positions of the molecules in the preaggregates become more restricted. A subsequent coilto-helix transition of the preaggregates results in the formation

Figure 9. (A) Structures of mono- (shown as a dimer) and bisureidotriazine derivatives 11 and 12, respectively. Their association mode via quadruple hydrogen bonds is also shown. (B) Schematic illustration of helical stacks of 11 and 12 through the formation of the quadruple hydrogen bonds and the subsequent π−π stacking. (Reproduced with permission from refs 80 and 81. Copyright 2000 Nature Publishing Group and Copyright 2002 The National Academy of Sciences, respectively.)

sites. The complementary monomers, L-10a and L-10b, consist of an L-tartrate unit in the middle and two peripheral pyridine and uracil derivatives, respectively, which undergo hydrogenbond-mediated supramolecular polymerization to produce

Figure 10. Schematic illustration of the hierarchical self-assembly of ureidotriazine-functionalized oligo(p-phenylenevinylene) derivative 13 to form a supramolecular helical assembly with a controlled handedness. (Reproduced with permission from ref 257. Copyright 2012 Nature Publishing Group.) 13758


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Figure 11. (A) Structures of deoxyguanosine oligomers 14 and lipophilic guanosine derivatives 15 and 16. (B) Schematic illustration of the diskshaped G-quartet formation (15)4 through intermolecular hydrogen bonds and the subsequent stacking of the G-quartets into helical columns in the presence of K+ ions. (Reproduced with permission from ref 88. Copyright 2000 American Chemical Society.)

Figure 12. Hierarchical self-assembly of 17b into hexametric rosettes, which further helically stack to form a rosette nanotube. (Reproduced with permission from refs 91 and 92. Copyright 2002 American Chemical Society and Copyright 2002 The National Academy of Sciences, respectively.)

cavity formed between the two G-quartets through the coordination with the eight carbonyl oxygens in the cavity (Figure 11B).87,88 Davis and co-workers, for the first time, confirmed the stacked structure of the K+ ion-bound lipophilic G-quartets composed of an analogue of 15 by X-ray crystallography.88 In contrast to the aforementioned G-quartets, the 8-oxo substituted analogues 16a and 16b self-assemble into a noncyclic, supramolecular helical structure mediated by the cooperative effect of hydrogen bonding and solvophobic interactions. This helical structure was observed both in the liquid crystalline phase, in the solution state, and at surfaces.89 Fenniri and co-workers have designed and synthesized G∧Cbased modules 17a and 17b possessing both the DDA hydrogen bonding array of guanine and its complementary AAD array of cytosine (Figure 12).90−92 These arrays allow 17a and 17b to self-associate to form six-membered supermacrocycles (rosette), which further self-assemble into stable helical rosette nanotubes (HRNs) with an excess handedness (17a)90

of a chiral nucleus composed of about 28 dimer units, at which point the elongation-growth pathway is initiated to form the supramolecular helical assembly with a controlled handedness. The stabilities of the intermediates and stacks are highly sensitive to the solvent structure. Gottarelli, Spada, and co-workers reported the hierarchical self-assembly of the sodium salts of deoxyguanosine oligomers 14 (Figure 11A) into chiral columnar stacks in water. Guanosine units of 14 form planar cyclic tetramers (Gquartets) through hydrogen bonding.84,85 The G-quartets further assemble into helical columns via hydrophobic interactions. The binding of alkali metal ions to the carbonyl groups of the G-quartet stabilizes the columnar stacks, where the K+ ion is more effective than the Na+ ion.86 The lipophilic deoxyguanosine derivative 15 in chlorinated organic solvents undergoes alkali metal ion-mediated self-assembly to form octameric or polymeric species depending on the amount of K+ ions in the solution, where the K+ ion is located in an internal 13759


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Chart 2

hydrogen-bonded discotic complex 23a3·22, which hierarchically organizes into fibrous columnar assemblies, as revealed by transmission electron microscopy (TEM) and AFM studies.96 The extended chiral stacks of the PDI moieties of 23a are present in the organized columnar assembly with a biased helical sense, as suggested by its remarkably high anisotropy factor (g = 1.5 × 10−3). This high chiroptical response is probably due to the position of the chiral centers that are located inside the discotic complexes rather than at their peripheries. Percec and co-workers utilized amphiphilic dendritic dipeptides composed of dipeptides (Tyr-Ala) possessing hydrogen bonding sites and a dendron as a solvophobic part to construct supramolecular helical assemblies with internal pores, where the dendrons are located at the periphery of the helical pores (Figure 13).97−99 While no self-assembly was observed in chloroform, dichloromethane, and tetrahydrofuran (THF), the hydrogen bond and solvophobically driven selfassembly of 24 took place in cyclohexane as a solvophobic solvent through the transition of a globular conformation of the dendron to an all-trans tapered form that facilitated the selfassembly. CD experiments of L-L-24 and D-L-24 revealed that the stereochemical information on the dipeptide moieties is transferred to the achiral dendritic moieties in supramolecular helical structures in both the solution and bulk states.97 Their helical sense is imposed by the stereochemistry of the Tyr residue, although the stereochemistry of the entire dipeptide determines the supramolecular arrangement of the dendrons.98 The helical porous structures were confirmed by TEM and XRD studies, and the pore diameter was determined to be ca. 13 Å for L-L-24.97

and an equal mixture of right- and left-handed helices (17b),91,92 respectively, through stacking and hydrophobic interactions between the rosettes in water (Figure 12). A similar approach utilizing the complementary hydrogen bonding units has been widely applied to construct supramolecular helical architectures (Chart 2). Kunitake and Lehn et al. reported that the complementary components “Janus” molecule 18 and alanine ester-appended melamine 19a in methylcyclohexane (MCH) undergo a 1:1 coassembly to form a supramolecular structure (Chart 2).93 An exciton-coupled CD signal was induced for the achiral chromophore 18 in the coassembly, probably due to the formation of either a helically stacked cyclo(18·19a)3, a helically grown structure, or a linearly extended tape. Similarly, an equimolar mixture of the achiral perylenediimide (PDI) 20 and chiral melamine derivative 19b in MCH results in the chiral superstructure formation through ADA/DAD-type complementary hydrogen bonding (Chart 2).94 An exciton-coupled CD spectral pattern observed for the assembly of 20·19b implies the arrangement of PDI moieties in an M-helical stack. Schenning, Würthner, Meijer, and co-workers demonstrated the formation of well-defined helical fibers formed from a 1:2 mixture of an achiral PDI possessing two DAD arrays (20) and a chiral OPV functionalized with diaminotriazine having an ADA array (21) in MCH (Chart 2).95 This supramolecular helix formation is involved in the collective and hierarchical self-assembly process including the formation of a 1:2 hydrogen-bonded complex 21−20−21, which further stacks into J-aggregates with a controlled helicity. A mixing of cyanuric acid (22) and 3 equiv of melamines equipped with two PDI chromophores and two chiral aliphatic chains (23a or 23b) (Chart 2) in MCH results in the 3:1 13760


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Figure 13. Self-assembly of dendron-functionalized dipeptides 24 into helical pores. (A) Top view of the porous column (L-L-24). (B) Tilted view of the helical pore without dendron (L-L-24). For simplicity, dodecyl chains are replaced with methyl groups. (Reproduced with permission from ref 99. Copyright 2006 The National Academy of Sciences.)

Figure 14. (A) Space-filling and (B) stick-representation of the X-ray structure of 25. Dashed lines represent intermolecular hydrogen bonding. (Reproduced with permission from ref 100. Copyright 2007 Wiley-VCH.) (C) Side and top views of the calculated inclusion complex of C60 inside the self-assembled nanotube 25. (Reprinted with permission from refs101 and 102. Copyright 2007 Wiley-VCH and Copyright 2012 American Chemical Society, respectively.)

which the carboxy groups of the amino acid residues adopt a syn conformation with respect to the NDI plane (Figure 14A and B).100,101 This conformation allows the peripheral S-trityl groups located at the same face of the helix to form interdigitated stacking and the carboxy groups to dimerize

Sanders and co-workers reported the formation of hydrogenbonded supramolecular helical nanotubes formed from the amino acid-derived naphthalene-diimide (NDI) derivative L-25 in less polar solvents and in the solid state. This helical tubular structure is clearly seen in the X-ray crystal structure of L-25, in 13761


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Chart 3

through two strong intermolecular hydrogen bonds, thereby producing the left-handed 31-helical supramolecular polymer that is further stabilized by additional CH···O hydrogen bonds between the i and i + 3th NDI moieties in the helix (Figure 14B). CD measurements of L-25 indicate that the helical structure observed in the solid state is retained in aprotic solvents such as chloroform. When fullerene (C60) was added to this chloroform solution, the helical nanotube of L-25 formed an inclusion complex with C60 while maintaining its helical structure (Figure 14C), in which an induced CD signal at a C60 absorption band was observed.101 Thermodynamic studies by 1 H NMR and CD measurements revealed that C60 acts as a template for stabilization of the supramolecular helical nanotube and thereby substantially increasing the degree of polymerization.102 Takeuchi, Sugiyasu, and co-workers demonstrated, for the first time, a living supramolecular polymerization utilizing a porphyrin-based monomer 26-M and an intramolecularly bridged porphyrin initiator 26-I of which the structure prevents a self-association (Chart 3).103 Supramolecular polymerization takes place to produce polymers with a controlled chain length and narrow polydispersity. Despite the supramolecular polymerization being noncovalent, the reaction kinetics is similar to that of conventional chain-growth polymerization. This is due to the formation of a very stable supramolecular fiber through H-type π−π stacking and hydrogen bonding

interactions in MCH, which strongly suppresses chain exchange/reshuffling. In this system, the spontaneous nucleation of 26-M without the initiator 26-I is also inhibited, because the monomer 26-M self-assembles into a kinetically trapped nanoparticle through a J-type stacking. The monomer dissociated from this metastable J-aggregated nanoparticle is consumed by polymerization prior to its self-association only when in the presence of the initiator or the elongated supramolecular polymer as a seed. Such seed-induced supramolecular polymerization has also been demonstared by Würthner and co-workers using PDI derivatives bearing tridodecyloxy benzamide units linked by alkyl spacers with different lengths (27-Cn, n = 2−5).104,105 In this system, a monomeric state of 27-Cn except for 27-C4 is kinetically trapped in low polar solvents such as MCH, toluene, and a mixture thereof, due to intramolecular hydrogen bonds between the amide protons of the benzamide moieties and the imide carbonyl oxygens of the PDI core, thus preventing their spontaneous supramolecular polymerization to form a nanofiber driven by intermolecular hydrogen bonds between the amide groups in addition to π−π stacking interactions between the PDI cores. Although these kinetically trapped monomeric PDIs, except for 27-C4, self-assemble to form nanofibers after their seed formations, whose nucleation processess are very slow under certain conditions, the monomeric PDIs undergo seeded supramolecular polymerization initiated by the addition 13762


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Figure 15. (A) Structures of C5-symmetric corannulene-based chiral initiators 28-I and monomers 28-M bearing chiral or achiral amide-appended thioalkyl side chains. (B) Schematic illustration of the chain-growth supramolecular living polymerization of monomer 28-M initiated with 28-I through the hydrogen bond reorganization of 28-M. The growing polymer carries an initiator unit 28-I at one chain end (the initiating end), whereas the other end [the growing (active) end] adopts a structure similar to 28-I, with free amide CO groups. This structural feature prevents bimolecular coupling between the propagating ends. (Reprinted with permission from ref 106. Copyright 2015 American Association for the Advancement of Science.)

Chart 4

of separately prepared, intermolecularly hydrogen-bonded aggregates of 27-Cn. In contrast, the intramolecularly hydrogen-bonded PDI (27-C4) possessing butylene spacers did not show such a kinetic stabilization of the monomeric state in low

polar solvents, but instead formed kinetically trapped nanoparticles driven by π−π stacking interactions without intermolecular hydrogen bonds.105 Interestingly, transformation of the nanoparticles of 27-C4 into the thermodynamically 13763


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Figure 16. (A) Schematic illustration of reversible transformation of bent-shaped amphiphile 32 through 2D sheets, discrete metallomacrocycles, and helical tubules in response to additives. (B) AFM image (scale: 450 nm × 350 nm) of self-assembled helical tubules formed from 32 with Ag(I) ions (0.03 wt %). (Reproduced with permission from ref 110. Copyright 2013 American Chemical Society.)

way. This extremely high stereoselectivity was also observed during the supramolecular polymerization of the dynamically racemic 28-M initiated by 28-IR or 28-IS, producing an optically active polymer composed of 28-MR or 28-MS units in the mainchain, respectively, indicating the complete induction of the bowl-shaped corannulene chirality of 28-M (∼100% selectivity).

more stable supramolecular polymer was promoted by stirring and catalyzed by separately prepared, intermolecularly hydrogen-bonded nanofibers of 27-C4. In contrast to the above-mentioned seeded supramolecular polymerization, Aida, Miyajima, and co-workers reported the complete living supramolecular polymerization using the initiator 28-I and monomer 28-M, which consist of a C5symmetric corannulene core and five amide-appended thioalkyl side chains (Figure 15A).106 These substituted corannulene cores themselves are chiral because of their bowl-shaped structures and undergo a bowl-to-bowl inversion (racemization).106,107 The dynamically optically inactive monomer 28-M adopts a metastable cagelike structure with a sufficiently large energetic barrier for the self-opening in a less polar solvent, such as MCH, due to the intramolecular hydrogen bonding network formed between the side-chain amide groups. This metastable closed structure is transformed into an open form by the side-chain N-methylated initiator 28-I through the hydrogen bond reorganization of 28-M, resulting in the formation of the intermolecularly hydrogen-bonded 1:1 complex 28-I·28-M. Subsequently, the free amide CO groups of the 28-M unit in this thermodynamically stable complex form intermolecular hydrogen bonds with the amide NH protons of the free monomer, thus producing an oligomer 28-I·(28-M)2 with the actively growing end. As a result of the elongation, supramolecular living polymers end-capped with the N-methylated initiator 28-I together with the active growing end are generated, leading to a living chain-growth polymerization that has been achieved via supramolecular polymerization (Figure 15B). Surprisingly, when the chiral initiator 28-IR was used for the polymerization of the racemic monomer (an equimolar mixture of 28-MR and 28-MS), only 28-MR was polymerized in an almost complete enantioselective

2.3. Metal-Coordination-Assisted Helical Assemblies

Metal-directed self-assembly has been widely used as a strategy for the construction of various kinds of discrete chiral supramolecules. On the other hand, nondiscrete coordination polymers that fold into a helical conformation in the solution or gel state are rare, although there are a huge number of reports on the helical coordination polymers that can only exist in the crystalline state. In 1992, van Koten and co-workers reported the first example of a thermodynamically stable coordination polymer using a multidentate ligand and labile metal ions such as the Ag(I) ion.108 An equimolar mixture of Ag(CF3SO3) and a peptide-based multidentate ligand produced a stereoregular coordination polymer 29 (Chart 4) which adopts a helical conformation in both the solution and solid states. In this helical polymer, the imine N, carbonyl O, thioether S, and imidazolyl N donor atoms of the ligand coordinate to the Ag(I) ions. A bent-shaped bipyridine ligand containing a dendritic optically active aliphatic side chain that coordinates to Ag(I) ions forms various secondary structures depending on the size of the counteranions (30a−30c, Chart 4). As the size of the counteranion increases, the secondary structure changes from a helically folded polymeric chain (30a), via a discrete dimeric metallomacrocycle (30b), to an extended zigzag polymeric 13764


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Figure 17. Schematic illustration of (a) induction of a helical chirality by coordination-driven supramolecular polymerization of chirality-memorizing 33a with chiral PdII(BINAP) (34) and (b) transfer of its chiral information into the chiral conformation of monomer 33a (hydrogen-bonded with acetic acid) by chiral information retentive depolymerization in acetic acid containing achiral DPPP as a decomplexing agent. (Reproduced with permission from ref 112. Copyright 2008 Elsevier.)

Figure 18. Schematic illustration of the self-assembly process of the coordination polymer gels of ([36·Ag]NO3)n. (B) AFM height image of the ([36·Ag]NO3)n aggregates and (C) CD (upper) and UV−vis (bottom) spectra of ([36·Ag]NO3)n gels (0.8 wt % in CH3OH−H2O) obtained in different batches. (Reproduced with permission from ref 113. Copyright 2008 The Royal Society of Chemistry.)

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dimeric metallomacrocycles (Figure 16A), as indicated by the AFM measurement (Figure 16B). The transformation between these two structures is reversibly controlled by the addition of a fluoride anion or Ag(I) ion.110 Similarly, chiral coordination complexes (31a−31c) built from an axially chiral bipyridylbased biphenyl ligand and Ag(I) ions also formed a helical structure with a preferred-handedness in solution, as indicated by the 1H NMR, UV, and CD measurements. Interestingly, the cationic chains of these complexes were found to fold into onehanded 21-, 31-, and 41-helices with NO3−, PF6−, and ClO4− as the counteranion, respectively, in the solid state.111 The related helical coordination polymers that exhibit a spring-like motion will be described in section 7. Aida and co-workers reported a ladder-shaped helical coordination polymer with a preferred-handedness, which is assembled from a chirality-memorizing saddle-shaped porphyrin bearing 3,5-dipyridylphenyl side arms at the opposite meso positions (33) and the Pd(II) complex of 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) (34) (Figure 17).112 In this coordination polymer, chiral information on the BINAP moiety is transmitted not only to the helical backbone, but also to the dynamically racemic porphyrin moiety to form its preferred-handed chiral conformation. A subsequent decomplexation reaction of this polymer in acetic acid containing achiral 1,3-bis(diphenylphosphino)propane (DPPP) as the decomplexing agent takes place in a stereochemically retentive way to produce the optically active 33a stabilized by hydrogen bonding interactions with two acetic acid molecules. This 1:2 complexation leads to suppression of the racemization of 33a. In contrast to the preferred-handed helical coordination polymers containing chiral ligands, You and co-workers succeeded in constructing gels of a nonracemic tubular coordination polymer ([36·Ag]NO3)n formed from the simple achiral ligand 36 and Ag(NO3) as a result of chiral symmetry

Figure 19. Schematic illustration of self-assembly of nonracemic helicenebisquinone 37 into a helical column. (Reproduced with permission from ref 118. Copyright 1999 American Chemical Society.)

Chart 5

chain (30c).109 A similar shaped aromatic amphiphile containing m-pyridine units at the terminals and a hydrophilic dendron at the apex (32, Figure 16) self-assembles into twodimensional (2D) zigzag sheet structures through π−π interactions between the aromatic segments in an aqueous solution. The assembled sheets transform into preferredhanded helical tubular assemblies at a high concentration in the presence of Ag(I) ions through the formation of discrete

Chart 6. Aliphatic and Amphiphilic Hexabenzocoronene (HBC) Derivatives

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Figure 20. (A) Schematic illustration of self-assembled graphitic nanotube (upper) and helical coil (bottom) formations composed of HBC amphiphile 40a. The helical coil obtained from a suspension in a THF/water mixture undergoes a thermodynamic coil-to-tube transition. (Reproduced with permission from ref 127. Copyright 2005 The National Academy of Sciences.) (B) TEM image of a self-assembled nanotube of 40a obtained as a suspension in THF. (C) TEM image of self-assembled helical coil 40a obtained as a suspension in THF/water (8/2, v/v). (Reproduced with permission from ref 124. Copyright 2004 American Association for the Advancement of Science.) (D and E) TEM images of selfassembled helical coils formed from (S)-40b (D) and its enantiomer (R)-40c (E). (Reproduced with permission from ref 127. Copyright 2005 The National Academy of Sciences.)

Chart 7

Figure 21. Hierarchical assembly of merocyanine dye 41a. Helical supramolecular polymers of 41a formed by dipolar aggregation of the merocyanine units further assemble into rod-type H-aggregate composed of six helical polymeric strands. (Reproduced with permission from ref 128. Copyright 2003 Wiley-VCH.)

breaking (Figure 18A).113 The resulting coordination polymers further assembled to form helical bundles, as suggested by the AMF measurement (Figure 18B). In addition, the resulting polymers obtained in different batches exhibited a mirrorimaged CD (Figure 18C). 2.4. Solvophobic and π−π Interaction-Assisted Helical Assemblies

A C3-symmetrical disc-shaped molecule consisting of a trisubstituted benzene core extended by acylated 3,3′diamino-2,2′-bipyridine moieties with chiral aliphatic chains (BiPy-BTA) (38a or 38b, Chart 5) was also found to selfassemble into one-handed helical columns in apolar solvents.119 The molecular conformation is rigidified by intramolecular hydrogen bonding, leading to strong interactions between the discs. In this system, chiral information on the point chirality introduced at the peripheries is effectively transferred to the core moiety in the columns due to a propeller-like conformation of the bipyridine wedges that are tilted with respect to the central benzene core. Similarly, a water-soluble analogue 38d (Chart 5) bearing chiral oligo(ethylene oxide)

In addition to the simple amphiphiles described in section 2.1, aromatic molecules bearing hydrophobic or/and hydrophilic chains are also known to form supramolecular helical assemblies driven by solvophobic and π−π interactions in the appropriate solvent system.114,115 Katz and co-workers reported that the nonracemic helicenebisquinone 37, bearing long alkyl chains, self-assembles into a one-handed helical column in apolar solvents in which the molecules are stacked along their helix axes (Figure 19).116,117 The columnar assemblies stabilized by π-donor− acceptor interactions are further organized into very long lamellar fibers. On the other hand, unlike the nonracemic 37, its racemate did not form long columnar structures.118 13767


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Chart 8

Figure 23. Schematic illustration of the self-assembly of a paired dimer of 48 into a thin nanofiber which reversibly inflates into a helical hollow tubule triggered by the guest addition (p-phenylphenol). (Reproduced with permission from ref 136. Copyright 2014 American Chemical Society.) Figure 22. (A) Schematic illustration of the self-assembled helical tubular structure of amphiphilic rigid-flexible macrocycle 47. (B) TEM image of the negatively strained helical tubular structure (left-handed) of 47. (Reproduced with permission from ref 135. Copyright 2006 American Chemical Society.)

interdigitation of the alkyl chains (Figure 20A and B), in which the internal and external surfaces of the nanotube are decorated by hydrophilic TEG chains. In addition, an equal mixture of right- and left-handed helical nanocoils as a kinetic intermediate of the nanotube is also formed from 40a in a THF/water mixture (8/2, v/v) (Figure 20A and C).124 In the case of the norbornene-appended nanocoil formed from achiral 40d (Chart 6), a thermodynamic coil-to-tube transition is precluded after post-ring-opening metathesis polymerization of the norbornene pendants.125 However, the norborneneappended chiral derivatives, 40e and 40f, self-assemble into only nanotubular or fibrous assemblies.126 In contrast to these chiral HBCs, enantiomeric (S)-40b and (R)-40c (Chart 6) produce right- and left-handed helical coils, respectively (Figure 20D and E).127 Non-disk-shaped molecules also self-assemble into supramolecular helical assemblies. Würthner and co-workers found

side chains at the peripheries also self-assembles into the helical columns in water.120 Symmetrically substituted achiral and chiral derivatives of hexa-peri-hexa-benzocoronene (HBC) (39a121 and 39b,122 Chart 6) are known to form columnar mesophases.123 In contrast, Fukushima, Aida, and co-workers have designed and synthesized a series of novel amphiphilic HBCs, which form a discrete π-electronic nanotubular assembly. The amphiphilic achiral HBC 40a asymmetrically substituted with long alkyl chains and hydrophilic triethylene glycol (TEG) chains selfassembles into nanotubules in THF by rolling up of a 2D pseudographite tape composed of π-stacked 40a with 13768


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Chart 9

closed (44) chiral bridging group and oligoether dendrons selfassemble into cylindrical micellar aggregates with a controlled helicity in an aqueous solution (Chart 7).132 Although the linker groups have the same chirality, the CD signals of these assemblies are opposite to each other. Various shaped amphiphilic molecules based on oligophenylenes and chiral oligoether dendrons (45−48, Chart 8) have also been reported to form helical assemblies. A dumbbellshaped molecule 45 formed a preferred-handed supramolecular helical fiber in aqueous solution, which reversibly transforms into a spherical capsule by the addition of an aromatic guest such as 4-nitrobenzene.133 Similarly, helical coils self-assembled from the elliptical macrocycle 46 reversibly transform into rodshaped assemblies upon heating while maintaining their supramolecular chirality.134 A rigid-flexible macrocycle (47) composed of a hexa-p-phenylene rigid rod and a chiral poly(ethylene oxide) chain also self-assembles to form a preferred-handed helical tubular structure, as revealed by a TEM study (Figure 22).135 A bent-shaped aromatic amphiphile containing an m-linked pyridine and a chiral hydrophilic dendron at the apex (48) self-assembles into nonhelical thin nanofibers by the stacking of closed dimeric micelles.136 The pyridine-functionalized nanofibers encapsulate p-phenylphenol guests through hydrogen bonding interactions, leading to transformation of the closed nanofibers into hollow tubular helical assemblies via the molecular rearrangements of the

that highly dipolar merocyanine dyes show very high dimerization constants in low polar solvents when used as a building block to construct supramolecular architectures.128−130 Achiral tris(n-dodecyloxy)xylylene having two merocyanine dyes (41a) forms an oligomeric/polymeric supramolecular chain through electrostatic interaction between the dyes. The six helically preorganized chains intertwine to form long and stiff rod-type tubular assemblies jacketed by tridodecyloxy groups (Figure 21).128,129 When the chiral derivative, 41b (Figure 21), is used, the molecule self-assembles into kinetically controlled right-handed helical nanorods that further transform into thermodynamically stable right-handed helical nanorods with a significant change in the helical pitch.130 In this process, however, the bisignate CD signal based on the twist angle between the closest neighboring dyes in the helically wound stacks is inverted before and after the transformation. Ajayaghosh and co-workers found that π-conjugated rigidrod OPVs bearing one and two cholesterol pendant groups (42a and 42b, Chart 7) self-assemble into supramolecular helices that exhibit remarkable differences in optical, chiroptical, and morphological properties.131 In particular, although AFM measurements revealed that both assemblies adopt righthanded helices, the CD signals of these assemblies are different from each other. This is probably due to their different stacking modes, such as pseudo H- and J-aggregates. Chiral-bridged amphiphilic molecules consisting of two penta-p-phenylene groups linked through an opened (43) or 13769


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Figure 24. TEM images of self-assembled vesicles (A), double-helical strands (B), and superhelical structures (C) of 49. All scale bars represent 200 nm. (Reproduced with permission from ref 137. Copyright 1989 American Chemical Society.) (D) Proposed model for a tubular double-helical ribbon assembled from 50 and subsequent topochemical polymerization of the diacetylene groups of 50. The ribbons are formed from two parallel β-sheets 50 (shown in cross-section). (Reprinted with permission from ref 138. Copyright 2006 Wiley-VCH.) (E) SEM image of the xerogel formed from a mixture of 51 and 52 (1:1, w/w) in a water−methanol mixture (10:1, v/v). The blue arrow points to the double-helical structures. (F) A possible self-assembling model in the bilayered chiral fiber formed from a mixture of 51 and 52. (Reproduced with permission from ref 139. Copyright 2002 American Chemical Society.)

Figure 25. (A) Photochemical conversion of 56 to 57 through cleavage of the 2-nitrobenzyl group in 56. (B) TEM image of a quadruple helical fiber. (Reproduced with permission from ref 143. Copyright 2008 American Chemical Society.)

Yanagawa and co-workers reported a right-handed doublehelical structure assembled from a phospholipid−nucleoside conjugate (49, Chart 9) through hydrophobic interactions between the aliphatic double tails of 49 along with stacking between the nucleic acid bases in an aqueous solution. During the initial stage of the self-assembly, amphiphile 49 formed bilayer vesicles (Figure 24A), which were completely transformed into the double-helical strands after aging at 25 °C for 10 h (Figure 24B).137 Interestingly, the double helices were further transformed into a larger sized helical assembly, while maintaining its right-handed helix sense (Figure 24C).

closed pairs of 48 into hollow hexameric macrocycles (Figure 23). 2.5. Multiple Helical Assemblies

Although multiple-stranded helical structures are common in biological macromolecules (see section 4), well-defined multistranded helical assemblies consisting of two or more intertwined helical fibers formed from artificial small molecules are quite rare, whereas a huge number of helical assemblies of small molecules have been reported (sections 2.1−2.4). 13770


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system, although both aliphatic chains form similar interdigitated bilayer structures. Huang and co-workers demonstrated the unique morphological modulation of a self-assembled double helix composed of a glucose-based amphiphile 54 bearing an azobenzene group with a butyl chain by external stimuli, such as pH, light, and surfactant addition.141 The double-helical morphology can be transformed into a spherical micelle or vesicle by a decrease in pH or by trans-to-cis photoisomerization of the azobenzene group in an aqueous solution. Interestingly, the double helix is unfolded into single-stranded nanofibers by the addition of sodium dodecyl sulfonate. Stupp and co-workers reported double- and triple-stranded helices formed from one-dimensional (1D) helical nanofibers of a terthiophene-appended tripeptide lipid 55 in chlorocyclohexane.142 The single helical fibers remained unchanged with respect to its helical pitch and handedness (left-handed) after further assembly to form multiple-stranded helices, as revealed by TEM. The formation of the multiple helices was considered to be due to the terthiophene J-aggregate interactions between the fibers, in which the hydrophilic ammonium headgroups and terthiophene groups are located in the core and at the periphery, respectively, in organic solvents. As described in section 2.1, the gluconamide-based amphiphile 5 self-assembles into the quadruple helices in water.71 A peptide-based amphiphile (56, Figure 25A) containing a palmitoyl tail and a β-sheet forming -Gly-Val3Ala3-Glu3- segment with the photochemically cleavable 2nitrobenzyl group bound to the nitrogen atom of the Gly residue also self-assembles into supramolecular quadruple helical fibers (yellow arrow in Figure 25B), which are composed of two double-stranded helices (blue arrows) formed from two single fibrils (red arrows) (Figure 25B).143 The photochemical deprotection of the bulky 2-nitrobenzyl group (Figure 25A) and subsequent annealing of the fibers resulted in dissociation of the quadruple helices of 56 into cylindrical nanofibers of 57. This dissociation is probably due to an enhancement of the internal packing interactions between the peptide amphiphilic molecules as a result of the reduced steric hindrance. Multiple-stranded helices have been found not only for the sugar- or amino acid-based amphiphiles described above, but also for aromatic amphiphilic molecules. Nolte and co-workers reported the hierarchical self-assembly of a disc-shaped molecule with chiral tails into one-handed supercoiled structures.144,145 The crown-ether-appended phthalocyanine 58 (Figure 26) bearing the chiral aliphatic tails self-assembles into right-handed helical stacks in chloroform, which in turn self-assemble to form left-handed coiled-coil aggregates in order to maximize the van der Waals contact between the fibers (Figure 26A), as can be seen in the TEM image (Figure 26B and C). Similarly, left-handed helical π−π aggregates formed from the 1:2 hydrogen-bonded complex 21−20−21 (Chart 2, see section 2.2) in MCH further self-assemble into doublehelical or even larger intertwined assemblies with a righthanded helicity.146 In contrast, Yagai and co-workers reported that one-handed helical stacks derived from a chiral azobenzene dimer connected by a tris(n-dodecyloxy)xylylene spacer (59, Figure 27) self-assemble into double-helical coiled-coils while maintaining the supramolecular helical sense during the hierarchical self-assembly processes.147 This molecule first folds into a cone-shaped molecular structure by a π−π stacking

Figure 26. (A) Schematic illustration of hierarchical self-assembly of crown-ether-appended phthalocyanine 58 into right-handed helical stacks, which further assemble into left-handed supercoiled structures. (Reproduced with permission from ref 145. Copyright 2003 The Royal Society of Chemistry.) (B) TEM image (platinum shadowing) of left-handed coiled-coil aggregates formed from 58. (C) Schematic illustration of the helices in (B). (Reproduced with permission from ref 144. Copyright 1999 American Association for the Advancement of Science.)

A tubular right-handed double-helical ribbon is also formed from an amphiphilic molecule 50 (Chart 9) composed of a diacetylene unit and hydrogenated poly(isoprene) segment linked through a β-sheet forming a tetra(L-alanine) segment in which the ribbon structure consists of two parallel β-sheet assemblies of 50 with a head-to-head arrangement (Figure 24D).138 The diacetylene groups were then successfully topochemically polymerized by UV irradiation, producing the corresponding poly(diacetylene)s while retaining its supramolecular helical structure (Figure 24D). Shimizu and co-workers produced a double-helical fiber structure formed from a phenyl glucose amphiphile 51 possessing a long alkyl chain in the presence of 52 (Figure 24E).139 In this supramolecular structure, 52 is incorporated into an interdigitated bilayer structure made of 51 to form a tight chiral packing via the intermolecular hydrogen bonding interactions of the sugar residues as well as the π−π stacking of the phenyl moieties (Figure 24F). In contrast, 51 alone displayed a twisted fiber structure. A similar amphiphile 53a (Chart 9) composed of a phenyl galactoside and an unsaturated long alkyl chain with three cis double bonds also self-assembled into well-defined double-helical fiber structures with a lefthanded helical sense, as revealed by TEM, whereas 53b (Chart 9), possessing two cis double bonds, formed a single left-handed helical ribbon.140 Thus, the self-assembly properties are strongly dependent on the number of double bonds in this 13771


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Figure 27. Schematic illustration of hierarchical self-assembly of chiral azobenzene dimer 59 into left-handed coiled-coils (AggIV) via uniform nanotoroids (AggI) that stack into chiral nanotubes (AggII) and further twist into a left-handed supercoil (AggIII), which finally intertwines to form left-handed double helices (AggIV). (Reproduced with permission from ref 147. Copyright 2012 American Chemical Society.)

Figure 28. (A) TEM image of the triple-helical nanofiber of BTA 60. (Reproduced with permission from ref 148. Copyright 2006 The Royal Society of Chemistry.) (B) Schematic illustration of hierarchical self-assembly of amphiphilic BiPy-BTA 61 into a triple-helical bundle via the formation of a supramolecular fiber. (Reproduced with permission from ref 149. Copyright 2014 American Chemical Society.)

interaction and then self-assembles into uniform nanotoroids (AggI) through intermolecular hydrogen bonding. The resulting nanotoroids further stack into left-handed nanotubes (AggII) under the control by temperature, concentration, or light. Interestingly, the helically stacked nanotubes twist into left-handed supercoiled fibrils (AggIII), which finally intertwined to form left-handed double helices (AggIV) (Figure 27). These morphologies and their helix sense have been confirmed by AFM measurements. Banerjee and co-workers also found the hierarchical helicalassembly of BTA bearing chiral amino acid residues as the

pendants (60, Figure 28A), which produces intertwined triplehelical nanofibers in an aqueous methanol solution through the formation of bundles of the helical columnar stacks of the BTA molecules while maintaining the helicity of the single stacks.148 The triple-stranded helical structure and its handedness can be seen in the TEM image (Figure 28A). An asymmetrically substituted disc-shaped amphiphile 61 as an analogue of 38b described in section 2.4 has also been reported to form supramolecular triple-helical bundles consisting of the 1D helical columnar assemblies in an aqueous alcohol solution (Figure 28B), while only 1D helical fibers were formed in pure 13772


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Figure 29. (A) Chemical structures of nucleotide-appended OPV 62 and oligodeoxyadenylic acid 63 (20-mer), and schematic illustration of binary self-assembly of 63 and 62 (1:20) through the complementary hydrogen bonding between thymine and adenine groups. (Reproduced with permission from ref 150. Copyright 2006 American Chemical Society.) (B) Chemical structures of PyBOX-appended Zn-porphyrin 64 and poly(trimethylene iminium) 65, and schematic illustration of a double-stranded helix formation driven by intermolecular hydrogen bonding between the PyBOX and iminium moieties. (Reproduced with permission from ref 151. Copyright 2007 American Chemical Society.)

can be induced to adopt a preferred-handed helicity by introducing a few chiral units. The latter principle implies that the minority chiral units in the dynamic helical chain obey the helical sense of the majority enantiomeric units. Over the past three decades, these chiral amplification phenomena have been intensely studied for a wide variety of covalent and noncovalent polymeric systems and supramolecular helical assemblies.4,22,24−26,154 In this section, we will discuss the chiral amplification in the dynamic helical assemblies of small molecules and its related phenomena, such as induction of helical sense, memory of induced helix sense, and chiral selfsorting. 2.6.1.1. Sergeants and Soldiers Effect in BTA Derivatives. In 1997, Meijer and co-workers, for the first time, reported that the sergeants and soldiers principle is operative in the dynamic helical assembly utilizing chiral BiPy-BTA 38a (the sergeant) and achiral 38c (the soldiers) (Chart 5, see section 2.4).119 As described in section 2.4, chiral 38a forms one-handed helical columnar stacks in apolar solvents, which exhibit an intense CD signal corresponding to the π−π* transition of the bipyridine moiety. Obviously, racemic helical columns formed from the achiral 38c show no CD signal. However, the addition of only 2.5% of 38a to 38c in n-hexane generated a remarkable CD signal, of which the magnitude is comparable to that of the pure 38a in n-hexane. A theoretical model combined with the experimental data implies that one chiral molecule of 38a is sufficient to induce 80 molecules of achiral 38c to adopt the one-handed helical sense.119 Similarly, sergeants and soldiers experiments of the chiral 38d and achiral 38e mixtures in water

alcohol due to the absence of a hydrophobic interaction between the fibers.149 The triple-helical bundle may have an opposite supramolecular helical sense to that of the 1D stacked helical column (Figure 28B). A different approach to construct a supramolecular doublehelical structure has also been reported by Shimizu and coworkers using a nucleotide-appended OPV (62, Figure 29A) and its complementary single-stranded 20-meric oligodeoxyadenylic acid (63, Figure 29A).150 A 1:20 mixture of 63 and 62 produced a supramolecular double-helical ladder with a right-handed helical sense in an aqueous solution, in which the two single-stranded chains 63 are noncovalently connected by helical stacks of 62 through complementary hydrogen bonding between the thymine and adenine moieties (Figure 29A). In a similar approach, Sada and co-workers constructed a preferredhanded double-helical ladder assembled from a Zn-porphyrin carrying two 2,6-bis(2-oxazolyl)pyridine (PyBOX) units (64) and poly(trimethylene iminium) (65) through intermolecular hydrogen bonding between the PyBOX and iminium moieties along with π−π stacking of the porphyrin rings (Figure 29B).151 2.6. Chiral Amplification in Helical Assemblies of Small Molecules

2.6.1. Sergeants and Soldiers Effect and Majority Rule. Pioneering studies on the chiral amplification in dynamic helical polyisocyanates by Green and co-workers provided two fundamental concepts that are known as the “sergeants and soldiers” effect152 and “majority rule” principle.153 In this effect, a large number of the achiral units in a dynamic helical chain 13773


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temperature.78 Taking advantage of the remarkable chiral amplification in the BTA system, Palmans, Meijer, and coworkers further demonstrated the formation of a preferredhanded helical columnar structure of BTAs bearing alkyl side chains with only a deuterium/hydrogen isotope chirality (66a or 66b, Chart 10). This is the first example of supramolecular helicity controlled by isotope chirality.156 Asymmetrically substituted BTAs also exhibit the sergeants and soldiers effect. As expected, coassembly of the achiral BTA bearing one polymerizable sorbyl-containing alkyl chain (66c, Chart 10) with the C3-symmetric sergeant molecule 9a in cyclohexane results in a nonlinear response in the CD effect similar to that of the 9a/9c mixed system in n-hexane.157,158 The polymerization of the sorbyl side chains in the chiral columns will be discussed in section 2.6.4. Interestingly, even when the desymmetrized BTAs bearing only one chiral side chain of which the asymmetric carbon is introduced at the α-, β-, or γ-position with respect to the amide moiety (66d, 66f, and 66h, Chart 10) were used instead of 9a, a nonlinear response in the chiral/achiral mixed systems was also observed with a slight decrease in the nonlinearity as compared to that of the 9a/9c mixed system.158,159 Among these systems using chiral 66d,f,h and achiral 9c, an odd−even effect of the asymmetric carbon position on the degree of chiral amplification was observed.159 A sergeants and soldiers experiment involving mixtures of chiral 66j (Chart 10) bearing a phenylalanine octyl ester at the side chain and achiral 9c in MCH shows a similar degree of chiral amplification to that of the 9a/9c mixed system in nhexane.160 A similar behavior was also observed when mixtures of the desymmetrized BiPy-BTA (67a) and achiral 67b (Chart 11) were used.161 In sharp contrast, almost no chiral amplification was observed when mixtures of the bulky 66k (Chart 10) bearing the three phenylalanine-based side chains and 9c in a diluted MCH solution (1 × 10−5 M) were used. This is because 66k prefers to form a 1:1 heterocomplex with the achiral 9c in addition to a heterocomplex without the helixsense induction of the columnar stacks composed mainly of 9c.160 However, 66k and its analogue 66m (Chart 10) selfassemble into long rods at millimolar concentrations (2 mM) and display a strong chiral amplification effect when coassembled with 9c.162 Similar to the incorporation of the bulky side chains, replacement of only one out of three aliphatic side chains of 9a with an ethylene-glycol chain (66n, Chart 10) results in a significant decrease in the association constant in apolar solvents. This is due to the intramolecular hydrogen bonding between the ethylene glycol chains and the amide groups, which prevents the efficient intermolecular hydrogen bonding between the BTAs; thereby, the 66n/66o (Chart 10) mixed system showed a low degree of chiral amplification even at a high concentration (48 mM in decalin).163 Palmans, Meijer, and co-workers have applied the concept of chiral amplification to the transformation of rac-66p (Chart 10) bearing a racemizing rac-phenylglycine octyl ester into an enantio-enriched 66p via the deracemization reaction in a preferred-handed helical column in the presence of sergeants and a base. The coassembly of rac-66p and 8 mol % of the nonracemizing 9a in the presence of 1,8-diazabicycloundec-7ene (DBU) as the base produced the optically active 66p with a 32% ee at the thermodynamically equilibrium state.164 Elemans and co-workers designed and synthesized largesized BTA derivatives bearing three porphyrin groups with

Figure 30. (A) “Sergeants and soldiers” effects: plots of the anisotropy factor g as a function of the mole fraction of (S)-38d for helical columnar assembly of (S)-38d and achiral 38e in water at 5 °C at 10−5 (circles) and 10−4 M (squares). (Reproduced with permission from ref 120. Copyright 2000 The Royal Society of Chemistry.) (B) “Majority rule” effect: plots of g versus the ee of (S)-38a and (R)-38b for helical columnar assembly of (S)-38a and (R)-38b in n-octane at 20 °C (closed circles) and 50 °C (open circles). (Reproduced with permission from ref 189. Copyright 2005 American Chemical Society.)

at two different concentrations showed a nonlinear response in the CD effect for both concentrations, as shown in Figure 30A. At a 10−4 M concentration, it is estimated that the columns with a degree of polymerization as high as 200 are formed and its cooperativity length is 12 molecules.120 When n-butanol was used as a solvent instead of water, the cooperativity length becomes 400 molecules, meaning that one chiral molecule of 38d is sufficient to induce 400 molecules of achiral 38e in one helical column with complete homochirality.155 Similar to the BiPy-BTA derivatives, the chiral and achiral BTA derivatives self-assemble into stable helical columnar structures as described in section 2.2. Mixing of the chiral 9a and achiral 9c in n-heptane resulted in the observation of the sergeants and soldiers effect, in which the cooperativity length is estimated to be at least 80 molecules.77 Temperaturedependent UV−vis and CD measurements of this 9a/9c mixed system revealed that the sergeants and soldiers principle is a rapid process in which the exchange between molecules and stacks of the sergeant 9a and soldier 9c takes place on the second time scale at room temperature and interconversion between the right- and left-handed helices is fast at this 13774


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Chart 10. Chiral and Achiral N,N′,N″-Benzene-1,3,5-tricarboxamide (BTA) Derivatives (66a−v)

assemble into stable helical columns,167 chiral and achiral porphyrin-appended BTAs self-assembled into very stable onehanded and racemic helical columnar stacks, respectively, in apolar solvents, even at a very low concentration (70%) suppressed the supramolecular polymerization, thereby leading to no chiral amplification.185 Feringa, van Esch, and co-workers applied the sergeants and soldiers effect to the enantioselective photochemical ringclosure of an achiral diarylethene derivative 89a(o) through the coassembly with the chiral 89b(o) (Figure 33). The achiral 89a(o) adopts an excess of either a P- or M-helical conformation in the coaggregates with the chiral 89b(o) in toluene due to the sergeants and soldiers effect. The photochemical reaction of a 1:1 mixture of 89a(o) and 89b(o) resulted in the formation of the ring-closed 89a(c) in 94% ee.186 A similar result was also found when a single enantiomer of the ring-closed 89a(c) was used as the sergeant instead of 89b(o).187 2.6.1.4. Majority Rule Effect. In the middle of the 2000s, the groups of Shikata,188 Meijer,189 and Aida and Fukushima127 individually reported the majority rule effect in the supramolecular helical systems. Majority rule experiments using enantiomeric mixtures of BiPy-BTAs (S)-38a and (R)-38b (Chart 5, see section 2.4) with varying molar ratios exhibited a nonlinear relationship between the anisotropic factor g and the ee (Figure 30B) in which the g value at 50% ee is comparable to that of the enantiopure BiPy-BTAs in n-octane. The nonlinearity and the maximum g value decrease with the increasing temperature due to deaggregation.189 The chiral BTAs ((S)-9a/ (R)-9b) of various ee values coassemble into supramolecular helical stacks, whose CD intensity nonlinearly increases with an increase in the enantiopurity of the BTA.188,190 Such a

assembles into supramolecular helical assemblies through a π−π stacking interaction in aqueous dimethyl sulfoxide (DMSO), but not into the helical nanotubes. The sergeants and soldiers effect was also observed for mixtures of the chiral 79 and achiral 80.179 Although the norbornene-appended chiral HBC 40f (Chart 6, see section 2.4) alone self-assembles to form only nanotubular or fibrous assemblies, nanocoils with a defined handedness were obtained from a mixture of 40d and 40f based on the sergeants and soldiers effect. In contrast, the coassembly of 40d and 40e (Chart 6, see section 2.4) produced a mixture of ill-defined structures and nanocoils with no preferred helix sense.126 The sergeants and soldiers principle is also operative for a variety of helical assemblies formed from mixtures of achiral and chiral molecules in nonpolar solvents. The achiral PDI 27C2 (Chart 3) and 20 mol % of chiral PDI 81 (Chart 15) coassemble to form H-type aggregates with a controlled helical sense, of which the CD intensity is almost saturated.180 Similarly, the coassembly of achiral and chiral coronene bisimides (82a and 82b, Chart 15) displays chiral amplification with saturation at 30−50% of the sergeant 82b.181 An achiral bis-phenylethynyl thiophene derivative functionalized with pyridyl biscarboxamides (83a, Chart 15) self-assembles into racemic helical fibers, as revealed by TEM and AFM measurements, of which the supramolecular chirality can be amplified by the coassembly with the sergeant molecule 83b to bias the helical arrangement.182 Interestingly, Hong and co-workers demonstrated a remarkable degree of chiral amplification using chiral and achiral units with different structures (84 and 85, Chart 15). An exclusive formation of one-handed helical fibers assembled from achiral 85 with only 1 mol % of the chiral 84 having alanine residues was found by SEM observations.183 Similarly, the mixing of the achiral diynoic phospholipid 86 with a small amount of the chiral diynoic phosphatidylcholine 87 resulted in the formation 13780


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Chart 17

nonlinearity was also observed for their OPE-based analogues ((S)-71b/(R)-71c) (Chart 13).191 In the chiral BTA system, the temperature-dependence on the nonlinearity was observed.190 Interestingly, the replacement of the amide bond by a thioamide in the chiral BTAs results in an increase in the degree of chiral amplification as compared to that in the (S)9a/(R)-9b mixed system.192

A remarkable majority rule effect was observed for the chiral dynamic nanotubes coassembled from enantiomeric mixtures of the bisurea monomers (S)-90a and (R)-90b (Chart 16) with various ee values in which only 10% ee is sufficient to induce all of the minor enantiomeric molecules to adopt the chiral twist of the majority enantiomer.193 Similar to the sergeants and soldiers effect using the desymmetrized BTAs, enantiomeric mixtures of various BTAs 13781


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Figure 35. (A) SEM image of a mixed gel of L-93 and D-5 that have different chain lengths and enantiomeric headgroups from each other. The gel contains P- and M-helical fibers formed from D-5 and L-93, respectively. (Reproduced with permission from ref 205. Copyright 1990 American Chemical Society.) (B) AFM image of a mixed (four components) hydrogel of Fmoc-L-Glu-OH, Fmoc-D-Glu-OH, L-Lys, and D-Lys. The gel contains both P- and M-helical fibers. (C and D) The enlarged AFM images of green (C) and violet (D) marked regions in (B). (Reproduced with permission from ref 212. Copyright 2011 The Royal Society of Chemistry.)

Figure 36. Schematic illustration of chiral self-sorting, depolymerization, and atropisomerization of chiral/achiral amide-functionalized zinc/copper tetraphenylporphyrin aggregates. (A) Chiral self-sorting in a mixture of 74-R-Zn and 74−S-Cu through their columnar self-assemblies. (B) Chiral memory of the helical columnar assemblies of achiral 74-A-Cu through the selective removal (depolymerization) of chiral 74-R-Zn from a 74-A-Cu/ 74-R-Zn mixed aggregate. (C) Helix inversion of the supramolecular helical chirality through the selective removal of 74-R-Zn from the “dilutedmajority-rule” system (74-A-Cu/74-R-Zn/74−S-Cu ternary assemblies). (Reproduced with permission from ref 202. Copyright 2011 American Chemical Society.)

Figure 37. Schematic illustration of supramolecular pathways and the formation of preferred-handed helical rosette nanotubes of 17b induced by a chiral promoter (L-Ala). (Reproduced with permission from ref 91. Copyright 2002 American Chemical Society.)

and the (S)-73b/(R)-73c mixed systems also showed a small degree of chiral amplification.169,194 The majority rule is also operative for large-sized supramolecular objects such as the HBC nanotubes obtained from the mixtures of (S)-40b and (R)-40c with varying molar ratios (Chart 6, see section 2.4). The CD profile of the HBC nanotubes formed from the HBC amphiphile of 20% ee is almost identical to that of the enantiopure form as a result of the long-range cooperativity in the self-assembling process.127

bearing only one chiral side chain with various ee values, (S)66d/(R)-66e, (S)-66f/(R)-66g, or (S)-66h/(R)-66i (Chart 10), also display obvious chiral amplifications. Among these systems, the degree of chiral amplification depends on the position of the asymmetric carbon (odd−even) rather than the distance between the asymmetric carbon and benzene core.159 In contrast, mixtures of the bulky BTAs (S)-66j/(R)-66l displayed a small majority rule effect due to the incorporation of limited amounts of monomers into aggregates of the nonpreferred helicity.160 In addition, both the (S)-71h/(R)-71i 13782


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Chart 18

enantiomeric mixtures of (S)-91a and (R)-91b with various ee values in n-dodecane resulted in a small degree of chiral amplification due to the preferential formation of homochiral assemblies rather than heterochiral coassemblies under this condition.197 Similarly, PDI derivatives bearing six enantiopure or racemic aliphatic chains ((R)-92a, (S)-92b, and rac-92c that contains 21 diastereomers, Figure 34) have also been found to self-assemble into helical columns in solvophobic solvents and in the bulk, but irrespective of their optical purities.198 Majority rule experiments with (R)-92a and (S)-92b and with (R)-92a or (S)-92b and rac-92c showed a moderate extent of chiral amplification in a n-butanol/MCH mixture. However, an asprepared thin film of enantiopure (R)-92a or (S)-92b showed no CD signal, due to the formation of a low-ordered columnar hexagonal crystalline phase (Φhk1) containing 41 distorded helical columns with off-centered stacking of PDI dimers (Figure 34A) and without either a preferred-handed helix sense or well-defined handedness. This Φhk1 phase can be transformed into high-order Φhk2-containing 21 perfect helical columns with centered stacking of PDI dimers (Figure 34B) by appropriate heating and annealing under kinetic control, resulting in a siginificant increase in the CD signal. Similarly, rac-92c and a 1:1 enantiomeric mixture of (R)-92a and (S)-92b also formed both the Φhk1 and Φhk2 phases. Interestingly, the Φhk2 of this optically inactive PDI consists of domains containing single-handed helical columns, as evidenced by thin-film microspot CD and optical polarized microscopic studies, whereas the Φhk1 did not show such a chiral domain formation (deracemization). As described in section 2.3, 33b coordinates to the chiral Pd(II) complex 34 to form a ladder-shaped helical coordination polymer with a preferred-handedness (Figure 17). When enantiomeric mixtures of the Pd(II) complex 34 with various ee values were used instead of the enantiopure one, the resulting coordination polymers did not exhibit a chiral amplification behavior.112 On the other hand, a preferredhanded helical conformation of its analogous polymer coassembled from 34 and achiral Pt(II) complex (Pt(DPPP)) can be induced by using optically active mandelic acid (35) through hydrogen bonding interactions (Figure 17, see section

As described in section 2.6.1.3, a slow process of chiral amplification was observed for the chiral nanorod formed from bis(merocyanine) dyes in the sergeants and soldiers experiments. A similar behavior was also observed in the majority rule experiments. The addition of MCH into mixtures of (R)-41b and (S)-41c (Figure 21, see section 2.4) at various molar ratios in THF resulted in the formation of helical nanorods as a kinetic product, which showed no chiral amplification. However, the degree of the chiral amplification increases with the increasing time as a result of transformation of the kinetically formed helical nanorods into thermodynamically equilibrated ones with a decrease in the helical pitch. In addition, the rate of this transition increases with an increase in the ee of the dye.195 Similarly, MCH solutions (200 μM) of ureidotriazinefunctionalized OPV (S)-13a and (R)-13b (Figure 10, see section 2.2) mixed together at 20 °C in different ratios result in the formation of metastable assemblies which did not exhibit chiral amplification. Interestingly, upon slow cooling (1 °C/ min) from the molecularly dissolved state, the majority rule effect is clearly observed in which the g value at 20% ee is almost identical to that at 100% ee. However, at a lower concentration (10 μM), a slower cooling rate (0.1 °C/min) is necessary to obtain the thermodynamically favored assemblies that contribute to the observed chiral amplification. This is because fast cooling results in a metastable aggregate under this condition, as concluded by a detailed analysis of the UV−vis spectra and fluorescence lifetime measurements.196 As described in section 2.2, the 3:1 hydrogen-bonded discotic complex (S)-23a3·22 (Chart 2, see section 2.2) selfassembles into helical stacks. A simple mixing of the enantiomerically pure assemblies of (S)-23a3·22 and (R)23b3·22 in MCH at various ratios resulted in no chiral amplification due to the slow monomer−aggregate exchange. In contrast, when the mixtures were prepared from the molecularly dissolved state, the majority-rule was operative, although ternary mixtures of (S)-23a, achiral 23c, and 22 did not display the sergeants and soldiers effect.96 A hat-shaped dendronized cyclotriveratrylene (CVT) bearing chiral aliphatic chains at the peripheries ((S)-91a or (R)-91b, Chart 16) self-assembles into helical columns in solvophobic solvents and in the bulk. Majority rule experiments using 13783


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Figure 38. Proposed structure of a 2:2 host−guest complex between 100a and (S)-101, and schematic illustration of the chiral induction in the helical stacks of 100a with chiral 101 guests. The P-/M-helix interconversion of the elongated stacks is prevented at lower temperature (≪ Te; elongation temperature). (Reproduced with permission from ref 214. Copyright 2007 Wiley-VCH.)

known about supramolecular polymeric systems in solution as well as the gel states. As described in section 2.1, self-assembly of a racemic mixture of artificial phospholipid 4 (Figure 3) resulted in the formation of both right- and left-handed helices, probably due to homochiral self-sorting.68 Fuhrhop and co-workers found that a mixture of N-alkyl gluconamide D-5 (Figure 4, see section 2.1) and L-93 (Chart 17) with different alkyl chain lengths initially underwent chain-length-induced homochiral selfsorting to form P- and M-helical fibers made of D-5 and L-93, respectively, as observed by SEM (Figure 35A). However, these helices were short-lived and transformed into flat plates consisting of a mixture of D-5 and L-93.205 Ishida and Aida demonstrated the unique optical resolution of the enantioenriched p-xylylene-bridged bis(cyclic dipeptide) 94 based on homochiral self-sorting during supramolecular polymerization driven by intermolecular hydrogen bonding. By taking advantage of the homochiral self-sorting, the major enantiomer of L- or D-94 in aprotic solvents self-assembles into

2.3) in which its helix-sense excess nonlinearly increases with an increase in the optical purity of 35.199 In contrast to the above-mentioned binary systems, the mixing of the soldier BTA 66c (Chart 10) with 20 mol % of the enantiomeric mixtures of sergeant BTAs (S)-9a and (R)-9b at various ee values (diluted majority rule experiment)200,201 resulted in the formation of supramolecular helical stacks composed of these three components in cyclohexane. In this ternary system, both the majority rule and sergeants and soldiers principles are operative.158 Similar results were obtained for the 9c/(S)-9a/(R)-9b190 and 74-A-Cu/74−SCu/74-R-Zn ternary systems.202 2.6.2. Chiral Self-Sorting. Self-assembly of enantiomeric mixtures of small molecules through high-fidelity selfrecognition and self-discrimination processes results in the formation of homochiral and heterochiral aggregates, respectively. This “chiral self-sorting” behavior has been well studied for discrete supramolecular systems.203,204 However, little is 13784


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Chart 19

Figure 39. Typical SEM images of the helical assemblies of TPPS (108) complexed with (R,R)- (A) and (S,S)-109 (B). The molar ratio of 108/109 is 1/4. (Reproduced with permission from ref 219. Copyright 2013 The Royal Society of Chemistry.)

exclusion chromatography (SEC).207 This methodology can also be applied to the optical resolutions of the enantioenriched C3- and C4-symmetric bowl-shaped macrocycles (S)-95a/(R)95a (n = 0) and (S)-95b/(R)-95b (n = 1) (Chart 17),

supramolecular polymers with a higher molecular weight (molecular mass; terminology recommended by IUPAC)206 than that of the minor enantiomer. Thus, the difference in their molecular weights enables isolation of enantiomers by size13785


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During the course of the majority-rule study that showed a linear relationship between the CD intensity and the % ee, Pantoş and co-workers found a high degree of homochiral selfsorting in enantiomeric mixtures of chiral NDI 25, as indicative of the presence of the supramolecular helical nanotubes that form an inclusion complex with C60 (see also Figure 14 in section 2.2). In contrast, it is supposed that a heterochiral assembly of the NDI would be unable to take a helical conformation due to the opposite directionality created at the chiral center.178 Such a linearity was also observed for the enantiomeric mixtures of pyrrolidine-based molecules (S,S)-97 and (R,R)-97 with various molar ratios. An AFM study revealed the presence of right- and left-handed helical fibers obtained from a mixture of (S,S)-97 and (R,R)-97 in cyclohexane.210 Similarly, mixtures of the pseudoenantiomers, 74−S-Cu and 74-R-Zn, self-sorted into homochiral helical stacks (Figure 36A), as indicated by the majority rule experiments. This high extent of chiral self-sorting is ascribed to the high stereocenter loading of 12 methyl groups onto the monomers. Interestingly, the self-sorted homochiral stacks of 74-R-Zn can be selectively depolymerized by coordination of the Lewis base quinuclidine (QND) to the zinc atom that prevents stacking of the 74-R-Zn molecules (Figure 36A).202 When two achiral NDI derivatives are connected by enantiopure (1R,2R)- or (1S,2S)-trans-1,2-bis(amido)-cyclohexane ((1R,2R)- and (1S,2S)-98a, Chart 17), their enantio-

Figure 40. Schematic illustration of ATP-triggered morphological reorganization of the 112 assembly. (Reproduced with permission from ref 222. Copyright 2014 The Royal Society of Chemistry.)

respectively, that undergo supramolecular polymerization with a homochiral self-sorting through van der Waals and hydrogen bonding interactions.208 Dendron rodcoil molecules (S)-96 and (R)-96 (Chart 17) bearing a chiral aliphatic tail self-assemble into right- and lefthanded supramolecular helical fibers, respectively, through intermolecular hydrogen bonding between the dendritic headgroups in addition to π−π stacking interactions between the biphenyl rodlike segments in acetonitrile, as revealed by AFM studies. Interestingly, the self-assembly of rac-96 resulted in the formation of both the right- and left-handed helices, whose helical pitch and height are almost identical to those of the enantiopure forms, indicating that (S)-96 and (R)-96 in the racemic mixture self-sort into (S)- and (R)-nanostructures during the self-assembly.209

Figure 41. Schematic illustration of the formation of helical metal−organic nanotubes coassembled from 113 and AgBF4 in the presence of (+)- or (−)-114, and the possible helical and propeller-chiral motifs. (Reproduced with permission from ref 224. Copyright 2015 American Chemical Society.) 13786


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Chart 20. Achiral Porphyrin Derivatives (115−119)

Figure 42. Chemical structures of dendritic zinc porphyrin 120 and its J-aggregated form of hydrogen-bonded supramolecular polymer (nanofiber).

by intermolecular hydrogen bonding in an MCH/1,1,2,2tetrachloroethane mixture.211 Interestingly, (1R,2R)-98a, bearing electron-accepting NDI derivaties, coassembled with (1R,2R)-98b, carrying electron-donating 1,5-dialkoxynathalene drivatives, to form alternately organized p-n stacks with the appearance of a strong charge-transfer (CT) band, whereas a mixture of (1R,2R)-98a and (1S,2S)-98b, with the opposite chirality, did not show such a significant CT band. An equimolar mixture of N-Fmoc-protected rac-glutamic acid (Fmoc-DL-Glu-OH) (Fmoc: N-fluorenyl-9-methoxycarbonyl) (Chart 17) and the oppositely charged rac-lysine (Lys) also self-sorts to form right- and left-handed helical nanofibers made of Fmoc-D-Glu-OH/D-Lys and Fmoc-L-Glu-OH/L-Lys, respectively, as suggested by AFM studies (Figure 35B−D).212 A racemic mixture of the hat-shaped CVT (S)-91a and (R)91b (Chart 16) undergoes chiral self-sorting to form enantiomerically pure columns in the bulk state, as indicated by the differential scanning calorimetry (DSC) and fiber XRD

Figure 43. Schematic illustration of chiral induction of 115·117 coaggregates in the presence of a chiral template and memory of the induced supramolecular chirality. The chirality-imprinted 115·117 coaggregate acts as the chiral template for further chiral propagation of achiral 115 and 117. (Reproduced with permission from ref 242. Copyright 2002 American Chemical Society.)

meric mixture underwent chiral self-sorting to form homochiral helical assemblies during supramolecular polymerization driven 13787


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Chart 21

Figure 44. Schematic illustration of the right-handed helical nanotube formation of 123Py[(S)‑Pt] (60% de) and memory of its supramolecular helicity, which is retained after replacement of ((S)-BINAP)Pt(II) with its enantiomer. (Reproduced with permission from ref 245. Copyright 2013 American Chemical Society.)

Figure 45. Schematic illustration of chiral symmetry breaking in hierarchically self-assembled helices composed of an achiral trisubstituted benzene derivative 124 and chiral memory of the induced supramolecular helicity by the “add−remove chiral solvents” procedure. (Reproduced with permission from ref 246. Copyright 2015 American Chemical Society.)

(Figure 12), as described in section 2.2. At a high concentration of 17b in methanol, the addition of a chiral amino acid promoter (L-Ala), that binds to the crown ether moiety of 17b, to the racemic HRNs resulted in a rapid transformation of the racemic to the preferred-handed HRNs (Figure 37, fast pathway). In contrast, 17b mainly existed in a nonassembled state at a low concentration. However, the addition of L-Ala under this condition led to the slow formation of the preferredhanded helical nanotubes (Figure 37, slow pathway) in which almost all of the crown-ether binding sites should be occupied with a promoter for the complete formation of chiral nanotubes (all-or-none response). Interestingly, the nonracemic Ala also induced HRNs with a higher helix-sense excess than that expected from the ee of Ala, indicating that a significant chiral amplification (majority rule effect) takes place during the supramolecular helical assembly, which has the potential to sense the chirality of amino acids.91

results, although it is imperfect in solution (see section 2.6.1.4).197 In contrast to the above-memtioned chiral self-sorting based on the point chirality, diastereomeric mixtures of atropisomeric twin N-annulated perylenecarboxamides bearing the same chiral side chains ((P,S,S)- and (M,S,S)-99, Chart 17) with various molar ratios were found to self-sort to form homochiral aggregates with respect to the axial chirality independent of the point chirality at the side chains.213 2.6.3. Helix-Sense Induction. 2.6.3.1. Helix-Sense Induction by External Chiral Guests. As described in section 2.1, Oda, Huc, and co-workers demonstrated the handedness control of supramolecular helical assemblies of an achiral gemini surfactant 7 through noncovalent electrostatic interactions with optically active tartrate.73,74 The achiral G∧C-based module 17b, bearing a crown ether moiety, hierarchically self-assembles into the racemic HRNs 13788


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two ammonium groups of the chiral guest molecule bridge the two achiral 104 molecules, leading to induction and further stabilization of either right- or left-handed helical assemblies.216 A similar strategy has been further applied to other supramolecular helical assemblies. Li and co-workers reported a preferred-handed helicity induction of supramolecular aggregates formed from the positively charged achiral pyrene derivative 105 through noncovalent interactions with the NBoc-protected L-tryptophan sodium salt (L-106, Chart 19, Boc: tert-butoxycarbonyl) as a negatively charged chiral guest in aqueous solution.217 Similarly, an achiral cationic PDI (107) self-assembles upon charged-complexation with an anionic chiral phosphate surfactant into preferred-handed helical aggregates in chlorinated solvents.218 An acid−base chiral interaction is also useful to control the supramolecular helical chirality. A mixing of meso-tetrakis(4sulfonatophenyl)porphyrin (TPPS, 108) (Chart 19) with (1S,2S)-(−)- or (1R,2R)-(+)-1,2-diaminocyclohexane (109) (Chart 19) in a chloroform/methanol mixture resulted in the formation of mirror-imaged helically twisted nanorods with a controlled handedness (Figure 39), while long nanofibers were formed in the absence of the chiral diamine.219 An achiral NDI-based bola-amphiphile functionalized with two dipicolylethylenediamine moieties coordinated to Zn(II) ions (110, Chart 19) has been found to form supramolecular helical assemblies with a preferred-handedness in aqueous solution in the presence of divalent adenosine diphosphate (ADP) as a chiral guest. The ADP molecules bind to the two Zn(II)-coordination sites to stabilize the helical assembly. Thus, no assembly of 110 was observed in the absence of ADP. Interestingly, when adenosine triphosphate (ATP) was used instead of ADP, the opposite-handed supramolecular assemblies were induced. A similar trend was also observed for the amphiphile 111 (Chart 19) bearing one Zn(II)-coordination site in 70% aqueous HEPES buffer in THF in the presence of these phosphate molecules. In addition, the induced helicity of the bola-amphiphile 111 triggered by ADP can be inverted by the further addition of ATP.220 By taking advantage of this ATP/ADP-triggered helicity inversion, enzymatic hydrolysis of ATP to ADP resulted in the helicity inversion of the helical stacks of the bola-amphiphile 111.221 On the other hand, an achiral PDI-based bola-amphiphile 112 (Chart 19) self-assembles into 2D nonhelical nanosheets through H-type cofacial π−π stacking (H1-state) in 90% aqueous HEPES buffer in acetonitrile. The addition of less than

Figure 46. Schematic illustration of a preferred-handed helicity induction in supramolecular columnar assemblies of 66c bearing a sorbyl residue in the presence of chiral 9a and locking and memory of the induced supramolecular chirality after photochemical covalent fixation and subsequent removal of the chiral template 9a. (Reproduced with permission from ref 157. Copyright 2005 WileyVCH.)

Meijer, Schenning, and co-workers have also demonstrated induction of a preferred-handed helicity in supramolecular helical stacks formed from the complementary hydrogenbonded dimer of the achiral ureidotriazine-functionalized OPV 100a (Chart 18, n = 2) through noncovalent hydrogen bonding with enantiopure citronellic acid (S)-101 or (R)-101 (Chart 18) in MCH in a specific temperature range (Figure 38). In this system, temperature control is crucial for this helix-sense induction due to the kinetic stability of the helical stacks similar to that of the chiral analogue (S)-13a or (R)-13b already described above. Interestingly, sergeants and soldiers and majority rule effects were observed for the helical stacks of the achiral 100a when mixtures of achiral 102 and chiral (S)101 and of (S)-101 and (R)-101 with various molar ratios were used as the additives, respectively.214 Such a helicity induction was also observed for supramolecular assemblies of the shorter analogue 100b (Chart 18, n = 1) in the presence of dibenzoyl tartaric acid (L- or D-103, Chart 18), which strongly binds to the 100b assemblies.215 Shinkai and co-workers succeeded in control of the helix sense in helical aggregates formed from an achiral oligothiophene derivative 104 (Chart 19) bearing two crown ethers at both ends by complexation with chiral 1,2-bisammonium guests through crown−ammonium interactions. In this complex, the

Figure 47. Schematic illustration of reversible switching of the helical sense of helically assembled achiral 126 by r- or l-CP light irradiation and memory of the induced supramolecular helicity through photopolymerization of diacetylene units by r- or l-CPUL irradiation. (Reproduced with permission from ref 247. Copyright 2015 Nature Publishing Group.) 13789


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Chart 22

Figure 48. Schematic illustration of catalytic and kinetic/thermodynamic controls of W/M-chiromer formations and interconversion in methanol (left) and water (right). (Reproduced with permission from ref 250. Copyright 2007 American Chemical Society.)

preferred-handed H-type helical stack (H2-state) in an allosteric manner (Figure 40).222 An achiral ferrocene-cored tetratopic pyridyl ligand (113, Figure 41) coassembled with AgBF4 to form discrete metal− organic nanotubes through π−π stacking and Ag(I)−Ag(I) metallophilic interactions via the formation of C10-symmetric

0.4 equiv of ATP did not induce a supramolecular helical structure of 112, even though the ATP molecules preferentially bind to the Zn(II)-coordination site on one side of the assembled nanosheets (Figure 40). However, the further addition of ATP or achiral pyrophosphate resulted in a morphological transition from the 2D nanosheets into a 13790


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Interestingly, the handedness of the supramolecular Jaggregates of 116 has been efficiently controlled by the relative directions of rotation and magnetically tuned effective gravity, but the magnetic orientation of the aggregates is also essential. In this case, applying these physical forces only at the beginning of the aggregation process is sufficient to achieve chiral selection.233 Aida and co-workers found that the formation of a chiroptically active film prepared by spin coating from a benzene solution of a zinc porphyrin dendrimer 120 (Figure 42) self-assembles into a 2D J-aggregated polymer through πstacking interactions in addition to intermolecular hydrogen bonding between its carboxylic acid side groups (Figure 42). Interestingly, the sense of this chiroptical activity can be controlled by the spinning direction.234 Similarly, macroscopic helical alignment of nanofibers formed from this supramolecular polymer in benzene was induced upon rotary stirring. The sense of this helical alignment can be controlled by the vortex direction. In addition, the induced sense can be detected by CD spectroscopy, although the resulting CD spectra contain the LD contribution.235 A similar macroscopic orientation induced by vortex flow was observed for the aggregates of an achiral PDI 27-C2 (Chart 3).180 Meskers, Schenning, Meijer, and co-workers also reported the effects of convective flow, shaking, and stirring of a fully transparent and nonviscous solution of supramolecular fibers formed from achiral 100b (Chart 18, n = 1) on the CD signals. All of these physical forces caused LD and/or linear birefringence effects that lead to an artificial contribution in the CD spectra of the 100b assembly, as a result of the partial orientation of the self-assembled fibers in the cuvette. The shape, sign, and intensity of the CD spectra are sensitive to the type of flow as well as the angle between the two aligned films of 100b with respect to the incident light beam of the CD apparatus. Based on these observations, the authors suggested that the chirality at the molecular level seems unaffected by the stirring.236 2.6.4. Memory of Helical Chirality. The chiral memory effect is known as the relatively long-term storage of an induced one-handed chirality in supramolecular architectures constructed from achiral building blocks after removal of the chiral guest or template molecules. In order to realize the chiral memory effect, a kinetically trapped chiral conformation must be stable during the removal of the chiral guests. Although such a chiral memory has been successfully achieved in several discrete supramolecular237−240 and covalent polymeric systems through noncovalent bonding interactions,4 this section will focus on the chiral memory effect in the supramolecular polymeric system. Purrello and co-workers demonstrated the chiral memory effect in porphyrin aggregates stabilized by electrostatic and π−π interactions in an aqueous solution. The mixing of the protonated form of tetra-anionic 115 with a kinetically labile chiral binary complex formed from tetra-cationic 117 (Chart 20) in the presence of α-helical poly-L-glutamate resulted in the formation of chiral ternary complexes that show a remarkable kinetic stability. Interestingly, the original induced CD signal of the complexes remained almost unchanged even 5 days after the addition of a 4-fold excess of poly-D-glutamate.241 In addition, a similar induced CD was also observed when the oppositely charged 115 and 117 were individually added to an aqueous solution of large-sized aggregates of L- or Dphenylalanine (chiral template, Figure 43). After the removal

Figure 49. Schematic illustration of the formation of exclusively metastable P-helical aggregates of 13a (middle) through helix inversion of the thermodynamically stable M-helical aggregates formed via noncovalent interaction with chiral guest 103 (left). After removal of the chiral guest, the metastable P-helical aggregates (right) gradually transform into the thermodynamically stable M-helical aggregates when the temperature is raised from 273 to 298 K. (Reproduced with permission from ref 257. Copyright 2012 Nature Publishing Group.)

nanorings which stacked in a helical fashion.223 When this coassembly was carried out in the presence of the chiral (+)- or (−)-menthylsulfate (114−) tetrabutyl ammonium salt, a preferred-handed helical nanotube with a controlled propeller chirality was formed as a result of noncovalent electrostatic interactions between the chiral 114− and the coordinated Ag(I) ions (Figure 41). Interestingly, both helical- and propellerchiral structures of the nanotubes responded nonlinearly to the ee of 114−, demonstrating a typical majority rule effect with no mutual stereochemical communication.224 2.6.3.2. Helix-Sense Induction by Physical Forces. In 1958, McRae and Kasha reported an induced CD signal derived from achiral cyanine dyes in the J-aggregate state even in the absence of optically active molecules.225 Later, Honda and Hada reported a similar induced CD due to J-aggregated dyes, whose Cotton effect sign was largely reflected by the direction of stirring.226 However, it has been pointed out that this detected CD was caused by linear dichroism (LD).227 Nevertheless, De Rossi and co-workers concluded that induced CDs generated from the spontaneous formation of J-aggregates of an achiral benzimidocyanine derivative was not due to LD or light scattering contributions.228 Ohno and co-workers reported that the clockwise or counterclockwise stirring of acidified aqueous media of the diprotonated tetrasodium meso-tetrakis(4-sulfonatophenyl)porphyrin (115, Chart 20) resulted in the formation of CDactive J-aggregates without a significant contribution of LD. In addition, a similar induced CD was also observed when Jaggregates of 115 were grown in the presence of D- or L-tartaric acid.229 The effect of the vortex direction, caused by stirring or rotary evaporation, on the chirality induction in the J-aggregates of 115 was further statistically observed by Ribó and coworkers.230,231 In addition, a spontaneous symmetry-breaking of the J-aggregates of 115 was also reported without the vortex direction in which the statistical distribution did not show any chirality dominance.230 The same group also studied their morphologies by AFM. The AFM images of the J-aggregates obtained from both a concentrated solution of 115 without the vortex direction and rotary evaporation of a diluted solution did not show a helical morphology, whereas its analogue 116 (Chart 20) bearing three sulfonate groups self-assembled into helical ribbons.232 13791


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Figure 50. Time-dependent morphology transformation of the P-tubular assembly of 7 complexed with an enantiopure L-tartrate ((A) % ee =100) upon the addition of D-tartrate with different ee values observed by TEM ((B) % ee =0; (C) % ee = −33; (D) % ee = −60; (E) % ee = −90). (Reproduced with permission from ref 259. Copyright 2015 The Royal Society of Chemistry.)

of the phenylalanine template, the coaggregates of 115 and 117 were still optically active. Furthermore, self-propagation of the induced chirality took place when equimolar amounts of 115 and 117 were individually added to a solution of the chiralitymemorized assemblies (Figure 43).242 By taking advantage of this unique self-propagation behavior, the memorized chirality in the coaggregates of 119 and the protonated state of 118 has been reversibly switched “on and off” through reassembly and disassembly processes by adjusting the pH value when the chiral seeds were not completely disassembled.243 As described in section 2.6.1.3, the achiral 74-A-Cu and a small amount of chiral 74-R-Zn coassemble into a preferredhanded helical stack with a conformational inertness. The chiral auxiliary 74-R-Zn can be selectively extracted from the coassembled stacks by treatment with QND, leading to the chirality-memorized helical stacks composed of only achiral 74A-Cu (Figure 36B). Although an induced CD signal derived from the memorized chirality decreased with the increasing

temperature, a partial recovery of the CD signal was observed after cooling. This is because the remaining chiral assemblies likely act as chiral seeds for further enantiospecific reassembly of 74-A-Cu upon cooling.173 A similar behavior was also observed for the memorized chirality in the helical stacks of the achiral OPV 100b, whose preferred-handed helicity was originally induced by L- or D-103.215 Jiang and co-workers demonstrated a unique procedure to construct chiral supramolecular J-type assemblies through an in situ transformation from perylene-3,4,9,10-tetracarboxylate (121, Chart 21) to perylene dianhydride (122, Chart 21). A water-soluble 121 in an acidic cetyltrimethylammonium bromide (CTAB, Chart 21) micelle solution in the presence of L- or D-tartaric acid gradually transformed into 122, resulting in the formation of the 122 J-aggregates that displayed an induced CD signal. In addition, the majority rule is operative in this system when a tartaric acid mixture of various ee values was 13792


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Figure 51. (A) TEM image of 135 from a 0.01 wt % aqueous solution prepared at 60 °C (scale bar, 100 nm). The inset shows the TEM image prepared at room temperature. (B) Temperature-dependent CD spectra of 135 (0.01 wt %) in aqueous solution. (C, D) AFM phase images of 2D self-assembled left-handed (C) and right-handed 135 (D) on mica and HOPG, respectively. Schematic representations of the expanded and contracted tubules are also shown. (E) Schematic representation of the regulation of C60−C60 interactions within the tubular cavities when the tubule contains 0.4 equiv of C60. (Reproduced with permission from ref 260. Copyright 2012 American Association for the Advancement of Science.)

Figure 52. Processes for reversible helicity inversion of supramolecular helical aggregates of open form 89b(o) and closed form 89b(c) via photochemical ring closing/opening and disassembly/reassembly pathways. Open form 89b(o) in its aggregate state (A) dominantly adopts a Phelical conformation, which can be diastereoselectively transformed into closed form (R,R)-89b(c). Heat is required to irreversibly transform the resulting metastable P-helical aggregate (A′) into a stable M-helical aggregate (B′). Photochemical ring-opening/closing reactions at each state are fully reversible with the retention of configuration.

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used. After the removal of the tartaric acid, the J-aggregates of 122 with CTAB exhibited the chiral memory effect.244 Different from the aforementioned chiral memory system, Fukushima, Aida, and co-workers utilized a detachable chiral complex ((S)-BINAP)Pt(II) that coordinates to two pyridyl side chains of an HBC derivative (123Py[(S)‑Pt], Figure 44). The chiral 123Py[(S)‑Pt] self-assembles into preferred-handed helical nanotubes or nanocoils depending on the solvents. The diastereomeric excess (de) of the nanocoils was determined to be 60% ((P)-helix rich) by SEM observation. The ((S)BINAP)Pt(II) pendants were able to be selectively detached from (P)-123Py[(S)‑Pt] by treatment with ethylenediamine, resulting in metal-free nanotubes ((P)-123Py) whose optical activity remained upon heating while maintaining its CD spectral profile. The treatment of (P)-123Py with ((R)BINAP)Pt(II) bearing an opposite absolute configuration to that of the original one resulted in no helix inversion (Figure 44).245 As described in section 2.6.3.2, physical forces have been used to induce a preferred-handed helcal sense of supramolecular helical assemblies of achiral molecules. A C3symmetric achiral benzene derivative 124 bearing methyl cinnamate groups was found to self-assemble into preferredhanded helical nanofibers in cyclohexane even without any physical forces and chiral additives, resulting in the formation of an optically active organogel as a result of a supramolecular symmetry breaking (Figure 45).246 The stocastically appeared preferential helicity could be controlled by using a small amount of enantiopure terpinen-4-ol ((R)- or (S)-125) as a cosolvent. Furthermore, a preferred-handed helix sense of selfassembled 124 induced by (R)- or (S)-125 was memorized in the gel state after complete removal of the chiral cosolvent (Figure 45). A kinetically labile chiral helical assembly is rapidly racemized when chiral auxiliaries are removed from the assembly. However, such a kinetically labile helically assembled structure can be memorized by the 1,4-photopolymerization of the sorbyl residue of each achiral BTA derivative 66c in the preferredhanded helical stacks induced by a small amount of chiral 9a under UV irradiation. The resulting optically active linear polymer folds into a supramolecular helical stack while maintaining its original preferred-handedness in apolar solvents even after removal of the chiral template 9a through the

Figure 53. (A) Schematic illustration of reversible helicity inversion of supramolecular helical assembly of 136 via E/Z photoisomerization of azobenzene moieties and a disassembly/reassembly process. Δ = heating; * = cooling. (B and C) SEM images of before (B) and after (C) photoisomerization of the self-assembly of (S)-136. (Reproduced with permission from ref 263. Copyright 2012 Wiley-VCH.)

Figure 54. (A) Reversible photoisomerization between helicenes M-137 and P-137′. (B and C) TEM images of bundled helical superstructures of xerogels formed from M-137 (B) and P-137′ (C). (Reproduced with permission from ref 264. Copyright 2013 American Chemical Society.) 13794


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Figure 55. Chemical and molecular structures of chiral stilbene dimer 138 (A) and its corresponding [2 + 2] photocycloaddition product 139 (B). (C) Schematic illustration of the proposed hierarchical self-assembly process of 138 and the coassembly process of 138 and 139. (Reproduced with permission from ref 265. Copyright 2015 Nature Publishing Group.)

Chart 23

Figure 56. (A) SEM and (B) TEM images of helical silica obtained by sol−gel transcription in right-handed helical assemblies of cationic 140 with neutral (S,S)-8 (1:1, wt %) before (A) and after calcination (B). (Reproduced with permission from ref 266. Copyright 2000 American Chemical Society.) (C) SEM image of the spiral silica obtained from an equimolar mixture of cholesterol-appended aza-crown ether 141 and AgNO3 before calcination. (Reproduced with permission from ref 267. Copyright 2000 Wiley-VCH.) (D) TEM image of helical ribbon silica obtained from crown ether-appended organogelator 142 with acetic acid after calcination. (Reproduced with permission from ref 268. Copyright 2001 American Chemical Society.)

A similar strategy has been applied to the covalent fixation of the one-handed helical columnar structure formed from achiral triazine triamides 72c bearing six terminal olefinic groups in the presence of chiral 72a (Chart 14). Interestingly, the ring-closing metathesis of the olefinic groups in the coassembly of 72c with 10 mol % of 72a resulted in the perfect fixation of the helical structure, whose CD signal did not change even after removal

unfolding state (Figure 46). This unusual chiral memory effect could be considered to be due to the asymmetric sorbyl mainchain that may be chirally imprinted during the photopolymerization of the helically assembled precursors.157 13795


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Figure 57. (A) SEM image of chiral mesoporous silica obtained using a mixture of 144a and 144b as a chiral source and schematic illustrations of a structural model of the left-handed chiral mesoporous silica and its cross-section. (Reproduced with permission from ref 272. Copyright 2004 Nature Publishing Group.) (B) Typical SEM image of 100% right-handed chiral mesoporous silica obtained using 144a and L-Arg as the chiral sources. (Reproduced with permission from ref 273. Copyright 2011 American Chemical Society.) (C) Field emission SEM (top) and TEM (bottom) images of left-handed multiple helical mesoporous 1,4-phenylene-silica nanofibers obtained using helical assemblies of 145 as a chiral template. (Reproduced with permission from ref 274. Copyright 2009 American Chemical Society.)

formation of a preferred-handed supramolecular helical assembly whose helix sense induced by r- or l-CP light irradiation could be reversibly switched by CP light irradiation with the opposite rotational direction. Interestingly, such switching did not take place after photopolymerization of the diacetylene units in the preferred-handed helical assembly of 126 by irradiation of ultraviolet CP light (CPUL) with the same rotational direction as the CP light used for the helicity induction of 126, as a result of the covalent fixation of the supramolecular helical structure. 2.7. Helix Inversion

As described in the previous section, an excess of a one-handed helix sense of the dynamically racemic supramolecular helical assemblies formed from achiral small molecules can be induced by specific noncovalent interactions with optically active molecules. In addition to this unique helix-sense control, inversion of the preferred-handedness of such dynamic helices regulated by external stimuli, such as the solvent polarity and additives that may alter the chiral environment around the helices, is also an intriguing phenomenon, as can be seen in biopolymers as well as synthetic helical polymers (see section 5.1). In 1969, Tachibana and Kambara found that the lithium salt of D- or L-1 (Figure 1, section 2.1) and its carboxylic acid form self-assembled into twisted fibers with an opposite handedness to each other.248 A detailed study by the same group revealed that the preferred helix sense of the fibers is sensitive to the alkali-metal counter cations and the alkyl chain lengths of the naliphatic alcohol solvents.249 As described in section 2.2, the G∧C-based module 17a bearing the lysine residue hierarchically self-assembled into the HRNs with a controlled supramolecular chirality in both water and methanol, as evidenced by the CD activity.90 The HRNs formed from its analogue 127 (Chart 22) also displayed a lefthanded supramolecular chirality in methanol (M-chiromer, Figure 48). Interestingly, the initially generated supramolecular chirality in methanol undergoes inversion upon the addition of very small amounts of water (99:99 mol % P-137′ and 104. In contrast, self-assembled microfibers of 168a with the CT complexation, obtained from the vapor diffusion of methanol into a concentrated THF solution of 168a (1.2 mM), exhibited almost no photocurrent generation.306 Similar to the HBC-TNF coaxial nanotube, an HBC-C60 covalent donor−acceptor dyad (168b, Chart 28) also selfassembles into a nanotube, whose surface is fully covered with a C60 monolayer.307 Interestingly, this HBC-C60 nanotube with a coaxial donor/acceptor heterojunction exhibits an ambipolar character in the field transistor output and displayed a photovoltaic response upon light illumination. In addition, the intratubular hole mobility of an HBC nanotube coassembled from 168a and 10 mol % of 168b is comparable to the intersheet mobility in graphite. In contrast to such a coaxial heterojunction, Fukushima, Aida, and co-workers further succeeded in the construction of a semiconducting supramolecular linear heterojunction composed of two types of HBC-based nanotube segments by bottom-up supramolecular approaches using a morphologically stable HBC-based nanotube as a seed and an electron-deficient HBC as a second nanotubular component that can electronically adhere to the seed termini.308 An amphiphilic HBC derivative (168c, Chart 28) has two bipyridine (bpy) units in order to stabilize the HBC nanotube structure by wrapping with a metal coordination network and self-assembles to form bpy-wrapped nanotubes (NT168c, Figure 63B, first step), which were heavily bundled. A subsequent coordination of the bpy units on the surface of NT168c to Cu(II) ions resulted in the dissociation of the bundled nanotubes into the dispersed ones after sonication due to an electrostatic repulsion between the positively charged surfaces of the Cu(II)-coordinated nanotubes (NT168c·Cu, Figure 63B). Interestingly, the self-assembly of a fluorinated HBC derivative 168d in an acetone dispersion of the morphologically stabilized NT168c·Cu as the seed produced block-NT168c·Cu/NT168d with a linear heterojunction (Figure 63B, second step), as confirmed by its TEM image (Figure 63C). This is due to the preferential assembly of the electron-deficient HBC 168d on the nanotubular facets of the seed NT168c·Cu, so that multiblock heterojunction nanotubes,

hydrogen bonding interactions between the phthalhydrazide moieties in nonpolar solvents, such as chloroform and toluene, whereas no assembly took place in methanol. The resulting trimer further assembled into well-developed chiral fibers, as revealed by SEM and AFM measurements. Interestingly, the glum value at 476 nm (|glum| = 0.035) for fibrous assemblies of 163 in chloroform is greater than that of the monomeric state in methanol (|glum| = 0.021).300 An aggregation-induced enhancement of the glum value has also been observed for an (S)-1,1′-binaphtyl-linked bis-PDI derivative (164, Chart 27), although the aggregated particles would not have a precisely organized structure.301 In contrast, its derivative containing the (1R,2R)-1,2-cyclohexane diamide linker (165, Chart 27) self-assembled into a right-handed twisted fiber in MCH, where the PDI moieties are in a P-helical arrangement, as suggested by the CD measurement. Interestingly, the signs of its first Cotton effect and CPL signal in MCH are opposite to those of the monomeric state in chloroform. Moreover, the glum value (|glum| = 0.025) of the helical assembly of 165 is ca. 32 times greater than that of the monomeric state.302 As described in section 2.6.1.2, the OPE-based trisamides (71b and 71c, Chart 13) bearing the optically active alkyl chains were found to self-assemble into the helical columnar stacks with a controlled handedness in MCH,170 whose helix senses were determined by the CD measurements in combination with the theoretical simulations of their CD spectra.303 Interestingly, the right-handed helical assembly of 71b in MCH exhibits the CPL signal around 408 nm with the glum value of +0.028, whereas no CPL was observed for the molecularly dissolved 71b in chloroform. The photoisomerization-induced inversion of the CPL signal has been reported for the helical assemblies formed from the azobenzene-appended OPE 136 (Figure 53), which displayed the helix-sense inversion triggered by a cis−trans isomerization of the azobenzene moieties as described in section 2.7. The right-handed helical assembly of 136 before UV irradiation exhibited a positive CPL emission with a glum value of +0.008 at 503 nm, whereas the opposite, negative CPL with a decrease in the glum value (−0.002 at 503 nm) appeared after the photoisomerization.263 Haino and co-workers demonstrated a helical-assembly induced CPL emission using a chiral tris(phenylisoxazolyl) benzene bearing a PDI moiety (166, Chart 27).304 This molecule did not form an aggregate in chloroform, whereas it self-assembled into helical stacks with a controlled helix sense in decaline, as indicated by its CD signal. The 1H NMR studies suggest that the peripheral PDI moieties stack into a helical array, leading to an induction of the CPL emission with the | glum| value of 0.007 in decaline.304 Similarly, a chiral silole derivative bearing two sugar moieties (167, Chart 27) also exhibits a helical-assembly induced CPL emission (|glum| = 0.32) together with a remarkable enhancement of the fluorescence quantum efficiency.305 2.9.4. Miscellaneous Applications. Precisely designed πconjugated small molecules can self-assemble to form supramolecular fibers or nanotubes with well-defined alignments of the chromophores. As described in section 2.2, a 1:2 hydrogenbonded triad of the electron donor−acceptor−donor 21−20− 21 complex (Chart 2, see section 2.2) self-assembles into supramolecular J-aggregated helical fibers.95 The resulting fibers containing p-type and n-type molecules were used to prepare a photovoltaic device. However, the diode behavior with an open 13807


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Figure 65. Structural motifs of π-conjugated aromatic foldamers and schematic illustration of a preferred-handed helix induction in foldamers through noncovalent chiral interactions with optically active guests.

such as NT168c·Cu−NT168d−NT168c·Cu, were not produced. Quite interestingly, the two graphite-like nanotubular segments in the block-NT168c·Cu/NT168d efficiently communicate with one another by excitation energy transfer over the heterojunction interface. Lee and co-workers have demonstrated that a unique open− closed switching of tubular pores assembled from a disc-shaped chiral amphiphile 169 can be applied to reversibly control the catalytic activity for a dehydrative cyclization reaction of adenosine monophosphate (AMP) to produce cyclic AMP (cAMP) (Figure 64).309 The initially formed fibrils of 169 bearing a chiral oligoether dendron laterally assembled into tubular structures at high concentration in water (Figure 64A). Upon heating to 45 °C, the resulting tubles underwent the pore closing along with changing the stacking modes of 169 from the eclipsed to the slipped packing arrangements to reduce the tubular diameter (Figure 64A). This is due to thermally regulated dehydration of the oligoehter dendron moieties that occupy the tubular pores, so that water is squeezed out from the internal pores. In addition, this transformation is reversible, although it could be inhibited by intercalation of a rigid rod

molecule 170 between the disc stacking (Figure 64A and B). Interestingly, AMP can be encapsulated into hydrophobic pores filled by the dehydrated oligoether chains at 45 °C (closed state). Moreover, the dehydrative cyclization of the encapsulated AMP successfully proceeded to afford cAMP in the closed state (Figure 64C), whereas almost no reaction took place in the open state.

3. HELICAL ASSEMBLIES OF FOLDAMERS Foldamers, an interesting class of unique oligomers and polymers that preferentially adopt a specific compact conformation, have been recognized as one of the most attractive and active research areas in supramolecular chemistry and polymer chemistry since 1998.5 Since then, a number of synthetic molecular strands that fold into a helical conformation have been designed and synthesized by Moore,6 Hamilton,310 Lehn,311 Huc,7 Gong,312 Li,8,313,314 and others315 as summarized in Figures 65 and 66. The helical conformations of foldamers are dynamic in nature, and therefore, either a rightor left-handed helical conformation can be induced by introducing a chiral residue at the pendant or chain end or 13808


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Figure 66. Structural motifs of nonheterocyclic (A) and heterocyclic (B) aromatic amide foldamers and a preferred-handed helix induction in foldamers through noncovalent chiral interactions with optically active guests.

Chart 29

Chart 30

through intermolecular noncovalent bonding interactions with chiral additives (Figure 65, top), although their helical conformations change into random ones depending on the solvents used.

This section first briefly describes the progress of helical foldamers having a preferred-handed helical conformation induced by noncovalent chiral interactions with optically active 13809


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Figure 67. (A) Structures of capsules (208a−d) and chiral guests (D-209 and L-209). (B) Schematic illustration of the inclusion complex formation between L-209 (red) and a racemic helical capsule (P-208b and M-208b) (blue and red) and subsequent handedness inversion. (C) Schematic illustration of the binding and release of D-209. Snapshot pictures of a capsule (P-208b⊃D-209) extracted from MD simulation before (left) and after (right) heating at 1200 K for 2.3 ns. A pyridine unit in the right snapshot is highlighted in gold, and isobutoxy groups are omitted for clarity. (D) Schematic illustration of the inclusion complex formation between D- or L-209 and a one-handed helical capsule (P-208d). (Reproduced with permission from ref 352. Copyright 2012 American Chemical Society.)

Poly(m-ethynylpyridine)s (176 and 177) also fold into a one-handed helix upon complexation with saccharides, such as β-D-178 and 179, within the helical cavity via intermolecular hydrogen bonding in apolar and polar solvents.322,323 Abe, Inouye, and co-workers have recently synthesized alternating oligomers (180) of pyridine and phenol units connected through an ethynyl linker and found that 180 (n = 6) could selectively extract mannose among an equal amount of Dglucose, D-mannose, D-galactose, and D-fructose.324 Lehn et al. took full advantage of the s-trans conformational preference of bipyridine and rationally designed a molecular strand composed of a pyridine/pyrimidine alternate sequence (181) that forces formation of a helical structure in solution and in the solid state.325,326 An indolocarbazole-based oligomer (182) has a tendency to form an inclusion complex with anions.315 Jeong et al. found that chiral organic anions, such as 184, can induce an excess one-handed helical conformation in 182, thus exhibiting characteristic CD signals.327 Analogous aromatic foldamers (183) have also been synthesized by introducing a diethynylpyridyl residue between the indolocarbazole units.328,329 The oligomers 183 (n = 2 and 3) fold into a helical structure to generate cylindrical cavities, in which 3 and 5 water molecules are encapsulated in a 1D array via hydrogen bonding, respectively, as revealed by X-ray crystallographic analyses.328 Interestingly, the dynamically racemic foldamer 183 (n = 2) self-assembles into a supramolecular helical array to form P and M homohelices with a long cylindrical cavity

compounds, thus generating a cylindrical cavity suitable for encapsulating specific guests. Foldamer-based helical assemblies assisted by metal coordination, solvent, and guest inclusion, which result in the formation of 1D helical structures and further supramolecular helical assemblies, will then be described in detail with an emphasis on amplification of the helical chirality. 3.1. Helical Cavity Formation and Inclusion Complexation

3.1.1. π-Conjugated Aromatic Foldamers. Figure 65 shows typical π-conjugated aromatic foldamers developed during the past decade or two that fold into a helical conformation in certain solvents or in the presence of specific guests. The o- and m-phenylene oligomers and polymers (185 and 186) (Chart 29) are the simplest structural motifs in foldamers and are known to form a helical conformation in the solid state, which, however, are believed to unfold into a random coil in solution.316,317 In 1997, Moore et al. discovered that oligo(m-phenylene ethynylene)s (171) fold into a dynamic helical conformation through solvophobic interactions in polar solvents, such as acetonitrile, while they unravel into a random coil in chloroform.318 One of the dynamic helices with an excess helical handedness has been successfully induced in 172 in the presence of optically active hydrophobic (173319) and rod-like guests (174320 and 175321), once encapsulated in a hydrophobic cylindrical cavity of 172, and the inclusion complexes showed a characteristic Cotton effect in the mphenylene ethynylene chromophore region. 13810


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Figure 68. (A) Color-coded foldamer structures. Six sequences (210a−f) and structure of 210f. The two terminal Q units in 210a, 210c, and 210e have a nitro group at the 8-position. (B) Schematic illustration of the encapsulation of a guest into the cavity of a foldamer. Side view of the crystal structure of the inclusion complex of 210e with β-211a, in which two fluorine atoms in 210e are shown in CPK. Structures of monosaccharide guests in which three dominant forms of D-fructose are shown: the β-pyranose (β-211a), α-furanose (β-211b), and β-furanose (α-211c). Only one form is shown for other guests (α-212, β-213, β-214, β-215, β-216, and α-217). All exist as a mixture of α/β-pyranoses in solution, and β-214, β-216, and α217 also form α/β-furanoses. The proportions of the complexes in a mixture of either 210b or 210f and each of 211−217. (Reproduced with permission from ref 353. Copyright 2015 Nature Publishing Group.)

handed helical conformation has been induced in these oligomers as well as 192 via intermolecular hydrogen bonding once complexed with sugars, such as 193−195.337,338 In 1994, Hamilton et al. for the first time synthesized the hydrogen-bonding-induced heterocyclic aromatic amide foldamer (196), which formed a helical conformation of more than one turn (Figure 66B).339 A similar helical foldamer (197) with an alternating amide sequence has also been prepared by Zeng et al.340 Analogous oligomers bearing sticky end groups have been further applied to water transports as described in section 3.4.2. Since the first report by Huc and Lehn et al. in 2000,311 a number of heterocyclic aromatic oligoamide foldamers such as 198 have been designed and synthesized.341−346 An excess of either a right- or left-handed helical conformation can be induced in 198a and 198b through hydrogen bonding interactions with chiral guests (199, 200, and 35) and chiral solvents (L-201 and D-201) (Figure 66B).344,346 Interestingly, the single-stranded foldamers (198a and 198b) further self-assemble to form a double-stranded helix under equilibrium,311 which is discussed in section 4.1.1. Huc et al. have also synthesized helical foldamers (202−204), which encapsulate one and two water molecules within their helical

occupied with water molecules in the solid state, whereas 183 (n = 3) crystallizes, giving a pair of PM and MP heterohelices in a slipped way.328 3.1.2. Nonheterocyclic and Heterocyclic Aromatic Oligoamides. Aside from the π-conjugated aromatic foldamers described above, Gellman and Seebach et al. focused on non-natural peptides from a biological viewpoint, such as oligo(β-amino acids) (187) (β-peptide) (Chart 30), and extensively explored their structure- and sequence-dependent folding properties and functions.330,331 The details of their pioneering studies and further progress have been reviewed elsewhere.5,332−334 A series of hydrogen-bonding-driven nonheterocyclic aromatic amide foldamers have also been prepared (Figure 66A); a three-centered hydrogen-bonded amide oligomer (188) forming a folded structure was first synthesized by Gong et al.312 Analogous oligomers (189 and 190) bearing a methoxyl group at the 2-position were prepared by the groups of Li and Zeng, respectively.335,336 The same group also synthesized a new class of hydrazide foldamers (191a−c), in which longer 191b and 191c oligomers adopt helical conformations of one and two turns, respectively, whereas a shorter 191a forms a crescent conformation.337 A preferred13811


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Figure 69. (A) Schematic illustration of stabilization of an excess one-handed helical inclusion complex of a m-ethynylpyridine oligomer (176) with a saccharide (β-D-178) through coordination of Cu(II) inside the helix and subsequent memory of the helical chirality after replacement of β-D-178 with achiral Phen. (Reproduced with permission from ref 354. Copyright 2012 The Royal Society of Chemistry.) (B) Schematic illustration of the induction of a preferred-handed helix of a Zn(II) meso−meso linked porphyrin oligomer (218) upon complexation with 219 in the presence of (S)220 and subsequent memory of the helicity in the presence of an equimolar amount of (R)-220. (Reproduced with permission from ref 356. Copyright 2005 American Chemical Society.) (C) Schematic illustration of the induction of a preferred-handed helicity in dynamically racemic 310helical 221 complexed with L-222 and subsequent memory of the helicity in the presence of an excess achiral 223. (Reproduced with permission from ref 357. Copyright 2008 American Chemical Society.

cavities, as evidenced by X-ray crystallography.342,345 Other small polar solvents and organic molecules were also entrapped, but water is preferentially encapsulated within the helices. The foldamer (205) bearing a 1,8-diazaanthracene unit at the center of the sequence has been designed to possess a larger internal cavity,347 to which a series of achiral linear dialkanediols can be encapsulated in chloroform. The crystal structures clearly showed the 1:1 inclusion complexes of 205 with 1,3propanediol, 1,4-butanediol, and 1-amino-4-butanol. Moreover, racemic single helices of longer oligomers 206a and 206b composed of 2-quinoline δ-amino acid units as well as its analog 207 have been successfully separated into enantiomeric right- and left-handed helices by chiral high-performance liquid chromatography (HPLC), indicating that these oligomers have a sufficiently high helix inversion barrier.348−350

In order to enlarge the helix diameter for encapsulating tartaric acids (D-209 and L-209) with a helical cavity having a high affinity and diastereoselectivity, a series of dynamically racemic helically folded capsules with a different chain length (208a−c) and its optically active derivative bearing chiral terminal groups at both ends (208d) have been prepared by introducing a pyridine−pyridazine−pyridine (pyr-pyz-pyr) unit at the center (Figure 67A).351,352 Detailed NMR, X-ray crystallography, CD, and molecular dynamics (MD) studies suggest that the foldamers 208a−c form very stable singlestranded interconvertible P- and M-helices in solution. In the presence of L-209, both the diastereomeric inclusion complexes, M-208b⊃L-209 and P-208b⊃L-209, were formed under kinetic control. However, the helical sense of the P-208b⊃L-209 complex was slowly inverted to the opposite one, and finally, 13812


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Figure 70. (A) Coordination polymerization of pyridine-bound phenylene ethynylene oligomers. (Reproduced with permission from ref 362. Copyright 2006 American Chemical Society.) (B) Structures of chiral (n) and achiral (m) phenylene ethynylene cooligomers 225n,m. (C) Structures of achiral (172m) and chiral (226n) phenylene ethynylene homo-oligomers. Plots of Δε314 as a function of mol % chiral homo-oligomer of 22618 (n = 18) in a mixture of achiral and chiral homo-oligomers in 40 vol % water in acetonitrile. The inset shows the proposed aggregation model during the π-stacked oligomerization. (Reproduced with permission from ref 364. Copyright 2001 American Chemical Society.) (D) Structures of chiral (227) and achiral (228) cationic oligo(phenylene-1,2,3-triazole)s.

greater conformational stability than the others, does not necessarily result in a slower binding and release. On the basis of further studies on the conformational changes during the binding and release of D-209, it has been revealed that one terminal helical quinoline unit of 208b tilts away and opens a window in the capsule wall at the binding site and the neighboring pyridine unit plays the role of a hinge (Figure 67C). This mechanism can explain why guest binding and release was faster in the longest capsule. Importantly, the diastereoselectivity could be determined during the release of the guest and not during the binding. Huc and co-workers have further synthesized a series of designer aromatic oligoamides with a specific iterative sequence to provide a suitable helical cavity (210a−f) for selectively encapsulating a target sugar (fructose) among structurally similar sugars (211−217) (Figure 68).353

the overall helical sense of 208b was converted to the thermodynamically more stable M-helix fully controlled by L209, resulting in the formation of the M-208b⊃L-209 complex with a complete diastereoselectivity (Figure 67B). The chiral 208d exclusively forms a P-helix biased by the terminal (1S)-(−)-camphanyl groups in solution and in the solid state, as evidenced by a single set of 1H NMR signals and X-ray crystal structural analysis, respectively. In the presence of L-209, however, inversion of the P-helix slightly occurs to form a minor M-208d⊃L-209 complex together with the major P208d⊃L-209 one. The 1H NMR titrations of P-208d with L-209 and D-209 revealed that the inclusion complexation of P-208d with D-209 (P-208d⊃D-209) is 1 order of magnitude more stable than that with L-209 (P-208d⊃L-209) (Figure 67D). Further kinetics and MD simulations demonstrated that guest release and binding were the fastest in the shortest capsule 208a, as anticipated, but the longest capsule 208c, which has 13813


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Figure 71. (A) Schematic illustration of imine methathesis between oligo(m-phenylene ethynylene) imines (229, 230a, and 230b) and the equilibrium shift driven by folding and guest binding. Structures of a chiral rod-like guest (175) and the products (233a−c). (Reproduced with permission from ref 367. Copyright 2002 American Chemical Society.) (B) Schematic illustration of a helical organization of the foldamer 234 around the chiral oligoammonium threads (235a and 235b), in which * highlights the chirality of the terminal proline derivatives of the threads and the curved arrow represents chiral information transfer. (Reproduced with permission from ref 368. Copyright 2004 Wiley-VCH.) (C) Schematic illustration of the alternative binding modes of 236 with the cyanurate 237 generating units of composition 236·2372 (C- and S-shaped (left and middle) and helical (right) complexes) which further assemble into helical columns. (Reproduced with permission from ref 370. Copyright 2000 Wiley-VCH.)

A first-generation sequence (210a) very strongly binds Dmannose (212) and then D-fructose (211), whereas the binding affinity of 210a toward pentoses (216 and 217) and D-glucose, and its derivative (213 and 215) was weaker, as revealed by CD studies. A second-generation sequence (210b) bearing (1S)(−)-camphanyl groups (R*) at both ends, which quantitatively forms a P-helix, showed a similar selectivity to that of 210a (Figure 68B). However, when one H and one P unit in the sequence of 210a and 210b was removed and replaced with F bearing a fluorine atom, respectively, the resulting 210c and 210d exhibited a remarkable enhancement in the selectivity toward 211, while decreasing the selectivity to other sugars. This is because the introduced fluorine atom fills a small space in the host cavity without further strong attractive or repulsive interactions. A further point-mutation by replacing one N unit with QF (210e and 210f) further significantly improved the selectivity toward 211. Interestingly, the optically active 210f quantitatively formed a complex with 211 to exclusively form 210f⊃β-211a in a CDCl3/DMSO-d6 mixture (8/2, v/v) even in

the presence of 3 equiv of 211−214, 216, and 217 (Figure 68B), judging from the 1H NMR and calculated proportions of other complexes. The crystal structure of 210e⊃β-211a clearly showed that the cavity space is completely filled with one fructose by the additional fluorine atom, leading to the high selectivity toward β-211a (Figure 68B). 3.1.3. Memory of Helical Chirality. As described in section 3.1.1, an excess one-handed helical conformation can be induced in poly(m-ethynylpyridine)s (176 and 177) upon inclusion complexation with saccharides, thus exhibiting induced CDs (ICDs). Inouye and Abe et al. further synthesized a series of oligomers of m-ethynylpyridine (176) and found an interesting chain-length-dependent helical folding ability of 176 and its memory effect (Figure 69A).354,355 The helical structures of the oligomers (176a and 176b) induced by β-D178 were stabilized by coordination with a Cu(II) ion, accompanied by a significant enhancement of the CD intensities; in particular, the CD enhancement observed for 176b was noticeable. In contrast, a shorter oligomer (176c) 13814


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memory effect of 218 (Figure 69B).356 An excess one-handed helical structure was induced in 218 complexed with 219 in the presence of the optically active binaphthyl-based diamine ((S)220), as revealed by its characteristic ICD observed in the porphyrin chromophore region. Upon further addition of an equimolar amount of the opposite enantiomeric diamine ((R)220), which results in a totally racemic mixture of 220, nevertheless, the ICD signal due to the helically twisted porphyrin array still remained and slightly decreased in intensity after the further addition of (R)-220. This indicates that an excess one-handed helical conformation induced in 218 is memorized, although a chiral filter effect (see section 5.7.1.5), which may contribute to this phenomenon, is not completely negligible. Ousaka, Kuroda, and co-workers have demonstrated a unique memory of a 310-helical conformation of a dynamically racemic oligopeptide (221) consisting of achiral amino acids with an intramolecular cross-link at the i and i + 3 side chains (Figure 69C).357 A preferred-handed helix was first induced in the entire peptide chain of 221 via a noncovalent acid−base interaction at the N-terminal amino residue with a chiral carboxylic acid (L-222) in a domino-like manner (the so-called the “noncovalent domino ef fect”).358−361 A split-type CD pattern observed in the ΔZPhe chromophore region of 221 suggests the formation of a right-handed helix. After complete replacement of L-222 with the achiral carboxylic acid 223, the peptide retained its optical activity, which, however, slowly decreased with time and then disappeared. 3.2. Foldamer-Based Helical Assemblies

As described in section 3.1, a number of helical foldamers have been designed and synthesized, and their handedness can be biased by inclusion complexation with chiral guests. This section mainly describes the foldamer-based helical assemblies assisted by metal coordination, solvent, and guest inclusion, through which chiral amplification is also operative. 3.2.1. Metal- and Solvent-Induced Helical Assemblies. Moore and co-workers reported supramolecular helical polymers by the coordination polymerization of oligo(mphenylene ethynylene)s (224) bearing pyridyl units at both ends with Pd(II) (Figure 70A).362 Metal coordination-assisted assembly of the tetramer and octamer strands (n = 2, 6) took place in acetonitrile, resulting in the formation of racemic metallosupramolecular helical polymers (poly-224) via a nucleation−elongation mechanism. In contrast, the hexamer (n = 4) assembled into a shape-persistent macrocycle that further π-stacked to form a columnar polymer, indicating the important role of the oligomer length that directly affects the polymerization mechanism. Moore, Meijer, and co-workers prepared a series of chiral (n) and achiral (m) m-phenylene ethynylene cooligomers (225n,m) and investigated the sergeants and soldiers effect in acetonitrile during an intramolecular folding process (Figure 70B);363 the CD intensity of 225n,m in acetonitrile showed a nonlinear effect with respect to the chiral-side-chain content, which revealed cooperative interactions among the side chains and intramolecular chirality transfer to their backbones. It was also found that only one chiral side chain at the terminal end of the oligomer (2251,13) has a strong twist-sense bias to induce an excess one-handed helical conformation, although its handedness excess was lower than that of the fully chiral oligomer in acetonitrile.

Figure 72. (A) Top and side views (left) of the calculated helical structure of 239. (B) Proposed diastereoselective self-assembly of 239 templated by right-handed α-helical PBLG. TEM image of the selfassembled nanostructures of 239 twining around PBLG, whose lengths are 62.8 ± 9.1 nm. (C) Schematic illustration of an enantioselective self-assembly of M-S-238 from the racemic mixture of M-S-238 and PR-238 mediated by the chiral template PBLG in 13% aqueous DMF. (Reproduced with permission from ref 371. Copyright 2013 WileyVCH.)

showed a weak CD in the presence of β-D-178 and almost no enhancement of the CD with a Cu(II) ion. Interestingly, the further addition of achiral o-phenanthroline (Phen), which probably replaces the β-D-178 included in the helical cavities of the longer oligomers (176a and 176b), further enhanced their CD intensities, while maintaining their optical activities for a couple of weeks, indicating a memory of the induced helical chirality, although the memory eventually disappeared. Timedependent CD intensity changes suggested that the 176b/ Phen/Cu complex retained its memory for a longer time than the corresponding complex with the longer 176a, probably because the single Cu(II) ion efficiently coordinates with up to two turns in the helical structure of 176b, whereas 176a folds into a three-turn helix, and a Cu(II) ion may not sufficiently coordinate to all the pyridine residues to retain the induced helical structure. A meso−meso-linked Zn(II) porphyrin octamer (218) adopts a dynamic helical conformation when complexed with an achiral urea (219) via complementary hydrogen bonding interactions, whereas 218 itself takes a nonhelical conformation in solution. Osuka and co-workers have found a unique chiral 13815


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Figure 73. (A) Structure of an aromatic amide foldamer (240). (a) SEM image of 240 and (b) the enlarged SEM image of (a). (Reproduced with permission from ref 372. Copyright 2013 Wiley-VCH.) (B) Schematic illustration of the hierarchical self-assembly of a foldamer (241) (a) into protofibrils or filaments (b), and fibrils (c). (Reproduced with permission from ref 374. Copyright 2002 Wiley-VCH.) (C) TEM image (left) of 241 with a 2D array of 2.6 nm fibrils highlighted in the inset (after 14 days), and AFM phase image (right) of 241 with 24 nm fiber (after 4 days). The inset shows the AFM height image of 241 with a 20 nm height scale (after 7 days). (D) CD spectra of 241 in the absence and presence of L- or D201 in dichloromethane (after 15 min). (Reproduced with permission from ref 375. Copyright 2012 The Royal Society of Chemistry.)

When the corresponding homo-oligomers (18-mer) of chiral and achiral m-phenylene ethynylenes (22618 and 17218) were mixed in an acetonitrile/water mixture (6/4, v/v) at a different molar ratio, both oligomers self-assembled to form intermolecularly π-stacked supramolecular polymers. The twist-sense bias of the chiral 22618 is transferred to the achiral 17218 during the helically assembled process, thereby showing the nonlinear effect of the CD intensity on mole percent chiral oligomer, and displaying the intermolecular sergeants and soldiers effect with cooperativity (Figure 70C).364 As anticipated, such a chiral amplification hardly took place in 100% acetonitrile. A similar intermolecular sergeants and soldiers effect was also observed for chiral (227) and achiral (228) cationic oligo(phenylene1,2,3-triazole)s in a methanol/water mixture (25/75, v/v) during the helically assembled process (Figure 70D).365 3.2.2. Guests-Induced Helical Assemblies. The solventinduced helical folding and further assemblies of foldamers have been extended to the folding- and guest-driven synthesis of

oligo- and poly(m-phenylene ethynylene)s by Moore et al., in which the reversible imine metathesis equilibrium between helical oligomers (233a−c) has been successfully shifted to a particular oligomer through the inclusion complex formation with a rod-like guest (175) (Figure 71A).366,367 The imine metathesis of equimolar amounts of 4mer (229) bearing two N-terminal imines, and 6mer (230a) and 12mer (230b) bearing one C-terminal imine produced a mixture of the three starting substrates, five oligomers (232a, 232b, and 233a−c), and byproduct (231). The distribution of the resulting products 233a−c in chloroform did not change in the absence and presence of 2 equiv of 175 because 233a−c adopt a random conformation in chloroform. However, the imine metathesis in acetonitrile even in the absence of 175 promoted the formation of 233a−c in a 19:37:16 ratio due to the folding-driven assembly of the oligomers.367 In the presence of 2 equiv of 175, the equilibrium was favorably and remarkably shifted to the formation of 233b (66%). In this system, 13816


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Figure 74. Schematic illustration of the self-assembly of hydrazide-based foldamers (242a−f) into gels, and helicity induction of 242 in the presence of α-D- or α-L-178 with chiral amplification. SEM image of the dried gel of 242a obtained from n-octanol, and AFM image of that of 242f obtained from toluene. (Reproduced with permission from ref 376. Copyright 2008 American Chemical Society.)

(S-238 and R-238) or a racemic helix (239) in aqueous DMF, while a random-coil was predominant in DMF. In the presence of PBLG, a negative bisignate Cotton effect was observed in the absorption region of the achiral 239 in aqueous DMF, indicating that the achiral 239 strands fold into an excess one-handed helical sense along the chiral template PBLG to form densely packed self-assembled rotaxane-like inclusion complexes. TEM images of the self-assembled PBLG/239 complexes revealed well-defined nanostructures, being almost consistent with the length of the template (Figure 72B). On the other hand, optically active S-238 and R-238 fold into left (M)- and right (P)-handed helical conformations, respectively, thus showing the mirror-image CD spectra in aqueous DMF. Interestingly, upon the addition of PBLG to a 1:1 mixture of M-S-238 and P-R-238 (racemic), an apparent negative Cotton effect was induced in the solution mixture, indicating that the PBLG template-directed inclusion complexation proceeds in a highly enantioselective way and S-238 is preferentially included over the antipode R-238 (Figure 72C).

dynamic templation using a size-selective guest resulted in the length-selective formation of 233b because 233b possesses the highest affinity for 175 among the products. Related to the foldamer 181 described in section 3.1.1, an oligomer 234 composed of an alternating pyridine/naphthyridine sequence helically self-assembles along cationic chiral guests (235a and 235b) in a 2:1 and 3:1 stoichiometry, respectively, thus forming a rotaxane-like inclusion complex with an excess helical sense biased by the terminal proline derivatives of the guests (Figure 71B); the chirality of the guests may be transferred and amplified to the layered stacked 234 strands.368,369 Lehn, Huc, and co-workers demonstrated that the linear oligo-isophthalamide strand (236) forms a helical disk-like complex (236·2372) as well as C- and S-shaped complexes in the presence of cyanuric acid (237) as a template under equilibrium, which further hierarchically self-assemble into columns (Figure 71C),370 resulting in the formation of supramolecular fibers stabilized by solvophobic and aromatic stacking interactions as supported by cryo-TEM observations. Rowan, Klumperman, and co-workers constructed a novel tobacco mosaic virus mimic consisting of chiral and achiral 1,4aryl-disubstituted-1,2,3-triazole foldamers (S-238, R-238, and 239, Figure 72A) and α-helical poly(γ-benzyl-L-glutamate) (PBLG) as a template.371 The foldamers exist in dynamic equilibrium between a random-coil and a single-stranded helix

3.3. Foldamer-Based Supramolecular Helical Assemblies

In this section, the foldamer-based helical assemblies described in the previous section have been further extended to the foldamer-based supramolecular helical assemblies, resulting in the formation of bundle structures, such as fibrils and fibers in a hierarchical way, through which supramolecular helicity with an 13817


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Figure 75. (A) Schematic illustration of self-assembly of trans-(R,R)-243 into a molar tooth-shaped object and its overall self-assembling process. SEM images (a−c) of each self-assembled structure resulting from 8 g/L of P123 (poly[(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)]) aqueous solution with different agitation and aging times at 0 and 20 °C. A molecular network viewed from an arbitrary axis is also shown in (c). (Reproduced with permission from ref 377. Copyright 2011 American Chemical Society.) (B) SEM images of the foldectures obtained from trans(S,S)-243 (n = 7) solutions in the presence of different P123 concentrations (scale bars: 1 μm): (a) 0.01 g/L (WP); (b) 0.015 g/L (RP1); (c) 0.02 g/ L (RP2); (d) 0.025 g/L (RP3). (Reproduced with permission from ref 378. Copyright 2010 Wiley-VCH.) (C) SEM image of trans-(S,S)-243 (n = 4) microtubes (a). Enlarged SEM image of the yellow-circled region (b) in panel a, indicating rectangular cross section. TEM image of trans-(S,S)-243 microtube (c). (Reproduced with permission from ref 379. Copyright 2012 American Chemical Society.) (D) Structures of a C-terminal free βpeptide foldamer (trans-(S,S)-244) and an α/β-peptide foldamer (trans-(S,S)-245).

fibers resulting from strong intermolecular interactions of the folded lock-washer subunits of 241 (Figure 73B).373,374 Interestingly, supramolecular chiral fibers with an excess onehandedness were generated during the hierarchical supramolecular self-assembly process. A secondary nucleation growth mechanism has been proposed for this unique chiral amplification or deracemization, leading to the optically active fiber formations.375 TEM and AFM images reveal that the fibers composed of bundles of 1D molecular-scale fibrils with a diameter of ca. 25 Å further hierarchically assemble into fibers with a predominant helical sense (Figure 73C). As anticipated, in the presence of diethyl D- or L-tartrate (L-201 or D-201), one of the helical fibers could be selectively produced, thus showing an ICD (Figure 73D).

excess one-handed helical sense has been induced by chiral guests accompanied by amplification of the chirality. Jiang and co-workers synthesized an aromatic amide foldamer (240) composed of two helical oligo(8-fluoro quinoline carboxamide) strands bearing an achiral dodecyl chain at the terminal, connected with a bipyridine linker,372 in which the 2,2′-bipyridine moiety adopts an s-trans conformation.325 SEM images reveal that 240 self-assembles into welldefined twisted helical microfibers (Figure 73Aa) possessing a hollow interior (Figure 73Ab). An alternating pyridine/pyridazine oligomer (241) with 13 heterocycles folds into either a right- or left-handed helical structure because of its s-trans conformational preference,325 which spontaneously self-assembles into protofibrils, fibrils, and 13818


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Figure 76. (A) Structure of a chiral aromatic hydrazide foldamer bearing (R,R)-proline units at the termini ((R,R)-246). Energy-minimized structure of (R,R)-246 with replacement of the octyl chains by methyl groups (left). Schematic illustration of diastereoselective complexation of (R,R)-246 with alkylated glucoses (α-L-193 and α-D-193) in its helical cavity. (Reproduced with permission from ref 383. Copyright 2007 American Chemical Society.) (B) (a) Structures of salen (247) and Mn(III)- and Ni(II)-salen foldamers (248, 249, and 250). (b) Variable-temperature CD and absorption spectra of 250 in acetonitrile. The energy-minimized structures of homochiral (P−P) and heterochiral (P−M) helices of 250. Hydrogen atoms except for the amide protons are omitted for clarity. The side chains were replaced with −CONHCH3 groups to simplify the calculations. (c) Asymmetric epoxidation in the presence of a Mn(III)-salen foldamer (248 or 249). (Reproduced with permission from ref 384. Copyright 2014 The Chemical Society of Japan.)

Li and co-workers investigated the hydrogen-bonding-driven self-assembly of a series of achiral hydrazide foldamers (242a− f) bearing decyl side chains and two aromatic terminal groups, which strongly gelate apolar and polar solvents (Figure 74).376 The achiral hydrazide strands first fold into either a right- or left-handed helical conformer stabilized by intramolecular hydrogen bonds, which further self-assemble to form cylindrical columns through intermolecular dislocated “head-to-head” stacking of the aromatic terminal groups. The resulting helical columnar aggregates then form bundles, giving rise to fibrous networks via van der Waals interactions between the decyl side chains, which are responsible for the gelation, as revealed by SEM and AFM observations. A unique feature in this system is that the foldamer gelators are capable of complexing with chiral octylated glucoses (α-D-178 and α-L-178), which significantly enhance the capacity of the foldamers to gelate apolar solvents because of strong complexation, and at the same time, one of the dynamic helices is predominantly induced during the

gelation in a cooperative manner, thus showing chiral amplification. The control of the precise three-dimensional (3D) micrometer-sized structures via self-assembly of small molecules or foldamers remains very difficult, but Lee and co-workers have found that β-peptide foldamers, trans-(R,R)-243 (n = 6) and trans-(S,S)-243 (n = 4 and 7), self-assemble into an unprecedented 3D molecular architecture (“foldectures”) due to their rigid and predictable helical conformation in solution. Interestingly, the self-assembly of trans-(R,R)-243 (n = 6) results in the formation of a micrometer-sized, molar toothshaped architecture through a 2D planar shaped assembly in a controlled manner in the P123 aqueous solution in which a right-handed superhelix in a unit cell consists of four individual left-handed helical monomers (Figure 75A).377 On the other hand, SEM images showed that trans-(S,S)-243 (n = 7) transformed its morphology from the windmill shape (WP) to the square rods (RP1, RP2, and RP3) in a stepwise way with the 13819


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Figure 77. Artificial ion and water channels generated from tubes; self-assembled macrocycles (A) and helical foldamers (B). (Reproduced with permission from refs 394 and 391. Copyright 2006 and 2015 American Chemical Society, respectively.)

and protein guests, such as green fluorescent quantum dot, rhodamine B, and green fluorescent protein, can be encapsulated. The resulting 3D structures look more like achiral than chiral, but the one-handed helical chirality of the βpeptide foldamers may play a role in producing such unique micrometer-sized structures.

increasing P123 concentration (Figure 75B).378 In addition, SEM images revealed that a shorter foldamer, trans-(S,S)-243 (n = 4), which does not have a fully helical conformation in solution, self-assembled into well-defined microtubes with rectangular cross section by the evaporation-induced selfassembly of trans-(S,S)-243 (n = 4) in a methanol/water mixture (9/1, v/v), and its TEM image also showed the microtube morphology with a hollow interior (Figure 75C).379 An analogous β-peptide foldamer (trans-(S,S)-244) (Figure 75D, left) also self-assembles to form rhombic rods, which have been applied to a unique gold nanoparticle foldecture-based composite material.380 The rhombic rod foldectures having collective diamagnetic anisotropy can be uniformly aligned under a static magnetic field in hydrogel of cross-linked poly(ethylene glycol) chains and exhibited instantaneous orientational motion under a dynamic magnetic field.381 A novel α/β-peptide foldamer (trans-(S,S)-245) (Figure 75D, right) was observed to self-assemble into a series of unique trigonal bipyramidal foldectures.382 One of them possesses a large hollow interior, in which a variety of inorganic, organic,

3.4. Functions of Foldamers and Helically-Assembled Foldamers

As described in the preceding sections, numerous helical foldamers, including multistranded helical foldamers, have been synthesized in order to develop novel foldamer motifs, while their applications as chiral materials for chiral recognition and asymmetric catalysis remain limited but a challenging issue. 3.4.1. Chiral Recognition and Asymmetric Catalysis. Li and co-workers further synthesized chiral hydrazide foldamers bearing hydrophilic side chains and two R- or S-proline units at both ends (246), which adopt a preferred-handed helical conformation biased by the proline moieties in solution.383 Enantiomeric dodecylated glucoses (α-D-193 and α-L-193) are also efficiently encapsulated into the helical cavity formed from 13820


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Figure 78. (A) Structure of the dendritic dipeptide (24) and its trans-tapered low-temperature (left) and globular high-temperature (right) conformers. The molecular model (top view) of a single porous column layer assembled from 24 (n = 1) is also shown (right). (Reproduced with permission from ref 97. Copyright 2004 Nature Publishing Group.) (B) Structure of an aromatic hydrazide foldamer (256) bearing tripeptide side chains, and schematic illustration of the unimolecular transmembrane channel formation. (Reproduced with permission from refs 398 and 391. Copyright 2014 and 2015 American Chemical Society.) (C) Structure of aquafoldamer (257) and schematic illustration of the formation of a water channel created via the “sticky” end-mediated 1D chiral helical stacks, which enable alignment of hydrogen-bonded water molecules in a chain-like fashion. (Reproduced with permission from ref 399. Copyright 2014 American Chemical Society.)

dimer formation could not be observed because such interstrand hydrogen bonds at the interface are not possible (Figure 76Bb). The variable-temperature CD experiments of Mn(III) (248 and 249) salen-linked foldamers as well as 250 in acetonitrile indicated that the helical stability and helical sense excess were remarkably enhanced at low temperature due to the intramolecular hydrogen-bonded networks (Figure 76Bb). These results also suggest amplification of the helical chirality during the complexation of 247 with metals, resulting in the predominantly homochiral (PP) helical dimer formation assisted by intramolecular hydrogen bonds between the strands. The (PP) helical 248 and 249 possess the catalytic active, but achiral Mn(III)-salen residue favorably located inside the helical cavity, which catalyzed the asymmetric epoxidation of a chromene derivative in the presence of a series of achiral Noxide derivatives, thus producing the corresponding epoxide in

246 in a diastereoselective manner in chloroform. Based on quantitative fluorescent analyses, the association constant (Ka) of (R,R)-246 with α-L-193 was approximately 144 times higher than that of (R,R)-246 with α-D-193 (Figure 76A). A series of novel (metallo)salen-linked foldamers have been rationally designed and synthesized by connecting two foldamer strands bearing chiral amide side chains through an achiral (metallo)salen linker (Figure 76Ba).384 The 2D nuclear Overhauser effect spectroscopy (NOESY) analysis and semiempirical molecular orbital calculations of the Ni(II)-salenlinked foldamer (250) revealed that 250 forms a homochiral (PP) helical structure stabilized by ten intramolecular hydrogen bonds, in which the two P-helical strands communicate to each other through the four interstrand hydrogen bonds at the interface between the two strands connected by the achiral salen linker. In contrast, the other possible heterochiral (PM) 13821


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enantioselective permeation or a catalytic activity are also highlighted. Figure 77 summarizes typical synthetic ion and water channels developed during the last two decades. In 1993, Ghadiri et al. reported the first artificial transmembrane ion channel composed of an alternating D- and L-α-cyclic peptide (251) that self-assembles to form a hollow tubular structure.392,393 Later, Matile and Kato et al. synthesized a folate-based dendrimer which self-assembles to form a quartet (252) and further π-stacked rosettes with an ion channel functionality without extra stabilization by covalent capture.394 Gong et al. also prepared aromatic oligoamides (253) as a novel family of shape-persistent, planar macrocycles395 as well as hexa(m-phenylene ethynylene)-based π-conjugated macrocycles (254)396 bearing six amide side chains, which form ion and water channels in lipid bilayers, respectively.395,396 Helical oligocholate foldamers labeled with pyrene units (255) prepared by Zhao et al. have been utilized as carriers for hydrophilic molecules across lipid bilayers.397 Hou and Li et al. have applied hydrogen-bonded helical hydrizide foldamers (256) to unimolecular ion channels in the bilayer (see below).398 Almost at the same time, Zeng et al. have for the first time applied an aromatic amide foldamer (257) as a novel motif for constructing an intriguing hollow tubular aquapore as described below.399,400 A novel family of amphiphilic dendritic dipeptides (24) developed by Percec et al. self-assemble to form hollow helical columns in solution and in bulk, which further self-organize into a 2D hexagonal columnar structure (Figures 13 and 78A).97,401 These hollow columns jacketed by the hydrophobic dendrons within phospholipid liposomes can mediate water transport through a Grotthuss-type mechanism without eliminating H+ transport across the liposomes, whereas Na+, Li+, and Cl− ions cannot be transported in this system.97,401 Hou, Li, and co-workers synthesized a series of aromatic hydrazide foldamers (256) bearing Phe tripeptide side chains, which form helical structures with a cylindrical cavity via hydrogen bonding (Figure 78B).398 The helical channel of the longest, 256b (n = 4), selectively transports an NH4+ over K+, while that of a shorter, 256a (n = 1), efficiently transports a Tl+ cation across the lipid membranes. The NH4+/K+ selectivity and Tl+ transport efficiency observed for synthetic helical channels 256a and 256b are higher than and as high as those of natural gramicidine A, respectively. Zeng and co-workers synthesized a series of aquafoldamers (257), which self-assemble to form 1D chiral helical stacks composed of folded strands of 257 with the same handedness via complementary hydrogen bonds between the “sticky” end groups of each strand, thus generating hollow tubular aquapores being capable of aligning hydrogen-bonded water molecules in a chain-like fashion (Figure 78C).399 The pore diameter of these hollow tubules is very close to the diameter of the water molecule (∼2.8 Å), thus allowing proton transport as well as proton-gradient-induced water transport across the lipid membranes. A novel class of artificial enantioselective transmembrane channels for amino acids have been developed by Hou, Li, and co-workers.402 An oligopeptide-appended pillar[n]arene 258 (n = 6) forms a tubular pore within lipid vesicles. Based on kinetic measurements using the fluorescent labeling method, the channel 258 mediates the enantioselective transport of Damino acids (Ala, Ser, Thr, Val, and Leu) across the lipid membrane (Figure 79Aa) and the difference in the

Figure 79. (A) Structure of the peptide-appended pillar[n]arene (n = 6) derivative 258. (a) Transport activities of 258 for L- and D-amino acids. (b) Time-dependent changes of L- or D-Phe concentration ([Phe]) outside the vesicles after the addition of a DMSO solution of channel 258 or pure DMSO. (Reproduced with permission from ref 402. Copyright 2013 American Chemical Society.) (B) Trans catalysis in polarized membranes. Schematic illustration of the conversion of intravesicular anionic substrates by externally added SCP1 (259) into anionic products. L, Leu; H, His; R, Arg; V, valinomycin. (Reproduced with permission from ref 403. Copyright 2003 American Chemical Society.)

up to 5.6% ee (Figure 76Bc). Although the enantioselectivity was low, this is the first example of a helical foldamer-based asymmetric catalysis that certainly takes place inside the helical cavity even when the catalytic active site is achiral. 3.4.2. Artificial Ion Channels. Natural ion channels are composed of transmembrane proteins, of which the β-barrel and α-helix components participate to provide specific pores, which regulate a number of biologically important functions, such as ion flow, signal transduction, and molecular transport.385 Inspired by such biological ion channels, a variety of artificial ion and water channels has been extensively designed and constructed from synthetic tubes, self-assembled macrocycles, and helical foldamers (Figure 77).385−391 In this section, foldamer-based ion and water channels along with historically important synthetic ion channels are mainly described, and some representative artificial ion channels showing an 13822


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Figure 80. (A) Double- and (B) triple-stranded helices in biological macromolecules. (Reproduced with permission from refs 404, 408, and 411, 412; Copyright 1979 Nature Publishing Group and Copyright 1988, 1994, and 1987 American Association for the Advancement of Science, respectively.)

Figure 81. (A) Structural formulas of the Cu(I) helicates (260a and 260b). (B) Structures of the ligands (261a, 261b, and 262) bearing two 2,2′bipyridine and bis(2,2′-bipyridine) units linked through oligo(ethylene glycol)s. (C, D) Optically active helicates (263 and 264) with a twist-sense bias induced by chiral substituents (C) or resolved by chromatography (D). (E) Structure of a tetranuclear Cu(I) double-stranded helicate (265a) that can be resolved into enantiomers by diastereomeric salt formation. Top and side views of ball-and-stick drawing of 265b. (Reproduced with permission from ref 431. Copyright 2011 The Royal Society of Chemistry.)

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Figure 82. (A) Structures of oligopyridinecarboxamides (198, 266, and 267) and their equilibria between single and double helices. (Reproduced with permission from ref 433. Copyright 2004 Elsevier.) (B) Side views of the X-ray single-crystal structures of a series of homodouble helices (see Table 1 for their structures). (Reproduced with permission from refs 311, 432, 434−437; Copyright 2000 Nature Publishing Group, Copyright 2001, 2006, and 2010 Wiley-VCH, and Copyright 2007 Elsevier.)

handed DNA double-helix, such as B-DNA, inverts into the lefthanded Z-DNA under certain salt concentrations,404 the structures of which include a helix reversal between them that has been elucidated by an X-ray crystallographic analysis of an oligonucleotide duplex.405 A recent work by Zhou et al. revealed that a specific, positively charged ruthenium complex can efficiently induce the B to Z transition almost independent of the DNA sequence.406 Gramicidins and collagen are biological proteins that consist of a unique sequence of peptides, which self-assemble into double- and triple-stranded helices to form ion channels407−409 and microfibrils,410,411 respectively. Polysaccharides are another important class of biological polymers, and some of them also have multistranded helical structures. For example, amylose412−415 and xanthan416−419 adopt a left-handed double-helix, while schizophyllan420,421 and Curdlan422 form a right-handed triple-helix. In this section, the recent advances in the multistranded helical oligomers formed from the assembly of synthetic oligomers in the presence and absence of metals or others are described together with the pioneering studies in this emerging research area.

enantioselectivity increased with the increasing size of the amino acids. Interestingly, the 258-based pore transported only the L-isomer of Phe (Figure 79Aa,b), indicating that the difference in the binding between the enantiomeric amino acids and chiral pores created by the peptide chains contributes to the efficient enantioselective transport toward racemic amino acids across the lipid membrane. A quite unique artificial ion channel with a catalytic activity has been constructed by Matile and co-workers. A synthetic catalytic pore (SCP1) consisting of a rigid-rod β-barrel (259) bearing His and Arg residues forms a tubular pore within spherical lipid bilayers, through which a pyrenetrisulfonate ester injected in the vesicle transports across the pore, producing the hydrolyzed product (Figure 79B).403 In the presence of valinomycin, it has been found that the rate of the substrate binding and product release in SCP1 is accelerated due to the inside-negative potentials.

4. MULTISTRANDED HELICAL ASSEMBLIES As briefly described in section 2.5, multistranded helical structures are ubiquitous in nature and take the central role in elaborate biological systems (Figure 80). The right-handed double-helical DNA is the most prominent double-helix, playing a vital role in the storage, transmission, and translation of genetic information.2 It is also well-known that the right-

4.1. Double Helices

4.1.1. Homo-Double Helices. The seminal work on double-stranded helicates was reported by Lehn et al. in 13824


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Figure 83. Color-coded structural formulas and side views of the crystal structures (a−f) or an energy-minimized model structure (g) of a series of homodouble helices whose sequences are abbreviated using the colored codes. (Reproduced with permission from refs 438−445; Copyright 2007, 2008, and 2014 Wiley-VCH, Copyright 2008 and 2010 The Royal Society of Chemistry, Copyright 2011 American Association for the Advancement of Science, Copyright 2012 Elsevier, and Copyright 2014 American Chemical Society.)

Table 1. Dimerization Constants (Kdim) of a Series of Homo-Double Helices at 25 °C in CDCl3 198a 266a 198b 266b 266c 266d 266e 266f 266g 266h 266i 266j 266k 266l 266m 266n 266o 266p 266q 266r 266s 267a 267b 267c

R1

R2

R3

n

Kdim

ref

OC10H21 OC10H21 H OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OBn OBn OCH3 OiBu OiBu OiBu OiBu OiBu H H OC10H21 H H

H OC10H21 H OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OC10H21 OBn OBn OCH3 OiBu OiBu OiBu OiBu OiBu H H H H H

COC9H19 COC9H19 CO2tBu CO2Bn CO2Bn CO2Bn CO2Bn CO2Bn CO2Bn H H H H H CO2tBu CO2tBu CO2tBu CO2tBu CO2tBu CO2Bn H COC9H19 CO2tBu CO2Bn

3 3 3 2 3 4 5 6 7 3 4 3 4 3 2 3 4 5 6 2 4 2 2 1

2.5−3.0 × 10 6.5 × 104 1.1−1.2 × 102 2.1 × 102 1.5 × 103 5.2 × 103 6.5 × 102 6.5 × 10

311 311 311 433 433 433 433 433 433 436 434 436 434 436 437 437 437 437 437 435 432 435 435 435

1987,423 which was a landmark discovery of metal ion-directed self-assemblies in supramolecular chemistry, through which a huge number of helicates along with supramolecular metal- and nonmetal-architectures have been constructed mostly based on

6.9 × 106 6 × 104 5.3 × 105 >6 × 106 8.2 × 104 3.1 × 104 7 × 105 >106 >106

125 22

the concept of self-assemblies. Lehn and co-workers found that 6,6′-methyl-2,2′-bipyridine oligomer strands with ether linkages readily intertwine around Cu(I) ions to form a double-stranded helical metal complex (260a) (Figure 81A). Although the first 13825


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Figure 84. (A) Structures of feglymycin (268) and peptide (269). Two crystal structures of 268 and one of the energy-minimized MD structures of 269. (Reproduced with permission from ref 447. Copyright 2007 The Royal Society of Chemistry.) (B) Chemical and X-ray crystal structures of 270 and 271. (Reproduced with permission from refs448 and 449. Copyright 2007 American Chemical Society and Copyright 2012 Wiley-VCH, respectively.) (C) Formation of a hydrogen-bonded double-stranded helix ([(272)2H2](ClO4)2) and a space-filling model of the X-ray crystal structure of [(272)2H2](FeCl4)2. (Reproduced with permission from ref 450. Copyright 2014 The Royal Society of Chemistry.) (D) Schematic illustration of self-assembled double-helix formation of chiral phosphoric acid diesters ((S)-273a) and their DFT calculated homodouble helix model structure. (Reproduced with permission from ref 451. Copyright 2009 Wiley-VCH.)

of crown ether, presumably with an intramolecular helicate-like structure (Figure 81B).425,426 Optically active 2,2′-bipyridine oligomers have been prepared by introducing a chiral substituent with an (S)-configuration at the ether linkage, which spontaneously self-assemble into double-stranded helicates with an almost right-handed helical structure upon complexation with Cu(I) and Ag(I) ions, as supported by 1H NMR analyses and presumed by CD studies (263a and 263b) (Figure 81C).427 An ethylene-linked terpyridine trimer also self-assembles to form a double-stranded helicate (264) in the presence of Fe(II) ions, which is kinetically inert toward racemization resulting from the

double-stranded helicate (260a) was a mixture of both handed helices, an optically active double-stranded helicate has also been synthesized by introducing one of the deoxyribonucleotides, thymidine, into the periphery of the helicate (260b), the structure of which resembles the DNA double-helix, and was called a deoxyribonucleohelicate.424 At approximately the same time when the term “helicate” was proposed, Nabeshima et al. synthesized analogous bipyridine oligomers (261a, 261b, and 262) for selective alkaline metal ion transport using transition metals including a Cu(I) ion, which coordinates to the bipyridine residues, producing a type 13826


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Figure 85. (A) Structural formulas of oligoresorcinols. Schematic illustration of racemic double-helix formation of achiral oligoresorcinols in water; X-ray crystal structures of single- (left) and double-stranded helices of 274 (n = 5) are shown. (B) Schematic illustration of single- and double-helix formations of poly(m-phenylene)s bearing a chiral or achiral oligo(ethylene oxide) side chain at the 5-position (276) in methanol and water, respectively. (Reproduced with permission from ref 455. Copyright 2009 The Royal Society of Chemistry.) (C) Possible duplex and double-helix structures of 277. (Reproduced with permission from ref 456. Copyright 2008 American Chemical Society.) (D) Schematic illustration of single- and intramolecular double-helix formations of water-soluble ethynylpyridine polymers ((R)- and (S)-278). (Reproduced with permission from ref 457. Copyright 2012 The Royal Society of Chemistry.)

aromatic oligoamides with the same sequence, but with different chain lengths and substituents at the pendant or terminal ends (R1−R3) (198a, 198b, 266a−s, and 267a−c) (Table 1). The structures of the homodouble helices have been fully characterized by 1H NMR and X-ray crystallographic analyses (Figure 82B).311,432−437 The oligoamide strands take a single-stranded helical conformation via intramolecular hydrogen bonds between the amide NH protons and their adjacent pyridine N atoms as described in section 3.1.2, and further hybridize to form a double-helix in solution, mostly stabilized by interstrand aromatic−aromatic interactions between the pyridine rings of each strand. The Kdim estimated for a series of aromatic oligoamides (198a, 198b, and 266a−s) are summarized in Table 1. The homodouble helices are favorably formed at low temperatures, while they tend to dissociate into single strands at high temperatures. The duplex formation of the strands (266b−g) with decyloxy side chains and benzyloxycarbonyl (CBz) end groups exhibits a peculiar chain-length dependence; the Kdim values for 266b−d increased with an increase in their chain length, whereas after reaching a maximum value (n = 4, 266d), those for 266e and 266f decreased as the chain length further increased.433 However, the authors later noted that the Kdim values for 266e and 266f were incorrectly estimated, because a much longer time was needed for these longer oligomers to reach the thermodynamic equilibria between their single and double helices.437 As the result, the Kdim values for the

substantial feature of the terpyridine framework (Figure 81D).428 Thus, the double-stranded helicate (264) was, for the first time, resolved into enantiomers by chiral ion-exchange chromatography using an aqueous eluent containing an optically pure salt. Unlike Fe(II) helicates composed of terpyridine oligomers, such as 264, Cu(I) double-stranded helicates are mostly labile; therefore, successful examples of the optical resolution are limited to one example (see section 4.5).429,430 However, both enantiomers of a tetranuclear Cu(I)-bound double-stranded helicate (265a) composed of ketimine-bridged tris(bipyridine) strands have been successfully separated by conventional diastereomer salt formation followed by ion exchange with an achiral anion, showing mirror image CDs (Figure 81E).431 The double-helical structure of 265b in the solid state as well as in solution has been confirmed by X-ray crystallographic and 2D NMR analyses, respectively. In 2000, Lehn, Huc, and co-workers developed a novel class of metal-free double-stranded helices of aromatic oligoamides composed of alternating 2,6-diaminopyridines311,432 and 2,6pyridinedicarboxylic acids (198 and 266 in Figure 82A) (see also P1 and P2 in Figure 83). An equilibrium exists between the single and homodouble helices in solution depending on the temperature and concentration, and the structures of both helices were elucidated by X-ray crystallography and NMR measurements. Since then, Huc et al. have extensively investigated the homodouble-helix formation of a series of 13827


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Figure 86. (A) Racemic (upper) and preferred-handed (bottom) homodouble helix formations of 279 through interstrand self-association and inclusion complexation with optically active amines, respectively. (B) Preferred-handed homodouble helix formations of optically active (S)-280 through interstrand self-association (upper) and inclusion complexation with achiral amine (piperidine) (bottom), respectively. (C) X-ray crystal structures of (281)2 (a) and (281)2·(2-MP)2 (b). (Reproduced with permission from ref 458. Copyright 2013 Wiley-VCH.)

Interestingly, the double-helix formation of the strands (198a, 198b, and 266r) resulted in an inhibition of the N-oxidation of the central pyridine rings, while the N-oxidation of the peripheral pyridine rings in the sequence of the strands was significantly enhanced. X-ray and 1H NMR studies revealed that the N-oxidized strands (267a−c) (Table 1) can also selfassemble into a homodouble helix (Figure 82).435 Huc et al. further designed and synthesized another series of aromatic oligoamide strands with different sequences by replacement of the pyridine units (P) with different aromatic subunits, such as A, Q, and F, in order to enlarge the diameter of the homodouble helices in a controlled way, resulting in the enhanced stability of the duplexes. X-ray crystallographic

analogous oligomers 266n−q bearing isobutoxy side chains and tert-butoxycarbonyl (Boc) end groups monotonically increased up to >106 M−1 with the increasing chain length.437 The stability of the double helices is also highly influenced by interstrand interactions between the side chains as well as the terminal groups (Table 1).434,436 For instance, the Kdim values of 198a, 198b, 266a, 266c, 266h, 266j, and 266l (n = 3) with different side chains and terminal residues increased in the following order: 198a < 198b < 266c < 266a < 266l < 266j < 266h in chloroform at 25 °C, indicating that the fully aliphatic and aromatic side chains (R1 and R2) stabilize the double-helix formation, while the end groups (R3), such as an amide, Boc, and CBz groups, destabilize the double-helical structures. 13828


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Figure 87. (A) Structures of oligomers (282a−c). (B) Schematic illustration of equilibrium between oligomers as single and double helices, or inclusion complexes with rod-like guests with different chain lengths (283) and bulky end groups (283, 284b, and 284c). (C) X-ray crystal structures of 282b⊃284b, 282a⊃284c, 282a⊃284a, and 282a⊃285a. (D) (a) Structures of rod-like guests with two identical (285a) or different (285b) stations. (b) Scheme of helix sliding of 282a between two identical stations along a symmetrical guest, in which the yellow dots indicate the hydrogen bond acceptors. (c) Scheme of helix sliding along a nonsymmetrical guest, in which the green and red dots indicate the amine function of 285b and the corresponding ammonium, respectively. (Reproduced with permission from ref 440. Copyright 2011 American Association for the Advancement of Science.)

bonds between ClO4− and the two ligand strands (Figure 84C).450 The single-crystal X-ray analysis of its model, [(272)2H2](FeCl4)2, supports such a doubly hydrogen-bonded double-helical structure. Taking advantage of the strong and directional hydrogen bond formation of phosphoric acids, a novel biphenol-derived phosphoric acid diester ((S)-273a) has been synthesized (Figure 84D).451 NMR and CD measurements along with the density functional theory (DFT) calculations revealed the formation of a homodouble helix with a preferred-handed helical sense biased by chiral substituents introduced at the linker unit. Upon complexation with the corresponding achiral strand (273b), three double helices, including two homodouble helices ((S)-273a·(S)-273a and 273b·273b) and a heterodouble helix ((S)-273a·273b) were produced. Based on the detailed CD studies of the homo- and heteroduplex formations with different molar ratios between (S)-273a and 273b together with simulations, it was concluded that a significant degree of chiral amplification takes place during the heterodouble helix formation, giving rise to a preferred-handed heterodouble helix with ca. 70−80% de. The uniform oligoresorcinols (274) self-assemble into homodouble helices in water via interstrand aromatic−aromatic interactions, thereby showing a remarkable hypochromic effect and upfield shifts in their absorption and 1H NMR spectra, respectively. In contrast, they take a random-coil conformation in organic solvents, such as methanol (Figure 85A).452 The pentamer (274) (n = 5) was found to form single and double

analyses (Figure 83a−f) and an energy-minimized molecular modeling (Figure 83g) also supported the homodouble helical structures.438−445 X-ray crystallographic analyses of feglymycin in two crystal forms (268a and 268b) obtained from different crystallization conditions revealed similar double-helical structures (Figure 84A).446 However, these double-helical dimers in the crystals have a significantly lower symmetry when compared to those of gramicidin having nearly exact 2-fold symmetry, likely due to the D,L-alternating sequence for feglymycin that is not as regular as for gramicidin. The oligomeric peptides (269) (n = 3, 4) have also been reported to have an intertwined doublehelical structure similar to those for gramicidins and their analogs (Figure 84A), as elucidated by NMR and electron-spray ionization (ESI) mass spectral studies along with MD calculations.447 Wisner and co-workers synthesized an alternate ADADA pentamer (270) and AADD tetramer (271) using a pyridine unit, and 1,4-thiazine-1,1-dioxide and indole units as a hydrogen acceptor (A) and donor (D), respectively (Figure 84B).448,449 270 and 271 self-assembled to form homodouble helices via intermolecular hydrogen bonding interactions supported by 2D 1H NMR and X-ray crystallographic analyses. Kwong and co-workers found that a chiral biphenylene-linked bis(2,2′-bipyridine) (272) forms a homodouble helix upon monoprotonation of the terminal bipyridine residues in the presence of HClO4, assisted by the interior interstrand hydrogen bonds together with exterior CH/Cl hydrogen 13829


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Figure 88. (A) Structural formulas of foldamer (286) and dumbbell guests (L- and D-287). (B) (left) Equilibrium between P- (blue) and M- (red) single helices of 286 and its P- and M-double helices. (top) Equilibrium between P- and M-single helices of 286 and its 1:1 inclusion complex ((M)286⊃L-287). (bottom) Equilibrium between P- and M-double helices of 286 and its 2:2 inclusion complex ((P)-(286)2⊃L-(287)2). The structures of (M)-286⊃L-287 and (P)-(286)2⊃L-(287)2 are obtained by molecular modeling and X-ray crystallography, respectively. (Reproduced with permission from ref 460. Copyright 2012 American Chemical Society.)

active group at both ends of each strand (275), thus showing a Cotton effect in the m-phenylene chromophore region. However, the CD completely disappeared once dissolved in methanol because it unfolds into a random-coil as anticipated. The double-helix formation of the oligoresorcinols in water and its mechanism, including kinetics and thermodynamics, have been thoroughly investigated using a series of chiral and achiral oligomers from 2mer to 15mer.453 Interestingly, the stability of the double helices shows a significant chain-length dependence and increased with an increase in the chain length up to 11mer, while it remained the same for oligoresorcinols longer than 11mer.453 Similar observations have been reported by Huc et al. for aromatic oligoamides during their double-helix formations.437 The kinetics study of an optically active heterodouble helix formation of 11mer through the chain exchange reaction between the corresponding optically active and optically inactive homodouble helices investigated by the time-dependent CD intensity changes suggests that it takes place via a direct exchange pathway, not via a dissociation-exchange one because of a low ΔS‡ value during a very fast chain exchange process.453 In contrast to the oligoresorcinols, analogous m-phenylenebased polymers bearing oligo(ethylene oxide) pendants at the 5-positions (276) fold into a single-stranded helical conformation in methanol via solvophobic interactions but possess a random-coil in chloroform (Figure 85B).454,455 As anticipated, a preferred-handed helical conformation can be induced in the poly(m-phenylene) by introducing an optically active pendant (276a) in methanol, and its helix-sense excess

Chart 31. Heterotopic Helicates Consisting of Molecular Strands Complementary to Each Other

helices in the crystals grown from a chloroform/acetonitrile mixture and from water, respectively, although the single-helical conformation immediately unfolds once dissolved in organic solvents. The crystal structure of the double-helix indicates a series of π−π and CH−π interactions that contribute to stabilizing the duplex, which is consistent with the results of absorption and NMR studies in water. An excess one-handed screw-sense can be induced in dynamically racemic double helices by introducing an optically 13830


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Figure 89. (A) Structural formulas of a series of complementary hydrogen-bonded heterodouble helices. Solid-state and optimized structures of 290· 292 (left) and 291a·292 (right), respectively. (Reproduced with permission from refs 464 and 465. Copyright 2010 The Royal Society of Chemistry and Copyright 2012 Wiley-VCH, respectively.) (B) (a) X-ray crystal structure of heterodouble helix of 266r·267c. (Reproduced with permission from ref 435. Copyright 2006 Wiley-VCH.) (b) Homo- and heterodouble helix formations of 293a and 293b. (c) Structures of 294 and preferential heterodouble helix formation of 294a with 294b. The X-ray crystal structure of the heterodouble helix of 294a·294b is shown. (Reproduced with permission from ref 442. Copyright 2014 Wiley-VCH.)

Figure 90. Schematic illustration of unwinding of the homodouble helices of oligoresorcinols (274) followed by the [3]pseudorotaxane (right) and heterodouble helix formations (left) with CyDs and linear oligosaccharides (295−298), respectively. (Reproduced with permission from refs466 and 467. Copyright 2007 American Chemical Society.)

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a homodouble helix stabilized by hydrogen bonds, as supported by the 1H NMR and vapor pressure osmometer (VPO) analyses (Figure 85C).456 The oligomer exhibited an ICD upon inclusion complexation with monosaccharide derivatives, which was accompanied by dissociation of the duplex into single helices. Optically active poly(m-ethynylpyridine)s bearing chiral side chains ((R)- and (S)-278) have been reported to display a unique CD spectral change in water depending on saccharide recognition as well as their molecular weight and temperature. Based on the fact that the saccharide-induced CD spectral changes were similar to those upon heating, an intramolecular double-helix formation of 278 has been proposed for a higher molecular weight 278 that may occur at low temperatures, while, at high temperature, the duplex likely dissociates into single helices (Figure 85D).457 A racemic homodouble helix ((279)2) has also been constructed by an interstrand self-association of m-terphenylbased carboxylic acid dimer (279) through hydrogen bonding interactions between the carboxy groups (Figure 86A), as revealed by an NOE cross-peak clearly observed between the interstrand protons of 279 in CDCl3.458 The crystal structure of 281, a unit model for 279, revealed a self-associated, entwined homodouble helical structure stabilized by the hydrogen bonds between the two carboxy groups (Figure 86Ca). The doublehelix (279)2 is racemic and optically inactive, but it showed intense Cotton effects in the absorption region of pdiethynylphenylene linkages (300−370 nm) in the presence of optically active amines, such as (R)-1-phenylethylamine and (S)-2-methylpiperidine ((S)-2-MP). The CD intensity of 279 gradually increased with the increasing amount of the chiral amine and reached a constant value in the presence of 2 equiv of the chiral amine. The X-ray single-crystal analysis of the 281· rac-2-MP complex showed a unique double-stranded inclusion structure in which the two m-terphenyl units sandwich a pair of the enantiomeric R and S amines 2-MP through a 2 + 2 carboxylate-ammonium ion-pair composed of the two carboxylate anions and the two R and S ammonium cations (Figure 86Cb). Based on these results, 279 most likely formed a double-stranded helical inclusion complex with a preferredhandedness induced by included chiral amines (Figure 86A). The formation of such inclusion complexes of 279 with the chiral amines was also supported by cold-spray ionization mass and 1H 2D NMR spectra. The dimer 279 exhibited almost no chiral amplification in the presence of nonracemic 1-phenylethylamine (majority rule effect) or a mixture with benzylamine (sergeants and soldiers effect) in contrast to the result of the corresponding m-terphenyl-based polymer 521, which showed clear chiral amplification (see section 5.4.1). On the other hand, the optically active dimer (S)-280, bearing the chiral alkoxy substituents on the central phenylene ring, displayed an ICD in CDCl3 due to self-association of the dimer strands to form a preferred handed homodouble helix (Figure 86B).459 Interestingly, an inclusion complex formation of (S)-280 with achiral amines, in particular with piperidine and diethylamine, resulted in a significant enhancement of the CD intensities of (S)-280, indicating amplification of the helical chirality that occurs during the formation of the double-helical inclusion complex with achiral amines sandwiched between the carboxylic acid strands (Figure 86B). Although the CD intensity of (S)-280 also remarkably changed in the presence of chiral amines, its homodouble helical inclusion complex formation with enantiomeric mixtures of the chiral amines resulted in a linear increase in the CD intensity with the increasing %ee of the amines,

Figure 91. (A) Complementary double-stranded helix formation and X-ray crystal structure of (R)-299a·300a. (B) CD spectra of optically active amidine dimers ((R)- and (S)-299a) and complementary double helices of (R)-299a·300a and (S)-299a·300a in CDCl3. (Reproduced with permission from ref 470. Copyright 2005 WileyVCH.)

increased with the decreasing temperature, indicating that the induced single-stranded helix is dynamic. Unfortunately, 276a is insoluble in water, but the poly(m-phenylene) with achiral pendant groups (276b) is soluble in water, which further hybridizes to form a double-helix similar to the oligoresorcinols,452,453 as supported by the results of a series of spectroscopic measurements. Therefore, the poly(mphenylene)s provide a useful scaffold for the design and synthesis of both single- and double-helical foldamers. Alternate cooligomers of pyridine and pyridone units (277) have also been found to self-assemble into a dimer, presumably 13832


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Table 2. Association Constants of Duplexes at 25 °C475

Ka (M−1)

a

1

duplex

R

X1

X2

299a·300a 299b·300a 299c·300a 301a·302a 301b·302b 304a·305a 304b·305b

(R)-1-phenylethyl i-Pr c-Hex (R)-1-phenylethyl (R)-1-phenylethyl (R)-1-phenylethyl (R)-1-phenylethyl

none none none 1,4-C6H4 2,5-(MeO)2-1,4- C6H2 Pt(PEt3)2 Pt(PPh3)2

none none none 1,4-C6H4 1,4-C6F4 Pt(PEt3)2 Pt(PPh3)2

CHCl3 6.43 2.66 3.17 2.99 1.12 5.10 1.08

× × × × × × ×

Ka (M−1)

a

DMSOb 3.8 × 104

1013 1012c 1014c 1012 1013 108 107

6.5 × 103 6.6 × 104 ∼0 ∼0

Estimated by CD titration. bEstimated by 1H NMR. cEstimated by CD titration in the presence of an optically active duplex.

and diastereoselectively converted to a left-handed 1:1 inclusion complex ((M)-286⊃L-287) (foldaxane) biased by L-287. Interestingly, the 1:1 inclusion complex ((M)-286⊃L-287) gradually changes to a 2:2 right-handed double-helical inclusion complex ((P)-(286)2⊃L-(287)2) with time. This unusual supramolecular transformation is accompanied by an inversion of the helicity from a kinetically formed left-handed single-helix to a thermodynamically stable right-handed double-helix upon specific inclusion complexations with L-287 (Figure 88B). In contrast to the P- and M-single helices, the equilibrium between the P- and M-double helices ((286)2) is very slow under this condition; therefore, the helix inversion requires dissociation of the 1:1 inclusion complex ((M)-286⊃L-287) into 286 and L287, suggesting that L-287 also forms such an 2:2 inclusion complex with (286)2 in a highly diastereoselective fashion. 4.1.2. Complementary (Hetero-) Double Helices. The currently prepared homodouble helices are certainly reminiscent of the DNA double-helix, but they lack the most important key feature of DNA, that is, the complementarity of the strands with the sequence information, which plays an indispensable role in the genetic information storage and transmission. Taking advantage of the characteristic feature of Cu(II) ions that form a penta- or hexacoordinate geometry depending on the ligand structures, Lehn and co-workers designed and synthesized a series of molecular strands composed of bipyridine and terpyridine units with a different sequence, thus producing the corresponding heterodouble-stranded helicates complexed with Cu(II) ions (288 and 289) (Chart 31). The structures were elucidated by ESI mass spectrometry and confirmed by X-ray crystallography.461−463 Wisner and co-workers prepared two series of trimeric strands composed of three hydrogen bond donor (290 and 291) and acceptor (292) units, which formed heterodouble helices stabilized by complementary intermolecular hydrogen bonds (290·292, 291a·292, and 291b·292), as supported by an X-ray crystallographic analysis of 290·292 (Figure 89A).464,465 Particularly, the stability of the heterodouble helix (291·292) was remarkably enhanced when the electron-withdrawing

indicating no chiral amplification. In contrast, the corresponding polymer (S)-524, however, displayed a weak but apparent degree of amplification of the helical chirality when chiral amines were sandwiched between the strands (see section 5.4.1). Aromatic oligoamides (282a−c) with different chain lengths (Figure 87A) also fold into double helices, as supported by their crystal structures in Figure 83b (282c (m = 1) and 282b (m = 2)), which, however, unwind into single helices in the presence of rod-like guests with appropriate chain lengths (283 (n = 0−6) and 284a−c) to include the guest molecules within the helical cavity through intermolecular hydrogen bonds between 2,6-pyridinedicarboxamide units of the foldamers and the amide groups of the guests, thus producing unique [2]rotaxanes (foldaxanes) (Figure 87B and C).440 X-ray crystallographic analyses revealed that the foldaxane structures of 282b⊃284b, 282a⊃284c, 282a⊃284a, and 282a⊃285a, in which the benzyl and diphenylmethyl groups of 283−285b, except for 284a, at both ends of the guests are too large to pass through the helical cavities, thus indicating the unfolding/ refolding mechanism (Figure 87C). In contrast, 284a without bulky end groups was suggested to form the foldaxane with 282a through a simple threading mechanism. Interestingly, the helical capsules formed between 282a and guests, 285a and 285b, can shuttle between the two stations on the single chain (Figure 87D), in which the shuttling rate is much faster than the disassembly of the helices. Moreover, the dialkylamino unit of 285b is able to act as another station, so that the helix can favorably encircle over the station of the aliphatic chain. The protonation of the amino group induced the helix to move to the aliphatic station, which is reversibly switched back to the original equilibrium state by adding a base. An analogous helical foldamer (286), which contains no terminal quinoline units in the sequence (208, see section 3.1.2), folds into totally racemic single and double helices that are in equilibrium (Figure 88),460 while single helices are predominant (>99%) at a lower concentration (0.1 mM). In the presence of a dumbbell-shaped chiral guest (L-287), the dynamically racemic single helices of 286 were quantitatively 13833


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Figure 92. (A) Structure of the p-phenylene-linked double-helix (301a·302a). (B) Photoisomerization of (R)-trans-303 and (R)-cis-303. (C) In-situ synthesis of the double-helical 299a·300a from 304b·305b via the formation of 306. (D) Structure and its illustration of the double-stranded metallosupramolecular polymer ((R)-poly-307). (Reproduced with permission from ref 474. Copyright 2006 American Chemical Society.)

substituents were introduced on the trimeric donor strand (291b). Aromatic oligoamide strands bearing pyridine (198a, 198b, and 266r) and pyridine N-oxide units (267a, 267b, and 267c, respectively) at both ends formed homodouble helices, as described in section 4.1.1 (Figure 82 and Table 1). When 198 and 266, respectively, were mixed with 267 in solution, some of them formed heterodouble helices (198a·267a, 198b·267b, and 266r·267c) stabilized by hydrogen bonding interactions between the amide protons, pyridine, and pyridine N-oxide units, as revealed by an X-ray crystallographic analysis of 266r· 267c (Figure 89Ba).435 The 1H NMR spectra of an equimolar mixture of 293a and 293b showed the signals due to the homo((293a)2 and (293b)2) and hetero- (293a·293b) double helices with a statistical distribution, indicating no preference in this system (Figure 89Bb).438 A single-helical foldamer (294b) with a wide diameter in the center can intercalate with a pentameric strand (294a), which forms a stable homodouble helix ((294a)2), as supported by its crystal structure in Figure 83d in section 4.1.1, resulting in the preferential formation of a heterodouble helix (294a ·294b) with an interior volume of ca. 150 Å (Figure 89Bc).442

As described in section 4.1.1, the oligoresorcinols (274) with specific chain lengths form a racemic mixture of homodouble helices (2742) in water via interstrand aromatic interactions. Upon the addition of β- or γ-cyclodextrin (CyD), 2742 (n = 9) was immediately unwound to form [3]pseudorotaxanes with a twist-sense bias induced by the CyD chirality (Figure 90, right).466 When an adamantane derivative was subsequently added, the [3]pseudorotaxane with β-CyD reverted to 2742 due to an inclusion complex formation with β-CyD. Moreover, 2742 cross-hybridized with linear oligosaccharides (295−298) with a specific chain length and glycosidic linkage patterns, thus selectively producing excess one-handed heterodouble helices with α-1,6-D-isomaltooligosaccharides (296) (Figure 90, left).467 Although a number of duplexes have been synthesized based on the hydrogen-bonding-driven supramolecular assemblies, most of them possess rather nonhelical zipper or ladder structures.8,468,469 An interesting and new class of complementary double helices with an excess one-handedness has been rationally designed and synthesized using specific amidinium-carboxylate salt bridges (Figure 91A).470 The diacetylene-linked m-terphenyl-based dimers bearing optically 13834


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Chart 32. Structures of Chiral and Achiral Amidine and Carboxylic Acid Dimers Connected by a Variety of Linkers

Table 3. Rates of Chain Exchange Reactions in CDCl3475

a

duplex

X

k (M−1 s−1) at 25 °C

t1/2 at 25 °Ca

ΔS‡ (J mol−1 K−1)b

299a·300a 301a·302a 304a·305a 304b·305b

none 1,4-C6H4 trans-Pt(PEt3)2 trans-Pt(PPh3)2

0.48 6.4 57 379

2h 9.1 min 61 s 9s

−80.6 −32.5 +81.1 +53.5

Half-life time period. bEstimated based on the Eyring plot for the chain exchange rates.

13835


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Figure 93. Schematic illustration of possible mechanisms (Routes A and B) of the chain exchange between the (R)-duplex and (S)-strand. (Reproduced with permission from ref 475. Copyright 2012 American Chemical Society.)

Table 4. Rates of Helix-Inversion475

a

duplex

X

R1

solvent

Tc (°C)a

G‡298 (kJ/mol)

τ298 (s)b

299b·300a 299c·300b 301c·302a 304c·305a

none none 1,4-C6H4 trans-Pt(PEt3)2

i-Pr c-Hex c-Hex c-Hex

CDCl3 CDCl3 CD2Cl2 CD2Cl2

−47 −26 < −80c < −80c

45.8 50.0

1.71 × 10−5 9.37 × 10−5

Coalescence temperature. bLife time of the one-handed helical state at 25 °C. cCoalescence temperature was not observed.

The photoresponsive azobenzene-linked complementary double-helix ((R)-303) with a twist-sense bias underwent reversible structural changes between (R)-trans-303 and (R)cis-303 without dissociation into each single strand via trans−cis isomerization by alternative photoirradiation with UV and visible light (Figure 92B) with maintaining the duplex structure.473 The kinetics analysis and molecular mechanics (MM) calculations revealed that the photoisomerization brought about a change in the molecular length of (R)-303 by ca. 15%, indicating that the (R)-303 is capable of acting as a photoresponsive molecular spring (see section 7). The trans-Pt(II)-acetylide-linked complementary doublehelix (304b·305b) bearing achiral triphenylphosphine (PPh3) ligands coordinated on the Pt(II) atoms maintained its preferred-handed double-helical structure. Upon the addition of cis-1,2-bis(diphenylphosphino)ethylene (dppee), the intrastrand ligand exchange reaction took place, quantitatively producing (R)-cis-306. Interestingly, the further treatment of (R)-cis-306 with iodine resulted in the formation of the fully original (R)-299a·300a double-helix linked by the diacetylene linkers through the oxidatively induced reductive elimination of the (R)-cis-Pt(II)-dppee linkers (Figure 92C),472 demonstrating the first example of the in situ double helix-to-double helix transformation driven by the chemical reactions. The salt bridge-based structural motif has been successfully applied to the synthesis of complementary double-stranded

active amidine ((R)-299a) and optically inactive carboxylic acid residues (300a) spontaneously formed a complementary double-helix ((R)-299a·300a) in chloroform through the amidinium-carboxylate salt bridges, thus showing intense Cotton effects in the absorption region of the diacetylene linkages around ca. 300−370 nm; its enantiomeric counterpart, (S)-299a·300a, exhibited the perfect mirror image Cotton effects, while the corresponding single strands of the amidine dimers ((R)- and (S)-299a) showed very weak CD signals (Figure 91B). A single-crystal X-ray analysis unambiguously determined the right-handed double-helical structure of (R)299a·300a biased by the chirality introduced on the amidine residues (Figure 91A).470 Taking full advantage of the versatility of the amidiniumcarboxylate salt bridges having high association constants even in polar solvents due to double hydrogen bonding along with the unique structural feature of the crescent-shaped mterphenyl based rigid backbones that force the duplexes to take double helices other than nonhelical duplexes, a series of complementary double-helical dimers (Table 2), oligomers (sections 4.1.3 and 4.4), and polymers (section 5.4.1) have been synthesized by introducing various linkers, e.g., pdiethynylbenzene (301a·302a, Figure 92A) and trans-Pt(II)acetylide (304b·305b, Figure 92C) linkages,471,472 while maintaining the predominantly one-handed double-helical structures. 13836


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Figure 94. (A) Schematic illustration of the design strategy for a short complementary double-helix and synthesis of (R)-308·309 from the corresponding (R)-308 and 309 strands. (Reproduced with permission from ref 476. Copyright 2010 The Royal Society of Chemistry.) (B) A skewed right-handed double-helix formation through salt bridges between dimer strands bearing chiral flexible N-linked formamidine ((R)-310) and achiral carboxylic acid residues (300b). (C) X-ray structure of the model duplex derived from the corresponding monomeric strands: (a) top view and (b) side view. The light blue dashed lines represent the intermolecular N−H···O hydrogen bonds. (c) The geometric representation of the skewed salt bridge. (Reproduced with permission from ref 477. Copyright 2010 American Chemical Society.)

307) (Figure 92D).474 The 1H NMR, CD, and dynamic light scattering (DLS) measurements together with the direct AFM observation revealed the formation of an excess one-handed metallosupramolecular helical polymer (poly-307) with the average length of approximately 100 nm, which corresponds to ca. 40 repeating units.

supramolecular helical polymers. A novel amidinium-carboxylate duplex (307) was designed to possess pyridine groups at the four ends, to which metal-coordination readily proceeds, leading to the supramolecular polymerization in the presence of cis-PtPh2(DMSO)2 in 1,2-dichloroethane to yield a supramolecular double-stranded helical polymer ((R)- or (S)-poly13837


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Figure 95. (A) Synthesis of the [2]catenane (m-313) from complementary strands with longer allyl linkers at the m-position (m-311a and m-312a) via the amidinium-carboxylate salt bridges during the RCM reaction and (B) schematic illustration of control over on/off switching of the salt bridge between the two macrocyclic components using an acid/base or Zn(II)/[2.2.1]cryptand system. Photographs of a solution of m-313 in dichloromethane/THF (10/1, v/v) under irradiation at 254 nm. (Reproduced with permission from ref 478. Copyright 2010 Wiley-VCH.) (C) Synthesis of the [1 + 1]macrocycles (o-, m-, and p-314) from the corresponding complementary strands with shorter allyl linkers (311b and 312b) at the o-, m-, and p-positions via the amidinium-carboxylate salt bridges during the RCM reaction and (D) schematic illustration of stimuli-triggered reversible structural changes between double-helical and nonhelical macrocycles of 314. (Reproduced with permission from ref 479. Copyright 2013 The Royal Society of Chemistry.)

Figure 96. (A) Structures of m-terphenyl-based oligomers (n = 2, 3, 4) with amidine and/or carboxyl groups and their corresponding double helices through the complementary amidinium-carboxylate salt bridge formations. “A” and “C” of the strands represent the monomer units with the chiral amidine and achiral carboxyl groups, respectively. (B) Schematic illustrations of the sequence-specific sorting of the six trimers with amidine and/or carboxyl groups via complementary double-helix formation. (C) The sequence-specific double-helix formation of CCA with AAC among three strands with a different sequence and its isolation by HPLC. (Reproduced with permission from ref 480. Copyright 2008 American Chemical Society.)

of linkers, such as diacetylene, Pt(II)-acetylide, and pdiethynylbenzene linkages, have been synthesized by a modular strategy (Chart 32).475 Most of the optically active double helices with the same (R)or (S)-amidine residues, except for 304b·305b, exhibited intense Cotton effects in chloroform at 25 °C, whose CD spectral patterns were different from each other due to the

In order to investigate the effects of the linker structures and the substituents on the amidine residues on their chiroptical properties and the thermodynamic and kinetic stabilities of the heterodouble helix formations stabilized by the amidinium− carboxylate salt bridges, a series of dimeric strands composed of m-terphenyl skeletons bearing complementary chiral or achiral amidines and achiral carboxylic acid groups joined by a variety 13838


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Figure 97. (A) The imine-bond forming reaction between 315 and 316 in the presence of the chiral trans-azobenzene-linked amidine template (trans-317). (B) Conceptual illustration for the photoregulated synthesis of complementary double helices stabilized by salt bridges via the templatedirected imine-bond formation between 319 and 320 in the presence of trans-321 and cis-321. (Reproduced with permission from ref 482. Copyright 2013 The Royal Society of Chemistry.)

Table 5. Results of Diastereoselective Imine-Bond Formations between 316 and rac-109 in the Presence of Chiral Amidine Templates (323a−c and 324a)483 initial run 1 2 3 4 5 6

template with linker chirality c

(R,R)-323a (S,S)-323ac (R,R)-324ac (S,S)-324ac (R,R)-323bd (R,R)-323ce

at equilibrium

conv. of 109a (%)

dea (%)

sb

time (h)

conv. of 109a (%)

15 13 27 14 9 12

57 (R,R) 32 (R,R) 0 4 (R,R) 8 (R,R) 9 (R,R)

4.0 2.0 1.0 1.1 1.2 1.2

3 4 1 2 2 2

48 49 50 50 50 50

dea (%) 30 15 0 30 8 9

(R,R) (S,S) (S,S) (R,R) (R,R)

Estimated by 1H NMR. bSelectivity factor determined using s = krel(fast/slow) = ln[1 − C(1 + de)]/ln[1 − C(1−de)], where C is the conversion of 109 and taking the de of 3162109. c(R)-1-Phenylethyl amidines were used. dAchiral isopropyl amidines were used. eAchiral cyclohexyl amidines were used. a

diacetylene linker (299a·300a), which is consistent with the reverse order of their bulkiness. The substituents on the amidine residues also affected the stabilities of the double helices, and the Ka values increased in the following order: isopropyl (i-Pr) (299b·300a) < (R)-1-phenylethyl (299a·300a) < cyclohexyl (c-Hex) (299c·300a). As anticipated from the CD results in DMSO, the double helices were less stable in DMSO than in chloroform and the Ka values in DMSO-d6 at 25 °C decreased in the following order: 299a·300a > 301a·302a > 304a·305a. Interestingly, the stability of the double-helix (301a·302a) in chloroform as well as in DMSO was significantly enhanced by the introduction of either electrondonating (2,5-dimethoxy) or electron-withdrawing (2,3,5,6tetrafluoro) substituents at the p-phenylene linkers of the

structural difference in their linkages, but the Cotton effect signs were more or less similar to each other, suggesting that the (R)-duplexes probably adopt the same right-handed double-helical structures biased by the (R)-amidine residues like the (R)-299a·300a (Figure 91A) independent of the linker structures. In DMSO, (R)-299a·300a and (R)-301a·302a also showed intense Cotton effects, whereas the ICD of (R)-304a· 305a almost disappeared due to dissociation into single strands in polar DMSO that hampers the salt bridge formation.475 The Ka values estimated by CD titrations combined with competition CD titrations (Table 2) revealed a link to the observed solvent effects on the CD spectra. The Ka values for the duplexes increased in the following order: Pt(II)-acetylide linker (trans-Pt(PEt3)2 (304a·305a) and trans-Pt(PPh3)2) (304b·305b)) < p-diethynylbenzene linker (301a·302a) < 13839


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Scheme 1. Diastereoselective Imine-Bond Forming Reaction between 316 and rac-109 in the Presence of Chiral Amidine Templates (323a−c and 324a) via Complementary Double-Helix Formation

(R)-308·309 in chloroform was slightly less than that of (R)301a·302a, the m-phenylene-based heterodouble helix has a significant advantage over the m-terphenyl-based one due to its synthetic accessibility. An analogous m-terphenyl-based amidine dimer joined by the diacetylene linker was also synthesized by replacing the rigid C-linked (R)-amidines ((R)-301a) with flexible N-linked (R)-formamidines ((R)-310) (Figure 94B).477 In contrast to the thermodynamically stable, right-handed heterodouble helix ((R)-299a·300a), the duplex with 300b ((R)-310·300b) showed very weak CD signals below 370 nm because the Nlinked (R)-formamidine group has only one stereogenic center located far from the m-terphenyl backbone. The CD intensities, however, gradually increased with the decreasing temperature and became drastically enhanced at −55 °C, although the CD intensity at −55 °C was still much lower than that of the (R)299a·300a, indicating the dynamic nature of its helical conformation with a low helix-sense excess. The single-crystal X-ray analysis of the model monomeric duplex revealed that the (R)-310·300b duplex likely adopts a skewed right-handed double-helical structure (Figure 94C), which may be responsible for its weaker CD than that of the (R)-299a·300a stabilized by the face-to-face complexed salt bridges. The unique and versatile structural motif of the complementary amidinium-carboxylate salt bridges with an optical activity has been further applied to the rational design and synthesis of a mechanically interlocked [2]catenane and helically twisted [1 + 1]macrocycles (Figure 95). The synthesized m-terphenyl-based chiral amidine and achiral carboxylic acid monomers bearing two longer (m-311a and m-312a) and shorter (o-, m-, and p-311b and -312b) arms with terminal vinyl end groups at both termini connected via an mlinkage and o-, m-, and p-linkages, respectively, formed heterodouble helices through the salt bridges. The further ring-closing metathesis (RCM) reactions of m-311a·m-312a with longer arms and 311b·312b with shorter arms using the first-generation Grubbs catalyst efficiently and selectively produced the optically active [2]catenane (m-313) (Figure 95A)478 and helically twisted [1 + 1]macrocycles (o-, m-, and p-

duplex ((R)-301b·302b) (Chart 32) due to the favorable CT interactions between the paired phenylene linkers (Table 2).475 The chain exchange experiments between the (R)-duplexes with different linkers and the corresponding enantiomeric (S)amidine single strands using CD by following the CD intensity changes with time revealed the effects of the linker structures on the kinetic stabilities of the (R)-duplexes and the mechanism of the chain exchange processes (Table 3). The kinetic analyses of the chain exchange reactions for the diacetylene- ((R)-299a· 300a) and p-diethynylbenzene-linked ((R)-301a·302a) duplexes indicated that these were slow processes with negative ΔS‡ values (−80.6 and −32.5 J mol−1 K−1, respectively), indicating that the chain exchange proceeds through direct exchange pathways (Route A) (Figure 93). On the other hand, those for the Pt(II)-acetylide-linked duplexes ((R)-304a·305a and (R)-304b·305b) were fast processes with positive ΔS‡ values (+81.1 and +53.5 J mol−1 K−1, respectively), suggesting that dissociation of the duplexes takes place followed by a chain-exchange (Route B) (Figure 93). Consequently, the chain exchange rates (k) increased with increasing linker bulkiness (Table 3).475 The helix-inversion kinetics for the racemic dimer duplexes (299b·300a, 299c·300b, 301c·302a, and 304c·305a) bearing achiral amidines were also examined based on variabletemperature 1H NMR measurements (Table 4).475 The diacetylene-linked 299b·300a and 299c·300b exhibited a coalescence temperature (Tc) at −47 and −26 °C, respectively, in CDCl3, while the p-diethynylbenzene-linked 301c·302a and Pt(II)-acetylide-linked 304c·305a did not show such a Tc even at −80 °C in CD2Cl2, indicating that the barriers for the helixinversion were mostly determined by the linker structures and were not significantly influenced by the amidine substituents. Using a similar synthetic strategy, a more compact heterodouble helix stabilized by the amidinium-carboxylate salt bridges has been synthesized by replacing the m-terphenyl groups of (R)-301a·302a with m-phenylene ones ((R)-308· 309) (Figure 94A),476 which also likely possess the same righthanded double-helical structure biased by the (R)-amidine residues as that of (R)-301a·302a, thus showing intense Cotton effects above 250 nm in chloroform. Although the stability of 13840


Chemical Reviews

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Figure 98. Structures of (A) ethynyl[4]helicene oligomers, (B) their derivatives with flexible and rigid linkers, (C) amido[4]helicene oligomers, (D) sulfonamido[4]helicene oligomers, (E) oxymethylene[4]helicene oligomers, and (F) [4]helicene bidomain and tridomain oligomers. Schematic illustration of reversible structural changes of 332 in a two-state manner upon heating and cooling. (Reproduced with permission from ref 500. Copyright 2012 Wiley-VCH.)

314) (Figure 95C),479 respectively, in good yields (68, 67, 92, and 96%, respectively). Due to the optically active (R)-amidine residues, the salt bridges were twisted in one direction, thus showing distinct Cotton effects in the m-terphenyl-based π-conjugated chromophore regions; the CD spectral patterns were almost identical,

independent of the topology, except for o-314. The reversible switch between the “locked” and “unlocked” states of the two macrocyclic components of the [2]catenane (m-313) and that between the helical and nonhelical states of the macrocycles (o-, m-, and p-314) were achieved by the sequential addition of an acid and base or Zn(II) and [2.2.1]cryptand, which produced 13841


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Figure 99. (A) Structures of pseudoenantiomeric aminomethylene[4]helicene oligomers ((M)-333 and (P)-334). (B) Temperature-responsive structural change of (M)-333 between double-helix and random coil. Model of two-sided thermal hysteresis of (M)-333 in toluene. The Δε values at equilibrium are drawn between their points. The energy diagrams for the temperature-induced inversion of the relative thermodynamic stability between double-helix (333)2 (favored at low temperature) and random coil 333 (favored at high temperature), as shown in states c and a, respectively. The relative populations of molecules in the (333)2 and 333 structures are represented by bars. (Reproduced with permission from ref 504. Copyright 2014 Wiley-VCH.) (C) Heating and cooling stimuli generate three-state molecular switching in the pseudoenantiomeric (M)-333/ (P)-334 system. One of the three-state thermal hysteresis: (P)-334 and (M)-333 (1:1) in fluorobenzene (5.0 × 10−4 M) were cooled from 70 to 42 °C (333 and 334 to (−)-333·334), and then to 5 °C ((−)-333·334 to (+)-333·334), and heated from 5 to 70 °C ((+)-333·334 to 333 and 334). (Reproduced with permission from ref 505. Copyright 2014 American Chemical Society.)

dissociation and restoration of the salt bridges, respectively, resulting in chiroptical changes detected by CD and fluorescence color changes in the Zn(II)/[2.2.1]cryptand system (Figure 95B and D).478,479 Among the macrocycles, o314 was the most sensitive to the Zn(II) ion and exhibited a significant change in its fluorescent properties before and after the addition of the Zn(II) ions, indicating that o-314 is a promising photoluminescence zinc sensor. 4.1.3. Sequence-Specific and Templated-Directed Complementary (Hetero-) Double-Helix Formations. The double-helix DNA-templated polymerization and sequence-specific double-helix formation play an especially vital role in the replication of the DNA, through which genetic information can be stored in the DNA sequences as a code and transmitted. The sequence-specific complementary pairing between the template and monomers and between the strands is of key importance for this elaborated process. The complementary amidinium-carboxylate salt bridges resemble the complementary nucleic acid base pairs in DNA, which

allowed the fully artificial, sequence- and chain-length-specific double-helix formation. A series of homo-oligomers (AA, AAA, AAAA, CC, CCC, and CCCC) consisting of the chiral amidine (A) or achiral carboxylic acid (C) monomer unit, an AC dimer, and four trimers (AAC, CCA, ACA, and CAC) with a different sequence of A and C were synthesized in a stepwise manner. When the three trimer (AA, CC, and AC) or six trimer strands (AAA, CCC, AAC, CCA, ACA, and CAC) were mixed in chloroform, one-handed heterodouble helical dimers (AA·CC and (AC)2) or trimers (AAA·CCC, AAC·CCA, and ACA·CAC) were sequence-specifically and exclusively formed, respectively, via the complementary amidinium-carboxylate salt bridges (Figure 96A and B).480 Upon the addition of CCA to an equimolar mixture of AAA, AAC, and ACA, CCA completely discriminated the sequences and selectively formed the heterodouble helical AAC·CCA, which can be isolated from the mixture by HPLC (Figure 96C). Moreover, the homo-oligomer mixtures of the amidine or carboxylic acid (A, AA, AAAA, C, CC, and 13842


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Figure 100. (A) Schematic illustration of the kinetic resolution of racemic aromatic alcohols in the presence of optically active silica (P)-NPs grafted with a (P)-[4]helicene derivative. Optically active 335 up to 61% ee (S-rich) was obtained by a preferential precipitation of aggregates formed with (P)-NPs. (Reproduced with permission from ref 506. Copyright 2012 American Chemical Society.) (B) Schematic illustration of an equilibrium shift between a double-helix ((P)-325a)2 (n = 5) and a random coil (P)-325a induced by (P)-NPs. (Reproduced with permission from ref 507. Copyright 2014 Wiley-VCH.)

Figure 101. (A) Intramolecular homodouble helix formation of a cyclic ethynyl[4]helicene oligomer ((M)-336) and an intermolecular ternary heterodouble helix formation with (P)-325a (n = 5). (B) Polarized optical microscopy (POM) image of a 1:2 mixture of (M)-336 and (P)-325a in toluene at 25 °C. Formation of lyotropic liquid crystals (left) and turbid gels consisting of randomly oriented bundles upon cooling (right). (Reproduced with permission from ref 510. Copyright 2015 American Chemical Society.)

CCCC) were hybridized in a perfect chain-length-selective manner to form the corresponding double helices (A·C, AA· CC, and AAAA·CCCC), which were also separated by SEC chromatography.

The high stability of the amidinium-carboxylate salt bridges together with its DNA-like complementarity has been further utilized as a novel template for a template-directed synthesis of 13843


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Figure 102. Structural formulas of the molecular ligands that form the (A) triple- and (B) quadruple-stranded helicates and their single-crystal structures. (Reproduced with permission from refs 515 and 516. Copyright 2006 Wiley-VCH and Copyright 2012 American Chemical Society, respectively.)

carboxylic acid dimer (cis-321) (cis-content; ca. 47%) as the template because of its bent structure; the reaction along cis321 was reduced by one-fifth compared to that along the trans321 on the basis of kinetic studies (Figure 97B).482 By the alternative cis-trans photoisomerization of the template strand, the template-directed imine-bond forming reaction was for the first time repeatedly photocontrolled during the course of the same run with ca. 2-fold acceleration upon visible light irradiation. A series of optically active amidine dimers composed of (R)1-phenylethyl amidine (a) and achiral isopropyl (b) and cyclohexyl (c) amidines with an m-terphenyl skeleton joined by either an (R,R)- or (S,S)-cyclic amide with a different amide sequence (323 and 324) have also been synthesized to use as templates for diastereoselective imine-bond formations between two achiral carboxylic acid monomers (316) bearing an aldehyde group at one end and racemic trans-1,2-cyclohexanediamine (rac-109). The imine-bond forming reactions proceeded in a diastereoselective fashion in the presence of the templates except for (R,R)-324a (Table 5), producing a preferred-handed heterodouble helix (Template·3162109) via the salt bridges (Scheme 1).483 The time-dependent 1H NMR spectral changes and kinetic studies revealed that the template (R,R)-323a assisted (R,R)selective imine-bond formation toward 109 to yield the duplex

heterodouble helices, which may lead to a preliminary step toward totally artificial self-replication systems. It has been found that the imine-bond forming reaction between achiral carboxylic acid monomers (315 and 316) bearing an amino or a formyl group at one end was significantly accelerated 6- and 4-fold in benzene-d6 at 30 and 50 °C, respectively, in the presence of an optically active transazobenzene-linked amidine dimer (trans-317) as the template, compared to those in the presence of the monomeric amidine, thus producing an optically active heterodouble helix stabilized by the salt bridges (318·trans-317) (Figure 97A).481 In CDCl3, a similar 10-times acceleration of the imine-bond forming reaction was observed at 25 °C. On the other hand, an analogous trans-azobenzene-linked, achiral carboxylic acid dimer (trans-321) was used as the template, on which the imine-bond forming reaction between chiral amidine monomers (319 and 320) took place with remarkable acceleration of the dimerization by a factor of 34 in CDCl3 at 25 °C (Figure 97B).482 The templates possess photoresponsive azobenzene units as the linkers, which undergo a reversible structural change during photoirradiation (see Figure 92B), thus providing a possible photoswitchable template for photocontrolling the imine-bond formation. The imine-bond formation between 319 and 320 rather slowly proceeded in the presence of the cis-rich 13844


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Figure 103. (A) Schematic illustration of the formation of a [3 + 2] cylindrical triple-helix ((R)-344, left) and of a [4 + 2] cylindrical quadruple-helix coordinated with 346 ((R)-345·346, right). (Reproduced with permission from ref 517. Copyright 2007 Wiley-VCH.) (B) Structure of the naphthyridine tetramer 347. Side and top views in cylindrical and space-filling representations of two crystal structures of (347)3 as (a) parallel and (b) antiparallel triple helices. (Reproduced with permission from ref 518. Copyright 2010 Wiley-VCH.) (C) Structure of the quinoxalinecarboxamide oligomers (348a and 348b). Side and top views of the crystal structures of 348a as a quadruple-helix. (Reproduced with permission from ref 443. Copyright 2008 Wiley-VCH.)

diastereoselectivity toward rac-109 (run 3 in Table 5), while (S,S)-324a exhibited a behavior similar to that of (S,S)-323a, showing the opposite diastereoselectivity at equilibrium (30% de) (run 4 in Table 5). Further replacement of the chiral amidines of 323a with achiral ones ((R,R)-323b and (R,R)323c) caused a significant decrease in the diastereoselectivity (8 and 9% de, respectively) while maintaining the (R,R)-selectivity toward 109 (runs 5 and 6 in Table 5). These results indicated that the amidine chirality predominantly contributed to the present diastereoselective imine-bond forming reactions during the initial stage under kinetic control, whereas the linker chirality of the amide groups mostly played a role to determine the selectivity ((S,S)- or (R,R)-109) and its diastereoselectivity

(R,R)-323a·3162109 with a 57% de during the initial stage, while the de value gradually decreased with time and reached a 30% de at equilibrium (run 1 in Table 5). The selectivity factor (s) estimated during the initial stage based on the assumption that the diastereoselective imine-bond formation could be considered as a kind of kinetic resolution was 4.0. When the linker of (R,R)-323a was replaced by its (S,S)-enantiomer, the template (S,S)-323a showed the same (R,R)-selectivity (32% de) during the initial stage, but the selectivity reversed with time, and the opposite (S,S)-109 was predominantly inserted between 316 to form the (S,S)-323a·3162109 duplex with a 15% de (run 2 in Table 5). On the other hand, surprisingly, template (R,R)-324a, which is only different from (R,R)-323a with respect to the linker amide sequence, showed no 13845


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Figure 104. (A) Structures of aromatic oligoamide foldamer (349) and 1,10-decanediol (350) as a guest. Side view of the crystal structure of the inclusion complex ((349)2⊃350). (Reproduced with permission from ref 519. Copyright 2009 Wiley-VCH.) (B) Structures of aromatic oligoamide foldamer (351a) (m = 4), rod-like guests (283a−f) with various lengths, and 352 with two binding sites. A schematic representation of the screw-like motion of the two strands of a molecular duplex and that of the trapping of screwed (left) and unscrewed (right) double helices upon binding with short and long rod-like guests, respectively. Solid-state structures of inclusion complexes (a) (351a)2⊃283b, (b) (351a)2⊃283d, and (c) (351a)2⊃283e. Illustrations of the screw-like motion are shown in (d) (351a)2⊃283b and (e) (351a)2⊃283d. L and l represent the distance between the two pyridine clefts in each complex (351a)2⊃283b and (351a)2⊃283d, and are equal to 6.8 and 9.0 Å, respectively. (Reproduced with permission from ref 520. Copyright 2011 Wiley-VCH.) (C) Structures of bichromophoric rod (353a) and its precursor (353b). Schematic illustration of the parallel−antiparallel equilibrium of a double-helix and the assembly modulated photophysical properties. Solid-state structures of (a) the free antiparallel double-helix ((351b)2) and (b) the foldaxane ((351b)2⊃353b). (Reproduced with permission from ref 521. Copyright 2016 Wiley-VCH.) (D) Structure of an aryl-triazole foldamer 354 and a molecular model of the double-helix (354)2 with a Cl− anion inside the helical cavity. (Reproduced with permission from ref 522. Copyright 2013 American Chemical Society.)

highly depends on the solvent, temperature, and concentration. It has also been found that the ethynyl[4]helicene oligomers bearing hard and/or electron-withdrawing side chains, such as a perfluorooctyl group (325b), form stable homodouble helices.487 On the other hand, the heterodouble helices were predominantly formed between the pseudoenantiomeric (P)and (M)-ethynyl[4]helicene pentamers with different substituents, such as a decyloxycarbonyl chain ((M)-325a) and a perfluorooctyl chain ((P)-325b), and between the pseudoenantiomeric oligomers ((P)-325a (n = 4) and (M)-325a (n = 5)) with a different number of [4]helicene units (Figure

under thermodynamic control and the linker amide sequences also affected the diastereoselectivity.483 4.1.4. Homo- and Hetero-Double Helices Based on Helicene Oligomers. Yamaguchi et al. designed and synthesized a series of unique optically active acyclic [4]helicene oligomers and found that they dimerize to form homoor heterohybridized double helices depending on the linker functionalities (Figure 98).484,485 The ethynyl[4]helicene oligomers (325) show solvophobically driven self-assembly to form a homodouble helix, as suggested by 1H NMR, CD, and VPO analyses (Figure 98A).486 The formation of double helices 13846


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Figure 105. (A) Structures of PNA and DNA (left) and sequences of PNA, DNA, and modified PNA (PNA*1−4) strands (right). The N- and Ctermini of the PNA strands are indicated by -N and -C, respectively. Deoxyribonucleotides are shown by dA, dT, dG, and dC, and * denotes the chiral residues incorporated in the PNA strands. Lys- denotes an N-terminus L-Lysinamide and T is a backbone PNA Thy residue modified with an (R,R)-cyclohexyl group. (B) CD spectra of (a) PNA*1·PNA, PNA*1L·PNA, and PNA*3·PNA duplexes, and PNA*1 strand, and those of (b) DNA· PNA, PNA*2·PNA, and PNA*4·PNA duplexes, and PNA*2 strand. PNA*1 and PNA*1L contain D-dC and L-dC residues, respectively. (C) Remote enantioselection in PNA-directed PNA-DNA ligation. (c−i) HPLC elution profiles of the products formed by ligation of the PNA primer to the DD and LL enantiomers of a dinucleotide activated as a phosphorimidazolide (5′-d(GC)pIm-3′). The numbers 1 and 2 above the peaks indicate the starting PNA primer (C-TTTGTACGAGA-N) and the product (C-TTTGTACGAGAd(CG)-5′), respectively. The starting PNA primer is observed as the double peak because of an N-terminal rearrangement of the PNA at pH 12 before and during HPLC.533 (Reproduced with permission from ref 525. Copyright 2000 Wiley-VCH.)

98A).488,489 In the latter case, the heterodouble helix further aggregates to form a fiber, thus generating a gel.489−491 Moreover, bis[hexa(ethynyl[4]helicene)] strands linked by either a flexible hexadecamethylene unit (326a) or a rigid butadiyne unit (326b) form an intramolecular double-helix (Figure 98B).492 The heterodouble helix composed of pseudoenantiomeric 326c strands with different lengths also forms a gel.493 Both of the [4]helicene oligomers ((P)-327 and (P)-328) connected by amide linkers with -CONH- and -NHCO- sequences, respectively, form helical homodimers in nonpolar solvents (Figure 98C).494,495 The [4]helicene

tetramer ((M)-329) connected by a sulfonamide linker also forms a helical homodimer in m-difluorobenzene, in which (M)-329 shows thermal hysteresis during reversible structural changes between a helical homodimer and a random coil, and concentration hysteresis (Figure 98D).496−498 Pseudoenantiomeric [4]helicene pentamer ((P)-330) (n = 5) and hexamer ((M)-330) (n = 6) connected by an oxymethylene linker also form heteroaggregates, which further self-assemble into an 1D fibril film at the liquid−solid interface (Figure 98E).499 The [4]helicene bidomain500−502 and tridomain500,503 oligomers (331 and 332) connected by acetylene and amide linkages form 13847


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Figure 106. (A) Structures of chiral/achiral amidine and achiral carboxylic acid strands: All-chiral amidine (a), edge-chiral amidine (b), center-chiral amidine (c), and achiral carboxylic acid strands (d). (B) Schematic illustration of amplification of helical chirality in complementary double-stranded helical oligomers (5-mers) consisting of chiral/achiral amidine and achiral carboxylic acid strands via chirality transfer from the chiral amidine unit at one terminus or at the center. (C) CD intensities of the first Cotton effect of the complementary double helices in chloroform. (Reproduced with permission from ref 534. Copyright 2011 American Chemical Society.)

temperature. Moreover, a similar, but rather complicated thermal hysteresis has also been reported for a 1:1 mixture of the aminomethylene-linked pseudoenantiomeric pentamer ((M)-333) and tetramer ((P)-334), which undergoes a threestate molecular switching in fluorobenzene that involves the heterodouble helices ((−)-333·334) and (+)-333·334) with an opposite handedness to each other, and the random coil (333 and 334), which thermally interconvert in one direction during the cooling (e and f) and heating (g) processes (Figure 99C).505 Yamaguchi and co-workers have prepared optically active silica nanoparticles ((P)-NPs), with a 70 nm diameter, grafted with a (P)-[4]helicene derivative and found that the (P)-NPs can separate various racemic aromatic alcohols into enantiomers under kinetic control. In particular, the (P)-NPs exhibit a better chiral recognition toward the racemic 2,2-dimethyl-1phenyl-1-propanol (335), and preferentially adsorb (S)-335 with up to 61% ee on the surface of the (P)-NPs (Figure 100A).506 The same (P)-NPs also recognize the molecular shape of the double-helix and random coil of a (P)ethynyl[4]helicene pentamer ((P)-325a) in solution, which are under equilibrium, and the random coil is a major species (90%) at 25 °C. In the presence of the (P)-NPs, its equilibrium was shifted to the double-helix (58%) because of selective precipitation of the double-helix with the (P)-NPs (Figure

four well-defined aggregate states, that is, all-dimer, amidodimer, ethynyl-dimer, and random-coil states, in which the reversible structural changes between the all-dimer and amidodimer and between the ethynyl-dimer and random-coil take place upon heating and cooling in nonpolar and polar solvents, respectively (Figure 98F). These multidomain [4]helicenebased pseudoenantiomeric oligomers also form heteroaggregates, thus producing a gel, which shows a reversible gel-to-sol transition or shrinkage upon heating and cooling or upon the addition of additives.501,502 The same group has recently synthesized analogous [4]helicene oligomers linked by an aminomethylene unit and found an interesting thermal hysteresis in solution (Figure 99A). A two-sided thermal hysteresis of the aminomethylene[4]helicene pentamer ((M)-333) has been observed during its reversible structural change between a homodouble helix ((333)2) and a random coil (333) during cooling (a to b to c) and heating (c to d to a) processes, in which the cooling and heating curves follow two different pathways, whereas the equilibrium curve appeared between them (Figure 99B).504 This difference in the molecular population between states b and d along with their deviations from the thermodynamically equilibrated state is considered to be the origin of this two-sided thermal hysteresis, so that molecules adopt two different states (b and d) even at the same 13848


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Figure 107. (A) Supramolecular organogel formation of (R)-301a·302a in a THF/n-hexane mixture. Reversible sol−gel transitions of (R)-301a· 302a in a THF/n-hexane mixture (3/7, v/v) triggered by thermal (B) and chemical stimuli (C). Photographs of reversible gel−sol transitions upon heating at 100 °C (B) or by the addition of TFA (C) followed by cooling (B) or neutralization with iPr2NH (C). Photographs in (C) were taken under UV light irradiation at λ = 365 nm. (Reproduced with permission from ref 535. Copyright 2014 Wiley-VCH.)

100B).507 This equilibrium shift using chiral NPs has been further applied to the system for the selective precipitation of heterodouble-helix intermediates during gelation by selfassembly and also to the rhodium-catalyzed disulfide exchange reaction, through which an optically active diol disulfide was obtained by kinetic resolution combined with an equilibrium shift.508,509 A cyclic oligomer ((M)-336) consisting of two linear ethynyl[4]helicene oligomers connected through two flexible hexadecyl linkers (Figure 101A)510 undergoes a reversible structural transformation between random coil and an intramolecular homodouble helix in response to temperature and solvents. In the presence of a pseudoenantiomeric linear

oligomer ((P)-325a) (n = 5), (M)-336 formed an intermolecular ternary complex with heterodouble helices, which further self-assembled to form lyotropic liquid crystals, in which fibers were anisotropically aligned at high concentrations. The lyotropic liquid crystals changed into turbid gels with randomly oriented bundles upon cooling. These two states can be reversibly switched upon cooling and heating (Figure 101B). 4.2. Triple and Quadruple Helices

Since the pioneering work by Lehn et al. in 1987,423 a number of double-stranded helicates and a series of metal-free doublestranded helices have been prepared. A short time later, the first example of a triple-stranded helicate ([Co(II)2(337)3]4+) fully characterized by an X-ray analysis was reported by Williams et 13849


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Figure 108. (A) CD spectra of mixtures of S-QF8 and QCl8 in chloroform at a fixed total concentration of 10−5 M. Solutions were equilibrated at 23 °C for 5 days before measurements. (B) Plot of Δε values at 331 nm as a function of the proportion of S-QF8. (Reproduced with permission from ref 444. Copyright 2010 The Royal Society of Chemistry.)

thorium Th(IV) ions assembled into the homochiral actinide quadruple-stranded helicates independent of the spacer, the structures of which were, for the first time, fully characterized by X-ray crystallography (Figure 102B).515 Later on, Williams and co-workers reported the first example of the diastereoselective synthesis of a one-handed tetranuclear quadruplehelicate via spontaneous assembly of the chiral tridentate ligand ((S)-342), Co(II) perchlorate, and diphenylphosphinic acid (dppH), which produced an optically active quadruple-helix ([Co4[(S)-342-H]4(dpp)2]2+) with a left-handed helical sense in high yield whose chirality is controlled by the chirality of (S)342 (Figure 102B).516 The complementary amidinium-carboxylate salt bridges have also been proved to be versatile for constructing metal-free triple and quadruple helices by a multicomponent self-assembly (Figure 103A).517 In fact, the amidine dimer ((R)-343) quantitatively assembled with 1,3,5-benzenetricarboxylic acid and zinc 5,10,15,20-tetrakis(carboxyphenyl)porphyrin (ZnTCCP) to form unique [3 + 2] ((R)-344) and [4 + 2] ((R)345) cylindrical complexes when mixed in 3:2 and 4:2 molar ratios in chloroform, respectively. A single-crystal X-ray analysis of (R)-344 revealed that the [3 + 2] cylinder was twisted in one direction induced by the chiral amidine residues, thus generating a right-handed triple-helical structure. In contrast, the [4 + 2] complex ((R)-345) took an untwisted cylinder structure and showed a weak CD. However, the cylindrical (R)345 tilted in one direction upon the addition of the achiral 4,4′bipyridine (346) that was sandwiched between the porphyrin rings via a metal coordination, resulting in the formation of a one-handed quadruple-helix ((R)-345·346), which was retained in solution, as evidenced by the distinct Cotton effects induced in the porphyrin chromophore regions (Figure 103A). As described in sections 4.1.1 and 4.1.2, Huc and co-workers synthesized a variety of designer aromatic oligoamides with different sequences, which assemble to form homo- and heterodouble helices with a controlled helical architecture. Huc et al. also succeeded in constructing triple and quadruple helices via the self-assembly of aromatic oligoamides with

Chart 33. Optically Active Helicates with a Twist-Sense Bias Caused by Chiral Template

al. in 1991 (Figure 102A).511 The same group further converted the labile [Co(II)2(338)3]4+ into the kinetically inert ([Co(III)2(338)3]6+) and further successfully resolved them into the triple-helical enantiomers by chiral chromatography using an optically active aqueous eluent.512 Based on the synthetic strategy of the helicate, Lehn et al. rationally designed a new oligobipyridine linked at the 5-position instead of the 6-position to avoid steric crowding around the metal ions, which assembled with hexacoordinate Ni(II) ions into a triplestranded helicate ([Ni(II)3(339)3]6+), as revealed by a singlecrystal X-ray analysis (Figure 102A).513 Interestingly, the single crystals are composed of either one of the enantiomeric triple helicates, and the CD spectra of their solutions exhibited mirror images of each other, thus indicating the spontaneous resolution of [Ni(II)3(339)3]6+ during the crystallization. Quadruple-stranded helicates are still limited to a few examples, probably due to their complex architecture and the difficulty in controlling the geometric preferences of metal ions with organic ligands for an eight- or nine-coordination, but they have been constructed based on the same helicate strategy.514 For example, Raymond et al. found that a ditopic ligand having two separate bidentate metal-binding sites connected through a three (340) or four (341) (odd or even) methylene spacer and 13850


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Figure 109. Synthesis of enantiomeric double helices based on helicity induction and dual memory strategy. Schematic representation of diastereomeric double-helix formation of (R)- or (S)-360·305b induced by the chiral phosphine ligands (MOP) coordinating to the Pt(II)-acetylide residue linked between the achiral amidine units, solvent-induced inversion of the helicity, and its memory by replacement of the chiral and achiral monophosphine ligands with achiral diphosphine ligands (dppm) that generate the enantiomeric double helices 361 with controlled helicity. Capped-stick drawing of the crystal structure of the “fixed” double-helix (361). (Reproduced with permission from ref 536. Copyright 2007 WileyVCH.)

Figure 110. (A) Schematic illustration of temperature-dependent inversion of double helices of 2-aminopurine (Ap)-bound RNA and DNA, showing fluorescence switches. (Reproduced with permission from ref 537. Copyright 2005 American Chemical Society.) (B) Schematic illustration of coordination-driven inversion of handedness of right-handed modified PNA (362a·362b) in the presence of Cu2+. (Reproduced with permission from ref 538. Copyright 2011 American Chemical Society.)

(Figure 103B).518 Solution studies also support a mixture of the parallel and antiparallel triple helices in both CDCl3 and CD3CN. An oligoamide tetramer composed of 7-amino-8fluoro-2-quinolinecarboxamide units (348a) gives rise to a double-helix with a large helical pitch,443 which promoted the

specific sequences (Figure 103B, C). A tetrameric oligoamide (347) consisting of four 2-amino-5-isobutoxy-1,8-naphthyridine-7-carboxylic acid units forms two different triple helices ((347)3) with respect to the parallel (a) and antiparallel (b) orientations of the three strands, as revealed by X-ray analyses 13851


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Figure 111. (A) Synthesis of homochiral quadruple-stranded helicate cages ([Zn8363a4Cl8] and [Zn8363b4I8]) from H2363a and H2363b, respectively, for enantioselective recognition and separation. The crystal structure and space-filling model of [Zn8[(R,R)-363a]4Cl8]·5THF. (Reproduced with permission from refs 539 and 540. Copyright 2012 American Chemical Society and Copyright 2014 Wiley-VCH, respectively.) (B) Interstrand ligand exchange reactions of optically inactive and active Pt(II)-acetylide-linked complementary duplexes (364a·305b, 364b·305b, and (R)-304b·305b) bearing the PPh3 ligands with achiral DPPPr and chiral DPPPe diphosphines. (C) Schematic illustration of a proposed mechanism for the diastereoselective switching of duplexes (365c) during the interstrand ligand exchange reaction of (R)-304b·305b with racemic DPPPe. (Reproduced with permission from ref 541. Copyright 2015 The Royal Society of Chemistry.)

tetradecameric aromatic oligoamide strand (349) consisting of four different monomer units has been synthesized so as to generate a wide double-helical segment upon hybridization of two strands, thus creating a specific cavity to encapsulate linear guests, such as 1,9-nonanediol and 1,10-decanediol (350). In addition, a narrow single-helical segment consisting of two different monomer units was designed to form an end-capped double-helical capsule and isolate the guest from the solvent. The resulting pseudorotaxane-like inclusion complex (foldanxane) structure ((349)2⊃350) was unambiguously determined by X-ray crystallography (Figure 104A).519 Huc and Jiang et al. further synthesized an aromatic oligoamide-based homodouble helix ((351a)2) that exhibits a unique template-induced screw-like motion within the duplex when complexed with specific guests; the sliding of two strands (351a) along one another within a double-helix can be controlled by rod-like guests (283 and 352), so that the length of the double-helix matches with the length of the guest (Figure 104B).520 The 1H NMR studies revealed that (351a)2 hardly formed inclusion complexes with the shortest (283a) and longest (283f) guests, whereas the guests with intermediate sizes (283b−e) were effectively included within the duplex

formation of a unique quadruple-helix through further selfassembly of the duplex, the structure of which has been determined by X-ray crystallography and remains in solution (Figure 103C). In contrast, such a quadruple-helix formation of the corresponding octamer (348b) could not be observed even at high concentrations and low temperatures, although 348b adopts the double-helix (Figure 83e). 4.3. Inclusion Complexation

As described in section 3, a number of single-stranded helical foldamers form inclusion complexes with specific chiral and achiral guest molecules, and one of the helices can be predominantly induced in the foldamers when complexed with chiral guests. On the other hand, only a few double helices have been reported to include guest molecules in spite of a series of homo- and heterodouble helical foldamers being designed and synthesized (see section 4.1). Huc et al. proposed a smart design strategy for synthesizing supramolecular capsules using aromatic oligoamide strands composed of different monomer units, each of which encodes three levels of information: i.e., a specific cavity size, recognition groups for guest binding, and a propensity to take a single- or double-stranded helical structure. Based on this strategy, a 13852


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binding sites equivalent to 283b and 283e, and this templateinduced screw-like motion is not accompanied by the dissociation of the duplex into two single helices. By taking advantage of such foldaxane formation, Ferrand, McClenaghan, and co-workers have recently demonstrated the control of reversible electronic energy transfer (REET) capabilities and luminescence lifetime of a bichromophoric guest (353a) through the foldaxane formation between an antiparallel homodouble helix (351b) (m = 6) and 353a, as a result of changing the distance between the two chromophores of 353a before and after the foldaxane formation (Figure 104C).521 This distance control was clearly revealed by the single-crystal X-ray analysis of the foldaxane composed of (351b)2 and an analogous threading guest (353b), in which an encapsulation of a flexible alkyl spacer moiety of 353b inside the cavity of (351b)2 leads to an induction of its extended conformation (Figure 104C). Flood and co-workers synthesized an amphiphilic capsulelike aryl-triazole-based foldamer (354),522 which partially folds into a single-stranded helical conformation in acetonitrile. However, in the presence of Cl− ions, the 354 strand further folds into single and double helices to form 1:1 (354·Cl−) and 2:1 ((354)2·Cl−) inclusion complexes, respectively, stabilized by multiple CH···Cl− hydrogen bonds that are in dynamic equilibrium in acetonitrile, although the 1:1 complex is predominant. Interestingly, the equilibrium was significantly shifted to the double-helix formation with the increasing amount of water in acetonitrile even though hydrophilic anions, such as Cl− ions, are favorably hydrated in an aqueous solution (Figure 104D). This is mostly because the stability of the duplex ((354)2·Cl−) relative to the single-helix (354·Cl−) is remarkably enhanced, resulting from hydrophobic interactions between the strands, which eventually create a solventexcluding binding pocket for encapsulating a Cl− ion via multiple CH···Cl− hydrogen bonds. Importantly, the binding affinity of the duplex ((354)2) toward the Cl− ion is quite large (>1012 M−2) in 25 and 50 vol % water/acetonitrile solutions.

Figure 112. (A) Structural formulas of ligands (366a and 366b), the X-ray crystal structure of the Λ,Λ-[Co2((S,S)-366b)2Cl2]2+ cation, and homochiral self-sorting during the reaction of rac-366a with CoCl2. (Reproduced with permission from ref 547. Copyright 2004 American Chemical Society.) (B) Synthesis of a dicopper double-helicate (369) with helical Λ-chirality. Schematic illustration of homochiral selfsorting of a mixture of rac-367, 368, and Cu(NCMe)4BF4 into double helicates during crystallization. Only one enantiomer of each is shown. (Reproduced with permission from ref 548. Copyright 2007 American Chemical Society.) (C) Structures of homochiral strands composed of alternating phenanthroline and alleno-acetylene residues ((P)- and (M)-370 and (P2)- and (M2)-371).

4.4. Chiral Amplification during Double-Helix Formation

Peptide nucleic acids (PNAs) are DNA analogues consisting of a N-(2-aminoethyl)glycine-based backbone with nucleobases linked to the nitrogen of glycine through a methylene carbonyl linker, which were first designed and synthesized by Nielsen et al. in 1991 (Figure 105A).523 PNA strands are achiral due to the lack of stereogenic centers, but upon binding to their complementary DNA strands, they form right-handed helical PNA·DNA duplexes in a highly selective and affinity manner, as confirmed by their CD spectra (Figure 105Bb), which are similar to those of the corresponding DNA·DNA duplexes.524,525 Moreover, PNAs with complementary sequences can hybridize, thus forming a dynamically racemic helical PNA· PNA duplex, whose preferential helicity can be induced by incorporating chiral substituents into one of the two PNA strands.526 The influence of the structures, positions, and number of chiral substituents on the handedness excess of the PNA double helices has been extensively studied.525−532 The incorporation of a chiral group only at the N- or C-terminus of a PNA strand can induce a preferred-handed helix in the duplex (domino effect), as exemplified by PNA*1·PNA, PNA*2·PNA, and PNA*3·PNA duplexes (Figure 105A and B).525 In particular, judging from the CD intensity of the PNA*2·PNA duplex, as few as two D-deoxyribonucleotides incorporated at one end of a decameric PNA double-helix can perfectly control

((351a)2). The X-ray crystal structures of (351a)2⊃283b, (351a)2⊃283d, and (351a)2⊃283e) (Figure 104Ba-c) also support the formation of the antiparallel double-helical foldaxanes, which takes place via a helix unfolding/refolding mechanism similar to the single-helical foldaxane formations (Figure 87 in section 4.1.1). In addition, comparison of the structures of (351a)2⊃283b and (351a)2⊃283d indicates that the two strands of (351a)2 undergo a screw-like motion by sliding in order to adjust the distance along the helix axis between the two binding units to form such double-helical foldaxanes (Figure 104Bd and e). More detailed ROESY measurements of (351a)2⊃352 concluded that exchange and the further screw/unscrew motion proceed via the shuttling of the double-helix ((351a)2) along the guest (352) with two 13853


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Figure 113. Schematic illustration of the chiral self-sorting behavior of (A) rac-372, (B) rac-373, and (C) the complete chirality- and sequenceselective complementary duplex formation of (S,S)-373 or (R,R)-373 among a mixture of rac-372 and rac-373 in the presence of (R,R,S,S,R,R)-323a or (R,R)-323c with a 100% diastereoselectivity, respectively. X-ray crystal structures of (a) ((S,S)-372)2 and (b) ((S,S)-373)2 are also shown. (Reproduced with permission from ref 550. Copyright 2015 Nature Publishing Group.)

the handedness of the double-helix, which means that the chirality of the N-terminal dinucleotide is efficiently transmitted through the covalently linked first PNA strand to the noncovalently bound second PNA strand (Figure 105Bb).525 This is a typical example of the chiral amplification with a highly cooperative interaction through the duplex formation. However, Green and co-workers later pointed out that the structures of the PNA−PNA duplexes likely possess a highly dynamic and fluctuating ensemble of conformations, leading to heterogeneous conformational properties as a function of the terminal chiral residue, such as an amino acid, unlike the corresponding DNA·DNA or DNA·PNA duplexes.530−532

Nielsen et al. further demonstrated an interesting remote enantioselection during the elongation of an achiral PNA primer strand assisted by the double-helix formation using a chiral PNA as the template (Figure 105C).525 In the presence of the achiral PNA strand as a template, both the DD and LL dimer substrates produced the corresponding primer−substrate conjugates (peak 2) in ca. 15% yields (Figure 105Ce,f), while no detectable conjugate was obtained in the absence of the complementary PNA as a template (Figure 105Cc,d). However, when the chiral PNA*2 strand was used as a template, the yield of the reaction with the DD dimer doubled to ca. 30%, while that with the LL isomer decreased to less than 5% (Figure 105Cg,h). The presence of the LL isomer hardly inhibited the 13854


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Figure 114. Schematic representations of asymmetric reactions with DNA-based catalysts through (A and B) noncovalent and (C−F) covalent approaches. (Reproduced with permission from refs565 and 554. Copyright 2010 Nature Publishing Group and Copyright 2010 The Royal Society of Chemistry, respectively.)

primer) enantioselectively reacts with the DD dimer over the LL dimer, and chiral information at the terminal can be transmitted over long distances through both covalent and noncovalent bond interactions. Taking full advantage of the synthetic versatility based on the modular strategy (see section 4.1.2), a series of all-chiral ((R)355a−d) and edge- and center-chiral/achiral ((R)-356a−d, (R)-357a, and (R)-357b) amidine oligomers connected by the Pt(II)-acetylide linkages from 2mer to 5mer and their complementary achiral carboxylic acid oligomers (358a−e) have been synthesized in a sequential stepwise manner (Figure 106A).534 The all-chiral amidine strands form double helices with a helix-sense bias upon complexation with their complementary strands in chloroform, as revealed by CD and 1 H NMR studies (Figure 106B and C). It was unexpectedly found that the Cotton effect signs for the chiral/chiral and chiral/achiral 2mer duplexes ((R)-355a·358a and (R)-356a· 358a) were opposite compared to those for the longer oligomer duplexes even though the duplexes possess the same chirality on the amidine residues, indicative of adopting an opposite-handed double-helical conformation (Figure 106C). The effect of the molecular lengths and sequences of the chiral/achiral amidine units on the amplification of chirality, that is, the “sergeants and soldiers” effect, during the doublehelix formation was systematically examined based on the CD measurement results together with the MD simulations. As summarized in Figure 106C, the CD magnitudes of (R)-357a· 358b and (R)-357b·358e were larger than those of (R)-356b· 358b and (R)-356d·358e, indicating that an excess one-handed helical sense in the chiral/achiral longer duplexes tends to be more efficiently induced by the chiral amidine unit introduced at the central part than that at the terminal part. This is probably because the Pt(II)-acetylide linkages most likely generate a greater flexibility of the duplexes than the

Figure 115. Asymmetric cyclopropanation of styrene with ethyl diazoacetate catalyzed by (A) double-helical Cu(I) helicate 393b and (B) a complementary double-helical molecule with an excess onehandedness (361) prepared by the helicity induction and memory concept (see Figure 109). Space filling representation of the crystal structure of 393c2+ is also shown in (A). (Reproduced with permission from ref 581. Copyright 2007 The Royal Society of Chemistry.)

reaction of the DD isomer, as observed in Figure 105Ci when the racemic mixture was used as the substrate. These results indicate that the right-handed double-helix (PNA*2·PNA 13855


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Figure 116. (A) (a) Diastereomerically pure metallo-helical “flexicate” complexes derived from chiral ligands 394a and 394b. (b) Structure of ΔZnRC-[Zn2394a3][ClO4]4. Octahedral coordination structures are shown in gray. (c) Schematic showing the flexicate sitting in the major groove of BDNA. (Reproduced with permission from refs 582 and 583. Copyright 2012 Nature Publishing Group and Copyright 2013 The Royal Society of Chemistry, respectively.) (B) Structures of triple-stranded metallohelices in which the strands are arranged head-to-head-to-tail (HHT). A spacefilling model of the cationic unit in ΔZn-RC-HHT-[Zn2394d3][ClO4]4 from X-ray crystallography is also shown (right). (Reproduced with permission from ref 584. Copyright 2014 Nature Publishing Group.)

ing in gelation via further self-assemblies. The amplification of helical chirality, such as the majority rule and sergeants and soldiers effect, also takes place during the supramolecular gelation of (R)-301a·302a in the presence of the opposite handed (S)-301a·302a and a dynamically racemic 301c·302a duplex in a THF/n-hexane mixture, leading to apparent positive nonlinear relationships between the molar ratios of the (R)301a·302a and 301c·302a fractions and the CD intensities of the corresponding gels, respectively. In the latter case, the chirality of (R)-301a·302a is most likely transferred to the dynamically racemic 301c·302a to produce a supramolecular copolymer with the same twist-sense as that of poly-[(R)-301a· 302a]. The helically twisted supramolecular fluorescent organogels stabilized by noncovalent intermolecular salt bridges have a significant advantage such that the sol−gel phase transition can be reversibly controlled by thermal and chemical stimuli (Figure 107B and C). An 8-chloroquinoline oligoamide (QCl8) prepared by Huc and co-workers assembles into a homodouble helix assisted by the N−H···Cl hydrogen bonds both in solution and in the solid state (Figure 83f), and also undergoes cross-hybridization with analogous 8-fluoroquinoline oligoamides, such as chiral S-QF8 and achiral QF8 (Figure 83e). Mixtures of S-QF8 and QCl8 showed CD spectra whose intensities at 331 nm displayed a typical nonlinear relationship versus the proportion of chiral SQF8 (Figure 108A and B), indicating the chiral amplification during the chiral/achiral heteroduplex formation.444 On the other hand, a linear relationship was observed in a mixture of S-

corresponding diacetylene-linked ones, resulting from its dynamic nature in the double-helix formation (see Table 4 and Figure 93); therefore, chiral information at the optically active amidine unit in the longer oligomers may be capable of transfer to the next achiral amidine unit in a more effective way. As mentioned in the previous section, the p-diethynylbenzene-linked complementary dimer strands form a stable doublehelix ((R)-301a·302a) in chloroform, while in polar solvents, such as THF, the duplex formation becomes significantly weaker and its Ka value remarkably decreases as compared to that in chloroform.475 Interestingly, (R)-301a·302a forms a unique organogel in THF upon the addition of n-hexane as a poor solvent, probably resulting from the supramolecular polymerization of the duplex via intermolecular rearrangement of the salt bridges (Figure 107A).535 In contrast, an analogous racemic duplex (301c·302a) bearing achiral cyclohexyl groups on the amidine residues and an optically active diacetylenelinked duplex ((R)-299a·300a) (see Figure 91A in section 4.1.2) showed no gelation at all because of their more tightly salt-bridged duplex formations in THF. Therefore, the mechanism for gelation of (R)-301a·302a in a THF/n-hexane mixture can be proposed as follows: the sliding of the duplex strands along each other initially takes place without dissociation in THF to produce an imperfect duplex with “sticky ends”, which participate in the supramolecular polymerization through the intermolecular salt-bridge formation upon the addition of a poor solvent, such as n-hexane, thus giving a linear supramolecular polymer (poly-[(R)-301a·302a]), result13856


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Figure 117. (A) (a) Chemical structures of DNA and SNA. S and R termini of SNA are assigned according to the chirality of the terminal residue. (b) The mirror image of SNA with an asymmetrical sequence ((S)-AT-(R)) is identical to SNA with the reverse sequence ((S)-TA-(R)), while SNA with a symmetrical sequence ((S)-TT-(R)) is identical to its mirror image. (c) Schematic representation of the relationship between SNA duplexes. CD spectra of duplexes 395a·395b (blue), 395c·395d (red), and 395e·395f (green) at 20 °C. (Reproduced with permission from ref 585. Copyright 2011 Wiley-VCH.) (B) Racemic DNA crystal structures illustrating the relationship between L and D enantiomers. B-type DNA duplexes formed from the sequence CCGGTACCGG, cocrystallized with Co2+ ions. (Reproduced with permission from ref 589. Copyright 2014 Wiley-VCH.)

Q F 8 and Q F 8 . These results revealed a predominant heteroduplex formation between S-QF8 and QCl8 with a 1:1 stoichiometry resulting from a preferred-handed helical sense biased by the optically active terminal group of S-QF8, while the S-QF8 and QF8 mixture separately formed the corresponding homodouble helices.

clearly demonstrating the memory of helicity, although it was a 66:34 mixture of enantiomers after hydrolysis430 and its optical activity would eventually disappear due to racemization. The helical conformations of the Pt(II)-acetylide linked double helices composed of amidine and carboxylic acid dimer strands are dynamic, and helix-inversion rapidly occurs (Table 4 and Figure 93). Taking advantage of this dynamic feature combined with the previously developed “helicity induction and dual memory of enantiomeric helices” strategy developed for a racemic helical poly(phenylacetylene) (see section 5.1.5), both enantiomers of a Pt(II)-acetylide linked double-helix (361) consisting of achiral amidine and carboxylic acid dimer strands have been for the first time enantioselectively prepared (Figure 109).536 A preferred-handed double-helix was first induced in a dynamically racemic Pt(II)-acetylide linked double-helix ((R)or (S)-360·305b) consisting of achiral isopropyl-bound amidine ((R)- or (S)-360) and carboxylic acid dimer (305b)

4.5. Memory of Double-Helical Chirality

In 1996, Siegel et al. developed a versatile method to control the helicity of oligo(bipyridine)-based double-stranded helicates using a chiral template, such as an (R)- or (S)-biphenyl (359a) or binaphthyl group (359b) (Chart 33), which determines the twist sense (P or M) and controls the overall handedness of 359a and 359b along the helicate backbones in the range of 20 Å.429,430 Interestingly, after removing the biphenyl group from 359a by hydrolysis, the resulting helicate retained its optical activity despite the lack of a chiral template, 13857


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as 1-(methylsulfinyl)benzene, 1-phenylethanol, 1-phenylpropanol, and 1-phenylethylamine in the solid state and enantioselectively adsorbed one of the enantiomers ((S)-rich). However, [Zn8[(R,R)-363a]4Cl8] or [Zn8[(S,S)-363a]4Cl8] exhibited only a slight enhancement of the luminescence with a negligible enantioselectivity toward saccharides, such as sorbitol. On the other hand, the salalen-based [Zn8[(R,R)-363b]4I8] or [Zn8[(S,S)-363b]4I8] possessing a helical structure similar to that of [Zn8[(R,R)-363a]4Cl8] or [Zn8[(S,S)-363a]4Cl8] showed an enantioselective enhancement of luminescence by responding to the chirality of various saccharides, such as sorbitol, maltose, glucose, galactose, and fructose, in solution as well as amino acids and chiral amines in the solid state (Figure 111A).539,540 This remarkable chiral recognition ability of [Zn8[(R,R)-363b]4I8] or [Zn8[(S,S)-363b]4I8] toward saccharides and amines in solution and in the solid state is supposed to result from the steric confinement of the amphiphilic helical cavity as well as the conformational rigidity of the coordinated salalen ligands. As described in section 4.5, the helical handedness of the dynamically racemic Pt(II)-acetylide linked double-helix composed of achiral amidine and carboxylic acid dimer strands can be biased by chiral monophosphine ligands coordinated on the Pt(II) atoms (360·305b), and further memorized by an achiral diphosphine (dppm) which replaces the chiral and achiral monophosphine ligands through the interstrand ligand exchange reaction, thus producing the Pt(II)-bridged doublehelix with a static helical conformation (361, Figure 109 in section 4.5).536 When an optically active 1,3-diphosphine, such as (R,R)- or (S,S)-2,4-bis(diphenylphosphino)pentane (DPPPe) was used instead of the achiral dppm as a chiral cross-linker during the ligand exchange reactions toward dynamically racemic Pt(II)linked double helices (364a·305b and 364b·305b) composed of achiral amidine and carboxylic acid dimer strands (Figure 111B), the resulting Pt(II)-bridged double helices (365a and 365b, respectively) showed intense Cotton effects in chloroform as well as in DMSO, indicating the formation of a preferred-handed double-helical structure biased by the chiral cross-linkers that further stabilized the double helices, as supported by the facts that the non-cross-linked 364a·305b and 364b·305b duplexes exhibited a very weak CD in DMSO because of the dissociation into single strands.541 The Pt(II)linked double-helix (365d) bridged by achiral 1,3-diphenylphosphinopropane (DPPPr) is optically inactive, but exists as an equal mixture of interconvertible right- and left-handed double helices. The variable-temperature 1H NMR spectra of 365d in CDCl3 revealed the nonequivalent N−H proton signals that appeared at −60 °C via the coalescence temperature (Tc = −46 °C), which enabled estimation of the free energy of activation for the helix inversion (ΔG‡25) of the double-helical 365d (42.7 kJ mol−1),541 which was much higher than that for the Pt(II)-linked double-helix with the PEt3 ligands (304c·305a), but lower than that of the double-helix bridged by the dppm ligands.536 Interestingly, the interstrand ligand exchange with the racemic DPPPe toward the optically active Pt(II)-linked double-helix ((R)-304b·305b) consisting of chiral amidine and achiral carboxylic acid strands, which possesses a preferredhanded helix sense induced by the chiral amidine residues, proceeded in a diastereoselective manner to produce complete homochiral double helices ((R,R)-365c > (S,S)-365c) via a unique chiral self-sorting (Figure 111C).541 The time-depend-

strands using the chiral phosphine ligands (R)- or (S)-2diphenylphosphino-2′-methoxy-1,1′-binaphthyl ((R)- or (S)MOP) coordinated on the Pt(II) atoms in CDCl3. Interestingly, the helix sense was inverted in toluene. The resulting diastereomeric double helices induced in different solvents are dynamic, but can be converted to static double helices (361) by replacement of the chiral and achiral monophosphine ligands with an achiral bis(diphenylphosphino)methane (dppm) in each solvent, which bridges the complementary strands as revealed by its X-ray crystallographic analysis (Figure 109), thus producing the enantiomeric double helices that are stable toward racemization and exhibit the mirror-image CD signals. 4.6. Helix Inversion

Nucleic acids and biorelated helical polymers, such as polypeptides and polynucleotides with specific sequences, and some synthetic helical polymers undergo inversion of their helicity (helix−helix transition) regulated by external stimuli, such as salt concentration, temperature, pH, solvent, and light (see section 5.1.3). However, synthetic double helices showing such a helix-inversion remain rare except for the MOP-bound Pt(II)-linked double-helix (R)- or (S)-360·305b536 as described in the preceding section. Sugiyama and co-workers designed and synthesized unique RNA and DNA molecules bearing a fluorescent 2-aminopurine (Ap) unit at the middle that undergo a reversible helix−helix transition, i.e., a right-handed A-to-left-handed Z transition and left-handed Z-to-right-handed B transition triggered by temperature, respectively, which are accompanied by a fluorescence switch (on and off), reflecting their helical handedness, lefthanded and right-handed double helices, respectively (Figure 110A).537 As described in section 4.4, when an L-Lys is introduced at the C-end of one (362a) or both strands of a PNA duplex, the resulting PNA preferably adopts a left-handed helical structure.525−528,530 Achim et al. introduced one C-terminal LLys (362a, 362c, and 362d) and/or one chiral PNA monomer bearing a 2,2′:6′,2″-terpyridine (Tpy) unit instead of a nucleobase (362b and 362d) into the PNA strands (Figure 110B).538 The resulting PNA duplexes (362a·362b and 362c· 362d) formed a right-handed double-helical structure because of the chiral induction effect exerted by the (S)-Tpy-PNA monomer. However, the right-handed PNA duplex (362a· 362b) inverted to the opposite left-handed duplex in the presence of Cu2+ ions, which coordinated to the Tpy units of two different PNA duplexes to produce an interstrand [Cu(Tpy)2]2+ complex. A similar helix inversion also took place for the PNA duplex (362c·362d) bearing an achiral tetrazole (Tz)-bound monomer at the middle of the 362c strand. 4.7. Functions of Multi-Stranded Helical Assemblies

4.7.1. Chiral Recognition. Cui and co-workers have demonstrated that optically pure pyridyl-functionalized salan (H2363a) and salalen (H2363b) ligands prepared from an (S,S)- or (R,R)-trans-1,2-diaminocyclohexane self-assemble into homochiral quadruple-stranded helicate cages ([Zn8363a4Cl8] and [Zn8363b4I8]) upon complexation with ZnCl2 and ZnI2, respectively (Figure 111A). 5 3 9 , 5 4 0 The salan-based [Zn8363a4Cl8] possessing a large chiral pore and chiral NH groups showed a luminescent enhancement in an enantioselective manner by responding to the chirality of amino acids in solution.539 The crystalline [Zn8[(R,R)-363a]4Cl8] showed a chiral recognition toward some small racemic molecules, such 13858


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ent 1H NMR spectral changes indicated that this ligand exchange reaction produced an excess of (S,S)-365c over (R,R)-365c in 14% de during the initial stage (Figure 111C, first step), and then (R,R)-365c was preferentially formed in association with a gradual decrease in the amount of (S,R)-365c with time, leading to reversed diastereoselectivity up to 19% de ((R,R)-rich) (Figure 111C, second step). Therefore, it seems that the diastereoselectivity of DPPPe is initially determined by the preferred-handed helix sense of the double-helix under kinetic control, whereas the overall diastereoselectivity is governed by the kinetically formed (S,R)-365c. 4.7.2. Chiral Self-Sorting through Double-Helix Formation. As described in section 4.1, double helices have the potential for the precise self-recognition or “the recognition of self from non-self” during the double-helix formations based on a difference in chain lengths and sequences, as exemplified by the seminal examples of the self-sorting of double-stranded Cu(I) helicates.542 High-fidelity self-recognition and selfdiscrimination are of key importance in biological systems. In this section, chiral self-sorting that produces either homo- or heterochiral helical assemblies,197,543,544 in particular, double helices, will be described. The chiral self-sorting of small molecules via supramolecular assemblies to produce chiral macrocycles and capsules or cages rather than helices with a controlled size, shape, and topology is not included but has been reviewed elsewhere.203,545 Telfer and Kuroda et al. reported a new class of chiral ligands ((S,S)-366) that self-assembled with CoCl2 and NiCl2 to form double helix-shaped dinuclear complexes [M2(366)2Cl2]2+ (M = Co and Ni). Particularly, an extremely high diastereoselective complex formation (de >95%) was attained when CoCl2 was used, indicating that the chirality of (S,S)-366 efficiently controls the stereochemistry of the Co(II) centers.546,547 The 1 H NMR and X-ray crystallographic analysis revealed that the reaction of rac-366a with CoCl2 produced an equimolar amount of the Δ,Δ-[Co2((S,S)-366a) 2Cl2 ]2+ and Λ,Λ[Co2((R,R)-366a)2Cl2]2+ complexes, while no heterochiral complex [Co2((S,S)-366a)((R,R)-366a)]Cl2] 2+ was formed, leading to a complete homochiral self-sorting that took place due to the ligand chirality (Figure 112A). Nitschke and co-workers developed a unique chiral selfsorting during a double-stranded helicate-like complex (369) formation upon mixing 1-amino-2,3-propanediol (367), a phenanthroline derivative with two aldehyde residues (368), and Cu(II) ions at the molar ratio of 4:2:2 (Figure 112B).548,549 When rac-367 as a subcomponent was incorporated into a dicopper double-helicate, only a single pair of enantiomers with the lowest solubility was selectively self-sorted among six possible diastereomeric pairs during crystallization via a fast ligand exchange reaction followed by a slow covalent iminebond metathesis reaction. Recently, Diederich and co-workers have also synthesized homochiral strands consisting of alternating phenanthroline and alleno-acetylene residues ((P)and (M)-370 and (P2)- and (M2)-371) and demonstrated their metal ion-mediated self-sorting behavior (Figure 112C). As described in section 4.1.3, optically active amidine dimers composed of chiral and achiral amidines joined by chiral amide linkers function as a chiral template for the diastereoselective imine-bond formations, whose diastereoselectivities were highly dependent on the linker amide sequences. Further studies using amidine and carboxylic acid dimers linked by the same chiral amide linkers with different sequences have provided further

intriguing phenomena during the homo- and heteroduplex formations (Figure 113).550 A racemic mixture of carboxylic acid dimers linked by racemic-1,2-cyclohexane bis-amides with different amide sequences (NHCO (372) or CONH (373)) self-associate to form homoduplexes ((372)2 and (373)2) in a completely sequence-selective manner with no trace of the cross-hybrid duplex, in which the amide sequences play a vital role in determining the homoduplex structures. The detailed 1H NMR analysis revealed that rac-372 exists as both homochiral and heterochiral duplexes with a molar ratio of 2:1 (Figure 113A), whereas rac-373 perfectly recognizes the chirality of the linker amide residues to exclusively form the homochiral duplexes via the complete chiral self-sorting (Figure 113B). The X-ray crystal structures of the homoduplexes (((S,S)-372)2 and ((S,S)-373)2) unexpectedly determined their unique deeply intertwined duplex structures, which were different from the anticipated, self-associated homodouble helix structure stabilized by interstrand hydrogen bonding between the carboxy groups, as observed for the m-terphenyl-based dicarboxylic acid dimer joined by the phenylene linker (279) (Figure 86). The crystal structures revealed that each strand possesses a “Ushape” structure due to the trans-1,2-cyclohexyl linker moiety and binds together via interstrand hydrogen bonds; each amide group at the linker residue is sandwiched between the two carboxy groups of the other strand. These unique structures were retained in solution, as supported by 2D NMR measurements. Interestingly, when an enantiopure amide-linked amidine dimer (R,R,S,S,R,R)-323a was added to a mixture of the racemic carboxylic acid dimers ((S,S)- and (R,R)-372 and 373), only a single optically pure heteroduplex with (S,S)-373 with a 100% diastereoselectivity and complete sequence specificity was obtained, in which the amidinium-carboxylate salt bridges contribute to stabilizing the duplex formation (Figure 113C). As a result, an unprecedented complete chirality- and sequenceselective successive self-sorting through specific homo- and heteroduplex formations has been, for the first time, achieved. Also interestingly, (R,R)-323c, composed of achiral amidines and an (R,R)-amide (CONH) linker, formed a heteroduplex only with (R,R)-373, with a complete sequence specificity and diastereoselectivity (de >99%) in a mixture of racemic 372 and 373, indicating the critical role of the linker chirality and sequence rather than the amidine chirality in the present diastereoselective heteroduplex formations. Therefore, when the chiral linker amide residue of (R,R,S,S,R,R)-323a was replaced by its achiral meso one, the diastereoselectivity almost completely disappeared. 4.7.3. Asymmetric Catalysis. A right-handed doublehelical DNA is one of the most ubiquitous chiral objects as well as a right-handed α-helical structure of polypeptides in nature. Although oligo- and polypeptides are known to catalyze various asymmetric reactions,551−553 the DNA itself has rarely been used as an asymmetric catalyst, likely due to the lack of a useful catalytic moiety in the double-helical DNA. In order to develop DNA-based asymmetric catalysts,553−555 there are two possible ways to introduce the catalytic moiety into the DNA; one is a noncovalent supramolecular approach (Figure 114A and B),556−572 and the other is a simple covalent approach (Figure 114C−F).573−579 In 2005, Roelfes and Feringa et al. have, for the first time, developed the DNA-based asymmetric catalysis in which the chiral environment of the DNA double-helix can affect the 13859


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DNA exists as the single enantiomer comprised of Ddeoxyribose residues. Smietana and Arseniyadis et al. synthesized the mirror-imaged L-DNA composed of unnatural L-deoxyribose residues, thus allowing the production of both enantiomeric products using the enantiomeric L- and D-DNA as the chiral source.580 In contrast to the aforementioned noncovalent approach, covalent anchoring of a catalytic moiety to DNA would enable the catalytic site to be incorporated into the desired positions in an oligonucleotide strand. Thus, the resulting DNA-based catalysts would adopt a much more defined structure than those prepared by the noncovalent approach, with respect to the position of the catalytic moiety. However, chemical modification of the DNA strands is not a straightforward and time-consuming process, so that the systematic optimization of such catalytic systems seems to be difficult. In fact, the preliminary DNA-based catalysts bearing a covalently introduced catalytic moiety exhibited low catalytic activities and enantioselectivities.573,574 Later, Jäschke et al. rationally designed and synthesized DNA-based catalysts by introducing an Ir(I)−diene complex covalently linked to a DNA strand at the middle position (386, Figure 114C).576 The DNA-based catalyst showed a better catalytic activity for the Ir(I)-catalyzed allylic amination reaction in an aqueous medium, but producing the amination product 387 with low ee values. Interestingly, when the Ir(I)/ diene-carrying DNA strand was hybridized with its complementary RNA strand, the enantioselectivity was inverted due to the difference in the overall double-helical structures. Roelfes et al. reported a modular strategy to produce DNAbased catalysts carrying a covalently anchored metal complex (388, Figure 114D).575 In this covalent approach, the DNAbased catalysts are instantly formed by a three componentassembly, an oligonucleotide bearing a bpy/Cu(II) complex at the 5′ terminal phosphate moiety linked through an alkyl spacer (n = 3 or 6), an unfunctionalized oligonucleotide, and its template strand with a complementary sequence to both of the oligonucleotide sequences (Figure 114D), thereby enabling the catalytic site to be located at the desired position of the resulting duplex. Thus, the DNA-based bpy/Cu(II) complex 388 efficiently catalyzed the Dields-Alder reaction in water, giving the product with ee values of up to 93%. In addition, an analogous DNA-based bpy/Cu(II) complex with a cisplatin moiety anchored to the DNA through a metal coordination (389, Figure 114E) exhibited a good enantioselective catalytic activity for both the Dields-Alder and Friedel−Crafts alkylation reactions,577 although the cisplatin moiety binds the nonspecific sites of DNA. In contrast to the covalent anchoring, Park and Sugiyama et al. reported new types of DNA-based Cu(II)-catalysts containing a bpy ligand covalently incorporated into the middle of the phosphate backbone linked through propylene spacers (390, Figure 114F).578 The new catalyst 390 was applied to the asymmetric intramolecular Friedel−Crafts alkylation of 391, producing the cyclized product 392 with a good enantioselectivity (up to 84% ee), although the ee value strongly depends on the composition of the nucleobases in the catalytic pocket. In addition, 390 also catalyzed the asymmetric Diels−Alder reaction of 377 with cyclopentadiene, affording the Diels−Alder product 159 with a high enantioselectivity up to 97% ee and endo/exo selectivity.579 Quite interestingly, the enantiomeric preference in the Diels−Alder reaction catalyzed by 390 is opposite to that by the above-mentioned DNA/

catalytic reactions through a noncovalently DNA-interacting achiral catalytic moiety.556 The DNA complexed with the Cu(II)-ligand bearing a DNA-intercalating 9-aminoacridine unit linked through a propylene spacer (374a, n = 3) catalyzed the Diels−Alder reaction of azachalcone (377) with cyclopentadiene in water, resulting in the corresponding endo isomer (159) as the major product with ca. 50% ee (Figure 114A). The enantioselectivity was highly dependent on the alkyl spacer length and the substituent R. Interestingly, 374a with a shorter spacer length (n = 2) resulted in the predominant formation of the opposite enantiomer of the product, whereas a longer spacer (n = 4 and 5) resulted in only a decrease in the ee values as a result of the increased distance between the catalytic site and the chiral DNA scaffold. However, when the ligand having a 3,5-dimethoxybenzyl substituent (374b) was used instead of 374a, such an inversion of the enantioselectivity was not observed even by changing the spacer length. By taking advantage of such a supramolecular approach, further fine-tuning of the catalytic system is feasible without chemical modification of the double-helical DNA. The remarkable enhancement of the enantioselectivity of the Diels−Alder product 159 was achieved when an achiral 2,2′bipyridine derivative (375) was used as the ligand (up to 99% ee) (Figure 114A), in which the endo isomer was exclusively obtained (up to >99% endo).557 In addition, the rate of this reaction catalyzed by the DNA/Cu(II)-375 complex was 58 times greater than that catalyzed by the Cu(II)-375 complex in the absence of DNA,561 likely due to the reaction taking place in the DNA groove through nonclassical hydrophobic interactions rather than classical ones.561,572 Furthermore, both the rate acceleration and the enantioselectivity were found to be dependent on the DNA-sequence, although such a DNA-induced rate acceleration was not observed when using the ligand 374b.564 Surprisingly, the replacement of the ligand 375 with the tridentate terpyridine 376 in the DNA-based supramolecular Cu(II)-catalyst resulted in the formation of the opposite enantiomer of the Diels−Alder product with a high ee value (up to 92%) comparable to that observed in the reaction using the DNA/Cu(II)-375 complex, despite the identical DNA being used as the chiral source.567 The observed opposite enantioselectivity is considered to be due to the difference in the coordination modes of these Cu(II)-ligands with the substrate. As summarized in Figure 114B, the supramolecular DNAbased Cu(II) catalysts have been further applied to various catalytic asymmetric reactions in water, such as the Diels−Alder reactions of α,β-unsaturated 2-acyl imidazole derivatives (378) with cyclopentadien (product = 379),558 Michael addition of a nucleophile to 378 (product = 380 or 381),559,570 oxa-Michael addition of an alcohol to 378 (product = 382),568 hydration of 378 (product = 383),565 Friedel−Crafts alkylations of pyrrole or indole derivatives (product = 384 or 385),563,569 hydrolytic kinetic resolution of epoxides,562 the intramolecular cyclopropanation reaction,571 and the electrophilic fluorination reaction.560 For some of these reactions, the reaction rates were enhanced in the presence of an appropriate amount of a water-miscible organic cosolvent in the reaction mixtures, while maintaining their enantioselectivities as observed in water.566 This acceleration effect is most likely due to the faster dissociation of the reaction product from the Cu(II) complex. However, most of these reactions only afforded an excess of the enantiomeric products, because the naturally occurring 13860


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Figure 118. Representative examples of static and dynamic helical polymers that differ through their helix inversion barriers.

bridged double-helix 361 were expected to coordinate to a Cu(I) ion in a tweezer-like fashion. Interestingly, despite 361 having only a helical chirality, the resulting 361·Cu(I) complex was found to catalyze the above-mentioned cyclopropanation reaction in good yield and enantioselectivity, thus giving the optically active trans-cyclopropane as the major product with up to 85% ee (Figure 115B). 4.7.4. Miscellaneous Applications (Bioactivity). As shown in the previous sections, a large number of multistranded helicates have been synthesized, and some of them have also been applied to asymmetric catalysis and as chiral sensors. However, it has still remained difficult to use such helicates in the medicinal area because they often contain mixtures of isomers and are generally insoluble in water. Thus, Scott and co-workers have designed and synthesized a series of water-stable triple-stranded Fe(II) helicates, which is called “flexicate”, composed of two series of flexible diether linkers (394a and 394b) via a highly adaptable self-assembly approach (Figure 116Aa,b).582 These flexicates are stereochemically and diastereomerically pure (diastereomeric ratio (dr) > 200:1 in CD3CN). Moreover, some of the water-soluble flexicates with a Cl− ion as a counteranion specifically interact with DNA (Figure 116Ac),583 and showed antimicrobial activity toward

Cu(II)-375 catalyst. Therefore, the mirror-imaged L-DNA is no longer required to obtain the enantiomeric Diels−Alder products. In contrast to the DNA-based asymmetric catalysts, there are only a few examples of totally synthetic double-helix-based asymmetric catalysts. Kwong et al. reported a unique asymmetric catalyst based on optically active double-stranded dinuclear Cu(I) helicates 393a−c, which are assembled from terpyridine strands bearing chiral residues at both ends and Cu(I) ions (Figure 115A). The P/M de values of the helicates 393a−c increased from 29 to 95% with an increase in the steric bulkiness of the substituent on the chiral residues.581 Repeated recrystallization of these helicates resulted in their de values of more than 95%. The resultant Cu(I)-helicates can catalyze the asymmetric cyclopropanation of styrene with ethyl diazoacetate. In particular, 393b showed an effective asymmetric catalytic activity, thus producing both trans and cis-cyclopropanes with up to 76 and 83% ee, respectively, although the cis/trans selectivity was low. As described in section 4.5, the dynamic double-helix 360· 305b can be transformed into the static double-helix 361 by replacement of the (R)- or (S)-MOP groups with achiral dppm ligands, while maintaining its optical activity as well as its helical sense (Figure 109).536 The interstrand alkynyl units of the 13861


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Figure 119. (A) Helix-sense-selective anionic polymerization of achiral tris(trimethylsilyl)silyl methacrylate (396). (B) Helix-sense-selective polymerization of achiral phenylacetylenes (398a, 398b) (a) and cyclopolymerization of an achiral diyne (399) (b) with chiral Rh catalysts. Helixsense-selective polymerization of an achiral acetylene (400) in a chiral micelle (c). (Reproduced with permission from ref 617. Copyright 2011 Wiley-VCH.) (C) Helix-sense-selective polymerization of an achiral diphenylacetylene (401) in a chiral monoterpene. (D) Helix-sense-selective polymerization of an achiral phenyl isocyanide (402) with an achiral Pd(II) initiator in the presence of L-LA.

Gram-positive and -negative bacteria as well as a low toxicity toward a nonmammalian model organism. The same group has further developed a new system for the highly stereoselective asymmetric self-assembly of chiral ligands (394c and 394d) to form very stable triple-stranded metallohelices with an antiparallel head-to-head-to-tail (HHT) “triplex” strand arrangement (dr >98%) (Figure 116B).584 These metallohelices possess an amphipathic functional topology and showed a high, structure-dependent toxicity toward certain cancer cell lines, resulting in dramatic changes in the cell cycle without DNA damage. They also exhibited a lower toxicity toward human breast adenocarcinoma cells and no remarkable toxicity toward Gram-positive and -negative bacteria.

and co-workers newly designed and synthesized a series of serinol nucleic acids (SNAs), which are composed of “serinol” as not only a flexible, but also an achiral acyclic scaffold (Figure 117Aa).585,586 An SNA dimer with a symmetrical sequence (T → T) is achiral, whereas an SNA dimer with an unsymmetrical sequence (A → T) is chiral, and its enantiomer is identical to that with the opposite sequence (T → A), indicating that the chirality of the SNA oligomers can be perfectly inverted by reversing the sequence of the SNA monomers (Figure 117Ab). The melting temperatures of the chiral (395a·395b and 395c· 395d) and achiral (395e·395f) duplexes are almost similar to each other. The CD spectra of 395a·395b and 395c·395d showed split-type Cotton effects with mirror images (Figure 117Ac, blue and red lines), thus revealing that 395a·395b and 395c·395d form enantiomeric right- and left-handed double helices, respectively. As anticipated, 395e·395f does not show an ICD because of its symmetrical meso-structure (Figure 117Ac, green line). The SNA strands (395a−f) also form duplexes with their complementary strands of DNA and RNA,

4.8. Miscellaneous

Most of the artificial nucleic acids developed so far possess rigid cyclic scaffolds, except for PNA, and they tend to form duplexes with their complementary strands of native DNA and/or RNA, which reduce the entropy loss. On the other hand, Asanuma 13862


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Figure 120. Ribbon diagrams of ferritin (Fr): the 24-subunit assembled cage (A), the inner cavity (B), and the 3-fold axis channel (C). (D) Schematic illustration of insertion of [Rh(nbd)Cl]2 into the apo-Fr cage and polymerization of phenylacetylene with the Rh(nbd)·apo-Fr composite. (Reproduced with permission from ref 633. Copyright 2009 American Chemical Society.)

Figure 121. (A) Schematic illustration of control of PA helix (403) by the positional switching of the optically active wheel in pendant rotaxane. (B) Structures of PPAs bearing a planar chiral rotaxane pendant (404). (Reproduced with permission from ref 636. Copyright 2011 The Royal Society of Chemistry.)

seems to have significant advantages over the “enantiopure” approach because of its easiness of obtaining single crystals.

although their melting points were lower than those of the SNA duplexes. Recent advances in the total chemical synthesis of unnatural 587,588 D-proteins can provide racemic proteins for X-ray crystallography. However, racemic DNA crystallography has not been examined even though both L- and D-deoxyribooligonucleotides are commercially available. Recently, Huc and co-workers have, for the first time, reported the crystal structures of racemic DNA duplexes and a four-way junction cocrystallized with divalent cations (Ca2+, Co2+, or Mg2+) (Figure 117B).589 Moreover, racemic crystal structures of tetramolecular and bimolecular G-quadruplexes formed from different sequences (TG4T and G4T4G4) have also been revealed. This “racemic” approach for X-ray crystallography

5. HELICAL POLYMERS AND THEIR ASSEMBLIES As mentioned in the Introduction, a number of comprehensive reviews of synthetic helical polymers have been published,4,10,22,24,26,154,590−601 and we also previously reported the progress in synthetic helical polymers with respect to their synthetic methods, structures, properties, and functions mainly between 2001 and 2009 in conjunction with the historical background.4 Since then, further noticeable progress has been achieved in this emerging area, which prompted us to briefly describe the further progress in helical polymers since 2009 along with their assemblies in this section. 13863


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Figure 122. (A) Long-range chiral information transfer in polymer brushes consisting of a dynamically racemic helical PA backbone and poly(phenyl isocyanate) pendants through a chiral domino effect. (Reproduced with permission from ref 639. Copyright 2014 American Chemical Society.) (B) Structures of polymer brushes consisting of a helical PA backbone and either polyisocyanate (407, 408) or poly(L-lactide) pendants (409).

5.1. Helical Polymers

For some polymers, such as poly(quinoxaline-2,3-diyl)s (PQXs), polycarbodiimides, polyisocyanides, and polyacetylenes (PAs), both static and dynamic helical polymers with an excess one-handedness have been derived from the identical backbone frameworks with different substituents (Figure 118).4 Therefore, by taking advantage of the intramolecular hydrogen bonding and acid−base interactions as well as bulkiness of the substituents, the helical conformations of dynamic helical polymers can be transformed into those of static helical polymers due to an increase in the helix inversion barrier, as clearly demonstrated by the memory effect observed in poly(phenylacetylene)s (PPAs) (433)607 and poly(4-carboxyphenyl isocyanide) (436) (see section 5.1.5).608 5.1.2. Progress in Static Helical Polymers: HelixSense-Selective Polymerization. Since the first helical vinyl polymer (PTrMA) with a one-handed static helical conformation was prepared by the helix-sense-selective anionic polymerization of achiral triphenylmethyl methacrylate using chiral anionic initiators by Okamoto et al.,603 various achiral methacrylates bearing other bulky substituents have been designed and synthesized to find effective structures for helixsense-selective polymerizations.590,593,609 However, such structures were limited to the triarylmethyl substituents or their analogues. Recently, Kamigaito and co-workers reported the asymmetric anionic polymerization of a novel methacrylate bearing an extremely bulky tris(trimethylsilyl)silyl group (396) to produce an optically active polymer with a prevailing onehanded helical conformation (Figure 119A).610 9-Fluorenyllithium (FlLi) complexed with chiral ligands, such as (−)-sparteine ((−)-Sp), effectively initiated the asymmetric anionic polymerization of 396 to afford the insoluble but highly isotactic (mm > 99%) optically active helical polymer (poly396), as confirmed by DRCD spectroscopy.

5.1.1. Progress in Helical Polymer Synthesis. Starting with the pioneering works by Nolte et al.,602 Okamoto et al.,603 and Green et al.,32 an increasing number of synthetic helical polymers with a controlled helix sense have been developed, which can be basically categorized into two types in terms of their helix inversion barriers, that is, static and dynamic helical polymers (Figure 118). As represented by poly(triphenylmethyl methacrylate) (PTrMA),603 poly(tert-butyl isocyanide),602 and polychloral,604 the helix inversion barrier in static helical polymers is significantly high enough to maintain their helical conformations even in solution, due to the high steric repulsion between their bulky side groups. Therefore, helical polymers showing an optical activity solely due to their helical chirality can be synthesized in a kinetically controlled way by the helixsense-selective polymerization (asymmetric helix-chirogenic polymerization; terminology recommended by IUPAC)206,605,606 of the corresponding achiral monomers using chiral catalysts or initiators. On the other hand, dynamic helical polymers have a relatively low helix inversion barrier, so that rapid interconversion between the right- and left-handed helical conformations occurs in a single polymer chain in solution. However, energetically unfavorable helix reversals that separate rightand left-handed helical segments infrequently occur, which results in a long helical persistence length in dynamic helical polymers. Therefore, introduction of a tiny amount of optically active units into the polymer chains through covalent or noncovalent bonding interactions can significantly amplify the helical sense excess of the whole polymer chains with a high cooperativity under totally thermodynamic control. This unique feature of chiral amplification in dynamic helical polymers has been experimentally and theoretically revealed by Green et al. using polyisocyanates.22,24 13864


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Figure 123. (A) Schematic illustration of chiral amplification and helix-sense inversion of 410 induced by complexation with mono- and divalent metal ions in a nondonor solvent or induced by complexation with monovalent metal ions in the presence and absence of cation−π interactions through the sp and ap conformational changes of the pendants. (B) Schematic illustration of ON/OFF switching of the “sergeants and soldiers” effect in 411 upon complexation with Ba2+ (right) and Li+ (left). (Reproduced with permission from ref 646. Copyright 2014 The Royal Society of Chemistry.) (C) Schematic illustration of controlled modulation of the helix sense (M/P) and the elongation (loose/tight) of the polymer backbone in 412 by changing the donor and the polar character of the solvent through two independently tunable bonds in the pendants: (O)C−C(−O) bond (sp and ap) and (H−)N−C(O) bond (cis and trans), which are sensitive to the polarity and the donor ability of the solvents, respectively. (Reproduced with permission from ref 647. Copyright 2013 The Royal Society of Chemistry.)

Aoki et al. reported the first example of the helix-senseselective polymerization of achiral PPA derivatives using a rhodium catalyst (Rh-1) in the presence of (R)- or (S)-1phenylethylamine ((R)- or (S)-397), producing optically active helical polymers (poly-398a and poly-398b) (Figure 119Ba).611−613 The obtained polymers showed an ICD in chloroform due to a preferred-handed helical conformation stabilized by intramolecular hydrogen bonds between the pendants, but the ICD disappeared upon the addition of DMSO. Novel optically active zwitterionic Rh complexes bearing a C2-symmetric optically active norbornene ligand (Rh2)614 and those with tethered chiral amino (Rh-3) and ether groups (Rh-4) developed by Hayashi and Sanda et al. have also produced optically active helical poly-398a with an excess handedness in a helix-sense-selective way.615 Later on, Hayashi et al. reported that polymerization of a nitrogen-bridged 1,8-diyne possessing terminal and internal alkynes (399) with Rh-2 proceeds through an alternating reaction of the terminal and internal alkynes to form a predominantly one-handed helical PA (poly-399) with a 1,2dialkylidene heterocyclic unit (Figure 119Bb).616 Optically active helical PAs have also been prepared by the polymerization of achiral N-propargylamides 400 in chiral micelles consisting of D- or L-dodecylphenylalanine coordinated

to Rh-1 inside the micelles (Figure 119Bc).617 The resulting PAs maintain their optical activity in chloroform after isolation from the micelles, probably due to intramolecular hydrogen bonds and steric repulsions of the pendent groups that may stabilize the helical conformation kinetically produced during the polymerization in the chiral micelles. However, the polymers lose their optical activities in polar DMF due to breaking of the hydrogen bonds between the neighboring pendant urea groups. The first helix-sense-selective polymerization of achiral disubstituted acetylenes in chiral solvents was achieved using an optically active monoterpene α-pinene as the chiral solvent by Fujiki, Kwak, and co-workers (Figure 119C).618 The polymerization of an achiral diphenylacetylene containing a trimethylsilyl group at the para-position of the phenyl ring (401) in the chiral α-pinene with TaCl5-nBu4Sn as a catalyst produced an optically active poly(diphenylacetylene) (PDPA) (poly-401) showing an intense Cotton effect, despite the absence of a stereogenic center. The polymer maintained its optical activity for a long time at room temperature. The intramolecular stacked structure of the bulky aromatic groups may be responsible for its optical activity during the polymerization process. 13865


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Figure 124. (A) PQXs showing solvent-dependent helix inversion. Plots of screw-sense excesses (curve) and energy difference (straight line) in chloroform (left) and 1,1,2-trichloroethane (right) against mean number of chiral units ((R)- or (S)-413) in poly-413(R or S) or poly-413(R or S)/ 414 are also shown. (Reproduced with permission from ref 649. Copyright 2010 The Royal Society of Chemistry.) (B) PQXs bearing pyrene pendants (poly-415a−c) showing solvent-dependent changes of the fluorescence color (blue and green) and the screw-sense selectivity. Photographs of poly-415a dissolved in various solvents under UV irradiation (365 nm) are also shown. (Reproduced with permission from ref 650. Copyright 2012 The Royal Society of Chemistry.) (C) PQXs bearing various chiral units (“sergeants”): effect of the position and structures of the “sergeants” on the solvent-dependent helix inversion. (D) A PQX bearing L-lactic acid-derived pendants (poly-417) showing a perfect switch of helical chirality between two ether solvents (1,2-DME and MTBE). (E) A PQX showing a solvent-dependent helix inversion between n-octane and cyclooctane (poly-418). (F) PQX random copolymers with opposite helix sense by changing the structure of the achiral soldier units (poly-419, poly-420) showing unusual bidirectional induction of the helical sense upon changing the mole fraction of the chiral sergeant units (poly-419) and copolymers showing preferred-handed helix inversion by arranging the sergeant units through random (poly-419) or block (poly-421) polymerization protocols having the same structure and mole fraction of the sergeant units.

5.1.1, chiral amplification is one of the most intriguing features of dynamic helical polymers. Therefore, optically active polymers with a large excess of a single-handed helical conformation can be obtained by the copolymerization of achiral monomers with a small amount of optically active ones or the copolymerization of nonracemic monomers with a small ee. These two chiral amplification phenomena were for the first time discovered by Green et al. in polyisocyanates and are termed the “sergeants and soldiers” effect152 and the “majority rule” principle,153 respectively. Another unique and intriguing feature of dynamic helical polymers is the reversible helix inversion in response to external stimuli, such as temperature, solvent, additive, and light. Because the free energy difference between the right- and left-handed helical states in dynamic helical polymers is

Wu et al. have reported that an organopalladium(II) complex (Pd-1) could promote the living polymerization of isocyanides having various functional groups.619 Recently, they found that the polymerization of an achiral phenyl isocyanide (402) with Pd-1 in the presence of L- or D-lactide (L- or D-LA) proceeded in a helix-sense-selective way to afford optically active polymers (poly-402) with a preferred-handed helical conformation (Figure 119D).620 L- and D-LAs were not incorporated into the polymer chains, but may interact with the growing chain ends, thereby inducing the helix-sense-selective polymerization. Interestingly, the Pd-1 complex polymerized one of the enantiomeric isocyanides faster than its antipode while maintaining its living nature (see section 5.6.2.2). 5.1.3. Progress in Dynamic Helical Polymers: Chiral Amplification and Helix Inversion. As described in section 13866


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Figure 125. (A) Synthesis of a helical polyallene (poly-L-422) by living polymerization of an allene derivative bearing a chiral amide pendant (L-422) with an allylnickel complex. Schematic illustration of the conformational changes of poly-L-422 by the alternative addition of TFA and triethylamine (a). (B) Synthesis of enantiopure alleno-acetylenic oligomer (424) (n = 16). A depiction of an ensemble of right-handed helical conformers of 424 (n = 8) with 0° ≤ θ ≤ + 45° range (left), and view along the helix axis of the conformer at θ = 0° (right) (b). (Reproduced with permission from ref 661. Copyright 2010 Wiley-VCH.)

Figure 126. (A) Synthesis of spiro-connected quaterthiophene-based helical polymers with the spacer unit having switchable planarity (427). (Reproduced with permission from ref 662. Copyright 2015 American Chemical Society.) (B) Two-point-covalent-linking protocol that selectively produces coil-shaped 431 and screw-shaped 432 helical polymers. (Reproduced with permission from ref 663. Copyright 2015 The Royal Society of Chemistry.)

relatively low, the helical sense preference can be switched into the opposite handedness by external stimuli.4 Such helix-sense switchable polymers have potential applications as new chiral functional materials, such as chirality switchable asymmetric catalysts, elution-order switchable chiral stationary phases (CSPs), optical devices, and liquid crystals for displays.

In this section, recent representative examples of chiral amplification and helix-sense inversion in dynamic helical polymers are described. 5.1.3.1. Polyacetylenes. Among the dynamic helical polymers, monosubstituted PA derivatives are the most widely studied dynamic helical polymers. This is because high13867


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Figure 127. (A) Schematic illustration of a preferred-handed helicity induction in optically inactive PPAs (433−435) upon complexation with chiral amines and subsequent memory of the helicity after replacement by achiral amines. (B) Schematic illustration of a preferred-handed helicity induction in an optically inactive poly(phenyl isocyanide) (436) upon complexation with chiral amines, and memory of the induced macromolecular helicity after complete removal of the chiral amines through a configurational syn−anti isomerization of the CN double bonds.

Figure 128. Schematic illustrations of a preferred-handed helicity induction in optically inactive PPAs bearing phosphonic acid monoesters as the pendant groups (434b−d) with chiral amines (437) (A), helix inversion by temperature and/or solvent (B), subsequent memory of the diastereomeric macromolecular helices generated at different temperatures or in different solvents (C), and storage of the induced helicity or helicity memory by enantioselective esterification with diazomethane (D). (Reproduced with permission from ref 671. Copyright 2015 American Chemical Society.)

Rh metals (Rh(nbd)·apo-Fr) (nbd: norbornadiene), which produced PPA with a relatively narrower molecular weight distribution than that obtained using Rh-1 as a catalyst (Figure 120).633 Detailed theoretical studies suggest the mechanisms of the polymerization of phenylacetylene by Rh(nbd)·apo-Fr and the factors that control its regioselectivity and stereochemistry.634 However, a preferred-handed helical conformation was not induced in PPA during the polymerization in the chiral apo-Fr cage. Takata et al. reported an interesting method to control the helical conformation of a PPA bearing a pendant chiral rotaxane unit triggered by its motion (Figure 121A).635,636 403a exists almost as a racemic helix because the chiral wheels are located far enough from the polymer backbone to induce a helix-sense bias, thereby showing no ICD in the PA backbone region.

molecular weight, stereoregular helical PAs bearing various functional pendants can be easily prepared by the polymerization of the corresponding monomers with readily available rhodium catalysts due to their high tolerance toward polar functional groups.621−625 Some Rh catalysts have been found to promote the stereospecific living polymerization of monosubstituted acetylenes,624,626−629 and the polymerization mechanism with a Rh catalyst as well as the structures of the obtained PAs have been studied in detail by various spectroscopic techniques, including NMR and MS measurements of isotopically labeled samples630,631 combined with theoretical calculations.632 The polymerization of phenylacetylene has recently been conducted within a protein-assembled nanocage with an inner diameter of 8 nm, with apo-ferritin (apo-Fr) complexed with 13868


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Figure 129. (A) Schematic illustration of a reversible switching and memory of the macromolecular helicity of poly-438 and its axial chirality at the pendants in the solid state as well as in solution. Preferred-handed macromolecular helicity of poly-438 and its axial chirality are induced and subsequently memorized in poly-438 through noncovalent interactions with a nonracemic alcohol (439) followed by its complete removal. The helical handedness and axial twist-sense of poly-438 are switched reversibly in the presence of the opposite enantiomeric alcohol in the solid state. (B) CD and absorption spectra of poly-438 with (S)-439 in n-hexane at 25 (i) and −10 °C (ii) after standing at 25 °C for 6 h, and the isolated poly438 in n-hexane at −10 °C recovered from i (iii) and those in n-hexane at −10 °C after immersing poly-438 in (S)-439 at 25 °C for 6 h in the solid state followed by isolation (iv) and subsequent immersion in (R)-439 at 25 °C for 6 h (v). (C) Structures of the copolymer (poly-440) composed of an equimolar 438 and (4-dodecyloxyphenyl)acetylene and analogous PAs bearing 2,2′-bisphenol-derived pendants (poly-441−443). (Reproduced with permission from ref 675. Copyright 2014 Nature Publishing Group.)

Figure 130. (A) Schematic illustration of a preferred-handed helicity induction in an optically inactive PDPA (poly-444) in the presence of chiral amines by heating and subsequent memory of the helicity after complete removal of the chiral amines. (B) Structures of optically inactive PDPAs (poly-401 and poly-445a−d) used for solvent-to-polymer chirality transfer in chiral limonene ((S)-446).

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Figure 131. (A) Schematic illustration of synthesis of preferred-handed helical poly(BBPFA) by asymmetric anionic polymerization of BBPFA and photoinduced racemization of its preferred-handed helix without any rearrangement of chemical bonds. (B) Schematic illustration of the photoinduced helix−helix transition of poly(BMBPSt) from a preferred-handed slim helix to a racemic stocky helix. (C) Schematic illustration of the preferred-handed helical conformation induction to 447 in a thin film upon irradiation with R- or L-CP light. CD and absorption spectra of 447 in a film upon irradiation with R-CP light for 0 min (blue) and 6 min (red), upon irradiation with L-CP light for 6 min (purple), and further for 12 min (green). (Reproduced with permission from ref 684. Copyright 2012 The Royal Society of Chemistry.) (D) Schematic illustration of selective photocyclic aromatization (SCAT) of poly-398a exclusively yielding a 1,3,5-trisubstituted benzene derivative (CT) that forms self-supporting membranes. (Reproduced with permission from ref 686. Copyright 2013 American Chemical Society.) (E) Schematic illustration of helix-senseselective degradation of poly-398a by SCAT using R- or L-CP light.

(405-co-406)) consisting of optically active macro-405 and optically inactive macro-406 bearing poly(phenyl isocyanate) chains with an optically active group and an achiral group at the termini, respectively (Figure 122A).639 Poly(405-co-406) exhibited an ICD as intense as that of poly-405 in the PA backbone region, even when it contained as little as 25 mol % of the macro-405 unit (r = 0.25), suggesting that the preferredhanded macromolecular helicity induced in the PA backbone by the optically active macro-405 grafts further hierarchically induced an excess helical handedness in the dynamically racemic macro-406 grafted regions. Similar helical polymer brushes consisting of a PA backbone and poly(n-hexyl isocyanate) grafted-pendants (407 and 408)640,641 or poly(Llactide) pendants (409)642,643 have been synthesized (Figure 122B), and a preferred-handed helical conformation was also induced in the PA backbones. Freire, Riguera, and co-workers have found that a helical PPA with (R)-α-methoxyphenylacetic acid (MPA) residues as the pendants (410) shows a unique chiral amplification upon complexation with metal cations, and its helix sense can be tuned by the valence of the metal cations (Figure 123A).644 In the absence of metal cations, 410 consists of an almost equal mixture of interconvertible right- and left-handed helical segments in chloroform, as confirmed by no ICD in the absorption region of the polymer backbone. This is probably because the chiral MPA pendants exist in two different conformations with respect to the rotation around the

However, upon the addition of a base, such as DBU, the pendant tert-ammonium groups are neutralized to give 403b in situ; therefore, the chiral wheels approach the main-chain to induce a preferred-handed helical conformation, thus showing an ICD. 403b then reverts to a racemic helical 403a by the further addition of an acid, such as trifluoroacetic acid (TFA), and this conformational switch can be reversibly repeated by alternating treatment with acid and base. They also succeeded in a similar conformational switch using PPAs bearing a planar chiral rotaxane pendant (404) (Figure 121B).637 Helical polymer-grafted helical polymers (helical polymer brushes) consisting of a dynamically racemic helical PA backbone and optically active poly(phenyl isocyanate) pendants (poly-405) showed an intriguing hierarchical amplification of macromolecular helicity through the longrange transfer of chiral information (Figure 122A).638 The helical handedness of the PA backbone in poly-405 could be controlled through a covalent-bonding chiral domino effect that occurred at the grafted poly(phenyl isocyanate) chains in which an excess one-handed helical conformation was induced by an optically active group, such as (S)-2-(methoxymethyl)pyrrolidinyl ((S)-MMP), introduced at the termini. As a result, chiral information was successfully transferred to as many as 70 bond lengths away from the PA backbone. Amplification of the macromolecular helicity based on the “sergeants and soldiers” effect through long-range stereochemical communication has also been achieved in PPA-based copolymer brushes (poly13870


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Figure 132. (A) Schematic illustration of asymmetric polymerization of the azobenzene moiety-substituted diacetylene (448) LB films with circularly polarized UV light (CP UV light, 313 nm) and the reversible chirality switching of the obtained PDA with CP laser (442 nm). (Reproduced with permission from ref 691. Copyright 2009 The Royal Society of Chemistry.) (B) Synthetic route of the discotic hydrogen bonding complex (451) from 449 and 450 (a). Schematic illustration of the experimental setup for the helix-sense-selective synthesis of helical PDA by application of LPL and magnetic field (b). CD spectra of 451 films polymerized by application of LPL irradiation (10 mW/cm) and the magnetic field (0.5 T): (i) parallel, (ii) antiparallel to the beam of LPL, and (iii) with zero magnetic field (c). Schematic illustration of the helix-sense-selective polymerization mechanism of 451 in the liquid crystalline phase by application of LPL under a parallel or antiparallel magnetic field (d). UV−vis absorption spectra of the prepared left-handed PDA assemblies: (i) before treatment, immersed into (ii) D-Lys or (iii) L-Lys solution, and corresponding color changes (insets) (e). The colorimetric response (CR) value of the left- and right-handed PDA assemblies in response to D-Lys or L-Lys (f). CR = (PB0 − PB1)/PB0 × 100, PB = A540/(A540 + A640) × 100, PB0: initial value, PB1: after immersion. (Reproduced with permission from ref 695. Copyright 2014 Nature Publishing Group.)

and Ag+, this cation−π interaction can be regulated by appropriate donor solvents, which eventually determine the helix sense of 410, resulting in inversion of the helicity (Figure 123A). By taking advantage of this metal cation-triggered conformational change of the MPA pendants, an interesting switchable (ON and OFF) “sergeants and soldiers” effect in the presence and absence of metal cations has been achieved for the PPA copolymers consisting of the chiral MPA unit and achiral units (411) (Figure 123B).646 In this system, the chiral MPA units behave as “sergeants” only when coordinated to metal cations. The same group has further demonstrated that a helical PPA bearing the (R)-α-methoxy-α-trifluoromethylphenylacetic acid (MTPA) residue as the pendants (412) can respond to the solvent polarity as well as donor ability, resulting in inversion of the helical sense accompanied by extension and contraction

(O)C−C(−O) bond; that is, synperiplanar (sp) and antiperiplanar (ap) conformers. However, upon the addition of a monovalent cation (M+), such as Li+, Na+, and Ag+, the MPA pendants adopt an ap conformation by the coordination of the monovalent cation to the carbonyl group, which induces a predominantly left-handed helix (M-helix) in the main-chain of 410 through chiral amplification. On the other hand, the MPA pendants favor an sp conformation when coordinated with a divalent cation (M2+), such as Ba2+ and Ca2+, which results in the opposite right-handed helix (P-helix) formation in 410 in a highly cooperative fashion. Extensive spectroscopic studies revealed that cation−π interaction stabilizes the ap conformation at the pendant due to the coordination of M+ to both the MPA phenyl and the carbonyl groups, whereas the chelation of M2+ to both the carbonyl and the methoxy groups fixes the pendant in the sp conformation.645 In the case of Na+ 13871


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Figure 133. Stabilization of helical conformations of foldamers (452, 454−459) via covalent or noncovalent intramolecular cross-linking by photoirradiation (A), reductive amination after imine-bond formation (B), alkene metathesis (C, D), pseudopolyrotaxane formation (Ea), and coordination of metal cations (Eb, c). (Reproduced with permission from refs 696, 700−703; Copyright 2003 Wiley-VCH, Copyright 2012 Elsevier, Copyright 2015 Wiley-VCH, Copyright 2009 The Royal Society of Chemistry, and Copyright 2011 The Royal Society of Chemistry, respectively.)

Figure 134. (A) Schematic illustration of helix inversion of polysilane aggregates (460a) depending on the achiral side chain length and cosolvent fraction. (Reproduced with permission from ref 706. Copyright 2013 American Chemical Society.) (B) Schematic representation of solvent- and temperature-dependent helix inversion of the 461 aggregate.

motions of the polymer backbone (Figure 123C).647 This helixsense inversion is considered to be regulated by the two sets of

rotations around the (O)C−C(−O) bond (sp and ap) and the (H−)N−C(O) amide bond (cis and trans) in the 13872


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Figure 135. (A) AFM image of helical nanofibers formed by 462a upon natural evaporation of its methanol solution on mica. (Reproduced with permission from ref 711. Copyright 2001 American Chemical Society.) (B) AFM image of left-handed superhelical fibers formed by 462b upon evaporation of its THF solution. (Reproduced with permission from ref 712. Copyright 2008 American Chemical Society.) (C) TEM image of superhelical fibers formed by 462b upon evaporation of its THF solution on a carbon-coated copper grid. (D) High-resolution AFM image of the aligned 462b chains on the air/water interface. Regions labeled with 1 and 2 have different orientation and gap widths. (Reproduced with permission from ref 713. Copyright 2012 American Chemical Society.)

Figure 136. SEM images of self-assembled 463a (A) and 463b (B) aggregates prepared from slow evaporation of a solution of 463 (1 mg/mL) in water/DMSO (6/4, v/v) in the absence (A) and presence (B) of (S)-397. The magnified images (b and d) correspond to the areas indicated by the blue (a) and red (c) squares, respectively. (Reproduced with permission from ref 714. Copyright 2011 American Chemical Society.)

equilibrium of the (O)C−C(−O) bond in favor of the most polar sp conformer that is more sterically demanding and, therefore, induces the helical sense inversion in order to reduce

pendants, whose conformations can be independently tuned by the polarity and donor ability of the solvents, respectively. An increase in the solvent polarity can shift the conformational 13873


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Figure 137. TEM image (A) and a plausible structure (B) of a left-handed superhelix formed from a right-handed helical poly(styrene)-bpoly(isocyanodipeptide) (464). (Reproduced with permission from ref 715. Copyright 1998 American Association for the Advancement of Science.) TEM images of left- (C) and right-handed (D) helical nanofibers obtained from PBLG-b-L- or D-menthyl esters bound-poly(phenyl isocyanide) copolymers, LL- and LD-465, respectively. (Reproduced with permission from ref 717. Copyright 2015 Wiley-VCH.)

Figure 139. (A) Schematic illustration of the formation of selfassembled superstructures from an optically active poly(styrene)-bpoly(L-LA) (467). (B) TEM images of helical ribbon (a) and tubular superstructures (b) obtained from 467 after sonication. (Reproduced with permission from ref 720. Copyright 2010 American Chemical Society.)

Figure 138. AFM images of right- (A) and left-handed (B) superhelical nanofibers formed in spin-cast polymer-blend films consisting of (R)-466a/(R)-466c (1/1, wt/wt) and (S)-466b/(S)466c (1/1, wt/wt), respectively, (Reproduced with permission from ref 719. Copyright 2015 American Chemical Society.) Figure 140. SEM image of helically twisted self-assembled fibrils with a cyclic morphology prepared from L-Asp-based polyester (468). (Reproduced with permission from ref 721. Copyright 2015 American Chemical Society.)

the steric hindrance. A donor solvent can then associate with the amide bond, which triggers the conformational change in the amide bond from the trans to cis form, thereby inducing both the stretching of the polymer chain and the helical sense 13874


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Figure 141. (A) Structures of optically active polyphosphazene block copolymers (469). (B) Bright-field TEM image of the worm-like twisted aggregates and helical nanostructures (black arrows) of 469a. (Reproduced with permission from ref 722. Copyright 2013 Wiley-VCH.)

Figure 142. (A) TEM and (B) AFM images of helical aggregates obtained from a solution of 470 in THF/water (60/40, v/v) (a, c, d) and THF/ water (40/60, v/v) (b, e, f). (Reproduced with permission from ref 724. Copyright 2015 The Royal Society of Chemistry.)

Figure 143. Structures of polypeptoids containing amino and/or carboxy groups as the pendants (471). (A) AFM image of a helical aggregate prepared from a solution of 471a in aqueous solution at pH 6.8. (Reproduced with permission from ref 725. Copyright 2010 American Chemical Society.) (B) Molecular model of the 2D crystalline sheet assembled from 471c and 471d. The modeled conformation shows that hydrophobic groups face each other in the interior of the sheet and oppositely charged hydrophilic groups are alternating and surface-exposed. (C) SEM (a) and AFM (b) images of 2D crystalline sheets on Si substrate assembled from an aqueous solution of periodic amphiphilic peptoid copolymers 471c and 471d at pH 9.0. (Reproduced with permission from ref 728. Copyright 2010 Nature Publishing Group.)

substituted PPA is less dynamic, showing a weak response to external stimuli, and it exists as an equilibrium mixture of two different helices with a similar helical pitch to that of 410 and a more stretched one. In contrast, the ortho-substituted analogue is inert to external stimuli (quasi-static) and possesses a highly stretched, almost planar helical conformation, which is prone to form fibrillar aggregates. 5.1.3.2. Poly(quinoxaline-2,3-diyl)s. The helical structures of the PQXs bearing bulky substituents, such as the p-

inversion as visualized by red-shift and Cotton effect inversion in the UV−vis and CD spectra, respectively. Very recently, the same group has also demonstrated a remarkable effect of the aromatic substitution pattern (ortho, meta, and para) of the pendant (R)-MPA residues of PPAs on the helical conformation (dynamic or static) as well as the helical pitch (compressed or stretched).648 Compared with the dynamic para-substituted 410, capable of responding to external stimuli such as metal ions, the corresponding meta13875


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Figure 144. (A) AFM image of a helical superstructure formed by self-assembly of 472 with the switch-segment intact in dichloromethane/methanol (1/1, v/v). (Reproduced with permission from ref 731. Copyright 2009 Wiley-VCH.) (B) AFM (a) and TEM (b) images of superhelical rods selfassembled from PBLG-b-PEG (473) and PBLG (474). (Reproduced with permission from ref 732. Copyright 2009 The Royal Society of Chemistry.) (C) Optical micrographs of chiral morphologies of CaCO3. The right- (c) and left-handed (d) helical growth in the presence of LL-475 and DD-475, respectively. (Reproduced with permission from ref 735. Copyright 2003 Wiley-VCH.) (D) AFM images of helical nanofibers formed by hierarchical self-assembly of 476 (40 μM) for 8 h at pH 9.2 and room temperature (e). (f) Magnified image of a type I fiber with a left-handed helical structure, corresponding to the area marked with a black square in the image of (e). The average diameter and helical pitch are around 6.0 and 45 nm, respectively. (g) Magnified image of a type II fiber, corresponding to the area marked with a white square in the image of (e). The type II fiber (diameter ca. 12 nm) was constructed by the intertwisting of the two type I fibers. (Reproduced with permission from ref 737. Copyright 2006 Wiley-VCH.)

(S)-413) and achiral ones (414) with varying ratios (poly413(R) or 413(S)/414) display a nonlinear relationship between the estimated helix-sense excess and the number of chiral units, which can be rationalized by a linear relationship in terms of the free energy. Based on these results, it is concluded that PQXs are in equilibrium between the P- and M-helical conformations, and each polymer chain has no helix reversal. Solvent-induced conformational changes in the chiral alkyl chains in the pendants seem to be responsible for the helixsense inversion. In order to probe the conformational change of the chiral pendant residues regarding the mechanism of the solventinduced helix inversion, a series of PQX copolymers bearing pyrene units have been synthesized (Figure 124B).650 A chiral pyrene-modified PQX copolymer with achiral pendants (poly415a) possesses an M-helix and exhibited an intense green excimer emission in chloroform and 1,1,2,2-tetrachloroethane, while, in 1,1,1-trichloroethane, the CD and excimer emission

propylphenyl group at the 5- and 8-positions of the quinoxaline ring, are very stable, and optically active PQXs with an almost single-handed helical conformation can be prepared by the helix-sense-selective polymerization of the corresponding achiral 1,2-diisocyanobenzenes using chiral organopalladium complexes. However, the PQXs behave as a dynamic helical polymer when they have less bulky substituents, such as methyl groups, at the 5- and 8-positions of the quinoxaline ring.594 Taking advantage of the living nature during the polymerization of 1,2-diisocyanobenzenes catalyzed by palladium and nickel complexes, Suginome et al. synthesized a series of optically active PQXs with a well-defined structure and demonstrated interesting chiral amplification and helix inversion phenomena. An optically active PQX bearing chiral (R)-2-butoxy pendant (poly-413(R)) shows a unique solvent-dependent helical sense inversion between the P-helix in chloroform and the M-helix in 1,1,2-trichloroethane (Figure 124A).649 Copolymers consisting of chiral units carrying (R)- or (S)-2-butoxy pendants ((R)- or 13876


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Figure 145. (A) Cryo-TEM image of helically assembled peptide βAβAKLVFF. The inset shows a region from a different micrograph which shows a double-helix. (Reproduced with permission from ref 742. Copyright 2009 Wiley-VCH.) (B) AFM image of a typical SMA type I fibril. The average height for several type I fibrils was 8.3 ± 0.9 nm. (Reproduced with permission from ref 743. Copyright 1999 National Academy of Sciences.) (C) Representative cryo-TEM images of shadowed insulin fibrils with a left-handed helical structure. (Reproduced with permission from ref 744. Copyright 2002 National Academy of Sciences.) (D) AFM images of left- (a) and right-handed (b) HET-s (218−289) prion fibrils grown in pH 2.0 and 3.9, respectively. (Reproduced with permission from ref 745. Copyright 2014 American Chemical Society.)

Figure 146. (A) AFM image of an LB film of achiral random copolymer 477 bearing azobenzene chromophores with nitro groups. Scale = 3 × 3 μm2. (B) The magnified images of the areas indicated by the blue squares (a−c) in part A. (Reproduced with permission from ref 746. Copyright 2014 American Chemical Society.) (C) TEM images of micelle-like aggregates in water (d) and helical nanocylinders in water/methanol (e) of 478. (Reproduced with permission from ref 747. Copyright 2008 American Chemical Society.)

intensities were significantly suppressed, thus showing a blue fluorescence (see photographs in Figure 124B). Poly-415b, composed of the same chiral pyrene and additional chiral pendants, and poly-415c, consisting of achiral pyrene and achiral pendants, showed similar solvent-dependent fluorescence changes. Therefore, the fluorescence color switches mainly result from the solvent-induced conformational changes in the chiral pendants and are mostly independent of the helical sense. However, the present system can be used as a method for screening solvents for the efficient induction of one of the single-handed helices, and 1,1,2,2-tetrachloroethane was found

to be one of the most suitable solvents for the preferred-handed helix induction. A series of PQX copolymers bearing various chiral units with a common achiral unit (poly-416a−h) have been synthesized, and their chiroptical properties have been investigated in order to find the appropriate chiral unit capable of more efficiently inducing a single-handed helix in a solvent-dependent chirality switchable manner. (Figure 124C).651 Optically active 2alkoxymethyl groups at the 6- and 7-positions of the quinoxaline ring (poly-416a−d and poly-416a′) more efficiently induced a one-handed helix than the chiral 3methylpentyl (poly-416e) or 2-methylbutoxy (poly-416f) 13877


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Figure 147. (A) Tentative model of the hierarchical structure of 479 superhelices. (B, C) Tapping-mode SFM amplitude images of the rodlike superstructures formed by 479a (B) and 479b (C) on graphite. L and R represent superhelices with left-handed and right-handed screw senses, respectively. (Reproduced with permission from ref 748. Copyright 2004 American Chemical Society.) (D) TEM image (unstained, bar represents 250 nm) of a right-handed helical aggregate of 480 found in a water/THF mixture of 90/10 (v/v). (E) Schematic representation of the formation of a superhelix from the coiling of two helical strands. (Reproduced with permission from ref 749. Copyright 2000 American Chemical Society.)

A helix-sense inversion in the chiral/achiral PQXs can also be regulated by changing either the structure of the achiral soldier units or the arrangement of the sergeant units (random or block), while keeping the structure and mole fraction of the sergeant units unchanged (Figure 124F).654 Random PQXs consisting of a chiral unit bearing an (S)-3-octyloxymethyl pendant and achiral units having propoxymethyl (poly-420) or butoxy groups (poly-419) adopt an M- or P-helical conformation, respectively. An unusual bidirectional helixsense induction has been observed for a series of PQX copolymers (poly-419) by changing the mole fraction of the sergeant units; poly-419 with a mole fraction of sergeant units of 16−20% and more than 60% formed P- and M-helices, respectively. Random (poly-419) and block (poly-421) copolymers (250-mer) consisting of the same combination of chiral and achiral units in a 18:82 molar ratio formed almost complete P- and M-helical conformations, respectively (>99% de). Recently, the first clear example of a pressure-induced helixsense inversion has been reported for an optically active PQX

groups. It was revealed that the position of the chiral center in the pendants is important for the solvent-dependent helix inversion, and the polymers bearing a chiral center at the third atom in the side chain (poly-416a−f and poly-416a′) underwent the solvent-dependent helix inversion between chloroform and 1,1,2-trichloroethane. PQXs bearing (S)-1-(pentyloxycarbonyl)ethoxymethyl side chains derived from natural L-lactic acid (poly-417) also showed a helix inversion in nonhalogenated solvents, such as between 1,2-dimethoxyethane (1,2-DME, M-helix) and tertbutyl methyl ether (MTBE, P-helix) (Figure 124D).652 Interestingly, an optically active PQX bearing (S)-3octyloxymethyl pendants (poly-418) exhibited an almost perfect solvent-dependent helix inversion between n-octane (M-helix) and cyclooctane (P-helix) (Figure 124E).653 As a result of the screening of various alkane solvents with different structures, it was found that poly-418 forms an M-helical conformation in linear alkanes with high molecular aspect ratios, whereas the opposite P-helical conformation is induced in branched or cyclic alkanes with small molecular aspect ratios. 13878


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Figure 148. (A) The C2-symmetrized, 2D class sum image of cryo-TEM of the 481 fiber network embedded in a vitrified layer of water. (B) Calculation of the 3D volume obtained from (A). (C) The reprojection of (B) into the 3D volume. (Reproduced with permission from ref 751. Copyright 2005 Wiley-VCH.)

number of building blocks, suggesting the formation of a preferred-handed helical conformation. Quantum mechanical calculations indicated that 424 most likely possesses a predominantly right-handed helical conformation with the torsion angle across the butadiyne axes ranging from 0° to +45° (Figure 125Bb). Takata and co-workers have synthesized spiro-connected quaterthiophene-based helical polymers (427) by the Stille cross-coupling polymerization of optically pure 425 with 426 and found a unique redox-responsive reversible conformational change of the polymer (Figure 126A).662 Oxidization with FeCl3 caused the quaterthiophene spacer units to be in a planar structure, resulting in the conformation of the polymer taking a coil-shaped, rigid helix, while the neutral 427 possessed a onehanded helical conformation originating from the optically pure C2-chiral 425 units. Recently, the same group has proposed the two-point-covalent-linking protocol that produced two different helical polymers, coil-shaped (431) and screw-shaped (432) helical polymers by copolymerization of C2-chiral spirobifluorene 428 with C2-symmetric 429 and Cs-symmetric 430, respectively (Figure 126B).663 The resulting helical polymers are thermally stable because of their rigid spirobifluorene units together with the covalently two-point-connected structures. 5.1.5. Helical Polymers with Helicity Memory. As previously demonstrated, a preferred-handed macromolecular helicity can be also induced in optically inactive dynamic helical polymers through noncovalent bonding interactions using optically active compounds. For example, cis−transoidal PPAs bearing acidic functional pendants (433−435) form an excesshanded helical conformation upon complexation with nonracemic amines, thus showing an ICD in the absorption regions of the polymer backbones (Figure 127A).664−666 As can be inferred from its dynamic nature, the ICD derived from an

bearing chiral (S)-2-butoxy pendants (poly-413(S) in Figure 124A).655 In 1,2-dichloroethane and a mixed solvent of chloroform−1,1,2-trichloroethane (35/65, v/v), poly-413(S) formed an P-helical conformation at normal atmospheric pressure (0.1 MPa), while the polymer adopted an opposite M-helical conformation at high pressure (200 MPa). 5.1.4. Other Types of New Helical Polymers. Tomita and Endo et al. previously found that allylnickel(II) complexes could promote the living coordination polymerization of allene derivatives having a variety of functional groups.656,657 Taking advantage of this living polymerization, optically active helical polyallenes possessing sugar or phenylcarbamoyloxy-substituted (R)-binaphthyl pendants have been prepared using a [(πallyl)NiOCOCF3]2/PPh3 complex as the catalyst.658,659 Recently, Wu and Liu et al. found that the (π-allyl)NiOCOCF3/ PPh3 complex is also useful for the living polymerizations of an optically active allene (L-422), forming poly-L-422 with a controlled molecular weight and narrow molecular weight distribution as well as a helical conformation with a preferredhandedness that is stable in aprotic solvents (Figure 125A).660 Since the helical conformation of poly-L-422 is stabilized by the intramolecular hydrogen bonding between the amide pendants, it can be transformed into a random coil by the addition of TFA, which can revert back to the original helix by the subsequent neutralization with triethylamine (Figure 125Aa). Poly-L-422 was also found to self-assemble into well-defined helical fibrils with a distinct handedness, as observed by AFM and SEM observations. Diederich et al. synthesized an enantiopure alleno-acetylenic oligomer (424) from an optically pure building block (423) by repeating the homocoupling and partial deprotection procedures (Figure 125B).661 The CD intensity of 424 exhibited a remarkable nonlinear increase versus the increase in the 13879


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after the chiral amines are completely replaced by achiral amines (Figure 127A).607,667 This is the first example of the macromolecular helicity memory induced in optically inactive polymers. A similar macromolecular helicity memory has also been reported for other PPAs, such as 434a,b668 and 435,669 indicating that the dynamic helical conformations are frozen, transforming into kinetically controlled, static helical polymers. However, in these systems, the replacement by achiral amines is indispensable for the helicity memory in PPA derivatives; otherwise, the chiral memory cannot be retained. The helical senses induced in PPAs bearing prochiral phosphonic acid monoesters as the pendant groups (434b− d) upon complexation with nonracemic amines (437) could be inverted by changing temperatures and/or solvent compositions (Figure 128A and B).670,671 The resulting diastereomeric right- and left-handed helices were further memorized by replacement with achiral diamines to produce the corresponding enantiomeric helices (“dual memory”) (Figure 128C). However, the helicity memory is not inert and is gradually lost because of its inherently dynamic characteristics. However, it is possible to “store” such a dynamic helicity memory after the prochiral pendant group is converted to its chiral methyl ester using diazomethane that proceeds in an enantioselective way to generate a phosphorus stereogenic center with an optical activity resulting from the chirality transfer from the preferredhanded helicity induced or memorized in the polymer backbones (Figure 128D).671,672 The enantioselective-esterification took place more efficiently for 434d, affording the corresponding optically active helical PPA with an enantioselectivity of 33% ee. Importantly, the chirality of the pendants thus produced is further significantly amplified in the polymer backbone to form an excess one-handed helical structure whose helix sense excess reached 84% ee at −50 °C.671 A similar preferred-handed helical conformation can be induced in an optically inactive poly(phenyl isocyanide) (436) in the presence of optically active amines in water. Unlike the PPA derivatives, the induced helical chirality is automatically memorized after isolation of the polymer to completely remove the chiral amines. That is, achiral chaperoning molecules are not required for this helicity memory (Figure 127B),608,673 which is in significant contrast to the conformational memory of the induced helical PPAs.607,667−672 The helix formation of 436 with chiral amines in water is accompanied by a configurational isomerization of the CN double bonds (syn−anti isomerization) into a uniform configuration driven by hydrophobic and chiral ionic interactions in water, and the resulting configurational regularity most likely prevents racemization of the polymer backbone.673 This noncovalent “helicity induction and conf igurational memory strategy” provide a facile way to construct a static helical polymer with an excess helix sense from an optically inactive dynamic helical polymer via specific noncovalent chiral interactions. This unique and versatile “static” helicity memory strategy makes it possible to further modify the side groups with the desired functional groups, such as esters and amides, while maintaining its onehanded helical conformation.674 The optically active helicitymemorized polyisocyanides have been used as chiral catalysts for asymmetric reactions and a CSP for HPLC (see section 5.7). Recently, Maeda, Yashima, and co-workers discovered an unprecedented switchable helicity induction and subsequent memory of the induced macromolecular helicity of an optically inactive cis−transoidal PA bearing 2,2′-biphenol-derived

Figure 149. (A) TEM images of 482 aggregates sprayed from dichloromethane/methanol at f M = 82% after sample aging for 24 days (a) and 3 months (b). (B) TEM tomography images of double (c) and triple (d) helices of 482 sprayed from dichloromethane/methanol at f M = 82%. (Reproduced with permission from ref 753. Copyright 2009 Wiley-VCH.) (C) Structure of thiophene-based diblock copolymer 483 and schematic representation of its self-assembly into superhelical structures through crystallization in the presence of potassium ions. (D) TEM images showing the structural transformation of 483 containing KI (0.47 mM) from single- to double- (e) and to multiple(f) stranded helices. Inset: TEM image and schematic showing association of double helices into quadruple superhelices (scale bar, 100 nm). (Reproduced with permission from ref 754. Copyright 2011 American Chemical Society.)

induced helicity immediately disappears when chiral amines are removed from the polymers using a stronger acid, such as TFA. However, the induced helicity is retained, that is, “memorized”, 13880


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Chart 34

pendants (poly-438), which is possible in the solid state as well as in solution (Figure 129A).675 In sharp contrast to the previously observed chiral memory in PPAs,607,667−672 one of the helical conformations can be induced in poly-438 through noncovalent weak interactions with nonracemic alcohols (439) in the solid state. More surprisingly, further replacement of the chiral inducers with achiral molecules is no longer necessary for the memory of helicity, which can be fully achieved in the solid state. For instance, after poly-438 was immersed in liquid (S)439 at 25 °C, in which poly-438 is totally insoluble, the isolated poly-438 dissolved in n-hexane at −10 °C exhibited an apparent ICD with an intensity comparable to that induced in the n-hexane solution in the presence of (S)-439 (Figure 129B). Upon subsequent immersion in (R)-439, the helicity of poly-438 was completely inverted and automatically memorized after isolation, as confirmed by the perfect mirror image ICDs in n-hexane (Figure 129B). During the macromolecular helicity induction and memory in poly-438, either a right- or left-handed twist-sense is induced in the biphenyl pendants, as evidenced by the appearance of an intense couplet vibrational

circular dichroism (VCD) due to the pendant methoxymethoxy (OMOM) groups. Cooperative interactions between the adjacent biphenyl pendants might be necessary for the helicity memory in the solid state, because the copolymer poly-440 showed almost no memory effect under identical conditions (Figure 129C). Although poly-441 bearing ethoxymethoxy groups instead of OMOM groups showed a similar helicity induction and memory behavior, the analogous poly-443 and poly-442 exhibited no memory effect and no helicity induction, respectively (Figure 129C). Therefore, it is suggested that both the biphenyl and alkoxymethoxy groups in the pendants are indispensable for the present unique helicity induction and memory effect, and the biphenyl pendants bearing alkoxymethoxy groups seem to act as a geared molecular brake capable of preventing racemization of the helical polymer backbone. This remarkable switchable memory effect observed for poly-438 has been applied to the development of a novel switchable CSP for separating enantiomers whose elution orders can be controlled and switched (see section 5.7.1). 13881


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Figure 150. Synthesis of helical polyacetylenes in chiral nematic liquid crystals and helical graphite from iodine-doped helical polyacetylenes with the morphology-retaining carbonization method. (Reproduced with permission from refs 780 and 781. Copyright 2010 The Royal Society of Chemistry and Copyright 2011 American Chemical Society, respectively.)

Figure 151. Helix-sense-selective polymerization of achiral aryl isocyanides (510) in a cholesteric liquid crystal matrix. POMs of the cholesteric matrix before (A) and after (B) polymerization of 510c. (Reproduced with permission from ref 785. Copyright 2014 Elsevier.)

optically active one once dissolved in optically active limonene ((S)-446), resulting from a preferred-handed helical arrangement of the pendant phenyl residues along the polymer backbone induced by interactions with the chiral solvent (Figure 130B).677 PDPAs with a longer alkyl chain (poly-445a and poly-445b) and bulkier alkyl group (poly-445c) at the para-position and a trimethylsilyl group at the meta-position (poly-445d) are not favored for efficient solvent-to-polymer chirality transfer. The induced optical activity in poly-401 is retained after isolation of the polymer followed by redissolving in achiral solvents, indicating the memory of the induced helicity of poly-401.678 This observation together with the fact that an optically active helical poly-401 can be prepared by the helix-sense-selective polymerization of the corresponding monomer618 suggests that poly-401 has features of both static and dynamic helical polymers due to its appropriate level of the helix inversion barrier. 5.1.6. Photoresponsive Helical Polymers and HelixSense Control with Light. Photoresponsive helical polymers have been synthesized in order to develop novel photoswitchable functional materials. In this section, recent representative examples of the helix-sense control of helical polymers with chiral and achiral lights are described.

A similar helicity memory turned out to be possible for an optically inactive PDPA bearing carboxy groups at the pendants (poly-444) upon complexation with optically active amines in water followed by thermal annealing and complete removal of the optically active amines (Figure 130A).676 The optical activity of poly-444 due to the memorized helical chirality was retained after conversion of the carboxy groups to methyl esters. The 1H NMR resonances at the ortho- and metapositions on each phenyl ring of poly-444 measured in DMSOd6−D2O (20/1, v/v) at 80 °C showed two broad but largely separated sets of peaks, respectively, which almost coalesced into single ones at 130 (ortho) and 110 (meta) °C. These observations suggest that each phenyl ring of the poly-444 is restricted from free rotation. Based on the coalescence temperatures, the free energy of activation (ΔG) for the interconvertible protons corresponding to the rotation barrier of the phenyl rings was estimated to be 78.5 kJ/mol (18.8 kcal/ mol). This high energy for the rotation of the pendant phenyl rings seems to contribute to the present memory effect in poly444, although thermal annealing is necessary for the preferredhanded helix induction. Lee et al. also reported that an optically inactive PDPA having no functional pendants (poly-401) changes into an 13882


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Figure 152. SEM images of lipid aggregates of 511 prepared from a 10 vol % methanol aqueous solution (A), 511·487 composite film obtained under neutral pH conditions after 60 redox cycles (B), 511·486 film after 60 redox cycles under neutral pH conditions (C), and PANI composite film obtained from electrochemical polymerization in the presence of 511 after 100 redox cycles (D). (Reproduced with permission from ref 786. Copyright 2004 Wiley-VCH.)

conformational transition from twisted to coplanar in the pendant terphenyl moiety. It has been previously demonstrated by Schuster, Green, and co-workers that either a right- or left-handed CP light can be used to induce an excess of helical conformations in a dynamically racemic helical polyisocyanate bearing CP-lightsensitive axially chiral bicyclo[3.2.1]octan-3-one derivatives as the pendants.683 Recently, Nakano et al. have successfully induced a preferred-handed helical conformation in an optically inactive polyfluorene derivative (447) by irradiation with either a right (R)- or left (L)-handed CP light generated from an artificial light source and the sun in the film state (Figure 131C).684 Intense negative bisignate Cotton effects were observed at around 400 nm for a film of 447 upon irradiation with R-CP light for 6 min, which almost completely disappeared by subsequent L-CP light irradiation for 6 min. Further irradiation with L-CP light for an additional 6 min afforded positive mirror-imaged CD signals. This is the first example of the preferred-handed helix induction in a virtually achiral main-chain conjugated polymer by CP light. The freeenergy landscape reconstructed for 447 chains predicts that the chirality switching occurs only on amorphous silica.685 Actually, a preferred-handed helix induction in 447 by CP light was very effectively attained in a film deposited on a quartz plate compared with that in suspension, and it was not possible to induce one of the helices in a dilute solution by CP light, indicating that specific interactions of 447 with amorphous silica contribute to this helix induction and its switching. Aoki et al. found an interesting photoinduced aromatization of cis−cisoidal poly-398a that highly selectively takes place upon irradiation with visible light of its solid membrane, quantitatively and specifically producing the corresponding cyclic trimer, i.e., a 1,3,5-trisubstituted benzene derivative, while maintaining its self-supporting membrane state despite its low molecular weight (Figure 131D).686 The resulting membrane is composed of the 1,3,5-trisubstituted benzene derivative linearly linked by hydrogen bonds and π−π stacking, thereby forming a

Figure 153. Polymerization of acetylene using copper tartrate as a catalyst. TEM image of a single copper crystal with a grain size of 50− 80 nm, which is located at the node of the coiled fibers. (Reproduced with permission from ref 787. Copyright 2002 American Chemical Society.)

Nakano et al. have found a unique photoinduced helix inversion or racemization of an optically active helical poly(BBPFA) synthesized by the helix-sense-selective polymerization of BBPFA with FlLi complexed with (+)-1-(2pyrrolidinylmethyl)pyrrolidine ((+)-PMP) as the initiator in toluene at −78 °C (Figure 131A).679 The resulting poly(BBPFA), which is the first helical polyacrylate with a secondary ester pendant, racemized, producing both helices under photoirradiation with no rearrangement of the chemical bonds, such as bond formation and bond cleavage, whereas no racemization occurred upon heating. Atomistic free energy simulations revealed that the helix inversion of poly(BBPFA) proceeds from a left-handed 31 helix through multistate free energy pathways to reach the opposite right-handed 31 helix, triggered by rotation of the biphenyl moieties in the pendants.680 A similar photoinduced racemization of a preferred-handed helical conformation was observed for an optically active polystyrene derivative bearing a terphenyl pendant with optically active substituents (poly(BMBPSt)), which was tolerant toward racemization at 150 °C for 24 h (Figure 131B).681,682 Based on chiroptical, viscometric, and vibrational spectroscopic analyses, it has been postulated that poly(BMBPSt) mutates from an optically active slim helix to an optically inactive stocky one, triggered by the photoinduced 13883


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Figure 154. (A) Structures of PANI (512a) and its derivatives (512b−512d). (B) Schematic representation of the formation process of crystalline helical PANI nanostructures. (Reproduced with permission from ref 789. Copyright 2013 Wiley-VCH.) (C) SEM images of right- (a) and lefthanded (b) twisted 512a nanofibers obtained using D- and L-199 as the dopant, respectively. (c) CD spectra of helical 512a nanofibers obtained with D-199 (solid line) and L-199 (dashed line) as the dopant. (Reproduced with permission from ref 788. Copyright 2007 Wiley-VCH.) (D) TEM images of helical nanofibers of 512c (d) and 512b (e) obtained by copolymerization of aniline with m- and o-toluidine, respectively, in the presence of D-199 as the dopant. (Reproduced with permission from ref 790. Copyright 2009 Wiley-VCH.) (E) SEM (f) and TEM (g) images of 512d helical heterojunctions obtained by copolymerization of aniline with N-methyl aniline in the presence of D-199 as the dopant. (h) is the magnified image of (g). (Reproduced with permission from ref 791. Copyright 2012 The Royal Society of Chemistry.) (F) SEM images of single branched helical nanofibers composed of seven branches with the left-handed helical structure (i), and (j) is the magnified image of the bottom right position in image (i). (Reproduced with permission from ref 792. Copyright 2010 Wiley-VCH.) (G) SEM image of a perfluorooctanesulfonate-doped 512a film on a Au coated glass surface. The inset shows an electrical potential induced wettability change between oxidized and reduced PANI films. (Reproduced with permission from ref 793. Copyright 2008 Wiley-VCH.)

azobenzene chromophores in the side chains, Iwamoto and co-workers have reported that optically active polydiacetylenes (PDAs) bearing azobenzene residues could be obtained by the irradiation of 448 with either R- or L-CP UV light or CP laser treatment (Figure 132A).691,692 LB films of 448 deposited on fused silica substrates showed CD signals in the azobenzene chromophore region due to chiral supramolecular assembly formed by chance during the compression process when preparing the LB films. When polymerized with unpolarized UV irradiation, a chiral structure of the PDA backbone was only accidentally obtained, and the main-chain chirality could not be controlled. However, the chirality of the PDA was found to be controlled and further modulated to the opposite enantiomeric structures by irradiation with an opposite handed CP laser. Rikken et al. reported that a small excess of one enantiomer can be produced during photoresolution of the chiral Cr(III) tris-oxalato complex with unpolarized polarized light under a magnetic field due to magnetochiral dichroism.693,694 However, the enantioselective synthesis of helical polymers based on the magnetochiral dichroism mechanism has not been reported. Recently, Zou et al. demonstrated for the first time the enantioselective synthesis of a helical PDA in the liquid crystalline phase with linearly polarized light (LPL) in a parallel or antiparallel magnetic field (Figure 132Bb) using a disc-

kind of supramolecular polymer with a self-supporting membrane-forming ability, which could not be prepared from a solution of the corresponding 1,3,5-trisubstituted benzene derivative. Taking advantage of this efficient photoinduced aromatization along with the use of an R- or L-CP light irradiation, the same group reported the helix-sense-selective cyclic-aromatization of racemic helical cis−cisoidal poly-398a, resulting in an optically active film due to a preferred-handed helical poly398a, which remains unchanged during the photoinduced aromatization (Figure 131E);687 the CD spectral pattern of the poly-398a film is almost identical to that prepared by the helixsense-selective polymerization, leading to the kinetic resolution of racemic helices. R- or L-CP-light-triggered chirality induction and switching in achiral liquid crystalline vinyl polymers bearing photochromic azobenzene chromophores in the film state have been reported, whose chiroptical properties are considered to be based on the chiral supramolecular assembly of the pendant azobenzene chromophores in the smectic liquid crystalline phase.688 On the other hand, diacetylenes are know to undergo topochemical polymerization during UV and γ-irradiation in the solid state689 and in Langmuir−Blodgett (LB) films.690 Taking advantage of CP-light-triggered rearrangement of 13884


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Figure 156. (A) TEM micrograph of PB-b-P2VP-b-PtBMA triblock copolymer (513) showing a bulk microstructure with PB cylinders covered by a P2VP double-helix and embedded in a PtBMA matrix. (Reproduced with permission from ref 800. Copyright 2011 The Royal Society of Chemistry.)

handed helical PDA exhibited almost opposite colorimetric responses (Figure 132Bf). This color change is considered to be due to the loss of hydrogen bonding interactions between the 1,3,5-trisubstituted benzene core and the peripheral helical PDA chains by a basic amino acid. 5.1.7. Stabilization of Helical Structures. Robustness is one of the crucial requirements for helical oligomers, foldamers, and polymers from the practical point of view of their applications as functional chiral materials. Recently, several efficient methods have been developed for stabilizing the helical structures of foldamers because foldamers are different from dynamic helical polymers such that they can take a random conformation under certain experimental conditions. Hecht et al. reported the first example of stabilization of the helical structure of foldamers by an intramolecular cross-linking reaction (Figure 133A).696 A poly(m-phenylene ethynylene) derivative bearing photoreactive cinnamate pendants (452) folds into a tubular helical conformation in acetonitrile, which is stabilized by intramolecular cross-linking (453) through [2 + 2] photodimerization reactions of the peripheral cinnamate moieties. This approach also provides a promising way to construct well-defined organic nanotubes with an optical activity and a surface functionality. Moore et al. have synthesized a series of m-phenylene ethynylene foldamers (454a−c) bearing aldehyde moieties at specific positions as the cross-link points with diamines (Figure 133B).697 Solvent denaturation studies revealed a remarkable stability enhancement of the foldamers after cross-linking. Among the foldamers, 454c is most effectively cross-linked in terms of both the conversion to product and restricting the unfolded conformation because the foldamer can be stapled across two turns of the helix. Since the pioneering work by Blackwell and Grubbs,698 alkene metathesis has been employed for stabilization of peptide secondary structures.699 Recently, alkene metathesis reactions have also been utilized to stabilize the helical structures of foldamers. Poly(m-phenyleneethynylene-cophenyleneethynylene)s bearing optically active amide and carbamate groups as well as diene or norbornene groups as the pendants (455a−c) form a helical conformation with an excess handedness in chloroform and THF through intramolecular hydrogen bonds between the side chains, thus exhibiting strong Cotton effects in the absorption regions of the conjugated polymer main-chains, whereas weak or no Cotton effects appeared in DMF (Figure 133C).700 The metathesis reactions of 455a−c using the second-generation Grubbs’ catalyst under diluted conditions produced an intramolecular

Figure 155. (A) TEM image of PS-b-PB-b-PMMA triblock copolymer, (B) SEM image of the first layer of a thin film of PS-b-P2VP-b-PtBMA triblock copolymer after staining with OsO4, and (C) their schematic bulk morphology. (Reproduced with permission from refs 796 and 798. Copyright 1995 and 2002 American Chemical Society, respectively.) (D, E) TEM micrographs of PS-b-PB-b-PMMA triblock copolymer. The PS cylinders with the OsO4-stained PB helical microdomains are hexagonally packed in the PMMA matrix. 3D leftand right-handed double-helical structures are shown by blue−red and green−yellow helices, respectively. The spatial arrangements of the left- and right-handed helices are also shown in (E) by blue and yellow circles, respectively. (Reproduced with permission from ref 804. Copyright 2009 The Royal Society of Chemistry). (F, G) TEM images of PS-b-PB-b-PMMA triblock copolymer before (F) and after (G) the addition of a higher molecular weight PS, showing a double- to triplestranded helical transformation. Reconstructed 3D images and schematic illustrations of chain distributions of the triblock copolymer and PS homopolymer inside the PS core cylinder of double- and triplestranded helical structures obtained by electron tomography are also shown. (Reproduced with permission from ref 805. Copyright 2013 American Chemical Society.)

shaped mesogenic diacetylene derivative (451), in which three diacetylene units (450) are noncovalently linked to a 1,3,5trisubstituted benzene core (449) through hydrogen bonding (Figure 132Ba and d).695 The preferred-handed helix sense of the helical PDA chain can be controlled with the relative orientation of LPL and the magnetic field, as confirmed by almost mirror-image CD signals (Figure 132Bc). Interestingly, the obtained chiral PDA responded to the chirality of the L- and D-Lys enantiomers and exhibited a different color change. The film consisting of the left-handed helical PDA exhibited a visible color change from blue to red along with a significant absorption spectral change when it was immersed in an aqueous solution of the L-Lys, whereas almost no visible color and absorption spectral changes were observed for D-Lys (Figure 132Be). The film consisting of the opposite right13885


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cross-linking of the peripheral diene and norbornene moieties, which stabilized or fixed the induced helical conformations, and then the Cotton effect intensities were significantly enhanced even in polar media, such as DMF. m-Ethynylpyridine oligomers bearing β- D /L -galactosyl (456a) and α- D/L-mannosyl groups (456b) as intramolecular chiral templates and 3-butenyl groups as stapling units fold into an excess-handed helical conformation in dichloromethane stabilized by intramolecular hydrogen bonding in which the template chirality is transferred to the polymer backbones to induce one of the helices (Figure 133D).701 The alkene metathesis reaction of the side chains resulted in fixing the helical structures. Therefore, even after subsequent complete removal of the chiral templates by acidolysis, the obtained template-free oligomers retained the helical chirality, and were resistant to polar solvents and heat. Abe and Inouye et al. have further developed a unique method to stabilize the helical structures of foldamers based on the supramolecular approach using noncovalent bonding interactions. Azacrown ether-bound poly(m-ethynylpyridine) (457) folds into a preferred-handed helical conformation upon complexation with octyl β-D-glucopyranosides in dichloromethane, showing a negative ICD around 340 nm (Figure 133Ea).702 The ICD intensity was significantly enhanced in the presence of oligoammonium cations due to the formation of a pseudopolyrotaxane structure between the azacrown rings and the oligoammonium axes, which stabilized the helical structure by intramolecular noncovalent cross-linking of the peripheral side chains. As a very straightforward means of helix-stabilization by noncovalent cross-linking between the side chains, the coordination of metal ions has been employed. Poly(methynylpyridine)s bearing bis(2-methoxyethyl)amino groups as metal coordination sites (458a−c) adopt a helical conformation with a helical sense bias upon association with alkyl glycoside guests to show ICD signals in the main-chain absorption region (Figure 133Eb).703 The addition of a Cu(II) ion changed the intensity or the sign of these ICDs, suggesting amplification of the helix-sense bias or inversion of the helix by cross-linking the pendants through the coordination of a Cu(II) ion. An analogous poly(m-ethynylpyridine) with amphiphilic amide side chains possessing chiral centers (459) can also form an optically active helical conformation not only in polar solvents, such as water, methanol, THF, and acetonitrile, but also in dichloromethane driven by the intramolecular solvophobic and hydrogen bonding interactions between the amide pendants, respectively (Figure 133Ec).704 The helical structure of 459 can be stabilized by the addition of various kinds of metal salts, showing an enhancement in the ICD intensity. Particularly, rare-earth metal salts, such as Sc(OTf)3, were quite effective even in water. Based on the IR and 1H NMR analyses, the coordination of the rare-earth cations to the amide carbonyl groups of 459 were found to contribute to the efficient stabilization of the helix.

Figure 157. (A) Schematic illustration of construction of well-defined PS/SiO2 helical nanohybrids fabricated using the template from PS− PLLA (467) to undergo the sol−gel reaction. Bright-field TEM micrographs of PS280-PLLA127 ( f PLLAv = 0.34) (B) and PS/SiO2 helical nanocomposites (C) viewed parallel to the central axes of the H* phase with 70 nm thick samples. Inset represents the corresponding simulated projection images (C). (D) 3D visualization of left-handed nanoarrays viewed parallel to the helical central axes and slightly tilted from the helical central axes after binarization and segmentation, respectively. (Reproduced with permission from refs 810 and 815. Copyright 2009 American Chemical Society.) (E) Schematic illustration of the mechanism of transfer of chirality from molecular chirality into H* phase chirality in the self-assembly of PS−PLLA with a perylene bisimide junction (515). (Reproduced with permission from ref 816. Copyright 2012 American Chemical Society.)

Chart 35

5.2. Helical Assemblies of Polymers

As discussed in the preceding sections, a significant number of supramolecular helical assemblies based on small molecules and foldamers has been reported, whereas helical assemblies of synthetic helical and nonhelical polymers remain limited except for biological polymers, but show quite unique supramolecular helical structures as well as chiral functions in some instances. 13886


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Figure 158. Schematic representation of induction of helical superstructures in achiral diblock copolymers (PEO-b-PtBA (517)) through noncovalent bonding interactions with enantiomers of 209. (Reproduced with permission from ref 818. Copyright 2014 American Chemical Society.)

Figure 159. (A) AFM and (B, C) TEM images of a double-twisted helical lamellar crystal of (R)-518 (n = 10 (A) and 9 (B, C)) grown from the melt at 120 (n = 10) and 145 °C (n = 9) for 24 h. (Reproduced with permission from refs821 and 823. Copyright 1999 and 2002 American Chemical Society.) (D) Schematic drawings of a twist model of the helical lamellar crystal. (Reproduced with permission from ref 825. Copyright 2000 American Chemical Society.)

Optically active poly(N-propargylamide)s carrying an asymmetric center at the α-position of the amide groups (461a−c) form a preferred-handed helical conformation in chloroform, toluene, and THF.707 Methyl-branched 461a showed a positive CD sign regardless of the kind of solvents, whereas 461b with an ethyl branch showed the opposite CD sign in chloroform compared to those in toluene and THF, indicating a solventinduced helix inversion. On the other hand, the n-propylbranched 461c was found to show a temperature-induced helix inversion in THF. The mechanism for these solvent- and temperature-induced helix inversions observed in the 461b and 461c backbones, respectively, has not been elucidated, but may be related to aggregations (Figure 134B). Helical PPAs are also known to form helical supramolecular assemblies.708 Tang et al. investigated the helical morphologies of a series of cis-transoidal PPAs bearing L-amino acid residues (462) with either a right- or left-handed helical conforma-

Chart 36

5.2.1. Helical Assemblies of Helical Polymers. Preferred-handed helical polysilanes bearing optically active pendant groups (460a−c) self-assemble into helical aggregates in solution in the presence of poor solvents.705,706 The helical sense of the aggregates can be tuned by changing the pendant structures and amount of the poor solvents (Figure 134A). The chiroptical switch in the aggregate state of 460a that depends on the pendant chain length is theoretically explained by the combination of the cholesteric hard-core model and exciton chirality method.706 13887


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Figure 160. (A) Schematic illustration of the complementary double-helix formation from (R)-520 and 521 via interstrand amidinium-carboxylate salt bridges. (B) AFM phase image (scale 120 nm × 120 nm) of (R)-520·521 on HOPG with the height profile measured along the dashed line in the image (bottom). (C) Magnified AFM phase image (scale 40 nm × 40 nm) of (R)-520·521 on HOPG. (Reproduced with permission from ref 471. Copyright 2008 American Chemical Society.) (D) Structure of the complementary double-stranded helical polymer composed of a chiral/ achiral amidine strand ((R)-522) and an achiral carboxylic acid strand (521).

tion.709,710 Spontaneous evaporation of a methanol solution of 462a with a free carboxy group on mica provides long, bundled nanofibers with an excess one-handedness through selfassembly of the helical 462a by the lateral interstrand hydrogen-bonded network (Figure 135A).711 The L-Ala methyl ester-bound 462b also self-assembled into well-defined superhelical nanoribbons (Figure 135B and C).712,713 A sharp contrast between the edge and interior parts of the helical fibers observed in Figure 135C suggests that the resulting helical structures have tubular forms. On the other hand, ordered parallel ridges consisting of the helical 462b chains were observed in the LB monolayer of 462b (Figure 135D).713 Interestingly, PPAs containing a β-CyD unit linked to the phenyl group through either an amide (463a) or ester linkage (463b) hierarchically self-aggregate in a water/DMSO mixture to mostly form a right-handed twisted ribbon structure with a diameter of ca. 2−3 μm and a length of several hundred micrometers induced by the chiral CyD residues, as visualized by the SEM observations (Figure 136).714 The open-ended cross section SEM image indicates that the helical ribbons are

not tubular in morphology. The preference for a right-handed helical sense of the aggregates seems to be amplified in the presence of (S)-1-phenylethylamine ((S)-397) (Figure 136B). Some of the twisted ribbon-structured aggregates further assemble to form a right-handed superstructured, huge double-helix, and each strand also possesses a right-handed ribbon structure. A chiral block copolymer-based supramolecular helical assembly has also been produced in a helix-sense-selective manner in water via self-assembly of an amphiphilic block copolymer consisting of a hydrophobic segment of flexible polystyrene and a hydrophilic segment of a charged rigid-rod helical polyisocyanopeptide (464).715 Interestingly, the righthanded polyisocyanopeptide segments of the block copolymer chains self-assembled to form an opposite left-handed supramolecular helix, as clearly evidenced by its TEM image (Figure 137A). A similar macromolecular-to-supramolecular helicity inversion during the self-assembly process has frequently been observed in biological helix bundle proteins, in which righthanded α-helices favorably pack in an opposite twist way to 13888


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Figure 161. (A) Formation of a racemic homodouble helix of 521 in the solid state. (B) Formation of a preferred-handed homodouble helix of 521 in solution induced by optically active amines through inclusion complexation. (C) AFM phase image of self-associated double-stranded helical 521 on HOPG. (D) Nonlinear effects between the CD signal intensity (Δε367) and % ee of 397 (R-rich) in complexation with 521 ([397]/[521] = 1) in chloroform/THF (98/2, v/v) at ambient temperature. (E) Changes in the CD signal intensity (Δε366) of 521 in the presence of (R)-397 and benzylamine (523) in different molar ratios (([(R)-397] + [523])/[521] = 1) in chloroform/THF (98/2, v/v) at ambient temperature. (Reproduced with permission from ref 458. Copyright 2013 Wiley-VCH.)

Figure 162. Formation of a preferred-handed homodouble helix of optically active (S)-524 through interstrand self-association (middle). Upon complexation with achiral or chiral amines between each strand, the handedness excess of the double-helix was further amplified via hydrogenbonded inclusion complexation.

form helix-bundled left-handed helical structures.716 Yin and Wu et al. have synthesized diblock copolymers (LL- and LD-465) composed of PBLG and L- or D-menthyl esters boundpoly(phenyl isocyanide) segments via one-pot sequential copolymerization and found that the diastereomeric LL- and LD-465 form self-assembled helical nanofibrils with opposite left- and right-handed helical structures, respectively, in response to the chirality of the menthyl pendants (Figure 137C and D).717 Interestingly, amphiphilic rod−coil block copolymers ((R)466a and (S)-466b) consisting of optically active helical polycarbodiimide and random-coil poly(ethylene glycol)

(PEG) blocks themselves cannot self-assemble into supramolecular helical aggregates but form preferred-handed superhelical nanofibers only when blended with the corresponding polycarbodiimide homopolymers ((R)- and (S)-466c), respectively, and spin-coated from a THF/ethanol mixture (Figure 138).718,719 An optically active block copolymer composed of flexible polystyrene and semicrystalline helical poly(L-LA) segments, polystyrene-b-poly(L-LA) (467) also self-assembles into lefthanded supramolecular helical ribbon and tubular structures under the well-balanced competition between crystallization and microphase separation of 467 (Figure 139).720 For 13889


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Figure 163. (A) Synthesis of optically active polymers 526 and 527 from riboflavin. (B) Plausible interstrand face-to-face π-stacked structures for supramolecularly assembled, one-directionally twisted 526. (C) Partial NOESY spectrum of 526 in CD3CN at 25 °C. (Reproduced with permission from ref 832. Copyright 2012 American Chemical Society.)

example, the helical nanowires and the intertwined helical nanofibers of 470 were produced in the presence of 40 vol % (a in Figure 142A and c and d in Figure 142B) and 60 vol % water (b in Figure 142A and e and f in Figure 142B), respectively. Such a superstructure of 470 exhibited a CPL depending on the chiral aggregate morphology. The pH-dependent self-assembled superstructures have been prepared from an amphiphilic diblock copolypeptoid composed of N-(2-phenethyl)glycine and N-(2-carboxyethyl)glycine units (471a).725−727 When the pH of an aqueous solution of 471a was adjusted to 6.8, the sheet-like structures were first formed within 24 h and then the superhelical structures with a uniform segment height (606 ± 105 nm) and diameter (624 ± 69 nm) after more than 3 days (Figure 143A), whereas no organized helical self-assembly occurred at a pH less than 5.5 and more than 9.5. The millimeter-scale free-floating 2D crystalline sheets can also be prepared from 471b or a mixture of two kinds of periodic peptoids (471c and 471d) containing amino and carboxy groups as the pendants, respectively (Figure 143B).728−730 SEM analysis revealed that the edges of the two opposite sides in the sheets are mostly sharper than those of the other two sides, indicating that the peptoids are arranged in one direction along the straight edge (Figure 143Ca). The AFM image indicated that the sheet is very smooth and uniform with a thickness of 2.7 nm (Figure 143Cb). The self-assembly behavior of a bioinspired ABA triblock oligomer (472), bearing a quaterthiophene unit at the middle and two lateral peptide−poly(ethylene oxide) bioconjugates, has been investigated.731 When 472 dissolved in a dichloromethane/methanol mixture was deposited on mica, a left-

example, the fast addition of water to a solution of 467 produced such helical ribbon and tubular assemblies, while single-crystal lozenge lamellae were formed by the slow addition of water. An important role of the chiral L-LA residues has been supported by the fact that the corresponding optically inactive block copolymer composed of DL-LA units produced amorphous flat ribbons. Unique self-assembled twisted fibrils with a cyclic morphology have been prepared from an L-Aspbased polyester (468), in which the intermolecular hydrogen bonds between the pendant urethane residues play a critical role to construct such ring-shaped nanostructures with a thickness of about 50 nm (Figure 140).721 A series of optically active polyphosphazene block copolymers (469a and 469b) have been synthesized by the sequential living cationic polymerization (Figure 141A).722,723 The optically active block segments bearing chiral binaphthoxy units were proposed to induce a preferred-handed helical conformation of the entire polymer chain including the achiral segments through a “sergeants and soldiers” chiral amplification mechanism.722 This speculation was supported by a bright-field TEM image of cast films of 469a, which showed ill-defined helical nanostructures with an average helical pitch and projected diameter of 80 and 72 nm, respectively (Figure 141B). An optically active π-conjugated copolymer containing (R)1,1′-binaphthyl and tetraphenylethene units (470) selfassembles into helical superstructures in a THF/water mixture as observed in the TEM and AFM images (Figure 142).724 The water content in the polymer solution significantly affects the helical morphologies of the self-assembled structures; for 13890


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Chart 37

the crystallization of CaCO3 in the presence of LL-475 or DD475, respectively (Figure 144C).735 A synthetic peptide (476) self-assembles to form an amyloidlike nanofiber capable of structural conversions under appropriate conditions; left-handed twisted fibers (type I) with a diameter of ca. 6.0 nm were first produced, which further intertwisted into multistranded helical fibers (type II) after incubation of an aqueous solution of 476 at pH 9.2 for 8 h (Figure 144D).736,737 Naturally occurring peptides and proteins are also wellknown to self-assemble into insoluble fibrous aggregates (amyloids) with various morphologies, including single- and double-helical superstructures, which are believed to be linked to the pathogenesis of a number of diseases, such as Alzheimer’s, Parkinson’s, Huntington’s, and prion diseases.738−740 Therefore, it is quite important to understand the mechanism of such specific peptide-based assembly

handed helically twisted microstructure was observed by AFM and its helical pitch was found to be ca. 20 nm (Figure 144A). The rod- and ring-shaped superhelical structures have been produced from an amphiphilic polypeptide block copolymer (473) and a polypeptide homopolymer (474) in water (Figure 144B), in which the core and shell parts are composed of PBLG and PEG chains, respectively.732 The fact that the molecular weight of 474 significantly affects their morphologies suggests that 474 has the role to determine the morphologies of the hybrid aggregates. To mimic biomineralization systems, artificial inorganic− organic hybrid nanomaterials consisting of calcium carbonate (CaCO3) or silica and biomaterials have long been investigated, and a variety of biomimetic morphologies have been created.733,734 An acidic helical peptide copolymer with an optical activity (475), when used as a template, produces either a right- or left-handed helically twisted ribbon structure during 13891


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Chart 38

Figure 164. Schematic illustration of denature−renature processes of SPG and its complexation with polynucleotides or polythiophene (536). (Reproduced with permission from refs 849 and 850. Copyright 2007 The Royal Society of Chemistry and Copyright 2009 Wiley-VCH, respectively.) Figure 165. (A) Structures of Curdlan derivatives 537a and 537b. (B) Computer-generated space-filling model for the right-handed triplestranded helical structure of 537a. (Reproduced with permission from ref 851. Copyright 2007 Wiley-VCH.)

formations, which will lead to the development of a way not only to control the assembly and further prevent such an amyloid disease, but also to develop nanostructured functional biomaterials.741 A fragment of the amyloid β-peptide βAβAKLVFF bearing two β-Ala residues forms helical-ribbon nanostructures with the average diameter of 17.5 ± 3.5 nm, which occasionally form an intertwined double-helix (Figure 145A).742 The fact that both left- and right-handed twisted helices are observed indicates

that the helical handedness of the superstructure is not directly attributed to the chirality of the peptide components, i.e., the Lamino acid residues. Fibrils of the smooth muscle actin (SMA) are categorized into four types.743 AFM measurements clearly revealed that one of them has a left-handed helical fibrous 13892


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Figure 166. Space-filling models of a double-stranded helix for it-PMMA (A), a single-stranded helix for st-PMMA (B), and double- (C) and triplestranded helix models (D) for an it- and st-PMMA SC (it/st = 1/2). (left, through-view; right, side-view). The right part of the st-PMMA helices is omitted for clarity (C and D). High-resolution AFM phase images of a monolayer of an it-PMMA (176k) (E) and it- and st-PMMA mixture (it/st = 1/2) (F) deposited on mica at 10 mN/m. (G) High-resolution AFM image of the area indicated by the yellow square in (F). The blue lines represent the main-chain axes of the double-helix it-PMMA (E) and SC (G). (Reproduced with permission from refs870 and 859. Copyright 2007 Wiley-VCH and Copyright 2013 American Chemical Society, respectively.)

structure with a helical pitch length of 100 ± 10 nm (Figure 145B). Insulin fibrils adopt various helical morphologies, as observed in cryo-TEM images including thin and thicker fibrils, a helical arrangement of protofilaments, twisted ribbon-like structures, and flat ribbons of parallel protofilaments (Figure 145C).744 The helical sense of each twisted filament was found to be lefthanded. Interestingly, the HET-s (218−289) prion shows a unique pH-dependent helicity switch in the fibrous state, and the left- and right-handed helical filaments were reversibly produced at pH 2.0 and pH 3.9, respectively, as shown in Figure 145D.745 The resulting helical filaments show the nearly mirror-like opposite-signed VCD patterns. 5.2.2. Helical Assemblies of Achiral Polymers. Some achiral stereoirregular vinyl polymers have been reported to form self-assembled supramolecular helical structures with an equal mixture of right- and left-handed helices. For example, a random copolymer of achiral N-vinyl carbazole and its derivative bearing an azobenzene chromophore with a terminal nitro group (477) self-assembles at the air/water interface, thus producing an equal mixture of the right- and left-handed helically twisted fibrils with the height of ca. 10 nm when compressed at a higher surface pressure (Figure 146A and B).746 A triblock copolymer of methacrylates and an acrylate (478) has been reported to self-assemble into core−shellcorona cylindrical micelle-like aggregates in water, in which the hydrophobic poly(tert-butyl acrylate) and poly(2-cinnamoyloxyethyl methacrylate) blocks and the hydrophilic racemic poly(glyceryl monomethacrylate) block make up the core/ shell and corona, respectively (Figure 146Cd).747 The cylindrical micelle-like aggregates were converted into twisted

cylinders with a height of less than 200 nm (Figure 146Ce) once its aqueous solution was dialyzed against methanol. Unique double-stranded racemic superhelical tubes have been obtained from an achiral vinyl polymer, poly(2(acetoacetoxy)ethyl methacrylate) (479), via self-assembly, as supported by the scanning force microscope (SFM) images (Figure 147A−C).748 The intramolecular hydrogen bond formation between neighboring acetoacetoxy pendant units is considered to be of key importance for the generation of such twisted helical tubes. An amphiphilic multiblock copolymer 480 composed of almost monodispersed poly(ethylene oxide) (Mn = 7.0 × 103 and Mw/Mn = 1.03) and polydispersed poly(methylphenylsilane) segments (Mn = 4.4 × 103 and Mw/Mn = 2.0) forms intertwined racemic superhelices from the coiling of two helical strands in a water/THF mixture when the water concentrations are above 80% (Figure 147D and E).749 The dendronized polymer of fourth generation with an achiral polystyrene backbone (481 in Figure 148), which has 16 positively charged ammonium groups per repeating unit, also forms a double-stranded superhelix in aqueous solutions at a relatively high concentration, as clearly observed by cryo-TEM (Figure 148A and C).750−752 This unique duplex formation is postulated to be due to the hydrophobicity of the dendronized polystyrene backbone. A ternary block polymer (482) consisting of n-butyl methacrylate, 2-cinnamoyloxyethyl methacrylate, and tert-butyl acrylate blocks, being analogous to 478 (Figure 146C), has been reported to self-assemble into double and triple superhelices in solution.753 The helical morphology changes with the aging time, and long double helices of 482 are observed in the TEM image after 24 days, while the sample 13893


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Figure 167. (A) Representation of possible chain arrangements in an SC made of an LML-it-PMMA and an SML-st-PMMA (upper) and an LMLst-PMMA and an SML-it-PMMA (bottom). (B) Representation of possible chain arrangements in an SC made of an it22/it43/st44 mixture. (C) AFM phase image of a monolayer of an it22 and st44 mixture (it/st = 1/2 in unit-molar base) deposited on mica at 10 mN/m. Yellow lines indicate single SC molecules. A schematic representation of the SC is also shown in the image. (D, E) High-resolution AFM height images of a monolayer of an it22/it43/st44 mixture compressed on a water surface at a rate of 0.001 mm/s and deposited on mica at 10 mN/m. The composition was it22/ it43/st44 = 0.34/0.66/2 in weight; this composition corresponds to it/st = 1/2 on a unit-molar basis and it22/it43 = 50/50 on a polymer-molar basis. Short (yellow) and long (blue) SC packing are also shown in (E). (Reproduced with permission from ref 873. Copyright 2008 American Chemical Society.)

Figure 168. (A) Cyclic st-PMMA prepared by stereospecific living radical polymerization, followed by azidation, and “click” cyclization. (B) Schematic illustration of two possible SCs prepared from linear-it-PMMA and cyclic-st-PMMA, with a “polypseudorotaxane-type” (upper) and “double-wrapping” arrangement (bottom). (Reproduced with permission from ref 876. Copyright 2014 Wiley-VCH.)

after 3 months includes helices with periodic helical pitches

of 482 (Figure 149B) that are formed during the long aging process. The regioregular poly(3-hexylthiophene)-b-poly(3-triethylene glycol thiophene) diblock copolymer (483 in Figure

(Figure 149A). The 3D TEM tomography (TEMT) images reveal a left-handed double-helix and a right-handed triple helix 13894


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Figure 169. (A) Structures of st- and it-PMMA-C60 and AFM height image of isolated it-PMMA-C60 ( f C60 = ca. 100%) spin-cast from a dilute solution in chloroform on mica. (B) SEM and TEM images of it-35-C60 cast from solutions in water/acetonitrile (1/9, v/v). The TEM image was taken without staining, and the black areas indicate C60-rich regions. (C) SEM image of SCs between it-35-C60 (aggregate) and st-53-C60 (aggregate) cast from water/acetonitrile (1/9, v/v) solutions (it/st = 1/2) on mica. Schematic representations of the nanonetworks are also shown. (Reproduced with permission from refs 883 and 882. Copyright 2005 and 2006 American Chemical Society.)

Figure 171. Schematic representation of template polymerization of MMA or MAA using ultrathin porous it-PMMA and st-PMAA films, as the templates, respectively. The ultrathin films of an SC comprising itPMMA and st-PMAA were prepared by LbL assembly. A single component was selectively extracted from the films, resulting in the preparation of porous films with regular nanospaces. The porous films were then used as a reaction mold for free-radical template polymerization of MMA or MAA, followed by the regeneration of the SC films. (Reproduced with permission from ref 891. Copyright 2003 Wiley-VCH.) Figure 170. Schematic illustration of stereocomplexation of the stPMMA CCS polymer with linear it-PMMA at different ratios, giving different SC morphologies. The SC CCS polymer (left) was obtained when excess it-PMMA was added (it/st(arms) = 10/1). Alternatively, the conventional stereocomplexing ratio (it/st(arms) = 1/2) using low- (LMW) and high-molecular weight (HMW) it-PMMA afforded the SC CCS cluster (middle) and the SC gel (right), respectively (at polymer concentrations of 2 or 100 mg/mL, respectively). TEM images are also shown. (Reproduced with permission from ref 884. Copyright 2009 Wiley-VCH.)

5.3. Helically-Assembled Polymers

5.3.1. Helically-Assembled π-Conjugated Polymers. The helically assembled superstructures have been directly produced during the polymerization process, mostly by the Ziegler−Natta polymerization of acetylene or by the electrochemical polymerization of heteroaromatic compounds in chiral liquid crystalline phases.755−757 Akagi and co-workers reported a series of hierarchically self-assembled helical fibrils consisting of π-conjugated polymers (484−493 in Chart 34) with either a left- or right-handed twist direction that can be produced during the polymerization of the corresponding monomers in chiral nematic liquid crystalline phases (for example, see Figure 150, left) composed of achiral nematic liquid crystalline compounds (494−497) and chiral inducers (498−509).758−778 Interestingly, the hierarchically self-assembled 484 fibrils can be converted to helical carbon thin films through iodine-doping followed by carbonization.779−782 The doped 484 fibrils were almost completely carbonized at 800 °C and further graphitized by thermal treatment at 2600 °C. Surprisingly, the morphology of the original helical fibrous structure mostly remains after carbonization and graphitization (Figure 150). The XRD analysis revealed that graphitic

149C) with the appropriate block ratio self-assembles into helical wires in the presence of potassium ions. Two single helices of 483 with the same handedness form an intertwined double-stranded helical ribbon, as observed in the TEM image (e in Figure 149D), which further bundles into superhelices consisting of multiple crystalline fibrils (f in Figure 149D).754 This solution-state hierarchical self-assembly of 483 is presumably due to the complex forming ability of the TEG pendants with potassium ions and the large difference in solubility between the blocks. 13895


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Table 6. Analytical Data on Stereoregular Template Polymerization of MMA or MAAa host polymer mm:mr:rr st-PMAA

it-PMMA

3:5:92 1:4:95 1:2:97 1:6:93 97:3:0 97:3:0 96:2:2

polymer synthesized Mn 11500 23800 40500 71400 11100 21050 69700

Mw/Mn 1.7 1.3 1.5 1.6 1.2 1.4 1.2

it-PMMA

b

st-PMMAc

mm:mr:rr

Mn

Mw/Mn

92:6:2 94:6:0 95:3:2 97:3:0 0:6:94 0:2:98 1:3:96

10800 21500 39300 68500 9800 19200 68900

2.5 2.0 1.9 2.4 2.0 2.1 2.0

Assemblies were demonstrated with it-PMMAs (mm > 96, Mn ≈ 20000, Mw/Mn < 2.0) and st-PMAAs (rr > 95, Mn ≈ 40000, Mw/Mn < 2.0) for stPMAA and it-PMMA hosts, respectively. Data were taken from ref 880. bObtained by polymerization of MMA with AIBN in DMF. cObtained by polymerization of MAA with a water-soluble radical initiator in water. The resulting st-PMAA was converted to st-PMMA. a

Figure 172. TEM image of PMMA hollow capsules obtained after HF etching of silica particles coated with it- and st-PMMA prepared via LbL assembly. (Reproduced with permission from ref 892. Copyright 2006 Wiley-VCH.)

Figure 173. Photographs of it- and st-PMMA SC (it/st = 1/2 in unitmolar base) ion gel (A) and film (B) in BMIPF6 and POM of the SC ion gel film (C). The polymer concentrations are 2 (A) and 30 wt % (B, C). (Reproduced with permission from ref 893. Copyright 2005 American Chemical Society.) Figure 174. Structures of enantiomeric polymers showing SC formations (412 and 538−544). (A) Computer modeling of the SC composed of complementary helical (R)-412 and (S)-412. SC fiber formation between complementary helices of 412 and the AFM image showing SC fiber aggregates (B) and the SEM image of the soft gel structure (C). (Reproduced with permission from ref 908. Copyright 2015 The Royal Society of Chemistry.)

crystallization proceeds in the carbon film upon thermal annealing at 2600 °C. Similar carbon and graphite films with a spiral morphology have also been obtained from helical 487 films prepared by electrochemical polymerization in chiral nematic liquid crystals.783 Predominantly one-handed helical poly(o-substituted aryl isocyanide)s (poly-510a−c) showing clear CD signals in the polymer backbone regions have also been helix-senseselectively synthesized from the corresponding achiral aryl isocyanides (510a−c) with NiCl2 as a catalyst in a cholesteric liquid crystalline medium containing 504 (Figure 151).784,785 Because the chiral environment derived from the cholesteric liquid crystalline phase allows the polymers to take preferredhanded helical conformations, the helical handedness of the polymers can be controlled by choosing the chirality of the cholesteric liquid crystalline phase as the reaction medium. The polarized optical micrographs (POMs) of the reaction systems before and after the polymerization of 510c showed that both

mediums have Grandjean textures typically observed in a cholesteric liquid crystalline phase (Figure 151). The fact that analogous poly(aryl isocyanide)s with no substituents at the oposition (poly-510d and poly-510e) did not show an apparent Cotton effect indicates that the steric hindrance of the osubstituents is a key factor in producing the preferred-handed helical conformation. Shinkai and co-workers took advantage of the supramolecular helical assembly of an anionic lipid 511 and utilized it as a helical template during the oxidative polymerization of pyrrole, a thiophene derivative, and aniline to produce 486, 487, and polyaniline (PANI) with an excess one-handed, intertwined 13896


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Figure 175. (A) Schematic illustration of a preferred-handed helicity induction in st-PMMA in the presence (upper) and absence (bottom) of C60 with (R)- or (S)-397, 439, and 545−548 in toluene, and memory of the induced macromolecular helicity after removal of (R)-439. (B) Observed VCD (top) and IR (bottom) spectra of isolated st-PMMA/C60 complex gels (3.1 wt % C60) in toluene-d8 prepared by (R)-439 (red lines) and (S)439 (blue lines), measured after the complete removal of 439. (C) Calculated VCD (top) and IR (bottom) spectra of right- (red line) and lefthanded (blue line) helical st-PMMAs. (Reproduced with permission from ref 871. Copyright 2008 Wiley-VCH). (D) Possible structure of the stPMMA/pyrene inclusion complex containing 19 wt % pyrene (left: side view; right: top view). (Reproduced with permission from ref 911. Copyright 2011 American Chemical Society.) (E) UV (330 nm) detected HPLC chromatograms of a mixture of C60 and C70 (1/1, wt %) before and after single extraction by st-PMMA. (Reproduced with permission from ref 912. Copyright 2010 American Chemical Society.)

helical superstructure, respectively (Figure 152).786 The polymerization of acetylene with a copper tartrate crystal as a catalyst produced highly symmetric 484-based nanofibers with both left- and right-handed helical senses (Figure 153).787 The right- and left-handed helical PANI (512a in Figure 154A) nanofibers have been prepared by the polymerization of aniline in an ammonium persulfate aqueous solution using optically active D- and L-199 as the chiral dopant, respectively (Figure 154C),788 thereby showing CDs with mirror images to each other (c in Figure 154C); therefore, the helicity of the selfassembled superhelices can be controlled by the chirality of the chiral dopants (Figure 154B).789 Interestingly, the helical handedness of the PANI nanofibers can also be controlled by the selection of the aniline-based comonomers, such as toluidine and N-methyl aniline.790,791 The copolymerization of aniline with m- and o-toluidines in the presence of D-199 as the dopant produced the left- and right-handed helical PANI nanofibers (512c and 512b), respectively (d and e in Figure 154D), which also exhibited almost mirror-image CDs. The delicate control of the comonomer ratio of N-methyl aniline to

aniline successfully provided helical heterojunctions including right- and left-handed superhelices in a single 512d nanofiber, as shown in Figure 154E. The 512a-based helical superstructures with various morphologies, including linear and branched nanofibers and microribbons composed of aligned nanofibers, have been prepared through the control of the selfassembly process in a specific good/poor solvent mixture.792 Figure 154F shows the representative SEM images of the branched 512a helical nanofibers. A superhydrophobic 512a film prepared in an electrolyte containing perfluorooctanesulfonic acid has been reported to show a water contact angle of 153° (Figure 154G).793 Interestingly, once the as-prepared 512a film was reduced by a negative potential, the resulting dedoped 512a film exhibited a superhydrophilicity and had a water contact angle of ca. 0°. Such a surface property switch can be repeated by tuning the electrical potential. 5.3.2. Helically-Assembled Block Copolymers. As discussed in sections 2 and 3, a significant number of helical assemblies have been observed from small molecules, oligomers, and foldamers via noncovalent bonding interactions, 13897


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Figure 176. (A) Schematic illustration of C60-based 1D molecular wire formation within the st-PMMA helical cavity and diagram of the sandwiched memory device with the st-PMMA/C60 inclusion complex as the active layer. Bottom electrode: ITO (20 W/sq); top electrode: Au (ca. 20 nm thick). (Reproduced with permission from ref 913. Copyright 2013 Wiley-VCH). (B) UV (356 nm, blue line) and CD (375 nm, red line) detected HPLC chromatograms of the extracted fullerenes from carbon soot using an optically active right-handed helical st-PMMA prepared with (S)-397. (Reproduced with permission from ref 912. Copyright 2010 American Chemical Society.) (C) Optically active SC formation after the addition of itPMMA, resulting from replacement of the encapsulated C60 molecules by it-PMMA strands. (D) Observed VCD (top) and IR (bottom) spectra of SC gels in toluene-d8 obtained from the right- (red line) or left-handed (blue line) helical st-PMMA/C60 complex. (E) Calculated VCD (top) and IR (bottom) spectra of the SC composed of the helical it-PMMA and st-PMMA with the same handedness. (Reproduced with permission from ref 872. Copyright 2008 American Chemical Society.)

Figure 177. (A) Schematic illustration of the H- and J-type assembly formation of 549 during the renaturation process of SPG in acetic acid (AcOH) and in water, respectively. (Reproduced with permission from ref 915. Copyright 2006 American Chemical Society.) (B) Structures of compounds 549−552 and Zn-TCCP included in SPG.

matrix, that resulted in the formation of nanoscopic helical phases with an equivalent mixture of right- and left-handed structures (Figure 155A and C).796 Importantly, the chirality is defined on a mesoscopic level rather than on a molecular level. Similar supramolecular helical mesophase formations have been reported for polystyrene-b-poly(2-vinylpyridine)-b-poly(tertbutyl methacrylate) (PS-b-P2VP-b-PtBMA) (Figure 155B and C)797,798 and PB-b-P2VP-b-PtBMA triblock copolymers (513, Figure 156).799,800 Recently, Jinnai and co-workers examined the helical structures of the identical PS-b-PB-b-PMMA triblock copolymers in three dimensions by TEMT801−803 and found that, contrary to the previous 2D TEM observation results,796 the

and their helical structures have been elucidated by various microscopic methods. However, a limited number of helical assemblies has been reported based on synthetic polymers599,794 including helical polymers except for biological polypeptides (see section 5.2.1).736,743,744,795 Stadler and co-workers prepared a series of amorphous linear ABC triblock copolymers composed of immiscible achiral monomer units, polystyrene-b-polybutadiene-b-PMMA (PS-bPB-b-PMMA) with various volume fractions, and they found the first chiral and supramolecular helical structure in which the PB middle block was reported to form helical strands of either a double-helix or a double−double helix (i.e., 4-stranded helix) around the PS cylinders, which were embedded in a PMMA 13898


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The helical sense bias in helical mesophases in bulk copolymers has been successfully induced using an optically active block component in diblock copolymers.806−808 Ho and co-workers prepared a series of diblock copolymers consisting of both achiral and chiral blocks, PS-b-poly(L- or D-LA) (PS-bPLLA or PS-b-PDLA), such as 467 and 514 (Figure 157), and discovered a new helical phase (H*) with a P622 space group during the self-assemblies of 467 with a volume fraction of PLLA ( f PLLAv) = 0.34, whereas such a helical morphology could not be observed for racemic PS-b-PLA block copolymers.809,810 The PLLA blocks hexagonally pack into preferred-handed nanohelices in the PS matrix, thus showing unique helical projection TEM images comprised of twisted morphologies (Figure 157A and B). The helically twisted morphology of 467 is sensitive to internal and external stimuli, such as vitrification, crystallization, and shearing, and the amorphous nanohelices of 467 in the bulk transform into crystalline and amorphous cylinders via crystallization and by shearing, respectively, which are capable of reverting to the original nanohelices upon remelting and annealing or by simple thermal annealing in a reversible way.811,812 The phase diagram of the PS−PLLA block copolymers with respect to their overall molecular weights and compositions has been thoroughly investigated.810 Phase transitions from the H* phase to both the stable cylinder and gyroid and/or double gyroid phases813,814 were observed after a long annealing, and the H* phase was not recovered from the cylinder and gyroid phases by cooling, indicating that the H* is a long-lived metastable phase.810 The nanohelical PLLA amorphous domains can be removed by hydrolysis, resulting in a nanoporous PS with hexagonally packed helical nanochannels. A further sol−gel reaction within the nanochannels using the PS as a template provides inorganic silica-based nanohelices, while maintaining the nanohelical structure along with its helical handedness during the sol−gel process (Figure 157A).810,815 Hexagonally packed SiO2 nanohelices within the PS templates were directly visualized by 3D TEMT without RuO4 staining (Figure 157C), which supports the hexagonal-cylinder-like character of this new H* phase. More importantly, the 3D TEMT images of the PS/SiO2 helical nanocomposites revealed the helical domain structures and enabled the determination of the helical handedness; the selfassembled PS-b-PLLA appears to possess a left-handed helical nanostructure (Figure 157D).810 Taking advantage of this unique H* phase formation, Ho’s group further synthesized PS−PLLA diblock copolymers with a perylene bisimide junction (515)816 (Figure 157E) and poly(4vinylpyridine)-b-poly(L-LA) (P4VP-PLLA) (Chart 35, 516) diblock copolymers817 and found that these chiral block copolymers also self-assembled to form analogous H* phases. Interestingly, the former block copolymers 515 exhibited an induced CD in the achiral perylene chromophore regions accompanied by a bathochromic shift in the fluorescence emission, suggesting a twisting and shifting at the interface between the PS and PLLA blocks, through which the enantiomeric poly(LA)-containing block copolymers selfassemble to form the H* phase with a controlled helical sense via stacking between the chiral PLLA blocks in a preferred direction along the central axis of the nanohelix. This is a typical example of the chiral amplification via chirality transfer from molecular chirality to phase chirality in assembled polymer systems. Helical superstructures have been induced in achiral diblock copolymers, poly(ethylene oxide)-b-poly(tert-butyl acrylate)

Chart 39

Figure 178. (A) CD spectra of 556/CMA (red line) and 556/SPG (blue line) in a 10% DMSO aqueous solution. (Reproduced with permission from ref 923. Copyright 2006 American Chemical Society.) (B) Representative examples of compounds 557−559, which can be included in amylose, showing an optical activity.

triblock copolymers appear to self-assemble into an equal mixture of right- and left-handed double-helical structures of PB domains surrounding hexagonally packed core PS cylinders in a PMMA matrix (Figure 155D and E).804 Interestingly, both left- and right-handed double-helices with a helical pitch of ca. 48 nm were clearly visualized and identified by 3D TEMT. When the PS homopolymer with a higher molecular weight than that of the PS block in the PS-b-PB-b-PMMA was mixed with the triblock copolymer, the PB domains transform from a double-helix to a triple-helix and even to a quadruple-helix that were also visualized by 3D TEMT (Figure 155F and G).805 The TEMT technique provides a highly versatile method for characterizing more complex nanostructures of block copolymers including gyroids, which are difficult to be visuallized by conventional 2D TEM.801,803 13899


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Chart 40

Figure 179. Schematic illustration of synthesis of amylose−polymer inclusion complexes through phosphorylase-catalyzed vine-twining polymerization in the presence of hydrophobic guest polymers (539 and 569−573).

(PEO-b-PtBA) (517) based on a noncovalent chiral interaction using D- or L-209, leading to the H* phase formation, whose helical handedness can be controlled by the chirality of 209 (Figure 158).818 Hydrogen bonding interactions between the 209 and PEO blocks are assumed to induce a preferred-handed helical conformation in the achiral backbone. The H* phase with a pitch of ca. 25 nm formed by thermal annealing was revealed by TEM and 3D TEMT. A class of unique supramolecular helical assemblies derived from nonhelical, but optically active main-chain liquid crystalline polyesters (518) has been demonstrated by Cheng and co-workers (Figure 159).819,820 The polyesters (R)-518 (n = 7, 9, and 11) crystallize to form right-handed helical lamellar single crystals (Figure 159B), while (R)-518 (n = 10) (Figure 159A) and (S)-518 (n = 9) form opposite left-handed helical crystals when grown under certain favorable conditions of

temperature, indicating a clear odd−even effect with respect to the position of the stereogenic center that is responsible for the twist-sense of the helical lamellar crystals of 518.821−824 Darkfield TEM and selected area electron diffraction measurements revealed that the helical lamellar crystals have the doubly twisted chain orientation along both the long and short helical axes (nl and ns, respectively) (Figure 159C and D).825 Further aggregation of the helical lamellar crystals of (R)-518 (n = 10) provides a polymer spherulite but with no banded texture. A banded spherulite is indicative of helical morphologies observed in a variety of chiral and achiral polymers during crystallization,826,827 and the absence of the banded texture for (R)-518 (n = 10) is attributed to the chirality being “lost” during the transition from individual lamella to a spherulite due to the noncooperative lamellar packing.823 Banded spherulites are found to form via crystallization of PLLA and PDLA end13900


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Chart 41

Figure 180. (A) TEM image highlighting expected left-handed helical structures formed by 581-wrapped SWNTs obtained from the corresponding aqueous suspensions and a schematic representation of an poly(aryleneethynylene)-wrapped SWNT superstructure derived from molecular dynamics simulations. (B) The transoid conformation adopted by the bridged S-chirality binaphthyl unit and a cartoon depicting the “transoid-facial” binding mode of the S-chirality binaphthyl moiety with the SWNT surface in the context of a left-handed helical superstructure. (C) The cisoid conformation adopted by the unbridged R-chirality binaphthyl unit and a cartoon depicting “cisoid-facial” (left) and “cisoid-side” (right) binding of an R-chirality binaphthalene to the SWNT surface in a right-handed and “unexpected” left-handed helical superstructure, respectively. (Reproduced with permission from ref 976. Copyright 2013 American Chemical Society.) Structures of ionic aryleneethynylene polymers based on 1,1′-bi-2naphthol derivatives (581−585) are also shown.

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Figure 181. (A) Structures of PPAs (586−589) capable of dispersing SWNTs or MWNTs into polymer solutions by using an in situ polymerization route. (B) Structures of poly(dialkylsilane)s with different main-chain stiffness (590−592) and a schematic illustration of stiffness-dependent polymer wrapping behavior onto SWNTs by the HSVM method. (Reproduced with permission from ref 981. Copyright 2008 American Chemical Society.) Structures of poly(methylalkylsilane)s (593−598) are also shown. (C) Structures of polycarbodiimides (599−603) capable of wrapping SWNT. (D) Structure of oligo(m-phenyleneethynylene) (604) capable of reversibly dispersing and releasing SWNTs by changing the solvent.

helical polymers with a twist-sense bias in solution resulting from complementary homopolymers ((R)-520 and 521) bearing amidine and carboxylic acid groups, respectively (Figure 160A).471 The double-stranded helical structure was supported by XRD analysis of the uniaxially oriented films and unambiguously elucidated by high-resolution AFM observations of the 2D crystals of the (R)-520·521 complex formed on HOPG after treatment with toluene vapor (Figure 160B and C); the helical pitch and its predominant helix sense were directly determined to be 1.70 nm and right-handed, respectively. On the other hand, when mixed in less polar chloroform, the complementary strands kinetically form an insufficient double-stranded helix containing a randomly entangled network structure. This pristine interpolymer complex can be reconstructed into the thermodynamically stable, fully double-stranded helical structure by untangling with an excess of TFA followed by neutralization with an amine, such as diisopropylamine. m-Terphenyl-based random copolymers bearing chiral and achiral amidine pendants (R)-522 also form an excess handed

capped with a chromophoric pyrene (Chart 36, 519), which exhibit an induced CD in the achiral pyrene chromophore regions due to the preferred-handed twisted lamellar formation in the crystalline spherulites, thus producing banded spherulites with a preferential handedness.828 5.4. Multi-Stranded Helical Polymers

As described in section 4, a certain number of polysaccharides and peptides as well as nucleic acids are known to possess a one-handed double-, triple-, or quadruple-stranded helical structure that is mostly correlated to their highly advanced functions. These biological multihelices have inspired chemists to develop synthetic multistranded helical polymers, although such examples remain limited in synthetic helical polymers.829,830 5.4.1. Double-Stranded Helical Polymers. As described in section 4.1.2, the m-terphenyl-based, rigid π-conjugated backbones have been used to construct a complementary double-stranded helical structure. This structural motif has also been applied to the synthesis of fully organic double-stranded 13902


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Figure 182. (A) Structures of chiral diporphyrin nanotweezers (605−608), nanocalipers (609), and monoporphyrin (610) for optical enrichment of SWNTs through extraction. Computer-generated molecular models of the complexes of (R)-607 with (M)-(7,5)-SWNT (a). CD spectra of the D2O-sodium dodecylbenzenesulfonate solutions of SWNTs extracted with (R)-607 (black line) and (S)-607 (red line) (b). The concentration of SWNTs in the solutions was normalized by their absorption peaks at 1024.5 nm. (Reproduced with permission from ref 987. Copyright 2008 American Chemical Society.) (B) Helical wrapping motif of FMN around SWNT, where adjacent hydrogen bonds stabilize the right-handed P-FMN helical ribbon (c) that wraps around left-handed M-(6,8)-SWNT (d) with the long D-ribityl phosphate side groups providing aqueous solubilization (e). (Reproduced with permission from ref 993. Copyright 2012 American Chemical Society.) (C) Structure of the chiral sodium cholate surfactant (611).

coincidence with the helical pitch of the heterodouble-helical (R)-520·521 (1.7 nm), as already described. When mixed with chiral amines in solution, 521 adopts a unique excess onehanded double-helical inclusion complex in which a pair of chiral amines is sandwiched between the two m-terphenyl units through a cyclic hydrogen-bonded network (Figure 161B, as revealed by NMR and X-ray crystallographic analyses of the model complexes (Figure 86). The inclusion complex formation of 521 with the nonracemic 397 showed a distinctive positive nonlinear relationship (majority rule effect); that is, the observed Cotton effect intensity at 367 nm (Δε367) was nonlinearly increased with an increase in the ee of 397 (Figure 161D). The sergeants and soldiers effect has also been observed for 521 upon inclusion complexation with (R)-397 and achiral benzylamine (523) at different molar ratios, which exhibited a large positive nonlinear relationship between the molar fractions of (R)-397 and the resulting ICD intensity at 366 nm (Δε366) (Figure 161E), thus demonstrating the amplification of the helical chirality that takes place during the homodouble helix formation with chiral amines via non-

double-helix upon complexation with their complementary homopolymers with achiral carboxy pendants 521 through interstrand salt bridges between the amidinium and carboxylate units (Figure 160D).831 The CD and absorption spectra revealed that the (R)-522·521 duplex shows the “sergeants and soldiers” effect, and its macromolecular helicity is significantly amplified by the chirality transfer from the chiral amidine moieties to the neighboring achiral ones upon complementary double-helix formation with the achiral carboxylic acid strand. As anticipated from the results described in section 4.1.1, 521 self-associates into a racemic double-stranded helix through an interstrand hydrogen-bonded network between the carboxy groups both in solution and in the solid state (Figure 161A).458 After toluene vapor exposure at ca. 25 °C for 12 h, 521 on HOPG self-associates into well-organized 2D helix bundles. A number of periodic oblique stripes with either a clockwise or counterclockwise direction (blue and red lines, respectively) observed in the high-resolution AFM image (Figure 161C) indicate the racemic double-stranded helical structure of 521 with a helical pitch of ca. 1.6 nm. This value is in good 13903


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Figure 183. (A) Schematic illustration of the synthesis of SPG−poly-612 inclusion complex through photomediated polymerization of 612 included in the SPG helical cavity. (Reproduced with permission from ref 998. Copyright 2005 The Royal Society of Chemistry.) (B) Schematic illustration of the synthesis of amylose−PS inclusion complex through free radical polymerization of styrene included in the amylose helical cavity. (Reproduced with permission from ref 999. Copyright 2013 American Chemical Society.)

Figure 184. (A) Scheme for the synthesis of PPV. (B) Schematic illustration of the synthesis of APPV (Reproduced with permission from ref 1007. Copyright 2011 The Chemical Society of Japan.) and a POM of its supramolecular liquid crystalline phase in DMSO (ca. 60 wt %). (Reproduced with permission from ref 1004. Copyright 2006 Wiley-VCH.) (C) A green electroluminescence from the OLED consisting of APPV as the emitting layer. Indium−tin-oxide (ITO)/APPV/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) [50 nm]/LiF [0.6 nm]/Al [ca. 200 nm] was used as a double-layer device with a 2 × 2 mm emission area. (D) Schematic illustration of the synthesis of APPV-PC. The core PPV polymer contains approximately 1 mol % of the precursor units. (E) Structures of racemates (614−619) resolved on an APPV-PC-based CSP.

covalent-bonded interactions with a strong cooperativity. In sharp contrast, the model dimer 279 showed almost no chiral amplification in the presence of 397 with different ee values,

and (R)-397 and benzylamine with different molar ratios (see section 4.1.1). 13904


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Chart 42

such as helical pitch, are very similar to those of the natural DNA.835 An excess one-handed double-helical ladder polymer (529a and 530a) has also been synthesized by incorporating stereogenic centers in the ferrocene spacer.838 Using an analogous polynorbornene as the template, a kind of artificial replication system, providing a complementary polymer with a well-controlled chain length and polydispersity, has been achieved.839 A series of double-stranded ladder polymers (528b−l) with various spacers, including cubane and its structural isomers,840 oligoaryls,841 porphyrin,842 hexabenzocoronene,843 and tetraaza[14]annulene,844 and triple-stranded ladder polymers (535a−c, Chart 38)845 have also been prepared from the corresponding monomers with two and three norbornene units, respectively. A triple-stranded helical structure of schizophyllan (SPG) formed in water as described in section 4 can be denatured into a single-stranded random coil by dissolving in DMSO. This solvent-induced structural change reversibly occurs, and the addition of water to a DMSO solution of SPG induces a regeneration of its original triple-stranded helical structure due to the hydrophobic interaction (renaturation process).420,846,847 Sakurai and Shinkai et al. have found that when certain polynucleotides (polycytosine, polydeoxyadenosine, etc.) are present during the renaturation process of SPG, unique triplestranded hybrid polymer complexes composed of a single polynucleotide strand and double polysaccharide strands are formed (Figure 164).848,849 In the same way, when water was added to a DMSO solution of a mixture of SPG and L- or D536, the right-handed triple-stranded helical complex consisting of two SPG strands and one 536 strand was created regardless of the chirality of the amino acid residues in 536, and the complex showed an ICD in the 536 chromophore region.850 A Curdlan derivative bearing an aromatic pendant at the 6 position of the glucose unit (537a in Figure 165A) also forms a triple-stranded helical structure in a DMSO solution containing water or methanol, and the pendant groups are helically aligned to form an H-type aggregation (Figure 165B).851 Moreover, the reversible structural modulation of 537a between the singleand triple-stranded helical structures has been realized by the alternate addition of Zn2+ ions or 1,4,8,11-tetraazacyclotetradecane in a DMSO/methanol mixture (1/1, v/v). On the other hand, an amphoteric Curdlan derivative bearing both amino

By introducing an optically active alkoxy group on the phenylene residue of 521, an optically active carboxylic acid strand ((S)-524) was synthesized (Figure 162).459 Although (S)-524 showed a weak CD in the absorption region of the main-chain chromophore, the CD intensity dramatically increased in the presence of achiral amines, such as piperidine (525), due to the homodouble helix formation with a greater excess one-handedness through hydrogen-bonded inclusion complexation with the achiral amines between each strand, resulting in the amplification of the macromolecular helicity. Again, the dimer model of (S)-280 (Figure 86) showed no chiral amplification, displaying a linear relationship (see section 4.1.1). An optically active polymer containing cationic riboflavin residues in the main-chain (526 in Figure 163A) has been synthesized for the first time from naturally occurring riboflavin (vitamin B2) and found to form a double-helix with an optical activity, thus showing an intense CD in the absorption regions of the flavinium chromophore.832 The riboflavin units of 526 can be reversibly converted to the corresponding 4ahydroxyriboflavins (527) by hydroxylation/dehydroxylation reactions. Based on the 2D NOESY spectrum of 526 (Figure 163C) along with the CD measurement results, the 526 chains most likely self-associate into a π-stacked duplexlike structure (Figure 163B), in which the planar flavin repeating units stack face-to-face and the bulky ribityl groups are arranged antiparallel to each other to decrease the steric hindrance. The fact that analogous riboflavin and flavin derivatives tend to form similar face-to-face stacking geometries in crystals suggests two possible stacking interactions between the strands; the Re− Re (or Si−Si) and Re−Si (Figure 163Ba and b, respectively) face stackings between the interstrand flavin rings. The former Re−Re (or Si−Si) model is considered to be the most probable geometry based on the results for the asymmetric oxidation catalyzed by 526 (see section 5.7.2). Luh et al. reported a series of interesting structural analogues of DNA derived from a fully artificial framework.833,834 Ringopening metathesis polymerizations of bisnorbornene derivatives bearing a ferrocene spacer produced double-stranded ladder polymers (531a and 532−534 in Chart 37).835−837 Scanning tunneling microscopy (STM) investigations revealed a double-helical structure of 531a, whose geometric parameters, 13905


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Figure 185. (A) High-resolution STM image of a 2739 base-paired double-stranded plasmid DNA at ∼95 K, exhibiting an internal structure with periodicity of 2.6−3.6 nm along the strand. The illustration fitted to the same scale is also shown below. (Reproduced with permission from ref 1029. Copyright 1999 Elsevier.) (B) AFM image of DNA in propanol. (Reproduced with permission from ref 1030. Copyright 1995 Elsevier.) (C) Highresolution AFM image of a right-handed DNA double-helix (a) and standard model of the double-stranded B-DNA (b). (Reproduced with permission from ref 1031. Copyright 2003 Springer.) (D) High-resolution AFM with frequency modulation detection method (FM-AFM) image of the plasmid DNA in aqueous solution (50 mM NiCl2) and the molecular model of the B-DNA (inset). The positions of the major and minor grooves of B-DNA are indicated by red and blue arrows, respectively. The gray arrow indicates the local melting region of the plasmid DNA. (Reproduced with permission from ref 1032. Copyright 2013 American Chemical Society.) (E) AFM image of plasmid DNA (c) and digitally straightened trace (top) and retrace (bottom) images of c (d). The inset in d shows a space-filling representation of the B-DNA crystal structure. (Reproduced with permission from ref 1033. Copyright 2014 Wiley-VCH.) (F) STM image of a single-stranded M13mp18 DNA molecule on a Cu(111) surface and part of the base sequence of M13mp18. The guanine sites indicated by red characters are connected to the corresponding parts of the STM image by red arrows. (Reproduced with permission from ref 1034. Copyright 2009 Nature Publishing Group.)

and carboxyl groups (537b) has been reported to show pHdependent structural transformations among the singlestranded random-coil, triple-stranded helical structure, and aggregate structures.852,853 5.4.2. Stereocomplex Formation. 5.4.2.1. Poly(methyl methacrylate). it- and st-PMMAs have been reported to fold into a double-stranded helix in crystals (Figure 166A)854−856 and a single-stranded helix in some organic solvents (Figure 166B),857,858 such as toluene, based on their XRD analyses, respectively. The double-stranded helical structure of it-PMMA has recently been visualized by high-resolution AFM of the 2Dfolded chain crystals of it-PMMA formed upon compression of its LB monolayer on a surface of water (Figure 166E).859−861 Upon mixing amorphous it- and st-PMMAs in a 1:2 molar ratio, a unique polymer-based supramolecular helix possessing an apparent melting point, the so-called stereocomplex (SC), is produced in specific solvents, such as toluene and acetoni-

Figure 186. STM image of 630 on HOPG and the optimized model of the 20-mer of cis−transoidal 630. (Reproduced with permission from ref 1035. Copyright 2001 American Chemical Society.)

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Figure 187. (A) AFM phase image of individual 631 strands prepared by spin-casting a dilute THF solution on HOPG. A computer-generated model is also shown. (Reproduced with permission from ref 1036. Copyright 2002 Wiley-VCH.) (B) AFM phase image of a single strand of (R)-632 prepared by spin-casting a dilute DMF solution on mica. (Reproduced with permission from ref 1037. Copyright 2003 American Chemical Society.) (C) AFM phase images of 633 in the presence of L-Ala (left) and D-Ala (right) on mica. Schematic representation of possible helical structure (top) and superhelix (bottom) of 633 visualized with AFM on a mica surface is also shown. (Reproduced with permission from ref 1038. Copyright 2004 Wiley-VCH.)

trile.862−864 The history of the SC is traced back to 1958, when the first PMMA SC formation was reported,865 but the structure of the SC on a molecular level and the mechanism of the SC formation have not been resolved.866 Nonetheless, the SC has been practically utilized as hollow fiber dialyzers.867 In 1989, Schomaker and Challa proposed an intriguing model for the PMMA SC based on the XRD analysis of the stretched fiber, that is, an intertwined double-stranded helical structure, which is composed of a 91 it-PMMA helix wrapped within an 181 st-PMMA helix with a repeating monomer ratio of 1:2, giving a complementary double-stranded helical structure with a helical pitch of 1.84 nm (Figure 166C).868 Since this model was proposed, the PMMA SC is believed to have such a complementary intertwined double-stranded helical structure. PMMA SC also forms from a mixed monolayer of the it- and st-PMMA chains spread on a surface of water upon compression based on the surface pressure−area isotherms,869 resulting in the formation of a 2D SC monolayer. The highresolution AFM image of the SC monolayer (Figure 166F) revealed helix-bundle-like structures with a chain−chain distance of ca. 2.4 nm,870 which is consistent with the width of the proposed double-stranded helix model (ca. 2.4 nm). The high-resolution magnified AFM image (Figure 166G) provides important information about the helical structure of the PMMA SC; a number of periodic oblique stripes (yellow arrows) with a pitch of 0.92 nm are clearly observed, which are further tilted either clockwise or counterclockwise along the SC main-chain

(blue lines) (Figure 166G).870 Importantly, the helical pitch (0.92 nm) observed in the AFM image is identical to half the helical pitch of the proposed double-stranded helix model (1.84 nm), providing convincing evidence that the PMMA SC, a long-standing question in polymer chemistry, may not be an intertwined double-stranded helix, but is likely a supramolecular-assembled inclusion complex of a triple-stranded helix composed of an outer 181 st-PMMA single-helix with the observed 0.92 nm helical pitch, in which an it-PMMA doublehelix with a 1.84 nm helical pitch is included (Figure 166D).870 This triple-helix model satisfies the AFM results, in particular, the helical pitch (0.92 nm) of the SC, the stoichiometric ratio of it- and st-PMMAs = 1/2, and the reported helical structures of each component, it- and st-PMMAs (Figure 166A and B).854−858 This triple-stranded helix model is further supported by the facts that st-PMMA forms an inclusion complex with a variety of fullerenes within its helical cavity to produce a crystalline peapod-like complex (see section 5.5),871 and the included fullerenes are further replaced by it-PMMA to form an SC.872 The high-resolution AFM observations of SCs formed from it- and st-PMMAs with different molecular weights and a narrow molecular weight distribution reveal the mechanism of how such a multiple-stranded supramolecular helix forms from stereoregular PMMAs as well as the molecular structure of the PMMA SC. It has been found that it- and st-PMMAs with the longer molecular length (LML) determine the total length of 13907


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Figure 188. (A) AFM phase image of the epitaxial 634-L crystals prepared by spin-casting a dilute benzene solution (0.05 mg/mL) on HOPG. The cross-section profile shown by a white line and the typical 2D fast Fourier transform (FFT) are also shown. The yellow arrows in the image indicate the direction of the polymer main-chain axes. The white arrows indicate the amorphous polymer chains. (B) Schematic drawings of the hierarchical structure of the self-assembled 634 on HOPG. (C) High-resolution AFM phase images of 634-L (a) and 634-D (b) with antipodal oblique pendant arrangements (blue and green lines, respectively). Possible models (left and right) were constructed on the basis of the X-ray structural analysis. nDecyl side groups are replaced by methyl groups for clarity. (Reproduced with permission from ref 1039. Copyright 2006 Wiley-VCH.)

cyclization (Figure 168A).876 The cyclic st-PMMA forms an SC with a linear it-PMMA, leading to the formation of a unique “polypseudorotaxane-type” supramolecular assembly (Figure 168B), whose physical properties are remarkably different from those of the conventional PMMA SC, probably due to its unique polypseudorotaxane-type topology imposed by the cyclic st-PMMA components. The simulations of the observed XRD profiles support the “polypseudorotaxane” formation rather than the “double-wrapping” formation. 5.4.2.2. Miscellaneous Applications of PMMA Stereocomplex Formation. Since the discovery of the PMMA SC, the SC has been applied to advanced materials, such as microcellular foams877 and thermoplastic elastomers878 as well as a practically useful hollow fiber dialyzer.867 The complementary SC formation as well as its unique multistranded helical structure has also been utilized to develop ultrathin films by layer-by-layer (LbL) assemblies,879 template polymerizations,880,881 and nanostructured materials.882 Kawauchi et al. reported a novel and versatile method for creating a supramolecular nanosphere and nanonetwork using it- and st-C60-end-capped PMMAs (it- and st-PMMA-C60’s) based on their SC formation together with self-assembly of the terminal C60.882,883 The stereospecific living anionic polymerizations of MMA followed by end-capping with C60 were used to synthesize it- and st-PMMA-C60’s with a precisely controlled structure, including its molecular weight, distribution, tacticity, and the chain-end structure (Figure 169A), the structures of which were identified by AFM (Figure 169A), SEC, NMR, UV−vis, and matrix-assisted laser desorption/ionization timeof-flight mass spectroscopy (MALDI-TOF-MS) analyses. The

the SC, and the complementary PMMAs with the shorter molecular length (SML) associate until they fill up or cover the longer component (Figure 167A).873 Further concrete evidence has been obtained by using uniform it- and st-PMMAs.874 An AFM image of the SCs (Figure 167C) formed between the itPMMA 22mer (it22) and st-PMMA 44mer (st44) (it/st = 1/2 in unit-molar base) suggests that the SCs with a uniform length (yellow lines) are aligned in parallel to form regular lamellar structures with a uniform width, while other irregular intermolecular aggregates are not observed. More importantly, when two uniform it-PMMAs, it22 and it43, and st44 are mixed, it-PMMA recognizes the molecular weights of each other and selectively forms a double-helix of the same molecular weight (“molecular sorting”) (Figure 167B, D, and E), resulting in a pseudorotaxane (a so-called “molecular necklace”). Again, the LML determines the total length of the SCs and the heterodouble-stranded helices derived from two it22 chains and a single it43 chain (Figure 167B, right-middle) are not observed by AFM.873 Based on these results, it can be concluded that the doublestranded helix of it-PMMA is most likely included in a preformed single-stranded helix of st-PMMA, thus forming a supramolecular inclusion complex, i.e., a triple-stranded helix. The recent MD simulation results for the PMMA SCs suggest a triple-stranded helical structure that successfully and accurately reproduces the experimental XRD profiles for the PMMA SCs.875 Kamigaito, Qiao, and co-workers prepared a novel cyclic stPMMA by the stereospecific living radical polymerization of MMA, followed by azidation and the copper-catalyzed click 13908


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Figure 189. AFM phase images of self-assembled left-handed 634-D spin-cast on HOPG from a dilute THF solution (A) and right-handed 634-D after helicity inversion by benzene vapors (B). The white arrows indicate the direction of the polymer main-chain axes. The magnified AFM phase images correspond to the areas indicated by the red and green rectangles, respectively. Schematic drawings of the crystal structures of left- (A) and right-handed (B) helical 634-D with antipodal oblique pendant arrangements are also shown. Possible models were constructed based on the X-ray structural analysis results. n-Decyl side groups are replaced by methyl groups for clarity. (Reproduced with permission from ref 1040. Copyright 2006 American Chemical Society.)

it- and st-PMMA-C60’s formed a core−shell aggregate with C60 as the core and the PMMA chains as the shell in a water and acetonitrile mixture (1/9, v/v) resulting from the solvophobic interaction-driven self-assembly of the terminal C60 residues (Figure 169B, right). These it- and st-PMMA-C60 aggregates further assembled via iterative SC formation into nanonetworks in which the self-assembled C60 clusters were strongly connected with 2D and 3D arrangements (Figure 169C).882 Furthermore, when the it- and st-PMMA-C60’s were mixed together, self-assembly of the terminal C60 units and SC formation of the it- and st-PMMA chains simultaneously took place, leading to the formation of spherical nanoparticles with a uniform size and resistance to heat (Figure 169B, left). Taking advantage of the supramolecular SC formation, similar nanospheres and nanonetworks have been prepared using the core-cross-linked star (CCS) st-PMMA with the linear it-PMMA as the binder. The morphologies can be controlled by the mixing ratios of the star st-PMMA and linear it-PMMA as well as the molecular weight of the linear itPMMA (Figure 170).884 Chen and co-workers developed the first in situ stereocomplexing polymerization of MMA using a pair of in-situ-

generated isospecific and syndiospecific zirconocene catalysts at ambient temperature that produced crystalline PMMA SCs with high melting temperatures up to 217 °C in high yields.885 The SC formation is believed to be applicable to the stereospecific synthesis of it- or st-PMMA during the polymerization of MMA in the presence of its complementary PMMA as a template.886,887 This template-directed polymerization is ubiquitous in biological systems,888,889 but almost no successful examples were reported in artificial systems until 2004, when Serizawa, Akashi, and co-workers discovered an incredible template-directed stereospecific free radical polymerization of MMA or methacrylic acid (MAA) within porous films composed of template polymers, producing polymers with controlled stereoregularities and molecular weights as well as molecular weight distributions.880,890 The key process necessary for the present template-directed stereospecific polymerization is assumed to fabricate porous films of st-poly(MAA) (st-PMAA) or it-PMMA by LbL assembly of these polymers to produce SC films followed by selective extraction of st-PMAA or it-PMMA (Figure 171).891 The st-PMAA was used instead of st-PMMA because it also forms an SC with it-PMMA and can be quantitatively removed 13909


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size and controlled film thickness using silica particles as the template core and subsequent removal of the silica by treatment with aqueous HF (Figure 172).892 Replacement of the st-PMMA with st-PMAA is possible, as already mentioned, which, therefore, will allow construction of pH-responsive hollow capsules after the st-PMAA is selectively extracted from the it-PMMA/st-PMAA SC shell by an alkaline aqueous solution. An SC formation between it- and st-PMMAs has been found to efficiently take place in ionic liquids, such as BMIPF6. Interestingly, the SC formation resulted in gelation of BMIPF6 at ambient temperature (Figure 173) and was fully thermoreversible with a high melting point (Tm = 175.0 °C), as supported by the DSC and optical microscopy measurement results.893 5.4.2.3. Other Stereocomplex Formations. Pairs of enantiomeric polypeptides (538),894−896 polyesters (539− 541),897−900 and it-polymethacrylate (542)901 are also known to form SCs (Figure 174).902 Being distinctly different from the optically inactive it-/st-PMMA-derived SCs with multistranded intertwined or inclusion helical structures, specific chain-tochain interactions between the enantiomeric helices or pendant groups of the polymers are most likely responsible for the formation of these SCs. Among the SCs prepared so far, those formed between the enantiomeric poly(L-lactic acid) (L-539) and poly(D-lactic acid) (D-539) have been extensively investigated with respect to their structures, the SC formation mechanism, and applications, because 539-based SCs have been used as practically important biomaterials for tissue regeneration, drug delivery systems, and alternatives for commodity plastics due to their biodegradability, producibility from renewable resources, and nontoxicity to the human body and the environment.902 Recently, Coates and Auriemma et al. and Lu et al. reported unique SCs consisting of amorphous, enantiomerically pure polycarbonates (543 and 544, respectively), which can crystallize upon stereocomplexation with their complementary enantiomers.903−905 The formation of crystalline hetero-SCs has also been achieved by mixing two amorphous polycarbonates (e.g., (R)-544c and (S)-544d) with different chemical structures and opposite configurations in a 1:1 mass ratio.906 In addition, an intramolecular polycarbonate-based SC has been prepared from an optically inactive stereomultiblock 544e consisting of alternating (R)- and (S)-polymer segments, which show similar crystallinity and thermal stability compared with an SC prepared through cocrystallization of (R)- and (S)544e.907 Riguera and co-workers found a unique SC formation between enantiomeric helical PPAs bearing the (R)- and (S)-αmethoxy-α-trifluoromethylphenylacetamide pendant groups ((R)- and (S)-412).908 An equimolar mixture of (R)- and (S)-412 in THF gives rise to supramolecular fiber-like aggregates (Figure 174B and C) due to the favorable intermolecular hydrogen bond formations between the cis amide residues of the enantiomeric helical PPAs (Figure 174A), while at higher concentrations the solution forms a gel. The cisto-trans amide conformational change at the pendants can be regulated by temperature and solvent polarity, which allows control of the SC formation due to a significant change between the intermolecular and intramolecular hydrogen bond interactions.

Figure 190. AFM height images of 2D self-assembled 635 cast on HOPG from a dilute benzene solution. The cross-section height profiles (A and B) denoted by white and yellow arrows, which represent right- and left-handed helical blocks separated by a helical reversal (A) and a gap (B), along with helical blocks of opposite handedness are also shown. Polymer models (A and B) are constructed by molecular modeling and MM calculations on the basis of the X-ray structural analysis. (Reproduced with permission from ref 1041. Copyright 2007 Wiley-VCH.)

by an alkaline extraction from its SC film. Surprisingly, the porous films of st-PMAA prepared on silica gel polymerized MMA in a highly isotactic specific way to produce it-PMMA (mm > 92%) with relatively low molecular weight distributions, and the molecular weights of the it-PMMA tended to increase with an increase in the molecular weight of the template stPMAA. When porous films of it-PMMA on silica gel were used, st-PMAAs (rr > 94%) with a controlled molecular weight were also obtained by the free radical polymerization of MAA in water (Table 6).880 A specific nanospace provided by the porous template polymer matrices after extraction of either itPMMA or st-PMAA together with the complementary multistranded helical SC formation during the polymerization has been proposed to be indispensable for the templatedirected stereospecific free radical polymerization. Taking advantage of the LbL assembled PMMA SC formation on silica gel, Akashi and co-workers further developed hollow capsules of it-/st-PMMA SCs with a uniform 13910


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Figure 191. (A) AFM phase image of 2D self-assembled 636 (r = 0.5) cast from a dilute toluene solution on HOPG, where the polymer strands with clearly identifiable left-handed helical blocks are shown in red color (right) (a). Plots of the eeh values of 636 versus the content of chiral units in the cholesteric liquid crystalline state in toluene (triangles) and 2D crystals (circles) (b). The eeh values of 636 in concentrated liquid crystalline solutions were calculated using the maximum cholesteric wavenumber qc (defined by 2π/(cholesteric pitch)) value of 0.94 as the base value, and the eeh values of 636 in the 2D crystals were estimated by AFM on the basis of an evaluation of ca. 1000 helical blocks. (B) AFM phase image of 2D selfassembled 637 (r = 0.55) on HOPG, where the copolymer strands with clearly identifiable left- and right-handed helical blocks are shown in red and blue colors, respectively (right) (c). Plots of the eeh values of 637 versus the % ee of the copolymer components (LL-rich) in the cholesteric liquid crystalline state in benzene (triangle) and 2D crystals on HOPG (circle) (d). The eeh values of 637 in concentrated liquid crystalline solutions were calculated using the maximum qc value of 0.82 as the base value, and the eeh values of 637 in the 2D crystals were estimated by AFM on the basis of an evaluation of ca. 1500 helical blocks. (Reproduced with permission from ref 1043. Copyright 2012 The Society of Polymer Science, Japan.) (C) AFM phase image of a 634−635 mixture cast from a dilute benzene solution on HOPG ([634]/[635] = 1/1, w/w), where the polymer strands with clearly identifiable right- and left-handed helices are shown in red and blue, respectively (right) (e). Schematic drawings of the possible arrangements of right- and left-handed helical segments of a 634 and 635 mixture in the 2D crystals, where red and blue helical segments represent the right- (P) and left-handed (M) helical 635 and green helical segments stand for the left-handed helical 634 (f). (Reproduced with permission from ref 1042. Copyright 2011 American Chemical Society.)

5.5. Helical Cavity of Helical Polymers

(Figure 175B), suggesting that the induced helical conformation of the st-PMMA is definitely retained. The calculated IR and VCD spectra for the right- and left-handed helical 181 stPMMAs (Figure 175C) are in good agreement with the observed ones, leading to the assignment of the st-PMMA helix induced by (R)-439, which is most likely right-handed. A similar st-PMMA gel with an optical activity can also be prepared using optically active amines ((R)- or (S)-397 and 545−548) in toluene in the absence of C60.909 The induced helical conformation of the st-PMMA is also memorized after removing the optically active amines, and this helical st-PMMA can be employed as the template for the encapsulation of C60, showing virtually the same VCD spectrum as those in Figure 175B. Based on the reported helical structure of st-PMMA, the most plausible helical structure of the st-PMMA/C60 complex has been proposed as shown in Figure 175A, in which the C60 molecules are entrapped to form a regular 1D alignment with an intermolecular distance of 1.0 nm.871 The helical pitch and

5.5.1. Inclusion Complexation of Small Molecules. As described in section 5.4.2.1, st-PMMA forms racemic singlestranded helices in aromatic solvents, such as toluene, accompanied by gelation in which solvent molecules are encapsulated in a large helical cavity of ca. 1 nm.857 Taking advantage of the induction of one particular helical sense in the PPAs and subsequent memory of the induced helical chirality, a preferred-handed helical structure was also found to be induced in st-PMMA in the presence of optically active alcohols, such as (S)- or (R)-1-phenylethanol (439) in toluene along with the gelation, and at the same time, fullerenes, such as C60, C70, C76, and further higher fullerenes, can be entrapped within its helical cavity to produce a processable peapod-like robust complex (Figure 175A).871 The st-PMMA/C60 complex gel showed clear VCD and ICD signals in the PMMA IR and achiral C60 absorption regions, respectively, after complete removal of 439 13911


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Figure 192. (A) AFM phase images of poly(R)-638 on HOPG showing single chains (a) and multistranded helices (b). (Reproduced with permission from ref 1044. Copyright 2010 Wiley-VCH.) (B) AFM images of poly(R)-410/Ba2+ (c) and poly(R)-410/Ag+ (d) on HOPG and helical structures obtained from MM calculations (side view). (Reproduced with permission from ref 644. Copyright 2011 Wiley-VCH.) SEM images of nanospheres (e), nanotubes (f), and toroidal nanostructures (g) obtained from poly(R)-410/Li+ complex ([poly(R)-410]/[Li+] = 1) in chloroform, chloroform containing 2-butanone as a higher-boiling-point cosolvent, and chloroform−dioxane (1/100, v/v), respectively. Scale bar: 200 nm (e), 2 μm (inset, 1 μm) (f), and 200 nm (g). (Reproduced with permission from ref 1047. Copyright 2014 Wiley-VCH.) (C) AFM images of poly(R)-412 in chloroform (h) or THF (i) and helical structures obtained from MM calculations (side view). (Reproduced with permission from ref 647. Copyright 2013 The Royal Society of Chemistry.) (D) AFM images of the 2D-monolayer of poly(R)-410 on HOPG prepared by the Langmuir− Shaefer deposition technique showing separate macroscopically enantiomeric domains of right- and left-handed helical chains (j) and enantiomeric superhelices (k, l). (Reproduced with permission from ref 1045. Copyright 2016 The Royal Society of Chemistry.)

lateral spacing of the st-PMMA/C60 complex, estimated by AFM, are consistent with those of the proposed model. MD simulations revealed that the encapsulated C60 molecules remain within the st-PMMA helical cavity at 400 K for 200 ps, which indicates the high thermal stability of the st-PMMA/ C60 complex. The homogeneous st-PMMA/C60 film containing 23.5 wt % C60 retained the crystal structure with the definite melting temperature (Tm) of 212 °C after evaporating the

solvents and showed a birefringence, as visualized by polarizing optical microscopy (Figure 175A). The encapsulation of C60 in the helical cavity of st-PMMA also partially occurred in acetonitrile, resulting in the significant increase in the solubility of C60 in acetonitrile upon the addition of st-PMMA.910 In addition, polycyclic aromatic hydrocarbons, such as pyrene and phenanthrene, can be entrapped within the st-PMMA helical cavity, resulting in the formation of a 13912


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Figure 193. AFM phase images of self-assembled diastereomeric helical poly-L-639a (A) and poly-L-639b (B) obtained in toluene at 100 °C followed by annealing in toluene at 100 °C for 6 days and in CCl4, respectively, on HOPG. The magnified images correspond to the areas indicated by the blue and red rectangles, respectively, and schematic representations of the left-handed helical poly-L-639a (A) and right-handed helical poly-L639b (B) are also shown (C). (Reproduced with permission from ref 1048. Copyright 2006 American Chemical Society.)

Figure 194. AFM phase images of 2D self-assembled poly-L-639c (A) and poly-L-639d (B) on HOPG. The bars and dotted lines indicate the individual polymer chains and 2D smectic layer lines, respectively. Schematic representations of the left-handed helical poly-L-639c and right-handed helical poly-L-639d with periodic oblique stripes (pink and blue lines, respectively), which denote a one-handed helical array of the pendants and optimized 154 helical structures of poly-L-639c and poly-L-639d on the basis of X-ray structural analysis results, are also shown. (Reproduced with permission from ref 1049. Copyright 2008 American Chemical Society.)

Figure 195. Schematic illustration of the enantiomer-selective and helix-sense-selective block copolymerization of L-639 and D-639 with the lefthanded helical poly-L-639c as the macroinitiator, resulting in almost perfect single-handed helical block copolymers with different polymerization rate constants (kL and kD). AFM phase images of poly-L-639c-b-L-639 (right) and poly-L-639c-b-D-639 (left) on HOPG with periodic oblique stripes (pink lines), which denote a left-handed helical array of the pendants, are also shown. (Reproduced with permission from ref 1050. Copyright 2009 American Chemical Society.) 13913


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Figure 196. (A) Schematic representation of a typical 2D helix-bundle structure of diblock copolymers with head-to-head (H−H) bilayer molecular packing. (B) High-resolution AFM height image of 2D self-assembled poly(640107-b-64192) on HOPG (top) and the zoomed image of the areas indicated by the white rectangle (bottom). (C) AFM phase images of poly(640202-b-641184) obtained after SEC fractionation on HOPG. Poly-640 and poly-641 blocks are indicated by red and blue lines, respectively (bottom). (Reproduced with permission from ref 1051. Copyright 2010 American Chemical Society.)

crystalline inclusion complex.911 A possible structure of the stPMMA/pyrene inclusion complex containing 19 wt % pyrene is shown in Figure 175D; the pyrene molecules are stacked in the helical cavity of st-PMMA with an intermolecular distance of 4.12 Å, indicating their 1D alignment. MD simulations of the st-PMMA/pyrene inclusion complex using an NTV ensemble demonstrated that the included pyrene molecules remain within the helical cavity of the st-PMMA even at 400 K for 200 ps, suggesting its thermal stability. The film of the st-PMMA/ pyrene complex containing 10 wt % pyrene displayed an apparent fluorescence emission due to the excimer formation of the pyrene molecules within the st-PMMA helical cavity, supporting the fact that the included pyrene molecules in the st-PMMA helix are densely and one-dimensionally arranged. A single-handed helical st-PMMA can efficiently discriminate the size of higher fullerenes by an induced-fit mechanism.912 Therefore, larger C70 molecules were selectively extracted by stPMMA from an equal-weight mixture of C60 and C70 in toluene with an almost perfect selectivity (99.8%) (Figure 175E). The C60-encapsulated helical st-PMMA complex has been applied as the active layer for sandwich devices, which show an irreversible electrical switching effect to provide a nonvolatile write-once-read-many-times (WORM) memory (Figure 176A).913 Quantum chemical calculations indicate that a violent Coulomb explosion proceeds in the peapod C60 wires during the charge injection process, resulting in the WORM performance. A preferred-handed helical st-PMMA can be used to enantioselectively extract chiral fullerenes (C76, C84, C86, C88, C90, C92, C94, and C96) with optical activity from carbon soot.912 The extraction by an optically active st-PMMA affords a series of optically active fullerenes, as evidenced by the CD-

detected HPLC chromatograms. For example, the chromatogram of the extracted fullerenes (≤C84) by an optically active, right-handed helical st-PMMA induced by (S)-397 is shown in Figure 176B. When (R)-397 was used for the preparation of the excess single-handed helical st-PMMA, the antipodal fullerene enantiomers can be extracted, as supported by the mirror image CD signals. Interestingly, the optically active st-PMMA/C60 complex with a helicity memory functions as a novel template for the helix-sense-selective inclusion complexation with the complementary it-PMMA to provide a crystalline optically active PMMA SC, through which the encapsulated C60 molecules are replaced by it-PMMA (Figure 176C).872 The it-PMMA in the SC forms a double-stranded helix with the same handedness as that of the st-PMMA single-helix via the formation of a topologically unique triple-stranded helix, as supported by the observed and calculated VCD spectra of the SC, which are in good agreement with each other (Figure 176D and E). The VCD patterns are totally different from those of the optically active st-PMMA/C60 complex (Figure 175B); in particular, at around, the 1260 cm−1 regions corresponding to the characteristic vibration band derived from it-PMMA. Taking advantage of an elevated-temperature electrospinning technique, Bucknall et al. succeeded in preparing st-PMMA/C60 or C70 fibers in which the C60 and C70 molecules entrapped within the st-PMMA helix in a mixed solvent gel (1:1:1 mixture of toluene, o-dichlorobenzene, and DMF) are uniaxially aligned in the shear field generated during the electrospinning and a high degree of molecular-level alignment with an order parameter of 0.70 has been attained for the st-PMMA/C60 fibers.914 13914


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Figure 197. (A) AFM height (a) and phase (b) images of a monolayer of 639a deposited on mica at 1 mN/m. Magnified phase image of the area indicated by the yellow square in b (c); the yellow and pink lines, respectively, indicate left- and right-handed antipodal oblique pendant helical arrangements with respect to the main-chain axes. (Reproduced with permission from ref 1052. Copyright 2010 American Chemical Society.) (B) AFM phase image of the self-assembled smectic layer structure of 642 (Mn = 15.9 × 104) on HOPG (d). The yellow bars and red dotted lines indicate the individual polymer chains and 2D smectic layers with respect to the polymer backbone axis, respectively. Schematic illustration of a possible layer structure of 642 in the lat-Sm phase (e) and POM of the fanlike texture of the new lat-Sm liquid crystalline phase of 642 in chloroform (f) are also shown. (Reproduced with permission from ref 1053. Copyright 2011 American Chemical Society.) (C) Structure of a star polymer 643 and AFM phase image of 2D self-assembled 643 on HOPG (g). (Reproduced with permission from ref 1054. Copyright 2011 American Chemical Society.)

Figure 177B) into its 1D cavity assisted by the cooperative C− H···O and B−H···H−O interactions as well as a hydrophobic one.916 The porphyrin (Zn-TCCP) forms an inclusion complex with SPG and assembles in a chirally twisted way, thus showing an optical activity.917 An excess one-handed helical conformation can be induced in achiral oligosilanes (551 and 552) once encapsulated within a hydrophobic chiral cavity of polysaccharides, such as SPG, Curdlan, and amylose derivatives.918−920 Carboxylic Curdlan (553a) also serves as a helical host to form an inclusion complex with alkynylplatinum(II) terpyridine (554, Chart 39) and produces helical fibers with different lengths and handedness depending on the preparation conditions.921 This unusual behavior is ascribed to the competition between the kinetically and thermodynamically controlled assemblies, resulting in the formation of right- and left-handed helices, respectively. An acidic dye molecule (555) has also been encapsulated within a cationic Curdlan (553b), resulting in the formation of an optically active supramolecular assembly.922 The encapsulated dye shows a high durability against photobleaching due to the radical scavenger property of the exterior Curdlan derivative. α-Sexithiophene (556 in Figure 178) adopts chirally twisted conformations in a hydrophobic chiral channel generated in helical polysaccharides, such as right-handed triple-stranded

Figure 198. High-resolution AFM phase image of 2D self-assembled 644 prepared by casting a dilute chloroform solution on HOPG, where yellow and pink lines represent the main-chain axes and the helical pitch of 644, respectively (A), and typical 2D FFT of the image (B). (Reproduced with permission from ref 1055. Copyright 2010 American Chemical Society.)

The 1D supramolecular assemblies of dye molecules have also been achieved using SPG as a helical polymer host.915 The dye molecules (549) are arranged in a different aggregation manner in the supramolecular nanofibers depending on the preparation methods. When water was added to a DMSO solution of 549 containing SPG, the J-type assembly of 549 occurred (Figure 177A). In contrast, the H-type assembly took place in the helical SPG cavity during the neutralization process of an NaOH aqueous solution of 549 by adding acetic acid. SPG can also efficiently encapsulate m-carboranes (550 in 13915


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Figure 199. (A) AFM height (left) and phase (right) images of (S)-645 on HOPG (scale = 300 × 300 nm) prepared by casting a dilute chloroform solution (0.02 mg/mL). The cross-section profile denoted by the white line and the zoomed image corresponding to the area indicated by the square are also shown. (B,C) High-resolution AFM phase images of (S)-645 (B) and (R)-645 (C) on HOPG with antipodal periodic oblique pendant arrangements. Optimized helical structures of (S)-645 (B, 60-mer) and (R)-645 (C, 60-mer) calculated on the basis of XRD structural analyses followed by MM calculations are also shown. Each structure is represented by space-filling models, and six sets of hydrogen-bonded helical arrays (n and n + 6) of the pendants are shown in different colors for clarity. (D) AFM phase image of cyclic (S)-645 chains (left) and its molecular model (right). (Reproduced with permission from ref 1056. Copyright 2012 American Chemical Society.)

Chart 43

within the cavity formed in helical polymers. In particular, SPG has been reported to form inclusion complexes with several πconjugated guest polymers (560−565, Chart 40).933−938 The first example of such an inclusion complex has been reported for a single-stranded SPG, which self-assembles with an achiral water-soluble polythiophene (560) to produce a supramolecular molecular wire wrapped by two single SPG chains.933 A preferred-handed helicity was induced in achiral 560 when it was confined within the interior cavity of the double-stranded helical SPG in water, and its induced chirality was further stabilized by the immobilization through a sol−gel reaction using TEOS.934 Taking advantage of this unique supra-

helical SPG and left-handed single-stranded, partially carboxymethylated amylose (CMA) in aqueous solutions.923,924 The inclusion complexes show ICDs with Cotton effect signs opposite to each other, suggesting an opposite handed twisted structure of 556 which reflects the helical handedness of the polysaccharides.923 A series of 4-(4-(dimethylamino)styryl)pyridine-type dye molecules (557),925−928 oligo(p-phenylenevinylene) (558),929 and achiral ladder polysilane (559)930 also exhibit an optical activity when encapsulated in the helical cavity of amylose.931,932 5.5.2. Inclusion Complexation of Polymers. Besides small molecular guests, macromolecules can also be included 13916


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Figure 200. (A) Structures of PPA-based CSPs. (B) Separation of racemic 658 on CSP 657e (eluent, hexane/2-propanol (95/5, v/v); flow rate, 0.1 mL/min). (Reproduced with permission from ref 1069. Copyright 2015 Wiley-VCH.)

(570 and 571),944,945 polycarbonate (572),946 poly(esterether) (573),947 and poly(L-LA) (L-539),948,949 have also been used as template polymers during the vine-twining polymerization. Moreover, amylose can selectively encapsulate one of the template polymers among a mixture of polymers with similar structures950,951 as well as a specific range in molecular weights of the template PTHF during the vinetwining polymerization.952 The enantioselective inclusion of poly(LA)s (539) in amylose has also been achieved through the vine-twining polymerization.953 Similar amylose-based inclusion complexes have been reported to form just by mixing native amylose with 569b as a guest in an ionic liquid and an aqueous solution.954,955 Akashi and co-workers have also reported that a partially 2,3-Omethylated amylose 568 can readily form an inclusion complex with 569b and 570b by a simple mixing in a DMSO/water mixture (1/9, v/v).956 The inclusion ability of 568 was strongly affected by the degree of methylation of the amylose, and it was found that 568 with 8 and 20% methylations was more suitable for the inclusion complex formations with the guest polymers compared to 568 with more than a 33% methylation. In addition, LbL assembled films consisting of the inclusion complex between 568 and 569b have also been prepared on a solid substrate.957 5.5.3. Inclusion Complexation of CNT. Carbon nanotubes (CNTs), including single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs), have been attracting significant attention due to their unique electronic, optical, mechanical, and thermal properties. However, CNTs are insoluble in any solvents due to their strong bundle formation, which limits the use of CNTs in many

molecular inclusion complexation by SPG, SPG has been used to improve the air stabilities of polythiophenes (560 and 561) by changing their redox potentials.936 SPG has also formed an inclusion complex with a cationic poly(phenylene ethynylene) (562), and the electrostatic interaction between the cationic 562 and anionic quantum dots (QDs) further provides the supramolecular complex decorated with QDs.937 The CT complex formation mediated by tetranitrofluorenone allows the bandgap modification of 562, which results in the highly sensitive fluorescence quenching of QDs in response to a small amount of tetranitrofluorenone. The characteristic fluorescence properties observed in the complexes of SPG/560 and SPG/ 563 will be discussed in section 5.7.3.935,938 Furthermore, polymer/polymer inclusion complexes composed of an outer helical amylose and an inner poly(pphenylene) (564) or poly(4,4′-diphenylene-vinylene) (565) have also been prepared, and their highly luminescent features of the complexes have been applied to electroluminescent lightemitting diodes.939 α-D-Glucose 1-phosphate (567) can be enzymatically polymerized with a phosphorylase as a catalyst in the presence of maltooligosaccharides as a primer, such as 566, to produce amylose. Kadokawa and co-workers have found that this phosphorylase-catalyzed enzymatic polymerization of 567 in the presence of poly(tetrahydrofuran) (PTHF, 569b) produces a unique supramolecular inclusion complex (Figure 179) in which PTHF used as a template is encapsulated within the helical cavity of the resulting amylose during the polymerization.940−942 This kind of polymerization has been called “vine-twining polymerization”. In addition to PTHF, several hydrophobic polymers, such as polyether (569a),943 polyesters 13917


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Figure 201. (A) Synthesis of M- and P-poly-L-639-b-660-based CSPs through helix-sense-selective living polymerization and immobilization on silica gel. (B) Structures of racemates (614, 616, 661−663, 665, 666, 668−670) resolved on these CSPs and those not resolved on these CSPs (664, 667). (Reproduced with permission from ref 1072. Copyright 2011 The Royal Society of Chemistry.)

Figure 202. (A) Synthesis of optically active poly(phenyl isocyanide)s bearing achiral benzanilide pendants with one-handed helicity memory (671 and 672). (B) Structures of racemates (661−663, 668−670, and 673−675) resolved on the CSPs 671 and 672.

water and organic solvents without damaging the intrinsic electronic properties of the CNTs by selecting the wrapping polymers. Biological and synthetic helical polymers are of particular interest, because they have the potential for the selective separation of SWNTs with respect to their structures,

applications. To overcome this problem, various solubilizing methods of the CNTs have been developed. Among them, noncovalent functionalization by wrapping with various polymers around the CNT surface is particularly attractive because it enables us to easily and effectively disperse CNTs in 13918


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Figure 203. (A) Schematic illustration of a switchable CSP for the HPLC separation of enantiomers based on reversible helicity induction and subsequent memory in poly-438 by sequential treatment with (R)- and (S)-439 (ee >50%) in an alternating manner. (B) Chromatograms for the resolution of 50% ee of 661 ((−)-isomer rich) on P-poly-438 (a) and M-poly-438 (b) prepared by treatment with (R)- and (S)-439, respectively, at ca. 0 °C (eluent, methanol/water (75/25, v/v); flow rate, 0.025 mL/min). (Reproduced with permission from ref 675. Copyright 2014 Nature Publishing Group.)

Figure 204. Structures of a poly(biphenylylacetylene) derivative bearing an ester linkage (poly-676) and racemic compounds (668−670 and 677− 679) resolved on the poly-676-based CSP.

chirality, and handedness.958 Since the electrical and optical properties of SWNTs significantly rely on their structures, as designated by the chirality index or roll-up index (n,m) that is not directly associated with the handedness of the SWNTs, a great deal of effort has been made to isolate SWNTs with a specific (n,m) structure. 5.5.3.1. Inclusion Complexation with DNA and Polysaccharides. Biological helical polymers, such as DNA,959,960 amylose,961 and β-1,3-glucan polysaccharides,962−965 were previously found to disperse SWNTs in an aqueous solution by wrapping themselves around the SWNTs in a helical way. Zheng et al. achieved the separation of a mixture of SWNTs into 12 individual kinds of semiconducting and two kinds of metallic armchair (n,m) SWNTs using DNA oligomers with different sequences.966,967 Dukovic et al. reported that DNAwrapped SWNTs exhibited apparent CD signals in the absorption region due to the excitonic transitions of the SWNTs.968 However, after replacement of the DNA with achiral sodium dodecyl benzenesulfonate, the resulting complex became CD inactive, indicating that the observed CD signal is

due to the chiral SWNT−DNA interactions, and almost no enantioselective wrapping of the SWNTs took place. 5.5.3.2. Inclusion Complexation with Chiral and Achiral πConjugated Polymers. Several π-conjugated polymers (574− 580) (Chart 41) have been reported to helically wrap around SWNTs in order to disperse the SWNTs in solvents.969−973 In sharp contrast to the structurally similar 574−576, watersoluble 577 and 578 showed significantly improved selectivities in sorting the SWNTs with small diameters (d = 0.75−0.84 nm), probably due to the formation of the confined helical conformation in water, which provides a cavity suitable for encapsulating SWNTs of specific sizes.972,973 Chiral fluorenebased copolymers composed of chiral and achiral units (579 and 580) can wrap onto the SWNTs with a specific (n,m)selectivity, which can be regulated by changing the copolymer compositions through recognition of the chiral angles of SWNTs by the copolymers.974,975 Therien et al. established the requisite design for aryleneethynylene polymers capable of forming a complex with SWNTs having a specific handedness (Figure 180).976 13919


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Figure 205. (A) Structures of PAs bearing an optically active [6]helicene unit as the pendant groups (P-680 and M-680) and results of enantioselective adsorption of racemic compounds (677 and 681) on 680 are also shown using separation factor α. (B) Side view (left) and top view (right) of a possible right-handed (main-chain) helical structure of M-680 (20 mer). (Reproduced with permission from ref 1075. Copyright 2014 The Royal Society of Chemistry.)

Chart 44

Figure 206. (A) SEM image of Fe3O4−PS−PA (683) composite microspheres (St, DVB, AIBN, and PVA denote styrene, divinylbenzene, 2,2′-azobis(isobutyronitrile), and poly(vinyl alcohol), respectively. (B) Time-adsorption profiles of (R)- and (S)-397 on the obtained microspheres at room temperature in chloroform. (Reproduced with permission from ref 1080. Copyright 2012 Wiley-VCH.)

Figure 207. Results of enantioselective adsorption of racemic compounds (661, 663, and 665) and racemic helical polymer (688) on 643.

Chiral aryleneethynylene polymers consisting of helically twisted bridged (S)-binaphthalene units in a transoid conformation (581−583) preferentially bind the surface of the SWNTs in a facial geometry to wrap onto the SWNT in an almost exclusively left-handed selective fashion, as visually demonstrated by TEM and AFM images (Figure 180A and B). On the other hand, the corresponding polymers with unbridged (R)-binaphthalene units in a cisoid conformation (584 and 585) interact with the SWNTs in two distinct binding modes (cisoidfacial and cisoid-side), resulting in the major right-handed and minor (∼20%) left-handed helical wrapping of the SWNTs (Figure 180C). 5.5.3.3. Inclusion Complexation with Helical Polymers. Some synthetic helical polymers, such as PPAs, polysilanes, and polycarbodiimides, have been used to solubilize CNTs by wrapping with the polymer chains around the CNT surface.

Tang et al. reported that the in situ polymerization of phenylacetylenes (586−589) with WCl6−Ph4Sn and [Rh(nbd)Cl]2 as the catalysts in the presence of MWNTs afforded complexes of PPAs with nanotubes (PPAs/MWNTs) soluble in various organic solvents, in which the PPA chains helically wrap onto the nanotubes (Figure 181A).977,978 This in situ polymerization method was used to enhance the mechanical properties of chitosan rods, in which the chitosan rods were reinforced by the in situ precipitation method through deprotection of the acetone-protected monosaccharide units of the corresponding in situ polymerized PPAs/MWNTs hybrids.978 They also demonstrated that the pyrene-functionalized PPAs can effectively solubilize MWNTs in common organic solvents by simply mixing.979,980 13920


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Chart 46

polycarbodiimide−SWNT complexes (Figure 181C).983 The complexes showed up to 12-fold differences in photoluminescence efficiency depending on their pendant functional groups. Moreover, exciton energy transfer between individually encapsulated nanotubes was observed in the mixture of two oppositely charged nanotubes encapsulated with aminefunctionalized 601 (cationic protonated form) and acidfunctionalized 600 (anionic carboxylate form) due to a Coulombic attraction, which was reversibly controlled by the subsequent addition of 601. Moore et al. demonstrated the reversible wrapping and release of SWNTs using oligo(m-phenylene ethynylene) (604) by controlling its conformational state by changing the solvent (Figure 181D).984 In chloroform, 604 exists in a flexible unfolded conformation and can wrap onto the SWNT surface due to the preferable association between the surface of 604 and the SWNTs, resulting in a stable dispersion of the SWNTs. Upon the addition of acetonitrile to the dispersion solution, the dispersed SWNTs are rapidly released from the 604-wrapped SWNT complexes and then precipitated due to the folded helical conformation of 604 that is favorable in acetonitrile, but not suitable for stacking onto the surface of the SWNTs. When the 604-wrapped SWNTs were assembled into network fieldeffect transistors, an interesting phototransistor behavior was observed, probably due to the photoinduced charge separation between 604 and the SWNTs. 5.5.3.4. Optical Resolution of SWNT. Other than zigzag and armchair structures, all the (n,m)-SWNTs are chiral and possess either a right- or left-handed helical chirality. Recently, separation of the racemic mixtures of the SWNTs into enantiomers has been achieved. Komatsu and Osuka et al. developed a unique chiral gabletype diporphyrin (605), which is called a “nanotweezer”, and for the first time, succeeded in isolating enantiomeric SWNTs with an optical activity using 605 as an efficient chiral selector (Figure 182A).985 As a result of further structural optimization of the chiral nanotweezers (606−608), simultaneous enrichments of the optical purity and (n,m) abundance have become possible.986−989 Figure 182Ab shows the CD spectra of optically active enantiomeric (7,5)-SWNTs extracted with (R)-607 and (S)-607, which are mirror images to each other.987 The rationally designed chiral diporphyrin with a nearly parallel orientation of the two porphyrins connecting a much longer spacer (>1.4 nm) (609), called “nanocalipers”, can simultaneously recognize the helicity, diameter, and metallicity of SWNTs to produce optically active SWNTs with >1.0 nm

Figure 208. (A) Results of enantioselective permeation of racemic compounds Trp and Phe on poly-398a and poly-689. (B) Plots of quantity (Q) of permeated (R)- and (S)-Phe versus permeation time through membranes of poly-690a (J, B) and poly-690b (E, G). Circle symbol: (R)-Phe, square symbol: (S)-Phe. (Reproduced with permission from ref 1083. Copyright 2014 The Chemical Society of Japan.)

Chart 45

The relationship between the main-chain stiffness of the helical poly(dialkylsilane)s and their wrapping behaviors around the SWNTs have been systematically investigated by using three poly(dialkylsilane)s with a different main-chain stiffness (Figure 181B).981 After applying a mechanochemical high-speed vibration milling (HSVM) process, random-coiled (590) and flexible (591) poly(dialkylsilane)s were wrapped around the SWNTs by changing their conformation so as to fit the surface curvatures of the SWNTs, whereas the semiflexible poly(dialkylsilane) (592) could not form a complex with the SWNTs. Poly(dialkylsilane)s (590 and 593−598) also showed significant diameter-selective wrappings for SWNTs with a specific diameter (ca. 0.9 nm), resulting in the selective separation of the (7,6)- and (9,4)-SWNTs (Figure 181B).982 Helical polycarbodiimides (599−603) have been reported to helically wrap SWNTs to form water-soluble well-dispersed 13921


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Figure 209. (A) Schematic illustration of helix inversion of 463a bearing β-CyD pendants. (B) CD and absorption spectral changes of 463a in alkaline water (pH 11.7)/DMSO (7/3, v/v) in the presence of 0−100% ee of 397 at 25 °C. (C) Photographs of 463a (20 mg/mL) in the absence (a) and presence of (S)-397 (b) and (R)-397 (c) in water/DMSO (6/4, v/v) at room temperature. (Reproduced with permission from refs 1092 and 714. Copyright 2006 and 2011 American Chemical Society.)

Figure 210. (A) CD and absorption spectra of 698 with (S)- (a and e, red lines), (R)- (b and f, blue lines), and rac-677 (c and g, green lines) and without 677 (d and h, purple lines) in water (pH 7.5) at 25 °C. (B) POM of a cholesteric liquid crystalline phase of 698 in about 40 wt % water solution taken at ambient temperature. (Reproduced with permission from ref 1093. Copyright 2011 Wiley-VCH.)

Chart 47

Chart 48

diameters enriched in metallic ones.990 The chiral monoporphyrin (610) was also found to show a preferential extraction of the SWNTs through simultaneous recognition of the helicity and diameter to form optically active SWNTs with relatively larger diameters.991

Taking advantage of the high selectivity of polyfluorene toward semiconducting SWNTs and the excellent enantiomer recognition ability of binaphthol derivatives, Nakashima et al. demonstrated that the copolymers consisting of achiral fluorene and chiral binaphthalene units (580) (Chart 41) can also selectively extract either right- or left-handed semiconducting SWNT enantiomers with enrichment of the (6,5)- and (7,5)chiralities by a simple one-pot sonication method.975 The naturally occurring flavin mononucleotide (FMN) has been found to preferentially extract left-handed SWNTs through formation of a right-handed helical wrapping due to 13922


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Figure 211. Structures of 703 and cyclic trimer (704). Schematic illustration of helicity induction in optically inactive 703 assisted by a chiral amine ((R)-397) (A) and subsequent macromolecular helicity memory (B). The optical activity of 703 is retained in the presence of an equimolar amount of (S)-397 through “memory” of the macromolecular helicity (B) or by the “chiral filter” effect (C). (Reproduced with permission from ref 1099. Copyright 2011 American Chemical Society.)

5.5.4. Polymerization in Helical Cavity. Polysaccharides, such as SPG and amylose, can encapsulate polymers as well as small molecules within their helical cavities, producing supramolecular inclusion complexes as described in section 5.5.2. Alternatively, polysaccharide/polymer inclusion complexes have been prepared by polymerization of the corresponding monomers within the cylindrical helical cavity of the polysaccharides. Shinkai and co-workers found that SPG acts as a helical host to form an inclusion complex with 1,4-diphenylbutadiyne derivatives (612) within its interior 1D cavity during the renaturation process, and the encapsulated 612 can be polymerized by UV- or γ-irradiation to afford the poly(diacetylene)-nanofibers (Figure 183A).997,998 Amylose−polystyrene (PS) inclusion complexes were also prepared by the free radical polymerization of styrene encapsulated in the hydrophobic helical cavity of amylose in water (Figure 183B).999 The molecular weights of the PSs formed within the helical cavity can be controlled by changing the molecular weights of the amylose used as a 1D host. Moreover, their polydispersities were lower than those obtained by the standard free radical polymerization of styrene without amylose, probably due to the confinement effect of amylose, thus providing a potentially useful way to prepare commodity plastics with controlled molecular weights.1000−1002 Shinkai et al. also reported that SPG partially wrapped onto the hydrophobic PS chains of spherical PS aggregates in an oilin-water emulsion, which produced unique supramolecular polymer micelles composed of PS aggregates as a core and hydrophilic SPG chains on the surface.1003 A luminescent amylose−poly(p-phenylenevinylene) (PPV) composite (APPV) has been prepared by the polymerization of the precursor monomer (613) in the presence of amylose in a mixture of DMSO and alkaline water. The resulting APPV was

Figure 212. Enantioselective discrimination of D- and L-Phe by chiral PANI thin films: (A) a green (R)-199-doped PANI thin film; (B) a blue (R)-199-dedoped PANI thin film; (C) a dedoped PANI thin film turns green after exposure to L-Phe; (D) a dedoped PANI thin film stays blue after exposure to D-Phe. (Reproduced with permission from ref 1100. Copyright 2003 Wiley-VCH.)

the chiral D-ribityl phosphate chain of FMN (Figure 182B).992−994 Similarly, separation of the enantiomeric SWNTs has been achieved by density gradient ultracentrifugation (DGU) using sodium cholate (611) as a chiral surfactant (Figure 182C).995 Later, Weisman et al. separated seven (n,m)enantiomeric pairs of SWNTs by nonlinear DGU using mixed surfactants consisting of the chiral 611 and achiral sodium dodecyl sulfate.996 13923


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Figure 213. (A) CD spectra of (S)- and (R)-199-doped PANI. (B) Formation mechanism of PANI superhelical microfibers. (C, D) SEM images of left-handed (C) and right-handed (D) single-helical microfiber-based enantioselective sensors and their response measurements under exposure to 2-aminohexane (705) enantiomers and their racemic mixture. (Reproduced with permission from ref 1103. Copyright 2014 American Chemical Society.)

5.6. Determination of Helical Handedness of Helical Polymers

soluble in DMSO and could be further processed into luminescent films and fibers (Figure 184B), whereas PPV is totally insoluble in solvents and cannot be processed (Figure 184A).1004 Moreover, the APPV is rigid enough to form a lyotropic liquid crystalline phase in DMSO, although amylose itself is too flexible to form a liquid crystal. The photoluminescence efficiency of the APPV film (0.28−0.44) is higher than that of the conventional nonsubstituted PPV film (0.27), probably because the wrapping structure of APPV, in which amylose is an insulator, efficiently contributes to inhibiting the fluorescence quenching between the stacked PPV chains and preventing the exciton migration.1005 The APPV was then applied to an emitting layer for an organic light-emitting diode (OLED) and a green light was clearly observed from the double-layer device, as shown in Figure 184C, although the maximum luminance of the APPV-based device was not sufficiently high (52 cd/m2). The APPV can be further functionalized by introducing various pendant units into the hydroxy groups of the exterior amylose using aliphatic and aromatic isocyanates and acetic anhydride, while retaining its rotaxane-like structure and supramolecular liquid crystallinity.1006 A rod-like APPV derivative modified with 3,5-dimethylphenyl isocyanate (APPV-PC) has also been synthesized in order to develop a novel CSP for enantioseparations by HPLC (Figure 184D).1007 Interestingly, the chiral recognition ability of APPV-PC was different from that of the corresponding amylose tris(3,5dimethylphenylcarbamate) (ADMPC), which has no cavity inside the helix to accommodate a PPV molecule.1008 ADMPC is well-known as one of the most popular commercially available CSPs,1009−1012 and some racemic diamides and sulfoxide compounds (614−619 in Figure 184E) were resolved on APPV-PC better than ADMPC, suggesting that the helical structures of APPV-PC and ADMPC are different from each other in spite of the fact that both of the amylose hydroxy groups were completely converted to the identical 3,5dimethylphenylcarbamates.

The helical pitch and handedness (right- or left-handed helix) are key structural information on helical polymers, which closely and directly correlate with their functions. Therefore, numerous efforts have been made to developing versatile methods to determine their exact helical structures, including the helical pitch and absolute helical handedness. The helical structures of synthetic helical polymers have often been investigated by conventional electronic CD in solution and XRD of oriented films, which generally provide useful information about their helical conformations, in particular when combined with the established exciton-coupled CD method1013 or theoretical calculations, and about the helical pitch when they form an liquid crystalline phase based on their main-chain stiffness, respectively.4 However, these methods may not be straightforward to unambiguously determine their helical handedness. The details of such studies have been previously reviewed by Yashima et al.4,1014 In this section, remarkable progress using VCD and scanning probe microscopy (SPM) that have been used for this purpose will be mainly described. In contrast to electronic CD, the recently developed VCD technique has a great advantage such that it provides characteristic signals for both chromophoric and nonchromophoric helical polymers with an optical activity, and more importantly, their VCD spectra can be calculated based on the available DFT. Taking advantage of this high reliability of the DFT-based theoretical calculations of VCD spectra, Novak et al. determined the predominant helical sense of helical polyguanidines (620−622) (Chart 42) obtained by the helix-senseselective polymerization of the corresponding achiral monomers with a chiral catalyst by comparing the observed VCD spectra with the calculated ones.1015−1017 Based on a similar methodology, the preferred-handed helical structures as well as the handedness of the poly(phenyl isocyanide) with a 13924


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Scheme 2. Asymmetric Reactions Using Preferred-Handed Helical Polymers as Catalysts

macromolecular helicity memory (436)673 and those of a variety of other optically active helical polymers, such as poly(alkyl isocyanide)s (623 and 624),1018,1019 an aromatic oligoamide bearing a chiral end group (625),1020 oligo(mphenylurea) (626),1021 perfluoroalkyls bearing a chiral end group (627 and 628),1022 poly(L-lactide) (629),1023 stPMMA,871 and it-/st-PMMA SC obtained through the helixsense-selective supramolecular inclusion of it-PMMA within the helical cavity of an optically active st-PMMA,872 have been determined (Chart 42). In addition, the helix sense of an optically active PTrMA and its chiral propeller conformation of the pendant triphenylmethyl groups have been successfully determined by Raman optical activity along with VCD coupled with their DFT calculations.1024,1025 5.6.1. Microscopic Observations of Helical Polymers. SPM, such as STM and AFM, has been now extensively used for the direct observations of the helical structures of biological and synthetic helical polymers, which allow us to determine their helical structures, including the helical pitch and helical sense.

5.6.1.1. Microscopic Observations of DNA. Since the discovery of the DNA double-helix,2 many researchers have devoted much effort to observe the high-resolution doublehelical structures of DNA by SPM because of its biological importance, sufficiently large helical pitch, as well as welldefined right-handed helical structure. Although the first attempts to visualize the DNA double-helix by STM on a HOPG substrate were unsuccessful,1026−1028 recent remarkable progress in SPM instrumentation techniques has enabled the direct observations of the DNA helical structures along with their sequences. Typical STM and AFM images of an isolated single DNA duplex chain mostly measured in liquid are shown in Figure 185. The right-handed double-helical structures of the B-form DNA (B-DNA) with the helical pitch of 3.4 nm have been visualized by STM (Figure 185A)1029 and high-resolution AFM (Figure 185B and C).1030,1031 The major and minor grooves of the B-DNA double-helix and individual guanine bases along the single-stranded DNA molecules have also been observed and sequenced by high-resolution AFM (Figure 185D and E)1032,1033 and STM (Figure 185F),1034 respectively. 13925


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Figure 214. Palladium-catalyzed asymmetric reactions using helicity-switchable PQXs (713a−713g) as ligands. (Reproduced with permission from ref 1117. Copyright 2010 American Chemical Society.)

Figure 215. (A) Asymmetric Henry reaction of 4-nitrobenzaldehyde with nitromethane catalyzed by 651b and its helical and nonhelical PPAs bearing cinchona alkaloid pendants poly-651b before (poly-651b) and after (poly-651b′) cis-to-trans-isomerization. (B) CD and absorption spectra of poly-651b before (poly-651b, blue line) and after (poly-651b′, red line) grinding for 20 min measured in chloroform/CF3CH2OH (6/1, v/v, 0.02 mg/mL) at 25 °C. (Reproduced with permission from ref 1126. Copyright 2012 American Chemical Society.) 13926


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Scheme 3. Asymmetric Reduction of Ketimine Catalyzed by 714

5.6.2. Microscopic Observations of Synthetic Helical Polymers. The direct observations of synthetic helical polymers by SPM have the potential as a promising method to determine their absolute handedness, and have been extensively studied. However, the fact that the helical pitch of most synthetic helical polymers is smaller than that of the DNA double-helix suggests difficulty in visualizing the helical structures at the molecular level. Although there is a report on an STM observation of the helical structure of an optically active PPA on HOPG (630) (Figure 186),1035 the direct observations of helical structures by AFM are more practically useful because most polymer chains are nonconductive. 5.6.2.1. Polyacetylenes. The AFM images of chiral PPAs bearing optically active pendants (631 and (R)-632)1036,1037 and an achiral PPA complexed with optically active molecules (633),1038 deposited from a dilute solution on a substrate, showed extended right- or left-handed twisted polymer chains with a long helical pitch (Figure 187). The observed helical pitches (10−20 nm) estimated from the AFM images are one order longer than those calculated based on the computergenerated models. This is probably because each single helical PPA chain further twists into the coiled-coil or superhelix (Figure 187C) or self-assembles to form a multistranded superhelix on the substrate, leading to a longer helical pitch than expected. Therefore, the observed AFM images may not provide a real structure of the helical polymers because of unfavorable interactions with substrates as well as the broadening effect of the tip, although the predominant helical sense of certain optically active helical polymers can be roughly differentiated by observing isolated single chains by AFM. The first AFM observations of the detailed helical structures including the helical pitch and handedness have been achieved when the helical PPAs bearing chiral L- or D-Ala residues with a long n-decyl chain as the pendants (634-L and 634-D) are spincast on HOPG upon exposure to organic solvent vapors, such as benzene,1039 resulting in the formation of self-assembled well-ordered 2D crystals, which enables one to visualize the helical structures with a molecular resolution. The resulting regular 2D crystals of the polymer chains have significant advantages not only for their robustness against the damage by the tip during high-resolution AFM measurements, but also the flat surface suitable for optimizing the AFM settings for highresolution imaging. The helical 634-L and 634-D spontaneously formed a highly ordered flat monolayer (first layer) (Figure 188A),1039 on which amorphous polymers (second layer) were deposited and further self-assembled into 2D helix bundles with an almost constant height after subsequent exposure to benzene vapor (Figure 188B). The high resolution AFM images of 634-L and 634-D showed clear periodic oblique

stripes tilted counterclockwise or clockwise, respectively, with respect to the axes of the main-chains (blue and green lines in Figure 188C), which suggests that 634-L and 634-D have mirror-image left- and right-handed helical structures, respectively, with respect to the pendant arrangements. The XRD measurements of uniaxially oriented films of 634s prepared based on their lyotropic liquid crystallinity revealed that the 634s possess an 115 helix with a half pitch of the pendant helical arrangement of 2.33 nm, which is in good agreement with those estimated by the AFM images: 2.34 ± 0.21 nm for 634-L and 2.47 ± 0.18 nm for 634-D. All of these data combined with the computer modeling conclude that 634-L and 634-D have left- and right-handed helical arrays with respect to the pendant arrangements, respectively, while the main-chains have the opposite right- and left-handed helical structures, respectively (Figure 188C). Thus, the exact helical structures including their helical pitch and handedness have been, for the first time, unambiguously identified based on the high-resolution AFM images and XRD analysis. Interestingly, the Cotton effect signs of 634s in nonpolar solvents, such as benzene, inverted to the opposite signs in polar solvents, such as THF, suggesting the solvent-dependent inversion of the macromolecular helicity, as frequently observed in optically active dynamic helical polymers including DNA and polypeptides (see section 5.1.3). The direct evidence for this solvent-dependent helix inversion of 634-D has also been visualized by the high-resolution AFM images upon exposure to each solvent vapor during the sample preparations, producing diastereomeric helices with an opposite helix sense to each other (Figures 188Cb and 189A).1040 In addition, the lefthanded helix of 634-D induced by THF can be further switched to the opposite right-handed helix by subsequent benzene vapor exposure (Figure 189B), thus providing the first 2D switchable chiral surface. The 2D helix bundle or crystal formation of helical polymers under an organic solvent atmosphere via hierarchical supramolecular self-assembly on a substrate has been proved to be a versatile method and successfully applied to other varieties of helical polymers, such as an optically inactive PPA, optically active copolymers of phenylacetylenes and poly(phenyl isocyanide)s, and PMMA SC (see Figure 166). Dynamic helical polymers showing no optical activity have been considered to consist of an equal mixture of interconvertible right- and left-handed helical segments separated by a few helical reversals, as experimentally and theoretically revealed for helical polyisocyanates by Green et al.22,24 This unique and key structural feature of dynamic helical polymers has been for the first time proven by the direct visualization of the helical structure of an optically inactive 13927


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Figure 216. Asymmetric aldol and Diels−Alder reactions catalyzed by 715, 716, 718, 721, and 717, respectively. (Reproduced with permission from refs 1135 and 1136. Copyright 2013 and 2014 Wiley-VCH.)

dynamic helical PPA bearing achiral α-aminoisobutyric acid (Aib) residues with the same n-decyl chain as the pendants (635) by high-resolution AFM. As shown in Figure 190, enantiomeric right- and left-handed helical block segments (red and blue colors, respectively) as well as helical reversals between them (white arrows) in individual polymer chains are clearly observed, which can be differentiated from a gap along with helical blocks of opposite handedness (yellow arrows in Figure 190) based on the height profiles (Figure 190A and B).1041 A statistical analysis of a series of high-resolution AFM images of 635 revealed that helical reversals appear only once every 287 monomer units (ca. 60 nm) on average. Based on the number-average length, the free energy difference between the

helical reversal states (ΔGr) of 635 has been estimated to be ca. 14 kJ/mol. In the same way, the helical structures of a series of copolymers of chiral and achiral (Aib) phenylacetylenes (636, Figure 191A) and nonracemic phenylacetylenes (637, Figure 191B) as well as a mixture of chiral and achiral homopolymers of phenylacetylenes (634 and 635, Figure 191C) have been investigated by AFM, and their helix sense and one-handedness excess (eeh) have been estimated by the high-resolution AFM, which provides particularly important information on the hierarchical amplification of the macromolecular helicity (sergeants and soldiers and majority rule effects) of covalent and noncovalent helical polymer systems in a dilute solution, 13928


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Figure 217. (A) Oxidative kinetic resolution of rac-439 using peptoid oligomers bearing TEMPO groups (722 and 723) as catalysts. (B) Energyminimized structures of 439 docked in the cleft of peptoid 722S as viewed perpendicular to the helix axis (a) and down the helix axis with the Nterminus projecting forward (b). Structure of 439 approaching the catalytic TEMPO site of the sterically encumbered scaffold of peptoid 723 (c). The ovals represent the reaction site comprising a hydroxyl group of 439 and a nitroxyl radical of TEMPO group. (Reproduced with permission from ref 1137. Copyright 2009 National Academy of Sciences.)

liquid crystalline state (blue triangle).1042 Thus, the macromolecular helicity of the dynamic helical 636 consisting of chiral and achiral units is hierarchically amplified in the 2D crystals over in the cholesteric liquid crystalline state (sergeants and soldiers effect). A similar hierarchical amplification of the helical sense excess in the cholesteric liquid crystalline state and 2D crystals over that in solution has also been observed for helical 637 bearing nonracemic D- and L-Ala residues as the pendants (majority rule effect) (Figure 191B).1043 However, 637 exhibited amplification of the helicity at almost the same level in the liquid crystalline state and in the 2D crystals (Figure 191B). In sharp contrast, the mixtures of the corresponding homopolymers (634 and 635) showed almost no chiral amplification both in the liquid crystalline state and in 2D crystals (Figure 191C).1042 This means that the chirality transfer from 634 to the neighboring dynamically racemic helical 635 strands hardly occurred even in the 2D crystals. As described in section 5.1.3.1, Riguera et al. reported a series of intriguing PPAs (poly(R)-410, poly(R)-412, and poly(R)638) showing significant chiral amplification, helicity inversion, and helical conformational changes originating from the conformational changes of the chiral pendants in response to coordination of the mono- or divalent metal cations or variation in the solvent polarity and donor ability. The high-resolution AFM observations combined with the MM calculation results of these PPAs also have afforded important information on their helical conformations, including the helical pitch and helical sense, which are in good agreement with the results obtained from their CD analyses (Figure 192A− C).644,647,648,1044 The high-resolution AFM images of the 2Dmonolayer of poly(R)-410 on HOPG prepared by the Langmuir−Shaefer deposition technique showed the formation of separate macroscopically enantiomeric domains of right- and left-handed helical chains (helix-sense-selective packing) (Figure 192Dj) as well as enantiomeric superhelices (Figure 192Dk and l).1045 Moreover, SEM observations together with

Table 7. Oxidative Kinetic Resolutions of 439 Catalyzed by 722 and 723a peptoid

conversion (C)b (%)

selectivityc (%)

ee (%)

Sd

722S 722R 723

84 85 56

60 (S) 59 (R) ∼52 (R)

>99 (R) >99 (S) ∼5 (S)

5.6 5.4 1.1

a

All reactions were performed with a molar ratio of [catalyst]/[439] = 1/100. Data were taken from ref 1137. bConversion values are based on 2 h of reaction time. cSelectivity at the quoted conversion value is defined as (% preferred enantiomer/% conversion). dS is the enantioselectivity coefficient, as defined by S = ln(1 − C)(1−ee)/ ln[(1 − C)(1 + ee)].

Scheme 4. Asymmetric Addition of Methyllithium or Ethylmagnesium Bromide to Aldehydes in the Presence of 725

lyotropic cholesteric liquid crystalline state, and 2D crystals on HOPG.1042,1043 As shown in Figure 191Ab, even when the chiral unit content (r) of the copolymers of 636 is 0.5, the eeh value in the 2D crystals is greater than 99% (Figure 191Aa), demonstrating a greater excess of a single-handed helix (red circle in Figure 191Ab) over those in the cholesteric liquid crystalline state, as estimated on the basis of the cholesteric helical pitch in the 13929


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Figure 218. (A) Asymmetric oxidations of sulfides catalyzed by 526 and 726. (B) Reversible interconversion between nonfluorescent film a and fluorescent film b. Film a consists of nonfluorescent 727a and 526 immobilized in a PVC film, which becomes fluorescent film b upon exposure to primary and secondary amine vapors due to the formations of fluorescent 727b and nonfluorescent 4a-amine-adduct (728), which is further converted to the nonfluorescent film a by treatment with aqueous HClO4. Photographs of film a and film b (R1R2NH = 2-propylamine) under visible (left) and UV light (λex = 365 nm, right) are also shown. (C) Photographs of film 526 under visible and UV light (λex = 365 nm) during exposure to the saturated (S)- or (R)-397/diisopropyl ether vapors at about 25 °C. (Reproduced with permission from ref 1141. Copyright 2014 Wiley-VCH.)

structures of poly(phenyl isocyanide)s have also been determined by high-resolution AFM. The polymerization of an enantiomerically pure phenyl isocyanide bearing an L-Ala residue with a long n-decyl chain as the pendant (L-639) using NiCl2 as a catalyst produced diastereomeric right- and left-handed helical polyisocyanides when polymerized in different solvents at different temperatures (Figure 193).1048 Poly-L-639a obtained in toluene at 100

the CD measurements of the complexes of poly(R)-410 with metal cations revealed the generation of macroscopically chiral supramolecular nanospheres, nanotubes, toroids, and gels, which could be controlled by the solvent and metal cations (Figure 192Be−g).1046,1047 5.6.2.2. Polyisocyanides. By applying the 2D crystal formation technique upon solvent vapor exposure, the helical 13930


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differentiating; terminology recommended by IUPAC)206,605,606 and helix-sense-selective way (Figure 195).1050 Although poly-L-639c and poly-L-639d are composed of the same L-639 units, the left-handed poly-L-639c and righthanded poly-L-639d preferentially copolymerized L-639 and D639 by a factor of 6.4−7.7 and 4.0, respectively. Highresolution AFM observations together with the CD spectra of the block copolymers clearly indicated that the helical handedness of the poly-L-639c and poly-L-639d used as the macroinitiators determined the overall helical sense of the block copolymers regardless of the chirality of the monomer units of the macroinitiators; in other words, the macroinitiators polymerize L-639 and D-639 in an almost perfect helix-senseselective fashion to produce block copolymers while maintaining their helical handedness (Figure 195). The 2D crystal formation of helical polymers on a substrate is also useful for visualizing not only the helical structures of helical polymers, but also helical polymer-based supramolecular architectures by high-resolution AFM (Figures 196 and 197). The left-handed helical rigid rod−rod diblock polyisocyanides with a narrow molecular weight distribution consisting of L-Ala-bound phenyl isocyanides bearing short n-hexyl (640) and long n-tetradecyl chains (641) as the pendants (poly(640nb-641m)), which were prepared by the two-step living polymerization using the Pt−Pd catalyst, self-assemble to form the nanometer-scaled bilayer smectic-like ordering on HOPG (Figure 196).1051 High-resolution AFM observations of poly(640107-b-64192) showed unique 2D smectic-like bilayer structures with different height segments in which the poly(640107-b-64192) molecules mainly formed head-to-head (H−H) supramolecular arrangements (Figure 196B). The poly(640202-b-641184) block copolymer with longer molecular segments also formed similar 2D helix bundles, but the smectic H−H ordering was not favored for poly(640202-b-641184) (Figure 196C), probably due to the difficulty in an effective packing for the 2D smectic H−H assemblies due to the longer polymer chains. The chain packing in an amorphous polyisocyanide monolayer of 639a deposited on mica at a surface pressure of 1 mN/m has also been for the first time visualized by highresolution AFM, which reveals that the long polymer chains are packed in the 2D film without any chain stacking, but partially forced to adopt unusual hairpin-like conformations (Figure 197Aa and b).1052 Periodic oblique stripes resulting from predominantly left-handed helical arrays of the pendants of individual polymer chains (yellow lines) together with minor right-handed ones (pink) were also clearly observed (Figure 197Ac). A rigid-rod helical L-Ala-bound poly(phenyl isocyanide) with a narrow molecular weight distribution bearing rigid mesogenic biphenyl pendants (642) has been found to form a lattice-like new smectic (lat-Sm) liquid crystalline phase in a concentrated solution to show a fan-shaped texture (Figure 197Bf).1053 The AFM image of the thin films of 642 on HOPG showed a 2DSm-like assembly of the polymer chains with a controlled spacing in which the layer structure was nearly perpendicular to the polymer backbone axis (Figure 197Bd). Based on the results of AFM combined with X-ray scattering measurements, it was concluded that the tilted smectic layer structure of the helical backbone aligns perpendicular to the layer plane of the mesogenic pendants, which arrange in an antiparallel overlapping interdigitated manner (Figure 197Be).

Scheme 5. Asymmetric Benzylation of NDiphenylmethylidene Gly tert-Butyl Ester Catalyzed by 729

Chart 49

°C showed a negative first Cotton effect (Δεfirst = −11) after further annealing in toluene at 100 °C for 6 days and selfassembled to form regular 2D helix bundles upon spin-casting on HOPG. The AMF image revealed that poly-L-639a most consists of a left-handed helical structure (Figure 193A). In sharp contrast, poly-L-639b prepared in CCl4 at ambient temperature exhibited a positive first Cotton effect (Δεfirst = +8.1) and its AFM image of slightly irregular 2D helix bundles showed that poly-L-639b is composed of major right-handed helical segments as well as minor left-handed helical ones (Figure 193B). These results suggest that the predominant helical sense can be controlled during the polymerization of a single enantiomeric phenyl isocyanide (L-639) under either kinetic (poly-L-639b) or thermodynamic control (poly-L-639a) assisted by hydrogen bonds, which appear to be hampered at high temperature. In addition, poly(phenyl isocyanide)s showing a positive first Cotton effect sign are unambiguously assigned to a right-handed helix. The polymerization of the same L-Ala-bound isocyanide monomer L-639 using the living Pt−Pd catalyst simultaneously afforded diastereomeric right-handed (poly-L-639d) and lefthanded (poly-L-639c) helical polyisocyanides with different molecular weights and narrow molecular weight distributions (Figure 194).1049 Interestingly, solvent fractionation with acetone can separate its mixture into each single-handed, rodlike helical polyisocyanide with a controlled length and handedness. The isolated polymers self-assemble to form 2D smectic layer-like helix-bundle structures on HOPG, and their helical structures can be directly visualized by high-resolution AFM (Figure 194A and B). The AFM images combined with the XRD results suggest that both right- and left-handed helical poly-L-639d and poly-L-639c possess a stiff 154 helix with a helical pitch of ca. 1.3 nm. The isolated poly-L-639c and poly-L-639d maintain their living feature and can be used as the macroinitiators for further block copolymerization of L-639 and D-639 that proceeds in a living and highly enantiomer-selective (asymmetric enantiomer13931


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Figure 219. Structures of optically active π-conjugated polymers showing CPL in the molecularly dispersed, powdered, or film states. Poly-401 shows CPL in the presence of (S)- or (R)-limonene (446). A possible structure for the SPG−563 inclusion complex is also shown. The absolute glum values are also shown. (Reproduced with permission from ref 938. Copyright 2014 Wiley-VCH.)

2D helix bundles with a constant height of ∼1.8 nm on HOPG after deposition from a dilute chloroform solution followed by chloroform vapor exposure. Periodic oblique stripes with a pitch of ca. 1.0 nm with a chain−chain spacing of ca. 1.5 nm were observed along the main-chain (yellow lines) originating from the helical array of the pendants. The observed helical pitch and chain−chain distance estimated by AFM are in good agreement with those observed by XRD measurements (1.0 nm and 1.6−1.8 nm, respectively). However, the observed helical pitch was too short to unambiguously determine the absolute helical handedness (Figure 198). Although the preferred-handed helix formation of oligo(mphenylene ethynylene) foldamers (172, 225, and 226) has been extensively studied in the past decade (see section 3),6,7 their helical structures, especially helical senses, still remain unknown. Most of the optically active oligo(m-phenylene ethynylene)s exhibit bisignated CD signals, but which are not straightforward to unambiguously determine the helical sense.315 Recently, amphiphilic poly(m-phenylene ethynylene)s bearing an L- or D-Ala pendant with a tri(ethylene glycol) chain ((S)-645 or (R)-645) (Figure 199) are found to adopt remarkably stable, preferred-handed helical conformations in a variety of polar and nonpolar solvents stabilized by

Chart 50

An optically active star polymer bearing rigid-rod helical polyisocyanide arms (643) has been prepared based on the “arm-first” approach by cross-linking of the living left-handed helical L-Ala-bound poly(phenyl isocyanide) (poly-L-639c) (Figure 197C).1054 The star-shaped “spiny” structure of 643, including the number and length of the arms and its helical sense, has been directly observed by high-resolution AFM (Figure 197Cg). 5.6.2.3. Other Helical Polymers. The helical structure of a poly(triarylmethyl methacrylamide) (644) has been for the first time postulated by XRD measurements of the oriented films prepared from its liquid crystalline state in a concentrated solution (Figure 198).1055 644 self-assembles into well-defined 13932


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Figure 220. Structures of optically active (736−745) and optically inactive (447 and 746−751) σ- or π-conjugated polymers showing CPLs after the formation of self-assembled supramolecular helical aggregates. 447 and 746−751 show CPL in the presence of optically active additives (446 and 752). The absolute glum values are also shown.

The AFM image of (S)-645 deposited on HOPG from a dilute chloroform solution revealed a bilayer structure as observed for helical PPAs; a regular monolayer consisting of nonhelical (S)-645 chains with a planar conformation (an average height of 0.4 nm (first layer)) was first formed on HOPG, on which the (S)-645 chains self-assembled to form regular 2D helix bundles with a constant height of 2.6 nm (Figure 199A). The observed periodic oblique stripes indicate that (S)-645 and (R)-645 have enantiomeric right- and left-

intramolecular hydrogen bonding networks in the pendant amide residues, thus showing an intense Cotton effect.1056 Because of its exceptional stability, the helical conformation is retained in the solid state, which made it possible for the first time to determine the helical structures of poly(m-phenylene ethynylene) foldamers including the helical pitch and absolute handedness by high-resolution AFM observations combined with XRD measurements. 13933


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helical arrangements of the molecular model (1.05 nm) constructed on the basis of the XRD structural analyses (Figure 199B (right) and C (right)). Interestingly, in addition to the 2D-assembled helix-bundle structures, cyclic polymers with a similar height of ca. 0.4 nm to that of the nonhelical first layer, which could be inevitably generated during the polycondensation reaction of the monomers followed by the termination, were also observed during the high-resolution AFM observations (Figure 199D). The 2D helix-bundle formation on a substrate has recently been proved to be versatile for visualizing the helical structures of supramolecular helical assemblies based on small molecules (135) and those after inversion of the helicity by highresolution AFM (Figure 51).260

Scheme 6. Reversible Photoisomerization-Enforced Switching of 753 between Emission and Quenching of Circularly Polarized Fluorescence

5.7. Functions of Helical Polymers and Their Assemblies

A variety of synthetic helical polymers with a controlled helical handedness have been prepared to mimic helical biomacromolecules, as already mentioned. So far, tremendous attempts have been performed for developing helical polymer-based advanced materials inspired by the sophisticated functions of biological helices. In this section, practical applications of optically active helical polymers associated with the chiral recognition, asymmetric catalysis, and CPL, together with miscellaneous applications as innovative chiral materials for bio- and nanotechnologies, are described with a focus on recent progress. 5.7.1. Chiral Recognition. 5.7.1.1. Chiral Stationary Phase. Optically active PTrMA and 646 (Chart 43) with a one-handed helical structure synthesized by the helix-senseselective anionic polymerization show a high resolution ability for a wide range of racemic compounds when used as a CSP for HPLC.603,1057−1060 The recognition abilities of PTrMA and 646 are attributed to their rigid helical structures with the chiral propeller triarylmethyl pendants, and these CSPs coated on a silica gel are commercially available. The characteristic chiral recognition due to their helical chirality has also been reported for helical polyisocyanides (647 and 648) and PPAs (649 and 650).602,1061−1064 In addition, the phenylcarbamate and benzoate derivatives of natural polysaccharides, such as cellulose and amylose, with a one-handed helical conformation developed by Okamoto et al., display excellent chiral resolving abilities, and various kinds of polysaccharide-based CSPs have also been commercially available. These pioneering studies on the helical polysaccharide-based CSPs have been extensively reviewed elsewhere.1012 Since the one-handed helical structure appears to play an important role in achieving the high resolution abilities as CSPs for HPLC, a variety of helical polymer-based CSPs have been developed. Optically active helical PPAs bearing cinchona alkaloid pendants (poly-651 and poly-652) have also been applied as CSPs, and their chiral recognition abilities were investigated (Figure 200).1065 The CD spectral patterns of poly-651a and poly-651b were almost mirror images of poly652a and poly-652b, respectively, due to the pseudoenantiomeric relationships with each other, indicating an opposite helical sense induced by the amide-linked cinchona alkaloid pendants. Among these helical PPAs, poly-651a and poly-652a exhibited much higher resolution abilities for a variety of racemates, particularly for chiral alcohols, metal tris(acetylacetonato)s, and N-Boc-amino acids. Reversals of the elution orders were observed for the enantiomers resolved on the pseudoenantiomeric poly-651a and poly-652a, although

Chart 51

Figure 221. Difference reflectance spectra of 757 after solvent vapor annealing by using mixed solvents of 1,2-dichloroethane (1,2-DCE) and chloroform. Photographs of the annealed polymer films were shown in the inset. Scale bar = 1.0 mm. (Reproduced with permission from ref 1167. Copyright 2014 American Chemical Society.)

handed helical structures with respect to the pendant arrangements, respectively (blue lines in Figure 199B (left) and C (left)). The helical pitch obtained from the AFM images (1.03 nm) is in good agreement with those of the pendant 13934


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Scheme 7. Synthesis of Guanidinium Functionalized Polycarbodiimides through the Click Reaction

a

Reproduced with permission from ref 1168. Copyright 2012 American Chemical Society.

Chart 52

DMF. These results suggest that the optically active pendants play a dominant role in chiral recognition and the helix sense of the 654a backbone may not be an important factor for its chiral recognition, although the helical conformation likely contributes to arranging the chiral pendants in a one-handed helical array. Analogous helical PPAs bearing L-phenylglycinol and its phenylcarbamate derivatives as pendants (656 and 657) have also been prepared.1069,1070 The resolution abilities of 657 are significantly influenced by the substituents on the phenylcarbamate groups when used as a CSP, and 657e with the 3,5dichloro substituents exhibits a higher recognition ability than the others and can achieve complete resolutions of several enantiomers, including 658 (Figure 200B). This is probably due to the more acidic N−H protons of the 3,5dichlorophenylcarbamate residues, which likely act as the attractive interaction sites with the racemates through hydrogen bonding. Both diastereomeric left (M)- and right (P)-handed helical polyisocyanides with a different molecular weight and a narrow polydispersity have been synthesized from a single enantiomer of a phenyl isocyanide bearing an L-Ala residue (L-639) by the helix-sense-controlled living polymerization using the μethynediyl Pt−Pd catalyst (659),1071 followed by acetone

their recognition abilities were slightly different from each other. An important factor of the amide linkages to achieve a high enantioselectivity was revealed by the fact that the esterlinked cinchonine-bound helical poly-653 showed a poor chiral recognition. In addition, trans-enriched nonhelical poly-652a′ prepared from the corresponding cis-poly-652a by grinding has a much lower chiral recognition than the helical cis-poly-652a, indicating that the chiral recognition ability significantly relies on the macromolecular helicity induced by the optically active alkaloid pendants. Stereoregular PPAs bearing various L-amino acid ethyl ester pendants (654 and 655) can also be used as CSPs, showing different resolution abilities dependent on the structures of the pendant units.1066−1068 Interestingly, their chiral recognition abilities were significantly affected by the coating solvents because the helical conformations of PPAs are dynamic and significantly change due to the solvent polarity. Among these PPAs, 654a with an amide linkage exhibited a superior chiral recognition when coated with a mixed solvent containing methanol and chloroform.1066 The Cotton effect signs of 654a in a methanol/chloroform mixture were opposite to those in DMF, whereas the elution orders of the enantiomers on the 654a-based CSP coated with a methanol/chloroform mixture remained unchanged when compared with those coated with 13935


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Figure 222. (A) Synthesis of poly(isocyanopeptide)s bearing oligo(ethylene glycol) units as the pendant groups. (B) Schematic illustration of the intramolecular hydrogen bond formation between the amide groups in side chains n and (n + 4) of helical poly-766. (C) AFM image of isolated polymer chains of poly-766c, spin-coated from an organic solution on mica. (D) AFM image of a monolayer of bundles of the poly-766c gel transferred to mica. (E) Statistical height histograms of both isolated chains (pink) and bundles (blue). (F) Stiffness of the poly-766c gel versus stiffness of the constituent poly-766c polymer. Main panel: open circles (E) show the plateau modulus G0 of poly-766c as a function of the single chain persistence length, lp,0(T) at T = 10−60 °C and c = 1 mg/mL; filled squares (B) show G0 values of poly-766c calculated using ξ = lc at the same temperatures and using the bundle number N (average number of polymer chains per bundle) = 9.1, where ξ and lc represent the mesh size and the average length between the cross-links between bundles, respectively. The colored lines represent general trends, which can be used to correlate lp,0 to G0 at a set concentration and given N = 1−64. The dotted line at G0 = 1 kPa is shown for reference; it shows that 1-kPa gels can be prepared from a very stiff single polymer chain as well as from much more flexible, tightly bundled polymers (large N). The inset shows the variation of the calculated critical stress σc with lp,0, which is independent of N. The open circles (E) are experimental data points obtained at T = 30, 40, and 50 °C. The corresponding calculated points (filled squares (B)) overlap with the trend lines of N = 1−64. (Reproduced with permission from ref 1174. Copyright 2013 Nature Publishing Group.)

optically inactive 436 by the noncovalent ‘‘helicity induction and memory strategy’’ (see section 5.1.5 and Figure 127B).608 The optically active polymers due to the helicity memory were coated or immobilized on A-silica, which is the first demonstration of the CSPs with a macromolecular helicity memory (Figure 202).1073 The P-671-based CSP showed an excellent resolution ability among the CSPs prepared (671 and 672) and efficiently separated a wide range of racemates, such as ether (661), ketones (662, 663, and 673), metal tris(acetylacetonato)s (668−670), amine (674), and alcohol (675). The elution orders of the enantiomers resolved on the immobilized CSPs prepared from P- and M-672 are dependent on their helical senses, because the optical activities of these polymers bearing achiral pendant groups are totally derived from the excess one-handed helicity memory. From a practical viewpoint, the elution order of the enantiomers often becomes an important concern in chiral HPLC. In the case of the analytical resolutions, the minor enantiomer should elute first to improve both the limit of detection and the accuracy of the quantification. For the preparative enantioseparations, however, the desired enantiomer can be obtained with a higher optical purity if it is eluted first because a part of the first-eluting enantiomer tends to

fractionation (Figure 201).1049 These one-handed helical polyisocyanides (P- and M-poly-L-639) maintain their living features and work as a macroinitiator for the further block copolymerizations of isocyanides.1050 Taking advantage of this living polymerization system, the one-handed helical polyisocyanides (P- and M-poly-L-639-b-660) have been immobilized onto 3-aminopropyl-silanized silica gel (A-silica) to investigate their resolution abilities as CSPs.1072 The M-polyL-639-b-660-based CSP can resolve some racemates, such as ether (661), ketones (662 and 663), dianilides (614, 665, 666), and dibenzamide (616), whereas the P-poly-L-639-b-660based CSP exhibited a rather complementary chiral recognition and efficiently separated the enantiomers of metal tris(acetylacetonato)s (668−670), which were hardly separated on the former CSP. A cyclic dibenzamide (664) and Fe(acac)3 (667) were not resolved on both the CSPs. Interestingly, reversals of the elution orders were observed for some enantiomers resolved on these CSPs. These results suggest that the helical structure of the polyisocyanides rather than the chiral L-Ala pendants plays a major role in the enantioseparation. A series of preferred-handed helical poly(phenyl isocyanide)s with achiral benzanilide pendants have also been prepared from 13936


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Chart 53

Figure 223. Schematic illustration of enantioselective rotaxane formations of α-CyD toward L- and D-771. (Reproduced with permission from ref 1178. Copyright 2007 American Chemical Society.)

Taking advantage of the remarkable feature of poly-438 capable of the helicity induction, memory, and inversion in the solid state as mentioned in section 5.1.5, the unique CSP, whose helical chirality can be switchable in the column at will, has been developed and the reversible switching of the elution order of enantiomers has been achieved for the first time (Figure 203A).675 The as-prepared optically inactive poly-438 was first coated on silica gel, which was packed into a column. The obtained column was then treated with an acetone solution containing (R)-439 followed by washing with methanol to produce the CSP composed of poly-438 with a right-handed helicity memory (P-poly-438). As shown in Figure 203Ba, the P-poly-438-based CSP can almost completely resolve the 50% ee ((−)-isomer rich) of 661 with a separation factor (α) of 1.11, in which the minor (+)-enantiomer eluted first followed by the major, (−)-, one. When the P-poly-438-based CSP was further treated with (S)-439 in the column, the helicity of poly438 was inverted from the right- to left-handed helix, and thereby the M-poly-438-based CSP with an opposite helicity

Figure 224. Photographs showing the (S)- and (R)-773 droplets on the (S)-772 and (R)-772 films after 30 s. (Reproduced with permission from ref 1179. Copyright 2013 American Chemical Society.)

overlap with the second one. Therefore, control of the elution order is significantly required, particularly when the two peaks derived from a pair of enantiomers are close to each other. 13937


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5.7.1.4. Enantioselective Crystallization. An optically active poly(N-propargylamide) bearing (R)- or (S)-10-camphorsulfonic acid residues ((R)- or (S)-poly-691, Chart 45) and its composite with silica gel have been used as a chiral additive during crystallization of rac-Ala in aqueous solution, giving crystals rich in D- or L-Ala, respectively.1084−1086 Analogous chiral composites composed of graphene oxide or Fe3O4 were also prepared by emulsion copolymerization of (R)- or (S)-692 with the corresponding alkynylated oxide materials for the enantioselective crystallization of rac-amino acids.1087−1089 5.7.1.5. Chiral Discrimination. As described in section 5.1.3, one of the most characteristic features of dynamic helical polymers is inversion of the helicity, which frequently takes place by changing the temperature and/or solvent polarity as well as by other achiral stimuli, but the helix-sense inversion regulated by external chiral stimuli was not reported before 1998, in spite of its potential for developing a novel method to sense chirality based on the helix inversion. The first example of a helix inversion assisted chiral discrimination was reported for a PPA bearing an optically active (1R,2S)-norephedrin residue (693, Chart 46), which showed an inversion of helicity as supported by inversion of the Cotton effects in the presence of specific enantiomers of chiral acids, such as (R)-mandelic acid and its derivatives (35, 200, and 694−696), through diastereomeric, noncovalent acid−base interactions.1090 The dynamic helical PPAs bearing β-CyD residues as the pendants (463a and 463b) can also respond to the chirality of enantiomers, resulting in the inversion of the macromolecular helicity (Figure 209A). Their chiral recognition abilities are highly dependent on the size of the CyDs as well as the linkers between the CyD units and the phenyl rings.1091,1092 For instance, the helix sense of the amide-linked PPA bearing the βCyD pendants (463a) (see Figure 136) became inverted in the presence of the (S)- or (S)-rich (50% ee) chiral amine 397, as supported by inversion of the Cotton effect signs. Interestingly, the helix-sense inversion is accompanied by a significant visible color change from yellow-orange to red due to a change in the twist angle between the conjugated double bonds (tunable helical pitch) (Figure 209B), while the CD and absorption spectra of 463a hardly changed with (R)- and (R)-rich (50% ee) 397, thus providing a unique colorimetric chirality sensing system. In contrast, the amide-linked PPAs bearing α- and γCyD residues as the pendants (697b and 697c) and the etherlinked β-CyD-bound PPA (697a) showed almost no difference in their solution colors in the presence of (S)- and (R)-397 under the same experimental conditions, although 697b and 697c exhibited thermo- and solvatochromism accompanied by the inversion of the Cotton effect signs.714,1092 Quite interestingly, β-CyD-bound 463a and 463b with amide and ester linkages, respectively, exhibited a unique enantioselective gelation in response to the chirality of 397 (Figure 209C), and the polymers further hierarchically self-assembled into micrometer-scale superhelical aggregates with a controlled helical handedness as described in section 5.2.1.714 The optically active PPA containing an L-Pro pendant (698) possesses a predominantly one-handed helical conformation biased by the chirality of the L-Pro residues and forms a lyotropic cholesteric liquid crystalline phase in ca. 40 wt % water solution (Figure 210).1093 The water-soluble 698 can enantioselectively interact with 677 within its hydrophobic cavity in water and underwent a helix inversion in response to the chirality of one of the enantiomers ((R)-677). The helical PPAs bearing L- or D-Val residues as the pendant groups (699,

memory resolved the enantiomers of 661 with virtually the same retention factor (k1) and α values, but with a reversed elution order (Figure 203Bb). A similar helicity induction and its memory was possible for an analogous polymer poly-676 bearing ester groups (Figure 204),1074 and the resulting CSP showed a better chiral recognition ability than poly-438 and resolved several racemic compounds, including axially chiral compounds (677−679) and metal tris(acetylacetonato)s (668−670), probably due to an effective interaction between the ester groups of poly-676 and the enantiomers. 5.7.1.2. Enantioselective Adsorption. The stereoregular PAs (680 in Figure 205A) bearing optically active [6]helicene pendants directly linked to the polymer backbone form a preferred-handed helical conformation biased by the chirality of the helicene residues.1075 These helicene-bound P- and M-680 show a high chiral recognition ability as an enantioselective adsorbent, and can selectively adsorb one of the enantiomers of the 1,10-binaphthyl derivatives (677 and 681) mainly due to the one-handed helical array of the chiral helicene pendants (Figure 205B), suggesting its potential as a novel helical polymer-based CSP. Helical poly(N-propargylamide)s bearing optically active pendant groups (682−686, Chart 44), such as camphanic acid, abietic acid, and cholesterol residues, have also been used for the enantioselective adsorption of a chiral amine (397), an alcohol (687), and amino acids (Ala, Phe, and Trp) in various material forms, including microspheres, hydrogels, and polymer/gold composites.1076−1079 Moreover, the magnetic microspheres composed of a one-handed helical 683 and Fe3O4 nanoparticles showed a chiral recognition ability toward 397 as well as recyclability with the aid of an external magnetic field (Figure 206).1080 As described in section 5.6, the copolymerization of the lefthanded helical living polyisocyanide (M-poly-L-639) with a bifunctional cross-linker provides a unique optically active star polymer (Figure 197).1054 The resulting star polymer (643) exhibits an enantioselective adsorption toward not only small molecular racemates with a stereogenic center (661, 663, and 665), but also a racemic helical polyisocyanide without any chiral elements except for a macromolecular helicity (688) (Figure 207). Interestingly, the star polymer tends to exhibit a higher chiral recognition than the arm polyisocyanide, indicating that a chiral nanospace confined in the star polymer by the self-assembly of the helical arms may play an important role in its enantioselective adsorption. 5.7.1.3. Enantioselective Permeation. Optically active helical PPAs (poly-398a and poly-689 in Figure 208A) bearing two hydroxymethyl groups on the phenyl residues prepared by the helix-sense-selective polymerization of the corresponding achiral monomers using a Rh-based chiral catalyst exhibited an enantioselective permeability toward amino acids, such as Trp and Phe.1081,1082 Compared with the membrane of poly-398a bearing the dodecyloxy groups, the poly-689 membrane containing the trisiloxanyl groups showed a much higher enantioselectivity (PD/PL; PD and PL are the permeability coefficients of D- and L-isomers, respectively) probably due to the flexibility, hydrophobicity, and bulkiness of the oligosiloxane chains.1082 Interestingly, a poly-690b membrane prepared from its precursor, the optically active poly-690a-based membrane, by in situ removal of the optically active pinanyl pendants maintained its enantioselective permeability,1083 although its enantioselectivity decreased, while the permeability increased four times (Figure 208B). 13938


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for the racemic 705 falls between those for the two enantiomers in both devices. 5.7.2. Asymmetric Catalysis. Asymmetric catalysts derived from optically active polymers generally contain small-molecule-based chiral ligands covalently linked or noncovalently immobilized to achiral polymers, resulting in polymer-based asymmetric catalysts, whose enantioselectivities are solely dependent on the introduced chiral ligands, while the polymers are just used as supports.1104−1107 However, if the polymer supports could adopt a preferred-handed helical conformation, one would be able to expect a kind of synergistic effect on the enantioselectivity in combination with the helical chirality, affording a more powerful asymmetric catalyst than the original chiral ligand itself. Another promising approach for providing polymer-based asymmetric catalysts is to use onehanded helical polymers whose optical activities are totally due to an excess one-handed helicity as a novel scaffold or template, along which achiral but catalytically active ligands arrange in a one-handed screw-sense, thus showing a specific asymmetric catalytic activity. In fact, the static helical polymethacrylates bearing one or two pyridyl groups with a controlled handedness developed by Okamoto590,593 have been for the first time utilized as a chiral ligand (706a and 706b) for the catalytic asymmetric C−C bond forming reaction, producing products up to 60% ee (Scheme 2).1108,1109 A pyridine N-oxide-bound polymethacrylate (706d) has also been reported as an asymmetric organocatalyst for the allylation reaction of benzaldehyde.1110 A preferred-handed helical polyisocyanide (707) bearing an achiral piperazine unit as a catalytic active site prepared according to the “helicity induction and memory” strategy followed by modification of the side groups (Scheme 2)1111 (see also section 5.1.5) showed an enantioselective organocatalytic activity toward the direct aldol reaction between aldehydes and ketones. Although its enantioselectivity was not satisfactory (up to 12% ee), the fact that both the achiral secondary amine and carboxylic acid residues arranged in a onehanded helical array along the polymer backbone with the macromolecular helicity memory plays a key role in the enantioselectivity in a concerted manner will provide a new design strategy for developing further useful helical polymerbased asymmetric catalysts. Not only static helical polymers, but also dynamic helical polymers with an excess one-handedness, such as polyacetylenes and polyisocyanates, have been used as a scaffold for the helical polymer-based asymmetric catalysts. For this purpose, optically active or inactive monomers bearing a catalytic active site or a metal-binding site have been homopolymerized (708 and 710)1112,1113 or copolymerized with optically inactive monomers using a chiral catalyst (709) or with optically active monomers (711),1109,1114 respectively (Scheme 2), although their enantioselectivities were moderate (14.5−49% ee). A series of optically active helical PPAs (712a−e) bearing various oligopeptide pendants consisting of L-Ala/achiral Gly as a catalytic site have been synthesized to investigate their organocatalytic activities for the asymmetric epoxidation of chalcone (Scheme 2).1115 All the PPAs catalyzed the asymmetric epoxidation to produce the epoxide of 21−38% ee, and 712e showed the highest enantioselectivity (38% ee). Importantly, the corresponding monomers provided almost no enantioselectivity (250-mers) containing sergeant and soldier units in a 18:82 molar ratio have almost the pure, but opposite P- or M-helical conformation, respectively. Introduction of a small amount of metal-binding phosphino pendant groups into the random and block copolymers produced chiral macromolecular ligands, which then produced the (S)- or (R)-products with 94% ee for the hydrosilylation of β-methylstyrene (Figure 214). In addition, a purely single-handed helicity induction in a PQX 713g consisting of nonracemic (R,R)- and (S,S)-repeating units together with a meso one in 38.8:15.8:45.4 ratio and a small amount of an achiral phosphorus-containing unit has been achieved.1124 The resulting one-handed helical 713g has been used as a highly enantioselective chiral ligand in asymmetric hydrosilylation and Suzuki−Miyaura cross-coupling reactions, in which the enantioselectivity can also be switched through the solvent-triggered helicity inversion (Figure 214). A series of helical PPAs bearing cinchona alkaloid pendants, including poly-651 and poly-652, have been used as the polymeric organocatalysts for the asymmetric conjugate additions, desymmetrizations of prochiral cyclic anhydride, and Henry reactions.1125−1127 Among them, poly-651b, bearing the quinine residues through an amide linkage, exhibits a high level of enantioselectivity toward the asymmetric Henry reaction of 4-nitrobenzaldehyde with nitromethane, forming a product up to 94% ee (Figure 215A), which is much higher than that catalyzed by the monomeric counterpart, 651b (28% ee).1126 The trans-enriched poly-651b′, prepared by grinding the as-prepared poly-651b, which almost lost the CD signals in the polymer backbone regions, as shown in Figure 215B, suggesting the lack of an induced helical conformation, showed a poor enantioselectivity (18% ee). These results unambiguously indicate the importance of the helical chirality of poly651b in the expression of an excellent enantioselectivity. L-Val- or N-methyl-L-Val-bound PPAs with a predominantly right-handed helical structure (714) also catalyzed the asymmetric reduction of aromatic ketimines to produce optically active amines (Scheme 3).1128 The absolute configurations of the major products were mostly opposite to those obtained with the corresponding monomeric catalysts, demonstrating the competitive chirality induction between the optically active pendants and the induced helical structures of the polymer backbones. An optically active helical PPA carrying L-Pro moieties as the pendants (715a) exhibits an organocatalytic activity toward the asymmetric aldol reactions of cyclohexanone with p-nitrobenzaldehyde, thus providing an anti-aldol product with 64% ee (Figure 216); this enantioselectivity was higher than that catalyzed by L-Pro (40% ee).1129 N-Propargylamide-based copolymers bearing L-Pro and (S)-camphanic acid residues (715b) also effectively catalyzed the asymmetric aldol reaction to give the anti-product in 80% ee.1130 A synergistic effect between the induced helical conformation of 715b and the optically active Pro pendants seems to play a role in the asymmetric reaction. Analogous poly(N-propargylamide)s bearing optically active thiourea or quinine moieties as a catalytic site have also been applied to asymmetric organocatalysis.1131,1132 Recently, Wan and co-workers have synthe-

preferred-handed helical array, thus showing the enantioselective organocatalytic activities. Recently, helical polymer-based asymmetric catalysts demonstrating an excellent enantioselectivity of more than 90% ee have been successfully developed by Suginome and co-workers, who synthesized a series of preferred-handed helical PQXs (713a−c) bearing optically active 2-butoxymethyl groups together with metal-binding phosphino pendant groups as a polymeric ligand for several palladium-catalyzed asymmetric reactions (Figure 214).1116−1122 Interestingly, these enantioselective catalytic reactions are solely promoted due to their onehanded helical conformations with a high helix-sense excess of the polymer main-chains. In addition to the easily reusable feature of the chiral polymeric catalysts, the solvent-induced reversible helix inversion of the polymer backbones described in section 5.1.3 has made it possible to switch the chirality (R or S) of the products in asymmetric catalysis, while maintaining its high enantioselectivity. For example, the P-helical 713a recovered from its chloroform or toluene solution exhibited an excellent enantioselectivity (97% ee) for the (S)-product during the asymmetric hydrosilylation of styrene (Figure 214).1117 When the P-helical polymer was dissolved in a 1,1,2-trichloroethane/ toluene mixed solvent (2/1, v/v), gradual helical inversion took place and the almost pure M-helical 713a was obtained after heating at 60 °C for 6 h, which catalyzed the same asymmetric hydrosilylation of styrene in the mixed solvent, thus producing the opposite (R)-product of 93% ee. Helical 713b and 713c bearing 3,5-xylyl and 2-naphthyl groups on the phosphorus atom, respectively, also catalyzed the asymmetric silaborative cleavage reactions of meso-methylenecyclopropanes1119 and Suzuki−Miyaura coupling reactions of arylboronic acids with aryl bromides,1118,1123 respectively (Figure 214), giving both enantiomers of the products with remarkably high enantioselectivities (>90% ee) in specific solvents using a single catalyst, in which the helical senses of 713b and 713c can be totally switched. A one-handed helical copolymer (713d) containing (S)-1(pentoxycarbonyl)ethoxymethyl groups derived from natural Llactic acid and diphenylphosphino pendants has also been developed as a chiral ligand for the asymmetric Suzuki− Miyaura coupling (Figure 214).652 713d showed an almost complete solvent-induced helix inversion between 1,2-DME (M-helix) and MTBE (P-helix), allowing the asymmetric synthesis of either an (R)- or (S)-product with a high enantioselectivity (>91% ee). A very unique solvent-dependent helix inversion has also been observed for an analogous polymer (713e) bearing (S)-3octyloxymethyl side chains and diphenylphosphino pendants.653 Surprisingly, linear alkanes possessing higher molecular aspect ratios, such as n-octane, induced an M-helical structure, whereas branched or cyclic alkanes with lower molecular aspect ratios, such as cyclooctane, produced the opposite P-helical structure. Thus, this solvent-induced helical inversion between n-octane and cyclooctane has been utilized again to switch the enantioselectivity in the asymmetric hydrosilylation of styrene to produce the (R)- and (S)-products of 94 and 90% ee in noctane and cyclooctane, respectively (Figure 214). As described in section 5.1.3.2, the sergeants and soldiers effect has been observed for PQX-based chiral/achiral copolymers (713f) bearing achiral butoxy soldier units upon changing the mole fraction of the (S)-3-octyloxymethyl sergeant units.654 This chiral amplification has been, for the 13940


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in 723 is crowded (Figure 217Bc) and precludes such π−π stacking interactions with the substrate when the reactive alcohol is adjacent to the TEMPO unit, leading to a lower enantioselectivity coefficient (S = 1.1) compared to those catalyzed by 722 (S = 5.4−5.6) (Table 7). Yang and co-workers have found that an optically active poly[(R)- or (S)-3-(9-alkylfluoren-9-yl)propene oxide] (725) can catalyze the enantioselective addition of methyllithium or ethylmagnesium bromide to aldehydes (Scheme 4).1138,1139 Specifically, the addition of methyllithium to 4-methoxybenzaldehyde in the presence of 725 provided a relatively high enantioselectivity (88% ee).1138 The helical cavity of 725, in which the metal cations could be encapsulated, resulting in the generation of the alkyl anions as nucleophiles, is proposed to be responsible for the present high enantioselectivity. 725 can be readily recovered from the reaction system, and the recycled 725 can be repeatedly utilized without decreasing the enantioselectivity. An optically active polymer consisting of cationic 5ethylriboflavinium residues as the main-chain (526) has been prepared from naturally occurring riboflavin (vitamin B2) in four steps (see Figure 163).832 The cationic 5-ethylriboflavinium units can be reversibly converted into the corresponding 4a-hydroxyriboflavins via oxidation and reduction cycles. As described in section 5.4.1, the cationic 526 formed a supramolecularly twisted, duplex-like helical structure with a preferred-handedness through face-to-face stackings between the intermolecular riboflavinium units. The optically active 526 can efficiently catalyze the H 2O 2-mediated asymmetric oxidation of sulfides, yielding optically active sulfoxides up to 60% ee, which is much higher than the enantioselectivity (30% ee) of the corresponding monomeric counterpart 726 (Figure 218A). This significant improvement in the enantioselectivity of 526 probably results from the πstacked twisted helical structure with an excess one-handedness induced by the optically active ribityl pendants, which most likely assists in the generation of the oxidatively active intermediate, 4a-hydroperoxyflavins, with a high diastereoselectivity.1140 The π-conjugated cationic 526 is nonfluorescent, but highly reactive toward nucleophiles, such as primary and secondary amines, which enables direct visual detection of the amine vapors through the 4a-amine adduct formations in the film.832 In order to develop a fluorogenic sensor for chiral and achiral amine vapors using the nonfluorescent 526, the fluorescence quenching ability of 526 toward fluorescent riboflavins, such as tetraacetyl riboflavin (727a), has been utilized, while the 526 is readily converted into the nonfluorescent 4a-amine adducts (728) with a nonquenching capacity upon exposure to amine vapors in the film state (Figure 218B).1141 In fact, the combination of a protonated riboflavin (727a) and 526 immobilized in a polyvinyl chloride (PVC) film as a fluorophore precursor and a specific amine receptor, respectively, provides a versatile turn-on type fluorogenic sensory system to chiral and achiral primary and secondary amines. For example, a PVC film exposed to saturated vapors of a mixture of (S)- or (R)-397 and diisopropyl ether showed a visible color change (Figure 218C) and the time-dependent photographs taken under visible and UV light clearly indicate that the 4a-amine-adduct formations occurred enantioselectively after 24 h, and (S)-397 preferentially reacted over the antipode (R)-397; thus, the fluorogenic detection of chirality of the amine vapors has been achieved for the first time.

sized optically active poly(3-vinylpyridine) derivatives with LPro (716) and pyridineoxazoline (717) pendants and demonstrated that the 716- and 717-based catalysts showed a moderate enantioselectivity (up to 62% ee) for the asymmetric aldol and Diels−Alder reactions, respectively, in which a helical conformation with an excess one-handedness induced in 716 and 717 has been proposed to contribute to the observed enantioselectivity.1133,1134 Meijer and co-workers have designed and synthesized a novel methacrylate-based ternary random copolymer (718), which consists of L-Pro units as enantioselective catalytic sites, optically active BTA units as structuring elements, and oligo(ethylene glycol) units to ensure water-compatibility, and they found that 718 exhibits a catalytic activity in an enzyme-like fashion in water (Figure 216).1135 The exquisite combination of the self-assembly of BTAs into helical stacks with the hydrophobicity of the 718 backbone allows folding of the copolymer into a single-chain-based polymeric nanoparticle (SCPN) in water. The excess left-handed helical aggregate formation of the S-BTA pendants in the SCPN was revealed by a CD spectral analysis. The conformationally adaptive hydrophobic environment around the L-Pro units provides the SCPN with an efficient catalytic activity for an asymmetric aldol reaction between p-nitrobenzaldehyde and cyclohexanone in water, thus producing a product up to 74% ee, while 718 hardly catalyzes the aldol reaction in chloroform, in which the copolymer cannot fold into helical stacks. An analogous SCPN with catalytic activity has also been prepared by mixing an achiral, amphiphilic binary random copolymer (719) with no catalytic sites and a “free” BTA containing the catalytic active L-Pro unit in water (720).1136 Noncovalent supramolecular helical assembly between the hydrophobic chiral and achiral BTA units of 720 and 719, respectively, results in the formation of a unique 719-based SCPN in water in which the optically active 720 molecules are encapsulated in the hydrophobic core. The resulting supramolecular nanoparticle (721) shows an excellent enantioselectivity (up to 98% ee) toward the asymmetric aldol reaction with a good conversion even at low catalyst loadings and substrate concentrations due to the effective shielding of the catalytic active L-Pro moieties from the aqueous environment. Although 720 itself catalyzed the aldol reaction with a comparable enantioselectivity probably due to self-stacking in water, the catalytic activity was significantly lower than that of the 721. Kirshenbaum and co-workers have synthesized a series of helical peptoid oligomers (722 and 723) bearing a 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) group as a catalytic site and used them as asymmetric catalysts for the oxidative kinetic resolution of rac-439 (Figure 217A and Table 7).1137 The enantioselectivity of 722 and 723 seems to be governed by the one-handed helical structure of the peptoid scaffolds, the position of the catalytic group in the peptoid backbone, and the conformational regularity of the peptoids. The energyminimized structures of 722 and 723 provide the rational explanation of the differences in the enantioselectivity of the peptoids (Figure 217B). A sterically accessible reaction groove exists in the model of 722S, in which the substrate 439 can be complexed with 722S through π−π stacking interactions and the reactive (S)-439 can be favorably located within 3 Å of the terminal TEMPO unit in the chiral groove (Figure 217Ba and b) and, thus, selectively oxidized to give 724. On the other hand, the environment around the TEMPO unit at the middle 13941


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biased by the chiral pendants and optically active additives (446 and 752), respectively, are summarized in Figure 220. Among these helical aggregates, an optically active fluorene-based copolymer bearing bulky (1S)-neomenthyl groups (737) emits a very intense CPL upon thermal annealing, and its glum value reached the order of 10−1 at a maximum.1154 PDPAs (736) bearing optically active (R or S) and racemic alkoxy pendants also show a remarkable CPL in a lyotropic chiral nematic liquid crystalline phase in the absence and presence of 498 as a chiral dopant, respectively.1152,1153 Helical polysilanes with optically active side groups (741−743) display an efficient CPL emission once helically aggregated in a specific good/poor solvent mixture.1150,1151 In particular, 741 aggregates showed an intense CPL with a glum value up to −0.7 with a high quantum efficiency over 50% by tuning its molecular weight. Akagi and co-workers synthesized a series of photoresponsive, optically active poly(p-phenylene)s (753a) and poly(bithienylene-phenylene)s (753b) to develop novel photoresponsive CPL materials (Scheme 6).1163 The polymer films showed a right- or left-handed CPL due to preferred-handed, helically π-stacked polymer backbones induced by the chirality introduced at the terminal end of the photoresponsive dithienylethene units, which can be switched (quenching and emitting) by the reversible photocyclization of the dithienylethene units upon alternate UV and visible light irradiations, respectively. Cholesteric liquid crystals can selectively reflect either a rightor left-handed CP light when the wavelength (λ0) matches the pitch (p) of the cholesteric helical structure, such that λ0 = npcos θ, where n is the average refractive index and θ is the angle between the incident light and the cholesteric helical axis.1164 Therefore, when the cholesteric pitch falls within a range of visible light wavelengths, the selective reflection of CP light gives rise to characteristic structural colors as frequently observed in nature. Such selective reflection of CP light-based chiral materials can be applied to the development of electronic and photonic devices.1165 For example, thermally annealed and aligned films of polyfluorene (754, Chart 51) and two other fluorene copolymers (755 and 756) bearing optically active alkyl chains preferentially reflect the left-handed CP light in the visible region due to their left-handed cholesteric arrangements with a helical pitch length comparable to the visible light wavelength.1166 Taking full advantage of a unique solvent-dependent helix inversion of helical PQXs bearing optically active pendants in solution, Suginome and co-workers have developed a novel switchable film for selective reflection of either a right- or lefthanded CP light using a quinoxaline-2,3-diyl-based optically active copolymer (757 in Figure 221).1167 The 757 film annealed in chloroform vapor selectively reflected the righthanded CP light in the visible region, whereas the left-handed CP light was reflected after annealing it in 1,2-dichloroethane vapor. In addition to the handedness of the reflected CP light, its wavelength range can also be tuned by changing the volume ratio of a chloroform/1,2-dichloroethane mixture for solvent vapor annealing of the 757 film. 5.7.4. Miscellaneous Applications. Cationic and amphiphilic polycarbodiimides carrying bioactive guanidinium pendants (760a−d and 761) have been synthesized from 758a−d and 759, respectively, by a click reaction to examine their antibacterial activities (Scheme 7).1168 The hydrophobic/ hydrophilic balance of the polymers has a significant effect on both the antimicrobial and hemolytic activities, and all the

The optically active ionic polymers composed of chiral and achiral quaternary ammonium sulfonate moieties (729a−d) have been employed as a chiral catalyst for the asymmetric benzylation of a Gly derivative, giving (S)-Phe derivatives up to 94% ee when catalyzed by 729b and 729d (Scheme 5).1142 The ionic polymers may possess a helical structure, although the structures have not yet been elucidated. 5.7.3. Circularly Polarized Luminescence. CPL has attracted much interest not only due to its potential to obtain chiral structural information in the excited state, but also due to the practical applications in photonic devices, such as 3D displays and optical information storage. Since the CPL emission from conjugated polymers was first reported using a polythiophene derivative bearing optically active side chains (730, Chart 49) by Meijer et al.,1143 various kinds of CPL materials derived from helical polymers and helical assemblies of polymers have been developed.1144 Optically active π-conjugated polymers showing a CPL derived from a preferred-handed macromolecular helicity in the molecularly dispersed, powdered, or film state are summarized in Figure 219. Networked polysilynes are composed of chromophoric and fluorophoric Si−Si σ-conjugated backbones; therefore, a polysilyne bearing optically active β-branched alkyl side chains (732) showed an apparent CPL emission at around 570 nm in THF, resulting from a predominantly twisted helical Si−Si-bonded network structure induced by the chiral pendants.1145 An optically active poly(2,7-bis(4-tert-butylphenyl)dibenzofulvene) carrying L-menthyl residues (733) exhibits a white CPL emission in the film arising from the one-handed helical array of the pendant dibenzofulvene units along the polymer backbone.1146 An optically active poly(m-phenylene) (734) also shows a CPL signal in the annealed film, in which the polymer chains with either a right- or left-handed helicene-like helical conformation self-assemble to form a hexagonal columnar structure.1147 PDPAs a class of highly luminescent π-conjugated polymers; for example, 731 bearing perfluoroalkyl groups (Chart 50) is the first luminescent PDPA with a green emission.1148 Fujiki and co-workers found that a CPL can be induced in an achiral PDPA bearing trimethylsilyl pendant groups (poly-401) when dissolved in (R)- or (S)-limonene (446), thus showing a CPL emission at around 520 nm, and its absolute magnitude of the glum value reached ca. 0.6 × 10−3 (excited at 383 nm), which was similar to that of the absolute CD magnitude gabs (Δε/ε) at 383 nm (ca. 0.38 × 10−3).677 The axially chiral structure between the polymer backbone and the side phenyl rings of poly-401 has been proposed for the appearance of CPL in 446. The SPG/560 and SPG/563 complexes also show a CPL in solution and/or in the powdered state.935,938 Computational modeling has revealed that a helical structure was induced in a single 563 chain during a triple-stranded cohelical complex formation with two SPG chains (Figure 219). A one-handed helical PQX bearing (S)-2-methylbutoxy pendants (735) exhibits a blue CPL emission in solution in which 735 can switch the CPL handedness through a solventinduced helix inversion between chloroform and 1,1,1trichloroethane.1149 Optically active (736−745)1150−1155 and optically inactive (447 and 746−751)1156−1162 σ- or π-conjugated polymers showing CPLs after the formation of self-assembled supramolecular helical aggregates with a controlled handedness 13942


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Figure 225. (A) TEM image of helical tubes formed from a DNA tile system carrying 5 nm AuNPs. (Reproduced with permission from ref 1185. Copyright 2009 American Association for the Advancement of Science.) (B) (a) Left- and right-handed nanohelices (diameter 34 nm, helical pitch 57 nm) composed of nine gold nanoparticles (10 nm diameter) attached to the surface of DNA origami 24-helix bundles (each of diameter 16 nm). Each attachment site consists of three 15-nucleotide-long single-stranded extensions of staple oligonucleotides, and AuNPs possess multiple thiolmodified complementary DNA strands. (b) CD spectra of left-handed (red line) and right-handed (blue line) helices composed of nine AuNPs with 16 nm diameter. The insets show TEM images of left-handed (red frame) and right-handed (blue frame) nanohelices (scale bars, 20 nm). (Reproduced with permission from ref 1186. Copyright 2012 Nature Publishing Group.)

Figure 226. (A) Schematic illustration of the formation of multihelical DNA−silica fibers (MHDSFs) and nonhelical DNA−silica fibers (NHDSFs). Cationic ammonium groups of APS interact with phosphate groups of DNA (a), which self-assemble to form a DNA-based liquid crystalline phase (b). During the silica condensation using TEOS, the cholesteric-like crystalline phase of DNA is partitioned into separated fibers by silica walls (c). SEM images of the MHDSFs (d) and NHDSFs (e) synthesized with APS/DNA molar ratios of 6 and 3, respectively. (Reproduced with permission from ref 1187. Copyright 2013 The Royal Society of Chemistry.) (B) SEM image of the DNA−silica complexes synthesized with addition of Mg2+ ions at 0 °C. Left- and right-handed impellers are denoted by + and − , respectively. (Reproduced with permission from ref 1188. Copyright 2012 Wiley-VCH.)

polymers except for 760a showed an antibacterial activity and rapid interaction with red blood cells, leading to blood precipitation without significant hemolysis, which has the potential for polymer coatings or wound protection. Kobayashi and co-workers have prepared a series of saccharide-bound helical PPAs (762, Chart 52) and poly(phenyl isocyanide)s (763) and investigated their binding affinity toward lectins.1169,1170 The results suggest that helical arrays of the pendant saccharides along the polymer backbones contribute to a small extent to specific interactions with lectins due to the lack of flexibility of the main-chain to form multivalent glycoclusters, so that flexible styrene- and acrylamide-based glycopolymers showed much better interactions with lectins. 587 has been reported to show a cytocompatibility with living HeLa cells and assists with the growth of living cells.1171

Kakuchi et al. have also investigated the binding affinities of the analogous glycosylated PPAs (765) derived from 764 toward plant and bacterial lectins by a hemagglutination inhibition assay.1172,1173 765 provides a 2- to 8-fold increase in the binding affinity to lectins per one saccharide unit compared to the corresponding monomeric glyco-ligands. Mechanical responsiveness is indispensable to all living systems, including tissues and cells. The intra- and extracellular mechanics of such biosystems are controlled by a series of proteins that involve microtubules, actin, intermediate filaments, and collagen, which can form helical aggregates and self-assembled superstructures with a differing diameter and persistence length to cover the whole mechanical spectrum. Polyisocyanopeptide hydrogels bearing oligo(ethylene glycol)s as the pendant groups (poly-766, Figure 222A) show unique structural and mechanical properties resembling those of intermediate filaments.1174,1175 Poly-766, with a stiff and helical 13943


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Figure 227. (A) Left-handed twisted nanoribbon formation assembled from C12-L-PEPAu. (Reproduced with permission from ref 1189. Copyright 2008 American Chemical Society.) (B) TEM and electron tomography images of left-handed (a) and right-handed (b) AuNP double helices fabricated from C12-L-PEPAu and C12-D-PEPAu, respectively (scale bars: 20 nm). (C) CD spectra for the left-handed (blue line) and right-handed (red line) AuNP double helices. (Reproduced with permission from ref 1190. Copyright 2013 American Chemical Society.)

architecture stabilized by intramolecular hydrogen bonds (Figure 222B), forms transparent hydrogels upon heating at a concentration as low as 0.006 wt % when the molecular weights of the polymers are greater than ca. 1600 kDa (poly-766c and poly-766e). Bundle formations of the polymer chains via hydrophobic interactions of the thermoresponsive grafted oligo(ethylene glycol) residues are revealed by AFM (Figure 222C−E) and cryo-SEM observations of the gels. An intriguing relationship between the stiffness of the gels and the stiffness of a single polymer chain (Figure 222F) is proposed, which leads to access of an optimal hydrogel with biomimetic cytoskeletal networks based on a fully synthetic helical polymer. Optically active helical PPAs bearing hydrogalvinoxyl pendants (767a and 768a, Chart 53) prepared by the helixsense-selective polymerization were successfully converted into polyradicals (767b and 768b) with a high spin concentration via oxidation reaction, while maintaining their optical activity, which showed an antiferromagnetic interaction stronger than that of the dynamically racemic helical polyradical.1176 Similar helical polyradicals (769b and 770b) showing an antiferromagnetic behavior have also been synthesized from the mphenyleneethynylene-based π-conjugated polymers with galvinoxyl units (769a and 770a).1177 The enantioselective recognition of helical polymers using a small molecular chiral host has been achieved by Ohya and Yui et al.1178 The host α-CyD forms an inclusion complex with a poly(L-LA)-based helical polymer, L-771, upon mixing both components at 170 °C, whereas its enantiomeric counterpart, D-771, hardly forms a complex with α-CyD under the same conditions (Figure 223). Such a remarkable difference in the enantioselective complexation is assumed not simply to be due to the steric hindrance, but presumably due to the difference in the thermodynamic stabilities between the diastereomeric inclusion complexes. Tanaka et al. reported an intriguing enantioselective wetting of an optically active, nonhelical polymethacrylate film (772) bearing (S)- or (R)-chiral alkyl chain bound biphenyl units linked through a flexible alkyl linker (Figure 224).1179 When a drop of a chiral aliphatic diol ((S)-773) was placed on the (S)772 film, the initial contact angle (θ) (63°) hardly changed over time after placement. In sharp contrast, the θ value of a drop of (R)-773 decreased with time and reached a constant value of 41° within 30 s. As expected, an opposite enantioselective wetting was observed for the (R)-772 film. Hydrogen bonds between the carbonyl group in 772 and the hydroxyl group in 773 seem to play an important role in the enantioselective surface discrimination. The enantioselectivity

Figure 228. Schematic illustration of the coassembly approach of CNCs chiral rods and YVO4:Eu3+ NPs. (Reproduced with permission from ref 1191. Copyright 2015 The Royal Society of Chemistry.)

of the polymer surface may improve using one-handed helical polymers, which often show much higher chiral recognitions than the corresponding nonhelical polymers.

6. TEMPLATE-ASSISTED HELICAL ASSEMBLIES As mentioned in the previous section, a large number of helical assemblies of small molecules and helical and nonhelical polymers have been reported, and their helical structures have been observed by various microscopic techniques. This section mainly describes the template-assisted helical assemblies of metals and inorganic compounds, and huge spiral structures and microcoils mostly consisting of inorganic components. 6.1. Helical Arrangements along Biological Helical Polymer and Oligomer Templates

Biological polymers, such as DNA and fiber proteins, have been extensively utilized as the building blocks or templates for the construction of 2D and 3D ordered nanostructures, such as metal nanowires and periodic crystalline lattices by metal deposition onto the templates or precise arrangements of metal NPs using the templates as a molecular glue, respectively, which provide a promising approach for the development of nanoscale electrical devices and biosensors.1180−1184 Taking advantage of the DNA self-assembly based on its complementary base-pairing together with the recently developed DNA tile-mediated self-assembly technique, Yan and co-workers have successfully constructed helical tubes carrying 5 nm gold nanoparticles (AuNPs),1185 which possess a left-handed helical structure for both the single- and doublestranded tubes, as revealed by TEM images (Figure 225A). Liedl and co-workers have taken further advantage of the DNAbased nanostructured origami method rather than the DNA tile assembly and succeeded in controlling the handedness of helical arrays of AuNPs on the rigid DNA origami cylindrical 13944


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Figure 229. (A) Schematic illustration of cross-section of plant tissue in dicotyledoneae. (a) The spiral vessels exist in part of the xylem to support water transportation as the secondary wall of the vessel. (b) Fabrication of a left-handed metal microcoil by a silver coating on a spiral vessel of Nelumbo nucifera rhizome. (Reproduced with permission from ref 1192. Copyright 2011 Wiley-VCH.) (B) Fabrication of Spirulina-templated microcoil (μcoil) and its optical activity against THz wave and SEM images of left-handed (LH) and right-handed (RH) μcoils. (Reproduced with permission from ref 1193. Copyright 2014 Nature Publishing Group.)

nanostructures of 24-helix bundles (Figure 225Ba).1186 TEM images revealed that exactly nine AuNPs were arranged in a left- or right-handed helical array onto the DNA cylinders via a complementary duplex formation between single-stranded oligonucleotides introduced at the specific position of the DNA cylinder surface with a controlled helical geometry. Interestingly, the right- and left-handed helically arrayed plasmonic AuNPs exhibited split-type CD signals in the collective surface plasmon resonance region (Figure 225Bb), which are mirror images of each other, indicating that the onehanded helically arranged AuNPs are indeed optically active. Silver nanoparticles (AgNPs) fabricated by the electroless deposition of silver on the gold helices also showed CD signals with a remarkable enhancement, resulting from the increased plasmon−plasmon interaction of nanostructured silver, and the observed CD spectra were in good agreement with the calculated ones. The double-helical structure of DNA has also been utilized as a template for silica mineralization with APS followed by condensation with TEOS, which produced self-assembled multihelical DNA-silica fibers.1187 Electron microscopic and CD studies revealed that the silica fibers are composed of hierarchically assembled helix bundles with different morphologies that can be controlled by the APS/DNA molar ratio (Figure 226A). Che et al. also found that a minor modification of this DNA-templated silica-mineralization method results in the formation of unique impeller-like helical DNA−silica complexes (Figure 226B).1188

Rosi and co-workers prepared left-handed double-helical AuNPs based on the self-assembly of oligopeptides (C12-LPEP) with an affinity for AuNPs, followed by the peptide-based biomineralization of AuNPs (Figure 227A).1189 Using AFM, TEM, and electron tomographic analyses, a helical array of the nanoparticles was revealed to have a highly regular left-handed double-helical structure (Figure 227Ba). Later, AuNPs-based double helices with an opposite right-handed helical structure (Figure 227Bb) were also prepared by using the peptide template with the opposite handedness (C12-D-PEP).1190 These left- and right-handed superstructures exhibited mirror-image plasmonic CD signals due to the collective dipole−dipole interactions between the AuNPs (Figure 227C). Xu et al. developed a novel chiral nematic luminescent film composed of YVO4:Eu3+ NPs attached to the surface of twisted, rodlike cellulose nanocrystals (CNCs) through hydrogen bonding and van der Waals interactions, which further selfassemble to form a chiral nematic phase (Figure 228).1191 The composite film exhibited a strong CPL with a high glum value. Kamata, Iyoda, and co-workers reported a novel biotemplatedirected synthesis of a huge micrometer-scale metal helix with a controlled helicity (Figure 229A).1192 The helical vessel existing in vascular plants was utilized as a template, and a silver microcoil was produced by an electroless silver coating while preserving the original helical spring shape and helicity of the vessels. The versatile biotemplating electroless process has been further applied to Spirulina, a type of blue-green algae, thus producing 3D helical microcoils. Since the helical handedness 13945


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Figure 230. (A) TEM image of a mixture of right- and left-handed double-helix carbon coils. (Reproduced with permission from ref 1194. Copyright 1999 American Institute of Physics.) (B) SEM image of ZnGa2O4 nanosprings. (Reproduced with permission from ref 1196. Copyright 2005 American Chemical Society.) (C) SEM images of the right- (a) and left-handed (b) double-helical Si microtubes. (Reproduced with permission from ref 1197. Copyright 2010 Wiley-VCH.) (D) SEM image of double-helical Fe3O4-based nanocubes. Inset shows a TEM image of two belts wrapping around each other. (E) Wide-view SEM image of double helices. Yellow and red arrows indicate helical reversals. (F) Wide-view SEM image of double helices. Gray and green colors indicate patches of right- and left-handed helices, respectively. (Reproduced with permission from ref 1198. Copyright 2014 American Association for the Advancement of Science.)

interesting in terms of their chirality and helicity, although they are racemic (Figure 230A).1194,1195 Metal-oxide-based nanosprings have been prepared by the thermal evaporation of a nonhelical, rodlike ZnSe nanowire from the as-prepared ZnGa2O4/ZnSe nanovines, in which the ZnSe nanowire is surrounded by a helical ZnGa2O4 nanowire, producing right- or left-handed ZnGa2O4 nanosprings (Figure 230B).1196 Obviously, the helical handedness cannot be controlled in this process. The first double-helical silicon microtubes with both helicities have been synthesized by heating a Zintl compound NaSi as the starting material (Figure 230C).1197 A similar double-helical superstructure has been quantitatively constructed by the self-assembly of cubic nanocrystals of magnetite, Fe3O4, in the absence of a template at the liquid−air

of the Spirulina microcoil structures can be controlled by the cultivation conditions, an enantiomeric pair of left- and righthanded microcoils has been fabricated with 100% optical purities as supported by SEM images (Figure 229B). A microcoil dispersion sheet showed an intriguing optically active response in the terahertz (THz)-wave region due to its large helical pitch, which provides the potential for developing chiral metamaterials with electromagnetic functions.1193 6.2. Super-Structured Helices and Helical Assemblies

Motojima et al. prepared a series of carbon coils with ordered structures but different morphologies upon the decomposition of acetylene on Ni catalyst particles. Among the carbon coils already prepared, the double-helical carbon coils are particularly 13946


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Figure 231. Self-assembly of CdTe nanoparticles into twisted nanoribbons induced by irradiation with circularly polarized light at 543 nm. SEM images of CdTe nanoribbons assembled with left- (left column) and right-handed circularly polarized light (right column) as a function of time exposure for 1, 12, 28, and 50 h. All scale bars are 1 μm. (Adapted with permission from ref 1199. Copyright 2015 Nature Publishing Group.)

Figure 232. (A) SEM image of a MgF2 sculptured thin film on a glass substrate. The film has 17.3 helical turns of pitch 360 nm. (Reproduced with permission from ref 1200. Copyright 1996 Nature Publishing Group.) (B) Schematic illustration for the fabrication process of 3D arrays of gold helices by direct laser writing and electrochemical deposition. (C) Normal-incidence measured (left column) and calculated (right column) transmittance spectra of freestanding 3D gold helices used as photonic metamaterials (no analyzer behind sample). Left- and right-handed circular polarizations are depicted in red and blue lines, respectively. (a) Slightly less than one pitch of left-handed gold helices, (b) two pitches of left-handed gold helices, and (c) two pitches of right-handed gold helices (see insets). For wavelengths longer than 6.5 μm, the glass substrate in the experiments becomes totally opaque, and therefore, transmittance cannot be measured. For wavelengths below 3 μm, light can be diffracted into the glass substrate (refractive index n = 1.5), giving rise to Wood anomalies. (Reproduced with permission from ref 1202. Copyright 2009 American Association for the Advancement of Science.)

interface in the presence of external magnetic fields (Figure 230D).1198 The computer simulation suggests spontaneous formation of chiral nanocube clusters. Interestingly, symmetry breaking and chiral amplification take place during the assembly process of chiral nanocube clusters to form single helices and further double- and triple helices, leading to the formation of a large domain consisting of one of the helices along with helix reversals (Figure 230E and F). Kotov et al. investigated the effects of circularly polarized light on the assembly of water-soluble NPs under ambient conditions and found that the irradiation of dispersions of racemic CdTe NPs with right- or left-handed circularly polarized light can induce the formation of right- or lefthanded twisted nanoribbons, respectively, with an ee exceeding 30%.1199 The time-dependent chiral transformations of NPs under irradiation of left- or right-handed circularly polarized light revealed the progress of right- or left-handed twisted nanoribbon formations (Figure 231). These results will provide a means to construct chiral photonics materials.

successfully constructed unique chiral-sculptured thin films composed of right- or left-handed MgF2-based nanohelices using the so-called glancing angle deposition method (Figure 232A), which showed an optical activity and can rotate the polarization plane of linearly polarized light when it is incident on a helical-sculptured thin film along the film’s helical axis. The enantiomeric pairs of the sculptured thin films exhibited optical rotations opposite to each other.1200 The sculptured chiral films composed of one-handed MgF2-based helical arrays

6.3. Applications

As already mentioned, metal-based nanostructured helices with a controlled helicity are particularly interesting because of their applications as chiral photonics materials. Robbie et al. 13947


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Figure 233. (A) Schematic diagram of the stable skewed conformation of 774. A space-filling model of the X-ray crystal structure of [Ag·774]NO3 showing a helical spring. (Reproduced with permission from ref 1203. Copyright 2000 American Chemical Society.) (B) Formation of supramolecular coordination polymers 775 and 776 with a thermally switchable helical pitch. (Reproduced with permission from ref 1204. Copyright 2007 American Chemical Society.) (C) Schematic illustration of the helical inversion dynamics of 777 in solution and in the solid state, demonstrating chiral symmetry-breaking in the solid state during crystallization of dynamically inverting helical 777, whose helical conformation becomes more stable after one-electron oxidation. (Reproduced with permission from ref 1205. Copyright 2011 Nature Publishing Group.)

expressing their motions to macroscopic work and also chiral materials showing switchable chiral recognition and asymmetric catalysis by responding to the chiral motions. In 2000, the first example of a self-assembled molecular spring (Figure 233A) whose helical pitch can be reversibly tuned by incorporating guest anions with a different size was prepared in high yield from a pyridine dimer (774) in the presence of silver ions that coordinate to the pyridyl residues in such a way to form a linear supramolecular metallopolymer [Ag·774]NO3 via the N−Ag(I)−N bond formation.1203 An Xray crystallographic analysis revealed that the polymer framework is an ideal cationic cylindrical helix and that its counteranions are sandwiched between the two columns inside the helix. The helical pitch of [Ag·774]NO3 is reversibly stretched through the counteranion exchange from 7.430(2) to 9.621(2) Å in exact proportion to the volume of the anion guests. This pitch-tuning is caused by subtle changes in the nonrigid dihedral angles between the two pyridyl groups around the O and Ag atoms that act as hinges within the helical subunit.

are porous, in which rodlike nematic liquid crystalline molecules can be embedded to show a chiral nematic phase.1201 Gansel et al. established a method to fabricate 3D arrays of gold helices with a one-handed helix sense using a polymer thin film by 3D direct laser writing and the subsequent electrochemical deposition of gold (Figure 232B).1202 The resulting 3D arrays of the gold helices after removal of the template showed a chiral response in such a way that one of either the right- or left-handed circularly polarized light with the same handedness as the gold helices is selectively blocked, while the opposite-handed circularly polarized light transmits (Figure 232C), thus providing the potential as a compact broadband circular polarizer.

7. MOLECULAR SPRING 7.1. Supramolecular Polymers, Foldamers, and Duplexes

Molecular springs capable of switching the helical pitch (helical springs) in response to external stimuli may undergo mechanical springlike motions that will provide intriguing prospects for constructing molecular nanomachines by 13948


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Figure 235. Structural formula and capped-stick representation of the single-crystal structure of the spiroborate-based helicate 783.

tional change results in switching of the fluorescence driven by the breakup of the intramolecular π−π stacking interactions in the polymer backbone. Aida and co-workers reported o-phenylene-based oligomers (777) that undergo a redox-responsive dynamic motion accompanied by a springlike motion along the helical backbone (Figure 233C).1205 In solution, 777 undergoes a rapid helixinversion and is converted into 777•+ by a one-electron oxidation. 777•+ adopts a more compact conformation and does not interconvert, thereby locking the helix into either a right- or left-handed helical conformation, leading to a longlasting chiroptical memory. Moreover, 777 undergoes a chiral symmetry-breaking phenomenon during recrystallization, affording enantiopure crystals. Crystals of both enantiomeric helices are formed, but not in equal quantities; thus, the overall mixture is not racemic. Takeuchi et al. synthesized a unique macrocycle (778) consisting of two different rotating modules, cerium(IV) bis(porphyrinate)s and ferrocene units, which are connected through rigid p-phenylene spacers (Figure 234A).1206 Upon cooperative and allosteric binding of (R,R)-779 to a pair of pyridyl moieties, the macrocycle 778 exhibits an ICD derived from the predominantly twisted pyridine pairs, which is accompanied by the intramolecular extension-to-contraction motion along with a decrease in the length between the Fe−Fe centers and the increase in length between the Ce−Ce centers. As described in sections 4.1.1 and 4.2, the synthesis and structures of a series of double helices (198, 266, and 267) (Figure 82A) and triple-helix 347 (Figure 103B) based on aromatic oligoamides have been reported. In these systems, the duplex and triplex formations require a springlike extension of each strand from its single helical conformation to reach double- and triple-helical pitches during the course of the hybridization. The extension is accommodated by an increase

Figure 234. Structural formulas of double-stranded helical springs. (Reproduced with permission from refs 1206−1208. Copyright 2009 Elsevier Ltd., Copyright 2010 Wiley-VCH, and Copyright 2013 The Royal Society of Chemistry, respectively.)

Lee and co-workers reported helical supramolecular coordination polymers (775 and 776) with a switchable pitch in an aqueous solution (Figure 233B).1204 The springlike metallosupramolecular structures showed elementary cylindrical fibers with lengths of several micrometers and a uniform diameter of ca. 6.5 ± 0.4 nm, as revealed by CD, fluorescence, DLS, and TEM experiments. The helical supramolecular springs demonstrated temperature-driven reversible extension and contraction motions. Moreover, this dynamic conforma13949


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Figure 236. Schematic illustration of anisotropic twisting motion observed in a right-handed double-helical helicate 784E by removal of the central Na+ ion from 784C using cryptand [2.2.1] (785) followed by the addition of Na+ ion. Space-filling models of X-ray single-crystal structures of the contracted (784C) and extended (784E) forms. (Reproduced with permission from ref 1210. Copyright 2010 Nature Publishing Group.)

in the twist angle between the adjacent rings in each strand.438,518 The triple-helix cavities become slightly narrower than that of the single-helix as a consequence of the springlike extension. Maeda et al. also found the first example of the enantiomerically pure, ZnII-coordinated bidipyrrin-based double helices (780 and 781) that display temperature-driven springlike motions (Figure 234Ba).1207 Variable-temperature CD spectral changes in 2-methyl-THF showed that the CD intensities of 780 and 781 significantly increased at low temperatures, suggesting a change in the double-helical pitches. Similar temperature-dependent springlike motions have also been observed for analogous ZnII-coordinated bidipyrrin-based double helices (782a−d), in which the two bidipyrrin units are linked by flexible alkene strapping (Figure 234Bb).1208 As described in section 4.1.1, the helicates are generally prepared from the oligopyridine-derived molecular strands in the presence of transition metal ions as templates, but those embracing typical metal ions still remain rare. In 2006, the first boron-containing double-stranded helicate was synthesized by the reaction of o-linked hexaphenol with sodium NaBH4, producing a spiroborate-based helicate (783) (Figure 235). The X-ray crystal structure of 783 reveals its unique doublestranded helical structure; the terminal biphenol units of the two strands participate to form spiroborates with the boron atoms, and a Na+ ion is encapsulated within the helicate at the middle stabilized by negatively charged boronates as well as by eight oxygen atoms. Attempts to remove the Na+ ions using crown ethers or cryptands were unsuccessful. The spiroborate helicate is chemically stable, and the racemic helicates can be easily separated into enantiomers by diastereomeric salt formation using an optically active ammonium salt, followed by cation exchange with an achiral ammonium salt.1209

The crystal structure of 783 also indicates that the central biphenol units are not necessary for the spiroborate helicate formation, because the spiroborate bridges are formed with the terminal biphenol units. Thus, a novel spiroborate helicate (784C) composed of two similar hexa(m-phenylene)-based strands that lack the two central hydroxyl groups has been designed and synthesized.1210 In the same way, the enantiomers of 784C can be resolved by a countercation exchange using an optically active ammonium salt. The central Na+ ion in 784C that shields the electrostatic repulsion between the two negatively charged spiroborate groups can be completely removed by the addition of a cryptand[2.2.1] (785) to produce the dianionic helicate (784E) with a remarkably extended helical structure (Figure 236). The X-ray crystallographic and 1 H NMR spectroscopic analyses indicated that the helicate was extended by almost 2-fold and partially unwound by ca. 100°. Furthermore, the addition of NaPF6 to 784E readily regenerates the original contracted 784C, quantitatively accompanied by partial rewinding. The extension−contraction motion could be repeated many times simply by the sequential addition of Na+ ions and 785, leading to a unique anisotropic twisting motion without racemization. Taking advantage of an intriguing feature and potential as intelligent chiral materials of molecular springs that reversibly and unidirectionally twist, novel double-stranded spiroborate helicates bearing functional units in the middle have been synthesized (Figures 237 and 238).1211−1214 Unlike the charged spiroborate helicates (783 and 784), a newly prepared bipyridine-bound spiroborate helicate 786 (Figure 237A) has a neutral feature because 786 accommodates two Na+ ions coordinated in the center of the helicate, as revealed by its Xray crystal structure.1211 Therefore, both enantiomers can be separated by chiral HPLC under normal phase eluent 13950


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Figure 237. (A) Structural formula and capped-stick representation of the single-crystal structure of the spiroborate-based helicate bearing bipyridine units (786). (B) Chemical structures of the helicates bearing 4,4′-linked 2,2′-bipyridine (787) and its N,N′-dioxide (788) units. Schematic illustration of allosteric regulation of proton-assisted reversible extension−contraction motion of 787, in which the cooperative binding and release of protons to the 2,2′-bipyridine units induce anti-to-syn and syn-to-anti conformational changes of the 2,2′-bipyridine units, respectively. (Reproduced with permission from ref 1212. Copyright 2016 American Chemical Society.)

conditions. Since the Na+ ions are strongly bound by the bipyridine nitrogen atoms as well as the spiroborate anions, the bound Na+ ions could not be released in the presence of 785. Thus, an ion-triggered springlike motion could not be achieved. In contrast to the helicate 786, bearing the 6,6′-linked bipyridine units in the middle, an extended helicate 787 with 4,4′-linked bipyridine in the middle instead of the 6,6′-linked bipyridine units and its N,N′-dioxide analogue 788 did not display a high binding affinity for the Na+ ion.1212 Interestingly, the contraction and extension motions of these two helicates could be allosterically regulated by the cooperative binding and release of protons to the two covalently linked bipyridine or its N,N′-dioxide units, respectively (Figure 237B), as revealed by NMR and CD measurements and an X-ray crystallographic analysis in combination with theoretical calculations. This allosteric regulation is reversible while maintaining their onehanded helical chirality during cooperative anti−syn conformational changes of the two bipyridine or N,N′-dioxide moieties. Furthermore, these local conformational changes at the linkages are amplified into a large-scale conformational transition of the overall structure of the helicates, thus leading to reversible

unidirectional springlike motions with a sufficiently largeamplitude conformational change in a highly positive, homotropic allosteric manner (Figure 237B). Similarly, such elastic motions of 787 triggered by the cooperative anti−syn conformational changes of the bipyridine units could also be controlled by the binding and release of Cu2+ ions to the bipyridine units. A double-stranded spiroborate helicate 789 with a bisporphyrin unit in the middle adopts a pseudo-D2 symmetric conformation, in which the two tetraphenol strands are intertwined with each other via two spiroborate bridges.1213 Upon the addition of an electron-deficient aromatic guest 790, the helicate forms an inclusion complex by an induced-fit mechanism. An X-ray crystallographic analysis suggested a sandwich structure in which the guest 790 is included between the bisporphyrin in a parallel fashion via face-to-face stacking interactions, leading to a significant expansion between the porphyrin rings from 4.1 to 6.8 Å. This expansion is accompanied by the unidirectional rotation of the porphyrin rings and, at the same time, unwinding of the spiroborate helix to reduce the twist angle from 368 to 330°, and a further slight 13951


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Figure 238. (A) Chemical structure of the spiroborate-based helicate bearing a bisporphyrin unit in the middle (789). Top and side views of the Xray crystal structures of 789 before (a) and after inclusion complexation with 790 (b) are shown in A. A right-handed double-helical structure is depicted. (Reproduced with permission from ref 1213. Copyright 2013 Wiley-VCH.) (B) Extension (E) and contraction (C) motions of a photoresponsive helicate bearing stilbene units (791). The contracted helicate (cis-791C) including a Na+ ion in its helical cavity extends upon the addition of cryptand [2.2.1] (785) (cis-791E). Further addition of NaPF6 induces the formation of a contracted form. cis-791E is further extended upon cis-to-trans photoisomerization. Optimized structures of right-handed helical cis-791C (monoanion), cis-791E (dianion), and trans-791E calculated by the Hartree−Fock method (6-31G*, CH3CN), in which all the tBu groups are substituted with hydrogen atoms for simplifying the calculations, are also shown. (Reproduced with permission from ref 1214. Copyright 2015 The Royal Society of Chemistry.)

7.2. Helical Polymers

contraction in length to a B−B distance (Figure 238A). As a result, the helicate 789 undergoes a unique guest encapsulationtriggered unidirectional dual rotary and twisting motions resulting from expansion of the bisporphyrin cavity. The control of unidirectional molecular motions by light irradiation is one of the emerging research fields. To this end, a new spiroborate-based double-stranded helicate carrying photoresponsive cis-stilbene units in the middle (cis-791C) has been synthesized (Figure 238B).1214 cis-791 adopts contracted (cis791C) and extended (cis-791E) forms under equilibrium in CD3CN, as revealed by the 1H NMR and NOESY measurements. The contracted cis-791C that accommodated a Na+ ion in the center exhibited extension and contraction motions in an almost reversible manner by the Na+ ion release and binding, respectively. UV irradiation at 295 nm could further induce an extension of the cis-791E via cis-to-trans photoisomerization, affording a mixture of cis,trans-791E (39%) and trans-791E (38%) helicates in the photostationary state. In this system, trans-to-cis photoisomerization of the trans-mixtures upon UV light (360 nm) irradiation was irreversible because of the photocleavage of the trans-stilbene moieties of trans-791E, resulting in the formation of photo-oxidated aldehyde species.

As described in section 5.7.1.5, the optically active helical PPAs bearing CyD units as the pendants, for example, 463a with βCyD pendants, exhibited visible color changes due to a change in their helical pitch arising from a change in the twist angle of the conjugated double bonds triggered by external stimuli, such as temperature (Figure 239A) and solvent compositions (Figure 239B) as well as upon the shape and chirality-selective inclusion complexation with achiral (Figure 239C) and chiral guest molecules (Figure 209) into the chiral β-CyD cavity.1092 Interestingly, these visible color changes are accompanied by an inversion of the helical sense as supported by the inversion of the Cotton effect signs, indicating that these helical PPAs behave like a molecular spring with a switchable helical pitch (extension and contraction) and helical sense that can be readily visible by the naked eye (Figure 239D). Similar stimuli-responsive visible color changes due to a springlike motion in solution are also observed for the PPA homopolymers (654a−b, 792, and 793) (Chart 54A)1215−1218 and 412 (Figure 123C in section 5.1.3.1).647 Tabata and coworkers found solid-state visible color changes for helical PAs (794−798) (Chart 54A) triggered by heat-1219−1221 or immersion-treatment with a solvent1222,1223 that likely induces the main-chain conformational change between the contracted 13952


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Figure 239. (A) CD and absorption spectral changes of 463a in DMSO with temperature; visible differences in color at 25 and 80 °C are shown in insets. (B) Visible color changes of 463a in DMSO/water mixtures at ambient temperatures. (C) Visible color changes of 463a in DMSO/water (8/ 2, v/v) at 25 °C induced by the addition of guest molecules with different shapes or structures (20 equiv to monomer units of the polymers). (Reproduced with permission from ref 1092. Copyright 2006 American Chemical Society.) (D) Schematic illustration of a springlike conformational change of 463a accompanied by a visible color change and inversion of the Cotton effect sign.

cis-cisoid and stretched cis-transoid structures, resulting in the transformation of the helical pitches and columnar crystal structures in the solid-state. An accordion-like helix oscillation along the main-chain axis in solution has been proposed for the aliphatic PA esters (799 and 800).1224,1225 Numata and co-workers have also observed a springlike motion for a semiartificial polysaccharide (801) bearing peripheral amphiphilic Zn-chlorophyll units (Chart 54B).1226 801 adopts a contracted helical conformation in aqueous media driven by the strong intramolecular π−π stackings of its chlorophyll units. Upon the addition of bipyridyl ligands (802− 806), the stacking of the chlorophyll units diassociates into monomeric states due to the strong Zn−N coordination, resulting in the expansion of the helical pitch. Percec and co-workers reported that the self-organizable dendronized helical cis-PPA (807) undergoes thermoreversible cisoid-to-transoid conformational isomerization in the bulk due to the bulky dendronized pendants that eliminate the intramolecular electrocyclization that often takes place in other cis-PPAs (Figure 240A and B).1227 This thermally induced conformational change resulted in the extension and

contraction motions of the individual polymer backbones cylindrically packed in the bulk, as elucidated by XRD measurements of the extruded fibers. The springlike motion of the fiber was demonstrated by the lifting performance of a U.S. dime that weighs 250-times the weight of the fiber (Figure 240C). 7.3. Super-Structured Helices

Stupp et al. have found that an optically active peptide-lipid 808 bearing a trans-azobenzene unit at the terminal self-assembles into superhelical nanofibers with a controlled helicity in cyclohexyl chloride, whose helical pitch can be manipulated by UV light irradiation.1228 The left-handed helical structures with a uniform helical pitch (ca. 78 nm) have been visualized by AFM (Figure 241Aa). Upon irradiation by UV light, the helical pitches of the nanofibers dispersed in cyclohexyl chloride were shortened to 40−70 nm due to an increase in the torsional strain from the trans-azobenzene to the less planar cisazobenzene, while maintaining its handedness (Figure 241Ab). A photochromic diarylethene crystal obtained from 809 has been found to twist into either a right- or left-handed helical structure upon UV light irradiation, accompanied by a color 13953


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Chart 54. Structures of Helical Polyacetylenes Bearing Functional Groups (654 and 792-800) (A) and a Chlorophyll-Bound Curdlan Derivative (801) (B) Showing Springlike Motions

Figure 240. (A) Structure of a thermoresponsible dendronized helical PPA (807). (B) A cis−cisoid (left) to cis−transoid (right) conformational change of the PPA backbone of 807 at low and high temperatures. (C) Visual performance of the oriented fiber that works in concert and lifts a dime on the inclined plane up an 8° of a Mettler hot stage via the macroscopic scale contraction/expansion motion; images were taken at 25 and 80 °C of the oriented fiber. (Reproduced with permission from ref 1227. Copyright 2008 and American Chemical Society.)

reversibly takes place upon alternating irradiation by UV and visible light, which could be repeated for over 30 cycles. In this system, the twisting motion of the crystal was induced by the

change from colorless to blue that is stable in the dark (Figure 241B).1229 Upon visible light irradiation, the twisted crystal relaxes to its original shape in a few seconds. This process 13954


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Figure 241. (A) AFM height images of the helically assembled fibers from photoresponsive 808 (a) before and (b) after exposure to UV light. The helical pitch changes from 78(±6) nm (trans) (a) to 56(±)4 nm (cis) (b). Scale bars: 100 nm. (Reproduced with permission from ref 1228. Copyright 2007 Wiley-VCH.) (B) Photoreversible twisting of the crystal obtained from 809 upon irradiation with (a) UV (λ = 365 nm) and (b) visible light (λ > 500 nm). (Reproduced with permission from ref 1229. Copyright 2013 Wiley-VCH.)

8. CONCLUSIONS AND OUTLOOK This review shows that the research area of helical assemblies in macromolecular and supramolecular helical systems has seen notable progress over the past decades, and a large number of single-stranded helical assemblies as well as multistranded ones with optical activity have been synthesized by the self-assembly of chiral and achiral small molecules, foldamers, and polymers through a variety of noncovalent bonding interactions, thus producing dynamic helical architectures. In addition, this review also briefly summarizes the further progress in the synthesis of helical polymers, their properties, structures, and functions mainly since 2009, when our last review was published. Of particular importance is the concept of dynamic helices first proposed for polyisocyanates that has been further proved to be valuable and applicable to the development of a variety of macromolecular and supramolecular helical systems, in which a

anisotropic shape change of the unit cell upon photoisomerization of the diarylethene molecule. Fletcher, Katsonis, and co-workers reported the photoresponsive chiral materials capable of converting light energy into mechanical work, including the springlike motion on a macroscopic scale. These unique chiral materials are prepared by photopolymerization of an azobenzene-containing monomer (trans-810 and cis-810) and an achiral liquid crystalline monomer as the host matrix in the presence of chiral dopants ((S)-811 and (R)-811) in a glass cell, resulting in liquid crystalline polymer springs doped with 811 after cutting in a specific direction. The molecular motions of these springs induced by reversible cis−trans photoisomerization are converted into reversible large-scale morphological changes, such as winding, unwinding, and helix inversion (Figure 242).1230 13955


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Figure 242. (A) Chemical structures of a photoresponsive azobenzene monomer (trans-810) and chiral dopants 811. (B) Under irradiation with UV light, the ribbons contract along the director and expand in the perpendicular directions, as is consistent with an UV induced increase of disorder. (C) After photopolymerization of trans-810 and an achiral liquid crystalline monomer as the host matrix in the presence of chiral dopants in a glass cell, the dried films were cut into ribbons (width of 0.7−0.9 mm) with a variety of shapes in a direction characterized by the angular offset ϕ, defined as the angle between the orientation of the molecules. The direction is a parameter which determines not only the pitch and handedness of the helical shapes, but also their photoresponsive behavior. Ribbons irradiated for 2 min with UV light (λ = 365 nm) display isochoric winding, unwinding, and helix inversion as dictated by their initial shape and geometry. (Reproduced with permission from ref 1230. Copyright 2014 Nature Publishing Group.)

small chiral bias introduced in the monomeric units or components through covalent or noncovalent bonding formations is significantly amplified with a high cooperativity, resulting in a large helical sense excess of the entire polymer chains and helically assembled supramolecular polymer chains, respectively. The underlying principle for such an intriguing chiral amplification in covalent and noncovalent systems has been theoretically and quantitatively interpreted at the macromolecular and supramolecular levels, which significantly contributes to provide a general method for the developments of the designer helical polymers and helically assembled supramolecular polymers with specific functions.

The variation of structures of helical polymers has been significantly broadened, but they can still be classified as either static or dynamic helical polymers with respect to their helix inversion barriers. Importantly, by taking advantage of the wellestablished helicity induction and memory strategy through noncovalent interactions as well as by introducing bulky substituents on monomers followed by the polymerization, the helical conformations of an increasing number of dynamic helical polymers can be transformed into those of static helical polymers due to an increase in the helix inversion barrier. The former helicity memory effect discovered in dynamic helical polymers has also been extended to dynamic supramolecular 13956


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helical assemblies and helical polymers and foldamers is significantly expanding, the supramolecular helical systems that we have reviewed may be able to contribute to rationally designing new chiral/achiral molecules and helical foldamers and polymers in a rather predictable way to construct multicomponent helical architectures with a controlled helical sense, which will open up new opportunities for the construction of advanced chiral materials with definitive functionalities that may not be achieved by biological systems.

helical systems. The noncovalent helicity induction and subsequent memory of the helicity strategy has also been recognized as a practically useful method to develop a helical polyacetylene-based chiral packing material for the separation of enantiomers in which the elution order can be switched due to its switchable memory effect in the solid state. The control of the handedness of dynamic helical polymers, foldamers, and helical assemblies has, in general, been achieved by using chiral residues at the pendants or terminal ends or chiral additives. It has been demonstrated that either a right- or left-handed CPL can be used to induce a preferred-handed helical conformation in a dynamically racemic helical polymer or a virtually achiral main-chain conjugated polymer in the solid state, which appears to be one of the promising approaches for the helix-sense-selective synthesis of helical polymers with an optical activity. As also shown in this review, remarkable progress has been made since 2009 in developing synthetic double-stranded and multistranded helical polymers and foldamers. In particular, an increasing variation in the homostranded and complementary double-stranded helical foldamers with a controlled handedness and DNA-like sequence information has been developed; some of them form a unique helical cavity suitable for encapsulating specific guests and also for asymmetric catalysis. Apart from the master molecule of life, the double-helical DNA, possessing a “complex and yet elegantly simple” structure, the next challenge will be to find a way to break nature’s monopoly on selfreplication and copy using totally artificial double-helical systems. Impressive progress has also been achieved in applications of helical assemblies and helical polymers and foldamers as chiral materials that involve chiral recognition, enantioseparation as a CSP, enantioselective permeation, and asymmetric catalysis together with chiral sensory systems for bio- and nanotechnologies. It is noteworthy that some helical assemblies and helical polymers and foldamers catalyze asymmetric reactions to produce products with an enantioselectivity of more than 90% ee, which is higher than that catalyzed by their monomeric counterparts. In addition, switching the chirality (R or S) of the products in asymmetric catalysis using a single helical polymeric catalyst composed of a dynamic helical poly(quinoxaline-2,3diyl) has been achieved, in which the solvent-induced helixsense inversion plays a dominant role. Although synthetic helical materials that are in practical use still remain limited to a few helical polymers as a CSP for HPLC, this finding together with the helical polymer-based switchable CSP implies that dynamic helical architectures in synthetic polymers, foldamers, and their assemblies showing unique helix-inversion and/or memory effect combined with the amplification of chirality that are unique characteristic features for dynamic helical systems will provide emerging opportunities for applications as practically useful chiral materials. Nature utilizes multicomponent helical architectures based on hierarchical self-assembly rather than monomeric helices in biological systems as a consequence of a long evolution that holds extremely important clues to understanding their extraordinary functions related in their structures to which chiral amplification via supramolecular helical assembly might play a definitive role to realize the homochirality in each component biopolymer. Although we know that there are still gaps between the natural and synthetic helical systems with respect to their structures and functions even at the present, as the field of

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2016, 116, issue 3, “Frontiers in Macromolecular and Supramolecular Science”.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (E.Y.). Notes

The authors declare no competing financial interest. Biographies Eiji Yashima received his B.S. (1982), M.S. (1984), and Ph.D. (1988) degrees from Osaka University in the group of Prof. Yoshio Okamoto. In 1986, he joined Kagoshima University, working with Prof. Mitsuru Akashi as Assistant Professor. After spending one year at UMass with Prof. David A. Tirrell, he moved to Nagoya University and was promoted to a full Professor in 1998. He was the project leader of the ERATO Project (JST) on “Yashima Superstructured Helix” (2002− 2007). He received the Wiley Polymer Science Award from the SPSJ in 2000, the Japan IBM Science Award in 2001, Molecular Chirality Award in 2005, Thomson Scientific Research Front Award in 2007, the Award of the Society of Polymer Science, Japan in 2008, Chirality Medal in 2013, and the Chemical Society of Japan Award in 2015. He is a fellow of the Royal Society of Chemistry, an associate member of the Science Council of Japan, and a senior program officer of Research Center for Science Systems (JSPS). His current research interests are in the design and synthesis of helical molecules, supramolecules, and polymers with unique structures and novel functions. Naoki Ousaka received his B.S. (2002), M.S. (2004), and Ph.D. (2007) degrees from Nagoya Institute of Technology in the group of Prof. Yoshihito Inai. After spending two years at the University of Tokyo with Prof. Reiko Kuroda, he joined the group of Prof. Eiji Yashima in Nagoya University as a postdoctoral researcher in 2009 and was promoted to Associate Professor in 2014. From 2011, he spent one year and six months at University of Cambridge with Prof. Jonathan Nitschke. Daisuke Taura received his B.S. (2004) degree from Nagoya University in the group of Prof. Eiji Yashima, and M.S. (2007) and Ph.D. (2010) degrees from Osaka University in the group of Prof. Akira Harada. After spending one year and four months at ETH Zürich with Prof. François Diederich, he joined the group of Prof. Eiji Yashima at Nagoya University as Assistant Professor in 2011. Kouhei Shimomura received his B.S. (2009), M.S. (2011), and Ph.D. (2014) degrees from Kanazawa University in the group of Prof. Katsuhiro Maeda. Currently, he is a postdoctoral researcher in the group of Prof. Eiji Yashima at Nagoya University. Tomoyuki Ikai received his B.S. (2003), M.S. (2005), and Ph.D. (2008) degrees from Nagoya University in the group of Professors 13957


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Yoshio Okamoto and Masami Kamigaito. In 2008, he joined Nagoya University as Assistant Professor. After spending five months at MIT with Prof. Timothy M. Swager, he moved to Kanazawa University in 2009 and was promoted to Associate Professor in 2014. Katsuhiro Maeda received his B.S. (1993), M.S. (1995), and Ph.D. (1998) degrees from Nagoya University in the group of Prof. Yoshio Okamoto. In 1998, he joined the group of Prof. Eiji Yashima at Nagoya University as Assistant Professor and was promoted to Associate Professor in 2002. After spending six months at MIT with Prof. Tim Swager, he moved to Kanazawa University in 2008 and was promoted to a full Professor in 2015. He received Thomson Scientific Research Front Award in 2012.

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