Homochiral Evolution in Self-Assembled Chiral Polymers and Block

Mar 3, 2017 - Rong-Ming Ho received his Ph.D. degree from the Institute of Polymer Science, University of Akron (USA), in 1995 under the supervision o...
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Homochiral Evolution in Self-Assembled Chiral Polymers and Block Copolymers Tao Wen,†,§ Hsiao-Fang Wang,†,§ Ming-Chia Li,‡,§ and Rong-Ming Ho*,† †

Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan Department of Biological Science and Technology, National Chiao Tung University, Hsinchu 30010, Taiwan

‡

CONSPECTUS: The significance of chirality transfer is not only involved in biological systems, such as the origin of homochiral structures in life but also in man-made chemicals and materials. How the chiral bias transfers from molecular level (molecular chirality) to helical chain (conformational chirality) and then to helical superstructure or phase (hierarchical chirality) from self-assembly is vital for the chemical and biological processes in nature, such as communication, replication, and enzyme catalysis. In this Account, we summarize the methodologies for the examination of homochiral evolution at different length scales based on our recent studies with respect to the self-assembly of chiral polymers and chiral block copolymers (BCPs*). A helical (H*) phase to distinguish its P622 symmetry from that of normal hexagonally packed cylinder phase was discovered in the self-assembly of BCPs* due to the chirality effect on BCP self-assembly. Enantiomeric polylactide-containing BCPs*, polystyrene-b-poly(L-lactide) (PS−PLLA) and polystyrene-b-poly(D-lactide) (PS−PDLA), were synthesized for the examination of homochiral evolution. The optical activity (molecular chirality) of constituted chiral repeating unit in the chiral polylactide is detected by electronic circular dichroism (ECD) whereas the conformational chirality of helical polylactide chain can be explicitly determined by vibrational circular dichroism (VCD). The H* phases of the self-assembled polylactide-containing BCPs* can be directly visualized by 3D transmission electron microscopy (3D TEM) technique at which the handedness (hierarchical chirality) of the helical nanostructure is thus determined. The results from the ECD, VCD, and 3D TEM for the investigated chirality at different length scales suggest the homochiral evolution in the self-assembly of the BCPs*. For chiral polylactides, twisted lamellae in crystalline banded spherulite can be formed by dense packing scheme and effective interactions upon helical chains from self-assembly. The handedness of the twisted lamella can be determined by using rotation experiment of polarized light microscopy (PLM). Similar to the self-assembly of BCPs*, the examined results suggest the homochiral evolution in the crystallized chiral polylactides. The results presented in this Account demonstrate the notable progress in the spectral and morphological determination for the examination of molecular, conformational, and hierarchical chirality in self-assembled twisted superstructures of chiral polymers and helical phases of block copolymers and suggest the attainability of homochiral evolution in the self-assembly of chiral homopolymers and BCPs*. The suggested methodologies for the understanding of the mechanisms of the chirality transfer at different length scales provide the approaches to give Supporting Information for disclosing the mysteries of the homochiral evolution from molecular level.

1. INTRODUCTION

conformation with exclusive handedness (conformational chirality) (Figure 1B) due to intramolecular interactions. The chiral stereogenic centers can be either located in the main chain or the side chains. For polymers without an intrinsic chiral center (achiral polymers), a polymer chain with induced conformational chirality can be achieved by association with chiral dopants (Figure 1C); this behavior is referred as induced circular dichroism (ICD).1 As the chiral polymers or achiral polymers with ICD self-assemble through intermolecular interactions, the packing of helical chains may lead to the

Chirality, of which human hands probably are the most familiar example, means non-superposition of an object with its mirror image. Chirality is an important aspect of life due to the chiral bias of nature; all amino acids in proteins are “left-handed” and all sugars in DNA and RNA are “right-handed”. Moreover, the understanding of chirality is one of the vital factors in the chemistry and materials. From molecular to hierarchical levels, the structures with chirality can be recognized at different length scales. A molecule that cannot be superimposed onto its mirror image is referred to as possessing molecular chirality or configurational chirality (Figure 1A). With the introduction of asymmetrical chiral center, a polymer chain will exhibit helical © 2017 American Chemical Society

Received: January 10, 2017 Published: March 3, 2017 1011

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Figure 1. Schematic illustration of the chirality transfer from molecular chirality to conformation chirality and finally hierarchical chirality for mainchain chiral homopolymers and chiral block copolymers (BCPs*), and the methodologies for the examination of chiral structures at different length scales, in particular for the ones in the bulk state. (A) The molecular chirality as examined by electronic circular dichroism (ECD). (B) The conformation chirality is examined by vibrational circular dichroism (VCD) to distinguish the optical activity from the molecular chirality. (C) Schematic illustration of the stereoregular polymers with induced circular dichroism (ICD). (D) The self-assembled helical phase from BCPs* can be directly visualized by reconstruction image from three-dimensional transmission electron microscopy (3D TEM) (i.e., TEM tomography (TEMT)). (E) For the twisted lamellae in the banded spherulites from the crystallization of main-chain chiral homopolymers and stereoregular polymers with ICD, the handedness is determined by rotation experiment of polarized light microscopy (PLM).

dichroism (ECD),5 which is sensitive to the absolute configuration and also conformational features.6 The handedness of the H* phases in the self-assembled polylactidecontaining BCPs* can be directly visualized by three-dimensional transmission electron microscopy (3D TEM), also referred as TEM tomography (TEMT).5,7 As a result, the homochiral evolution in the self-assembly of the BCPs* is suggested based on the investigated chirality at different length scales. Also, the crystallization of the enantiomeric chiral polylactides provides another exemplary circumstance for the examination of homochiral evolution from self-assembly at which helical superstructures (twisted lamellae) (Figure 1E) in crystalline banded spherulite can be formed. The handedness of the twisted lamella can be determined by using a rotation experiment of polarized light microscopy (PLM); the morphological examination suggests the homochiral evolution in the crystallized chiral polylactides.8 Similar methodologies for the examination of homochiral evolution can also be applied to different self-assembled systems. By taking advantage of the behaviors of induced circular dichroism (ICD), poly(2-vinylpridine) (P2VP), in particular isotactic poly(2-vinylpridine) (iP2VP), can form helical polymer chains with preferred handedness by association of the polymer chain with chiral dopants.9 The helical sense of P2VP chains is defined by the optical activity of the chiral dopants; namely, the handedness of the helical chain with ICD can be determined by the induced optical activity. Consequently, the crystallization of the iP2VP with ICD can be guided by the chiral dopants for the growth of twisted lamella with preferred handedness to give a banded spherulite with specific optical activity.10,11 With the use of the rotation experiment of PLM for the examination of twisted lamellar handedness, the homochiral evolution from the molecular level to hierarchical superstructure can be verified.

formation of a one-handed helical superstructure or phase (hierarchical chirality). The transfer of chiral information (homochiral evolution) from molecular chirality to hierarchical chirality is critical not only for molecular processes in nature such as communication and replication, as well as enzyme catalysis, but also for the control of self-assembled structures for functions and complexity. It is essential to understand the mechanisms of chirality transfer at different levels, through the design and synthesis of molecules and the control of selfassembly. Block copolymers composed of chiral entities (designated as chiral block copolymers, BCPs*) were designed to fabricate helical architectures from self-assembly.2 A helical phase (denoted as H* to distinguish its P622 symmetry from that of the normal hexagonally packed cylinder phase, denoted as hexagonally packed cylinder (HC) with P6/mmm symmetry) (Figure 1D) was discovered in the self-assembly of polystyreneb-poly(L-lactide) (PS−PLLA) BCPs*, reflecting the chirality effect on the self-assembly of BCPs.3,4 The self-assembly of enantiomeric polylactide-containing BCPs*, PS−PLLA and polystyrene-b-poly(D-lactide) (PS−PDLA), thus provides an exemplary circumstance for the examination of homochiral evolution. How to determine the structures with chirality at different length scales is primary and critical. Well-developed spectroscopic theories and techniques provide convenient pathways to connect chiroptical spectra with the dissymmetry of molecules. How the chiral bias transfers from the dissymmetric molecule to the helical chain and then to the helical superstructure or phase from self-assembly is essential. Yet, the approaches for the handedness determination of the helical chains and the helical superstructures, as well as the helical phases, in particular in the bulk state, remain challenging. Here, the handedness of helical polylactide chain can be explicitly determined by vibrational circular dichroism (VCD) to distinguish the contribution of the optical activity from constituted chiral entities detected by electronic circular 1012

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Figure 2. (A) Chemical structures of L-/D-lactic acid, L-/D-lactide, and polylactides. ECD and corresponding UV−vis absorption spectra of lactic acids (B) and polylactide homopolymers (C) in dilute AcCN solution. Concentration of the solution is 0.1 wt %. VCD and corresponding FTIR absorption spectra of lactic acids (D) and polylactide homopolymers (E) in dilute CH2Cl2 solution. Concentration of the solution is 2 wt %. (F) VCD and corresponding FTIR absorption spectra of polylactide-containing BCPs in solid films. Adapted with permission from ref 5. Copyright 2012 American Chemical Society.

2. HOMOCHIRAL EVOLUTION FROM CHIRAL MOLECULE TO HELICAL CHAIN Poly(L-lactide) (PLLA) belongs to the family of aliphatic polyesters resulting from polymerization of L-lactide (Figure 2A). Polymerization of D-lactide gives the enantiomeric polymer of PLLA, poly(D-lactide) (PDLA). Copolymerization of racemic lactide, mixture of equimolar L-lactide and D-lactide, leads to achiral poly(D,L-lactide) (PLA). Despite that the structural symmetry on the molecular level can be detected by X-ray, the most convenient and widely used approach to analyze molecular chirality is ECD spectroscopy. The ECD is given by the differential absorption between left- and righthanded circularly polarized light, which may arise from inherently molecular asymmetry. On the basis of chirality sector rules, the absolute configuration (molecular chirality) could be determined by the corresponding chiroptical results.12 Figure 2B shows ECD and the corresponding absorption

spectra of L-lactic acid, D-lactic acid, and D,L-lactic acid in dilute acetonitrile (AcCN) solution. A positive Cotton effect can be observed in L-lactic acid solution, whereas D-lactic acid solution produces a negative Cotton effect. As expected, racemic lactic acid solution is ECD silent. The ECD signal of lactic acid is attributed to the n−π* transition of the carbonyl group in the lactic acid at approximately 210 nm.13 It had been clarified that the absolute configuration of L-lactic acid is (S)-type and that of 14 D-lactic acid is (R)-type. Figure 2C shows the ECD and corresponding absorption spectra of polylactides in dilute AcCN solution. The PLLA and PDLA solutions show a positive and a negative Cotton effect, respectively, whereas the PLA solution appears ECD silent. Notably, the ECD intensity of repeat units of PLLA and PDLA is approximately three times that of L- and D-lactic acids, respectively. This behavior is known as chiral amplification effect due to the formation of helical conformation (conformational chirality).15 However, for 1013

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self-assembly. To examine the microphase-separated morphologies from the self-assembly of the BCPs*, TEM experiments are usually carried out. Figure 3A shows the 2D TEM

helical polymer with main-chain chirality (the chiral entities located at the backbone) like the enantiomeric polylactides, it is not feasible to use the corresponding absorptions from the delocalized n−π* transition for the determination of the handedness of forming helical conformation by ECD results even though there is considerable effect of the neighboring chiral centers on the corresponding absorptions.16 By contrast, for the IR absorptions of the chiral polymers, the corresponding circular dichroism spectra can be simply attributed to the neighboring vibrational modes; as a result, VCD spectroscopy can be utilized to determine the handedness of the helical conformation of chiral polymers with main-chain chirality.17−21 Figure 2D,E shows the VCD and corresponding FTIR absorption spectra of lactic acids and polylactides in dilute dichloromethane (CH2Cl2) solution, respectively. As shown in Figure 2D, no significant VCD signal can be found in lactic acid solutions, due to the absence of coupled oscillating between adjacent chromophores.22 By contrast, a split-type VCD signal at 1760 cm−1 corresponding to the characteristic absorption of the CO stretching motion of carbonyl groups can be observed in the PLLA solution (Figure 2E). Also, the VCD spectrum of the PDLA solution appears as the mirror-image compared with the spectrum of PLLA, and the PLA solution yields no VCD signal. The observed VCD sign reflects the interference of the CO stretching vibrations of two carbonyl groups closely located along the main chain due to intramolecular chiral interaction. On the basis of the coupled oscillator model,12,22 PLLA and PDLA chains can be determined as left-handed and right-handed, respectively. Similar spectroscopic results can also be found in the dilute CH2Cl2 solution of PS−PLLA and PS−PDLA, reflecting the formation of helical chains for the constituted chiral polylactides in the BCPs*. Figure 2F shows the VCD and corresponding absorption spectra of the BCP* in a solid film in which an H* phase is formed (see below for detail). In the VCD spectra, mirror-imaged VCD signals with a split-type Cotton effect for PS−PLLA and PS−PDLA can be found, but PS−PLA is VCD silent. The VCD results are similar to those obtained from solution, suggesting that the BCP* chain in the H* phase indeed possesses preferentially a one-handed helical conformation.

Figure 3. (A) TEM micrographs of PS−PLLA, PS−PDLA, and PS− PLA. (B) Schematic illustration of left-handed and right-handed helices in 3D space (left column) and 2D projection (right column). (C) Section of 3D reconstruction image viewing along the helical axis for PS/SiO2 helical nanohybrids fabricated using the template from PS−PLLA for templated sol−gel reaction. (D) Three-dimensional visualization of left-handed helical nanoarrays in the reconstructed volume of the rectangular area in panel C and use of color separation process to identify each helical microdomain. (E) Left-handed (left side) and right-handed (right side) helical nanostructures reconstructed from PS/SiO2 helical nanohybrids fabricated using templates from PS−PLLA and PS−PDLA, respectively, for templated sol−gel reaction. Adapted with permission from ref 5. Copyright 2012 American Chemical Society.

3. HOMOCHIRAL EVOLUTION FROM HELICAL CHAIN TO HELICAL PHASE OF BCP* Self-assembly is the spontaneous organization of components into patterns or structures by cooperating secondary interactions (noncovalent bonding forces).23−25 Block copolymers (BCPs) are able to self-assemble into one- (1D), two(2D), or three-dimensional (3D) periodic nanostructures in bulk, because of the incompatibility and the chemical connection between constituted blocks.26,27 In the pioneering work reported by Stadler and co-workers, helical structures that have equal right- or left-handed order can be observed in the self-assembly of achiral triblock copolymer.28 Also, hierarchical superstructures with a helical sense were obtained by the solution casting of the amphiphilic BCPs that contain a charged chiral block, suggesting that the chiral block, aside from amphiphilicity, ionic bonding, and electrostatic effects, plays an important role in the formation of helical nanostructures from solution.29 To simplify the effect of secondary interactions on the self-assembly of block copolymers, enantiomeric polylactide-containing BCPs*, PS−PLLA and PS−PDLA, were synthesized to directly investigate the chirality effect on BCP

micrograph of self-assembled morphology of PS−PLLA (Figure 3A, left) and PS−PDLA (Figure 3A, middle), respectively. The RuO4-stained PS microdomain appears as a dark matrix, whereas polylactide microdomains appear bright. A helical texture having a crescent-like projection image suggests the formation of H* phase.3,4 In contrast to the BCPs* with the helical phase, the projection image of self-assembled PS−PLA (Figure 3A, right) suggest a typical HC phase. The formation of the H* phase can be found in the self-assembly of the enantiomeric polylactide-containing BCPs* due to the effect of chirality. However, the handedness of the helical structures cannot be simply determined by conventional 2D TEM images due to the projection limitation.30 After the microtoming of TEM sections, the helical nanostructure might be cut into halves, resulting in ambiguous TEM projection for determining 1014

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Figure 4. (A) Chemical structure of pyrene-labeled PLLA (I) and pyrene-labeled PDLA (II). (B) ECD and UV−vis spectra of banded spherulite of pyrene-labeled PLLA and PDLA isothermally crystallized at 110 °C as well as amorphous pyrene-labeled PLLA PLM images of (C) pyrene-labeled PLLA and (D) pyrene-labeled PDLA isothermally crystallized at 110 °C with the use of gypsum plate. The delimited areas in panels C and D represent the observed slices during the rotation experiment. Vertical sections of (E) pyrene-labeled PLLA and (F) pyrene-labeled PDLA spherulites observed by polarized optical microscopy during the rotation. The value at the bottom represents the angle of twist around the radial axis in the right-handed positive sense. Scale bar is 200 μm in length. Adapted with permission from ref 8. Copyright 2014 John Wiley & Sons.

handedness. As shown in Figure 3B, full-sized left-handed and right-handed helices exhibit the same 2D projected images in 2D projection. Moreover, while the helix is cut in half by microsectioning, the left-handed helix might give both righthanded and left-handed 2D projections, depending on which part of the helix is examined. To unambiguously determine the handedness of the H* phase, real-space imaging (i.e., 3D visualization) will be necessary; as a result, TEM tomography is engaged to acquire reconstruction images by tomographic technique. The word “tomography” means the visualization of 3D object from imaging by sections or sectioning through the

use of any kind of penetrating wave; for instance, in medical applications of computerized axial tomography (CAT scan), 3D images are taken from a series of 2D images around a single axis of rotation. For the basic principle of electron tomography, a tilt series of 2D TEM projections is used for the reconstruction of a 3D object through Fourier transform followed by inversed Fourier transform or filter back projection.31,32 Figure 3C shows a section of a 3D reconstruction image viewed along the helical axis for hexagonally packed SiO2 helices in a PS matrix using hydrolyzed PS−PLLA as a template followed by templated sol−gel reaction.7 As demonstrated in Figure 3D, 1015

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Figure 5. VCD and corresponding FTIR absorption spectra of (A) CO and (B) C−O−C vibration in pyrene-labeled PLLA and PDLA in amorphous and crystallized states. (C) Illustration of the formation of the banded spherulite and the hypothetical mechanism of chirality transfer in the crystallized pyrene-labeled polylactides. Purple ellipses, red arrows, and l indicate the chromophore, the radial growth of the twist lamellae in the spherulite, and lamellar thickness, respectively. Adapted with permission from ref 8. Copyright 2014 Wiley-VCH.

PLM observations is commonly observed.37 With crystalline lamellar twisting along the radial growth direction for polymer crystallization, banded spherulite can be formed.38 For chiral polymers with stereoregular configuration and flexible main chains (e.g., isotactic or syndiotactic C−C single bonds), the imbalanced surface stresses at opposite folding surfaces resulting from different folding structures or conformations for polymer crystallites may drive the formation of lamellar twisting.39 As the extinction rings of the banded spherulites can be observed in PLM attributed to the periodic change of crystal orientation along the radial direction,40,41 the handedness of twisted lamellae can thus be determined by the rotation experiment of PLM.42,43 To systemically investigate the chirality transfer in the crystallization process of chiral polylactides, an achiral pyrene moiety as a molecular probe is labeled at the chain end of polylactide (Figure 4A).8 Figure 4B shows ECD and the corresponding absorption spectra of amorphous and crystallized films of pyrene-labeled polylactides. ECD spectrum is silent in pyrene-labeled PLLA fast quenched from the melt state (i.e., in the amorphous state), reflecting the cryptochirality of pyrene moieties.44 By contrast, a negative Cotton effect at 345 nm followed by a positive Cotton effect at 337 nm can be found in pyrene-labeled PLLA after isothermal crystallization, whereas a mirror-imaged Cotton effect can be observed in crystallized pyrene-labeled PDLA. The ECD signal, which is attributed to the characteristic absorption of pyrene at 300−350 nm,45 is induced by the formation of twisted lamellae

the reconstruction image of the rectangular area in Figure 3C shows an interdigitated character of left-handed helical packing nanoarrays with coloring for identification. Moreover, as shown in Figure 3E, left-handed (left side) and right-handed (right side) helices can be clearly identified for the reconstruction images from PS/SiO2 helical nanohybrids fabricated using templates from PS−PLLA and PS−PDLA, respectively. Consequently, the results of ECD, VCD, and 3D TEM for self-assembled polylactide-containing BCPs* can be used for the examination of chirality at different length scales and suggest the behaviors of homochiral evolution from molecular chirality to hierarchical chirality. Specifically, it is envisaged that helical superstructures or phases with preferred handedness can be obtained by self-assembling chiral polymers or BCPs* due to the effect of helical steric hindrance and intermolecular chiral interactions on self-assembly through chirality transfer from molecular level.5,25,33−35 As a result, self-assembly of polylactide-containing BCPs* is a typical example of the chiral amplification via chirality transfer from molecular chirality to hierarchical chirality in assembled polymer systems.36

4. HOMOCHIRAL EVOLUTION FROM HELICAL CHAIN TO TWISTED LAMELLAE OF CHIRAL HOMOPOLYMER Among self-assembled helical morphologies from polymer crystallization, banded spherulites possessing the presence of spherulitic crystalline morphology with extinction rings under 1016

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Figure 6. (A) Chemical structures of LA, HMA, and MA and the corresponding ECD spectra of iP2VP complexing with the chiral dopants in dilute chloroform solution. (B) ECD spectra of iP2VP, aP2VP, and sP2VP complexing with MA in dilute chloroform solution. Concentration of the solution is 0.4 wt % and the molar ratio of P2VP monomer to chiral dopant is 1:1. (C) Hypothetic molecular dispositions of ICD for P2VPs. Adapted with permission from ref 9. Copyright 2012 Royal Society of Chemistry.

range of C−O−C stretching vibration (1000−1250 cm−1) can be found in crystallized PLLA and PDLA (Figure 5B), whereas the VCD is silent in the amorphous ones. Considering that the electron transition dipoles of the C−O−C stretching vibration are nearly parallel to the helical axis of the chiral polylactide, we suggest that the induced split-type VCD in the C−O−C absorption range is attributed to the formation of intermolecular interaction between helical chains. As a result, the tilted helical chains lead to the twisting of lamellar crystals with preferred direction along the radial direction of the spherulite, giving the formation of the banded spherulite with specific optical activity. The spectroscopic results of VCD on the chiral polylactides and the ICD of the labeled pyrene moiety as well as the morphological examination thus suggest a homochiral evolution from PLLA/PDLA helical chains (conformational chirality) to twisted lamellar crystals (hierarchical chirality) in the crystallized chiral polylactides. As illustrated in Figure 5C, the helical chains of PLLA/PDLA will fold into crystalline lamellae during isothermal crystallization, and the end-capped achiral chromophore moieties will be excluded (step 1). Owing to the different fold structures or chain conformation at the fold surfaces, the imbalanced stresses at the opposite folding surfaces result in the twisting of lamellar crystallites (step 2).46 The mirror-imaged helical conformation in PLLA and PDLA gives rise to opposite rotation direction along the growth direction, yielding a helical superstructure (twisted lamella) with preferred handedness (step 3).

of PLLA and PDLA with opposite twist sense (Figure 4C,D). To determine the twist sense of the lamellae in the banded spherulite, the sample is rotated by a goniometer along the axis parallel to the radial direction of spherulite. The mechanism is that the rotation of a right-handed lamella in the sense of its own twist will result in a descent of the extinction rings, and vice versa.42,43 The corresponding images of central vertical slices of banded spherulites of pyrene-labeled PLLA (Figure 4C) and pyrene-labeled PDLA (Figure 4D) subjected to a right-handed rotation are shown in Figure 4E,F. For banded spherulite of PLLA, a descent of the extinction rings with the rotation can be observed (lines AA′ and BB′), suggesting that the lamella is left-handed. Conversely, for banded spherulite of PDLA, an ascent of the extinction rings (lines CC′ and DD′) indicates that the twisted lamella is right-handed. The mechanism of chirality transfer in crystallized chiral polylactides can be further examined by VCD spectroscopy. Figure 5A shows the VCD and corresponding IR absorption spectra of isothermally crystallized pyrene-labeled PLLA and PDLA. In contrast to amorphous PLLA and PDLA, the normalized intensity of VCD signals in the CO stretching region is significantly enhanced by crystallization. Also, the hypsochromic shift of the corresponding absorption band can be observed. These results indicate the formation of intramolecular interaction of CO chromophores in the chiral polylactide due to the close packing of polymer chains in the crystal. Moreover, additional VCD signals in the absorption 1017

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Figure 7. (A) ECD and UV−vis spectra of neat iP2VP and iP2VP/HMA complex in thin film and PLM images of isothermally crystallized iP2VP with (R)-HMA (B) and (S)-HMA (C) by using of gypsum plate. The delimited areas in (B) and (C) represent the observed slices during the rotation experiment. Vertical sections of (D) iP2VP/(R)-HMA and (E) iP2VP/(S)-HMA spherulites observed by PLM during the rotation. The value at the bottom represents the angle of twist around the radial axis in the right-handed positive sense. Adapted with permission from ref 10. Copyright 2015 John Wiley & Sons.

introduction of chiral dopants, split-type Cotton effect in the ECD spectrum can be observed in the absorption range of isotactic P2VP (iP2VP), whereas the pure iP2VP is ECD silent. For iP2VP complexed with (R)-type chiral dopant, a positive Cotton effect at 275 nm followed by a negative Cotton effect at 255 nm can be found, suggesting the formation of right-handed helical chain. Consistently, a mirror image of the ECD can be found in the iP2VP complexed with (S)-type chiral dopant, giving a left-handed conformation. Note that the mixture of pyridine and chiral dopants merely shows a trivial CD signal, conforming that the observed ICD of iP2VP is attributed to the formation of helical conformation.9 Those results reflect a homochiral evolution from molecular chirality to conformational chirality. Also, the ICD of the iP2VP is strongly dependent upon the bulkiness of chiral dopants. The intensity of ICD in the iP2VP complexed with mandelic acid (MA) is stronger than that with hexahydromandelic acid (HMA) and lactic acid (LA) (Figures 6A and 2A). Most interestingly, the stereoregularity of the P2VP was found to play an important role in the formation of helical conformation. The magnitude of the ICD in iP2VP is stronger than that in syndiotactic P2VP (sP2VP) and atactic P2VP (aP2VP) (Figure 6B). Figure 6C shows a schematic illustration of hypothetic mechanisms with respect to the dependence of the ICD on the isotacticity of the stereoregular P2VP. For the flexible main chain, it is necessary to provide significant steric hindrance to maintain the induced conformaiton. The degree of steric hindrance will be determined by the tacticity of mainchain P2VP while the side chain (e.g., pyridyl ring in P2VP) is associated with chiarl dopants. For the P2VP with high isotacticity, the chirality transfer can be achieved due to its large steric hindrance. By contrast, owing to the low degree of isotacticity in the main chain of aP2VP and sP2VP, the ICD for the entire polymer chain will be limited as illustrated in Figure 6C. Yet, it is still feasible to provide ICD even for the aP2VP since there is still a certain degree of isotacticity for the P2VP with irregular configuration, giving the discrepancy on the intensities of ICD between aP2VP and sP2VP.

Eventually, the radial growth of the helical lamellae develops as the banded spherulite as evidenced in Figure 4 via the transfer of homochirality, from molecular chirality (L- or D-lactide) to hierarchical chirality (twisted lamellae) (step 4).

5. HOMOCHIRAL EVOLUTION FROM CHIRAL DOPANT TO STEREOREGULAR POLYMER VIA ICD In polymers, the chirality transfer from molecular level to conformational level could be achieved through either backbone or side chain. The essential chirality of the former one is attributed to the chiral center in the main chain of the polymer such as enantiomeric polylactides as introduced above. In contrast to the main-chain chirality, the helical conformation of polymers can also be driven by the chiral center in the side chain. In addition to the intrinsic chirality of chiral polymer, one-handed helical conformation of dynamically racemic helical polymers can also be formed by association of optically active molecules through noncovalent bonding (ICD).47 In general, the maintenance of the induced helical conformation will be strongly dependent upon the rigidity of the polymer backbone. For rigid polymers such as polyacetylenes, both the main chain helicity and axial chirality of the pendants can be induced by using nonracemic alcohols.48 However, the examples of flexible polymers such as polyolefins and vinyl polymers with ICD are rare.49−51 The stereoregularity plays a vital role in the formation of helical conformation for the vinyl polymers with flexible C−C bond. For instance, one-handed helical chain of syndiotactic poly(methyl methacrylate) (sPMMA) can be induced using chiral alcohols, and the sPMMA with ICD can be used to encapsulate isotactic PMMA (iPMMA) within the induced helical cavity through stereocomplexation.52,53 Also, the ICD behaviors can be found in poly(2-vinylpyridine)s (P2VPs) with ortho-position of pyridyl ring. Note that the presence of the ortho-position of pyridyl ring in P2VPs would provide significant steric hindrance to stabilize the induced helical conformation, while the chiral dopants are introduced for ICD. Chiral α-hydroxy-carboxylic acids can be used to induce the formation of helical conformation for the P2VPs via the acid−base interactions between chiral dopant and pyridyl group (i.e., complexation).9 As shown in Figure 6A, with the 1018

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6. HOMOCHIRAL EVOLUTION FROM STEREOREGULAR POLYMER WITH ICD TO TWISTED LAMELLAE

7. CONCLUDING REMARKS AND PERSPECTIVES As demonstrated in chiral polylactides and polylactidecontaining BCPs*, homochiral evolution from molecular chirality to hierarchical chirality can be achieved through selfassembly. With the selection of molecular chirality, it is conceivable to control the handedness of self-assembled helical superstructures and helical phases through homochiral evolution. Similar behaviors can also be found in the selfassembly of stereoregular polymers with ICD. Nevertheless, this conjecture is not always truthful to follow since there are many exceptions. The information on chirality might be reversed or even lost due to the effect of self-assembling conditions on the forming textures, in particular for long polymer chains with slow mobility and relaxation. As a result, it is still a case-by-case phenomenon with respect to the suggested homochiral evolution; comprehensive studies are necessary to answer the question of whether it is a universal behavior. Although methodologies for systematic studies of the chirality transfer at different length scales are proposed, it is still not easy to examine the corresponding mechanisms with respect to homochiral evolution. The chiroptical method is certainly a convincing method to unambiguously determine the chirality transfer in self-assembled chiral polymers and BCP* from a molecular level. However, the analyses of the ECD spectra still require big efforts due to the effects of neighboring environments on corresponding absorptions, in particular in bulk that involves complicated intramolecular and intermolecular chiral interactions. VCD can be utilized as a powerful tool in determining the conformational chirality in self-assembled chiral polylactides and polylactide-containing BCPs*, but it is still in the early stages. For the determination of the handedness of helical phases in nanoscale from the selfassembly of BCPs*, the reconstruction imaging from electron tomography is essential. To identify the preferred twisting of the twisted lamella from the crystallization of chiral homopolymers or stereoregular polymers with ICD, the rotation experiment of PLM is straightforward and simple. Nevertheless, delicate faculties with easy operation are still in demand for characterization of self-assembled helical superstructures and helical phases in bulk. For practical applications, it is obviously advantageous to exploit the self-assembled systems for devices. In addition to the applications for separation and enantioselective catalysis, novel applications are to use the self-assembled phases with specific handedness for the development of chiral plasmonic nanostructures and chiral photonics, such as beam splitters, as well as chiral metamaterials for optical applications such as negative refraction. By taking advantage of the homochiral evolution in chiral homopolymers and BCPs* from self-assembly, it is feasible to fabricate chiral materials from the helical assembles with controlled chirality, in particular, in the thin-film state, with large-scale orientation. The demonstration for the chirality control in self-assembled systems thus provides a new concept for the design of optical memory devices, optical communications, three-dimensional biodetectors, chiral plasmonic nanostructures, and chiral photonics, as well as chiral metamaterials with tunable optical activities.

As demonstrated above, the one-handed helical conformation of iP2VP can be induced by complexation with the chiral dopant through homochiral evolution from molecular to conformational chirality. However, how to achieve the chirality transfer for such a polymer complex from conformational chirality to hierarchical chirality remains challenging. In general, the formation of helical superstructures requires a parallel packing model with effective interaction upon the packing of helical conformation.54,55 As demonstrated above, it is feasible to achieve chirality transfer from the helical polymer chain to the hierarchical twisted lamella through crystallization of chiral polylactides due to the effective intermolecular interaction for the packing of the helical chains. Considering the crystallizable character of stereoregular polymers, solid films of iP2VP/HMA complexes were prepared for crystallization to scrutinize the feasibility of homochiral evolution from the self-assembly of the iP2VP with ICD.10 As shown in Figure 7A, for amorphous films, split-type Cotton effect with mirror image for iP2VP/(R)HMA and iP2VP/(S)-HMA complexes can be found. Note that the results are similar to the ECD results from the solution state, indicating that the ICD character can be well preserved in the solid film. With the control of crystallization process, largesized spherulites of iP2VP with banded rings can be obtained.11 Specifically, the formation of twisted lamellae can be achieved by crystallization of the iP2VP with ICD. Subsequently, the handedness of twisted lamella in the banded spherulite of iP2VP crystallized with HMAs was examined by the rotation experiment of PLM. Figure 7D,E shows the corresponding images of central vertical slices of banded spherulites of iP2VP/ (R)-HMA (Figure 7B) and iP2VP/(S)-HMA (Figure 7C) subjected to a right-handed rotation at different angle, suggesting that the lamellae are right-handed for the iP2VP/ (R)-HMA and left-handed for iP2VP/(S)-HMA. To clarify the mechanism for the growth of the twisted lamella with preferred handedness, wide-angle X-ray diffraction experiments were conducted; the results reveal that the crystal structure of the iP2VP crystallized with the HMAs is identical to that of intrinsic structure of neat iP2VP.10,11 On the basis of the diffraction results, we suggest that decomplexation of dopant molecules from the iP2VP will occur during crystallization. On account of the isochiral structure of iP2VP crystals, the initial helicity of iP2VP chains driven by ICD can be memorized upon nucleation and the following growth for crystallization. As a result, the twist sense of lamellar crystals of the iP2VP is consistent with the chirality of chiral dopants. Namely, the chiral dopants should be excluded from the crystal lattice of the iP2VP. After the exclusion of the HMAs, iP2VP chains can closely pack into the lattices, and the corresponding intermolecular interactions with specific steric hindrance due to the induced helical conformation with one-handed helicity give rise to the formation of twisted lamella with preferred handedness. Note that the lamellar twisting exhibits a relatively high proportion of preferred handedness even though there is only a small bias of chiral inducement in the initial complex. It reveals that the crystallization process apparently amplifies the effect of chirality transfer.56

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Corresponding Author

*E-mail: [email protected]. 1019

DOI: 10.1021/acs.accounts.7b00025 Acc. Chem. Res. 2017, 50, 1011−1021

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Accounts of Chemical Research ORCID

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Rong-Ming Ho: 0000-0002-2429-7617 Author Contributions §

T.W., H.F.W., and M.C.L. contributed equally.

Funding

Authors gratefully acknowledge the financial support the Ministry of Science and Technology, Taiwan (Grants MOST 105-2119-M-007-011 and MOST 105-2811-E-007-014). Notes

The authors declare no competing financial interest. Biographies Tao Wen received his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS), in 2013 under the supervision of Profs. Fosong Wang and Dujin Wang. He is currently working as a postdoctoral researcher in the group of Prof. Rong-Ming Ho at the Department of Chemical Engineering, National Tsing Hua University (Taiwan). Hsiao-Fang Wang received his Ph.D. degree from the Department of Chemical Engineering, National Tsing Hua University (Taiwan), in 2016 under the supervision of Prof. Rong-Ming Ho. He is currently working as a postdoctoral researcher in the group of Prof. Rong-Ming Ho. Ming-Chia Li received his Ph.D. degree from the Department of Chemical Engineering, National Tsing Hua University (Taiwan), in 2013 under the supervision of Profs. Rong-Ming Ho and Yu-Der Lee. He then spent one year at Nagoya University (Japan) with Prof. Eiji Yashima. Currently, he is an assistant professor in the Department of Biological Science and Technology, National Chiao Tung University (Taiwan). Rong-Ming Ho received his Ph.D. degree from the Institute of Polymer Science, University of Akron (USA), in 1995 under the supervision of Prof. Stephen Z. D. Cheng. He then moved to Minneapolis and worked as a postdoctoral fellow in the Department of Chemical Engineering and Materials Science, University of Minnesota (USA), with Profs. Christopher W. Macosko and Frank S. Bates. Currently, he is a distinguished professor in the Department of Chemical Engineering, National Tsing Hua University (NTHU) (Taiwan).

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DOI: 10.1021/acs.accounts.7b00025 Acc. Chem. Res. 2017, 50, 1011−1021