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Hexaarylbenzene: Evolution of Properties and Applications of Multitalented Scaffold Varun Vij, Vandana Bhalla,* and Manoj Kumar* Department of Chemistry, UGC Centre for Advanced Studies, Guru Nanak Dev University, Amritsar, Punjab 143005, India ABSTRACT: The easily rotatable peripheral aromatic rings around central benzene in hexaarylbenzene (HAB) derivatives create a very intriguing nonplanar, propeller-shaped geometry. Because of the very low susceptibility toward self-aggregation, HAB derivatives are much stronger candidates among various polyphenylenes/hetero-oligophenylenes when poor molecular cohesion and inefficient packing is required. However, the native properties of hexaphenylbenzene (HPB) can be varied by proper tailoring and substitution of the HAB core. The cohesion and packing in the structures of HAB aggregates induce effective structural variations so as to modify the fundamental features. Recently, HAB derivatives attracted a lot of research interest because of their significant role as liquid crystalline materials, organic light-emitting diodes, photochemical switches, redox materials, and molecular receptors. Herein, detailed attention is given to the pioneering work based on synthetic optimization of different HAB cores, elaborated study of their crystal engineering, various interesting applications of HAB derivatives, and future possibilities and capabilities of this still underexplored scaffold.

CONTENTS 1. Introduction 2. Synthesis and Structural Parameters 2.1. Synthetic and Structural Optimization of HPB 2.2. Synthetic and Structural Optimization of Other HABs 3. HAB Derivatives for Organic Light-Emitting Diodes 3.1. Red Light-Emitting Diodes and Color Tuning in HAB Derivatives 3.2. Blue Light-Emitting Diodes and Color Tuning Based on HAB Derivatives 3.3. HAB Derivative As Hole-Blocking Layer for White Emitting Diodes 4. Molecular Inclusion and Receptor Properties of HAB 4.1. Role of Cation−π Interactions in Molecular Inclusion and Receptor Properties 4.2. Molecular Inclusion in HAB Derivatives Due to π−π and C−H···π Interactions 4.3. Fluorogenic Receptors Based on HAB Using Coordination Interactions 5. Self-assembly, Aggregation, and Supramolecular Interactions 5.1. Star-Shaped HAB Derivatives Undergoing Columnar Self-assembly 5.2. Self-assembly in Low-Symmetric HAB Derivatives 5.3. Hydrophobic Aggregation in HAB Derivatives and Their Applications 5.4. Metal-Induced Self-assembly in HAB Derivatives © 2016 American Chemical Society

6. Molecular Network and Intrinsic Microporosity 6.1. One-Dimensional Network of HAB Derivatives 6.2. Two- and Three-Dimensional Networks of HAB Derivatives 6.3. HAB Derivatives as Templates for Generating Porous Macrocyclic Molecules 7. Inter-/Intramolecular Electron/Charge-Transfer and Redox Properties of HAB Derivatives 7.1. Donor−Acceptor Properties of HAB Derivatives 7.2. Redox-Active HAB Derivatives 8. Photochemical/Chemical Molecular Switches Based on HPB Derivatives 9. HAB-Based Dendrimers and Their Applications 10. Summary and Future Prospects 10.1. Summary 10.2. Future Prospects a. Energy Materials b. Heteroatom Doping c. Graphene Nanoribbons d. Metal/Covalent Organic Framework Author Information Corresponding Authors Notes Biographies Abbreviations References

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9584 Received: February 24, 2016 Published: August 8, 2016

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DOI: 10.1021/acs.chemrev.6b00144 Chem. Rev. 2016, 116, 9565−9627

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Figure 1. General synthetic schemes for HPB 1 by (A and B) Diels−Alder and (C) cyclotrimerization reaction.

1. INTRODUCTION Hexaarylbenzene (HAB) derivatives have attracted a lot of research interest because of their unpredictable geometry and significant role in generating liquid crystalline materials,1 molecular-scale devices,2,3 and molecular receptors.4 Although applications of HAB derivatives were not much explored during the 20th century, their intriguing nonplanar complex skeletal and challenging synthetic optimization became an exciting area for synthetic chemists, stereochemists,5−7 and crystallographers.8,9 In the last one and a half decades, a lot of work dealing with different applications of newly designed HAB-based functional materials has been reported. HAB has a remarkable nonplanar structure due to easily rotatable peripheral aromatic units with respect to the central ring, mimicking the shape of a propeller. Almenningen et al.,10 on the basis of an electron diffraction study of hexaphenylbenzene (HPB) in vapor phase, proposed that its peripheral rings were almost orthogonal to the central benzene. However, the low restriction to rotation of the outer rings reportedly brings equilibrium with steric congestion between hydrogens of adjacent outer rings to end up having oscillating peripheral phenyl groups within ±10° from being completely orthogonal to the central ring.10 In contrast to this generalization, the X-ray diffraction (XRD) study of single crystals of HPB by Bart revealed that the orthogonality of its HPB is deviated by ∼25°, which depends upon intermolecular forces that vary with the solvents used for crystallization.11 The conformational inclination of HAB toward nonplanar topology becomes the reason for poor intramolecular π-conjugation as well as intermolecular π−π and C−H···π interactions.12 This leads to the higher highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap, low degree of self-aggregation, higher solubility of HAB derivatives, and lower crystalline character.13 Thus, HAB derivatives are strong candidates where poor molecular cohesion and inefficient packing is required.14,15 However, the conventional properties of HAB can be improved by proper tailoring and radial substitution of the HAB core. For instance, the radial substitution of noncrystalline HAB core with flexible paraffinic chains can partially introduce the order in the layered structure to yield columnar mesomorphic phase to generate liquid crystalline materials.1 The introduction of

amphiphilic character in HAB derivatives has shown a great tendency to form stable nanoaggregates like micelles and bilayers in water.16,17 Despite the numerous publications based on synthetic optimization, stereochemistry, and applications of HAB derivatives, there is no major review article exclusively covering all these reports. Therefore, in this Review, we have tried to cover almost all the reports relevant to HAB until now. This Review will commence with the reports on synthetic optimization of HAB core and its elaborated crystal engineering, which will be followed by various interesting applications of molecules of this class. It is well-established that highly delocalized π-e−’s of HABs raise the HOMO energy levels, thus enhancing HAB’s electrondonor ability.13 To extend the functional limits of native HABs, the suitable radial substitution of HAB with electron-withdrawing groups can help to control the HOMO energy levels of the materials to regulate the electron transport and emission properties, which can help to develop new materials for fabrication of electronic devices and organic light-emitting diodes.18−20 Most of the reports on steering energy band gaps and resulting transformations in electron-transport properties of materials based on functionalized HAB have been covered quite broadly in this Review. HPB derivatives, which represent the most contemplated branch of the HAB family, had earlier been extensively used just as precursors for the synthesis of respective planar coronene derivatives.21−29 Nonetheless, the moderately good emissive amorphous HPB and its heteroderivatives later emerged as much more promising amorphous materials than their highly crystalline planar analogues in the field of material and supramolecular chemistry. A large part of the Review will cover reports on HPB derivatives with a major emphasis on the role of star-shaped topology of these derivatives in formation of thin amorphous films with relatively high value of Tg and better charge-transfer properties in organic devices.30−32 Furthermore, the reports where star-shaped HAB macromolecules have been used as a template to generate highly porous macrocycles have been discussed widely, which can bring in a new prospect of material application of HAB derivatives.33,34 In addition to this, the HAB derivatives have also been used as key precursors for the bottom-up approach for designing and 9566


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Figure 2. HPB derivatives 2a−d, 3a−c, and 4a−d.

finally produce HPB derivatives 2a−d (62−86% yield) (Figure 2). In 1968, Bart11 studied the single-crystal structure of HPB and reported two different kinds of structures, i.e., pyramidal and orthorhombic, of modified HPB. The orientation of the molecule was obtained by three-dimensional Patterson function. According to this report, the peripheral aromatic rings, which were earlier presumed to be perpendicular to the horizontal plane of the central ring, are actually twisted by 25° from its expected position. However, no substantial changes in C−C bonds were observed. Besides, this crystal structure study explained well the intramolecular arrangement and role of type of bonding between peripheral aromatic groups in this profoundly functionalized benzene derivative.38 Interestingly, the crystal structure of HPB showed that its peripheral rings are perpendicular with respect to the central ring on the NMR time scale. However, actual structural parameters of HPB were demonstrated by the X-ray crystal structure of orthoor/and meta-HPB studied by Gust, which showed a propeller conformation with peripheral aryl rings, each making an angle of ∼65° with the plane of the central ring,6 which corroborates the results of Bart.11 Gust substituted HPB in ortho or meta positions of the peripheral rings, which results in restriction in rotation of peripheral rings about the single bonds joining the central benzene ring. This restriction in rotation further results in complex stereoisomerization. In this report, the rotation barriers of different HPB derivatives 3a−c bearing substituents in ortho and/or meta positions were studied (Figure 2). Also, the isomerization tendency and patterns of HPB bearing methyl and methoxy groups were studied. According to the X-ray diffraction studies of 3a−c, the isomerization involves rotation of the ring bearing the methoxy group because methyl groups have a much stronger steric effect than methoxy groups. In derivative 3b, the phenyl ring bearing methyl groups at the ortho position requires 38 kcal mol−1 to undergo rotation to finally give two diastereomers as a DL pair. However, the enantiomerization of derivative 3a requires much higher activation energy as it requires both peripheral rings bearing methyl and methoxy groups to rotate simultaneously. On the other hand, the meta-substituted HPB 3c requires 16.4 kcal/mol of energy to undergo diastereoisomerization due to relatively less steric hindrance created by the meta-substituted methyl groups. This keenly studied work discussed the mechanism of the observed transformations and provided data concerning the relative motions of the substituted rings. However, its probable effects on the unsubstituted rings were left untouched.

synthesizing dendrimeric architectures that, due to their porous structure, can encourage guest binding for detection, purification, transportation, and drug-delivery purposes. The wide range of applications of HAB derivatives would be of keen interest to synthetic, biomedical, analytical, supramolecular, and material chemists in the future to design new functional HAB derivatives with more interesting properties.

2. SYNTHESIS AND STRUCTURAL PARAMETERS 2.1. Synthetic and Structural Optimization of HPB

In 1933 HPB 1 was first reportedly synthesized by Trosken and co-workers35 using a Diels−Alder reaction between 1,2,3,4tetraphenylcyclopentadienone 1b and stilbene 1a, which also resulted in evolution of hydrogen and carbon monoxide gases as byproducts (Figure 1A).35 However, the same product was obtained by Hoeschen and co-workers without the evolution of hydrogen gas by a Diels−Alder reaction of diphenylacetylene 1c as a dienophile in the place of stilbene 1a (Figure 1B).36 Even earlier, the HPB was claimed to have been prepared by the reaction of phenylmagnesium bromide with hexabromobenzene, but it turned out to be 1,2,4,5-tetraphenylbenzene instead of HPB.37 Later in 1950, Grummitt and Fick synthesized HPB by a Diels−Alder reaction between 1,2,3,4-tetraphenylcyclopentadienone 1b and stilbene 1a under moderate pressure in a sealed tube at 250 °C, followed by dehydrogenation of the hexaphenyldihydrobenzene with bromine.5 In the present scenario, a Diels− Alder reaction is most frequently used to synthesize unsymmetrical HABs, where 1,2-diphenylacetylene derivative 1c is preferred as a precursor over analogue 1,2-diphenylethene derivative (Figure 1).38 However, the C3- or C6-symmetric, star-shaped HAB is more frequently synthesized by metal-based catalytic cyclotrimerization of corresponding 1,2-disubstituted alkynes in the presence of activated transition metal complexes under inert atmosphere (Figure 1C).39,40 The unsymmetrical substitution of HPB derivatives can be synthesized by heterosubstitution of precursors tetraphenylcyclopentadienone 1b and diphenylacetylene 1a. Potter and Hughes reported the productive synthesis of heterosubstituted 1,3-diarylacetones using asymmetric carbonylative couplings of two different para-substituted benzyl halides in the presence of iron tetracarbonyl disodium.41 The resulting 1,3-diarylacetones were made to react with benzil to give the desired heterosubstituted tetraphenylcyclopentadienones. These heterosubstituted tetraphenylcyclopentadienones then undergo a Diels−Alder reaction with substituted diphenylacetylene to 9567


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Figure 3. Bishexaphenylbenzene derivatives 5a−e, 6a−e, and 7a−d.

solid and solution state. This clearly showed that the substitution on HPB derivatives brings a very small change and only in the ground and excited states, whereas no significant alteration in electronic transitions was observed.

In a continuation of this work, Gust and Patton further kinetically studied the NMR and other classic methods to conclude that a peripheral ring at a time rotates by ∼π radians with respect to the central benzene ring.7 However, the experiments did not describe here the direction of rotation of a ring. Nevertheless, rotation by π radians in either direction would have yielded the same overall result. The kinetic study exhibited that the measured free energy of activation for isomerization is 33 and 17 kcal/mol, respectively, for rotations of rings bearing orthoand meta-methoxy groups. Later Hoogzand and co-workers8 widely studied the stereochemistry of substituted HPB derivatives bearing methyl and methoxy groups at their ortho and meta positions by NMR in achiral solvents. The ortho substitution of peripheral phenyl rings of HPB not only results in stereoisomerization but also strongly affects the cohesive forces, i.e., C−H···π interactions between two molecules, which directly affects the crystal packing as measured by packing indices, densities, solubilities, temperatures of sublimation, melting points, and ratios of H···H, C···H, and C···C contacts. Thus, we can say that the stereochemical behavior of HAB derivatives can be tuned by changing the substituents as well as the position of the substituents. Gagnon et al.13 studied the crystal structures of various ortho-substituted HPB derivatives 4a−d (Figure 2) to show increasing resistance to the formation of intermolecular C−H···π interactions and to the systematic increase of the ratio of H···H to C···H interactions. The standard analyses of Hirshfeld surfaces42 to quantify the intermolecular interactions in 4a−d showed that the substitution of progressively large groups at the ortho position of the peripheral phenyl groups results in a monotonic increase in H···H interactions accompanied by a decrease in C···C and C···H interactions. In addition to this, the crystal structure analysis of these derivatives suggested that the central benzene in HPB is strongly involved in intermolecular C−H···π interactions that can be altered by ortho substitution so as to increase useful properties like inefficient packing, high solubility, and molecular cohesion without altering other valuable features of the HPB unit. This elaborated study of crystal illustrated that molecular alterations can assist in establishing the patterns of molecular association that can guide the search for better materials. In another report studying the effect of substitutions on HAB derivatives, Harvey and Ogliaruso43 compared absorption properties of HPB and bis(hexaphenylbenzenes) (BHPBs) 5− 7 (Figure 3). It was observed that the absorption spectra of the substituted derivatives shift to slightly longer wavelength accompanied by a decrease in molar extinction coefficients in comparison to the corresponding unsubstituted derivatives. However, no significant shift in the position of the major peaks was observed in the electronic spectra of these compounds in

2.2. Synthetic and Structural Optimization of Other HABs

To get a better insight into the structural parameters of HAB in a broader fashion, Foces-Foces et al.9 studied the stereochemistry of hexakis(1-pyrazolyl)benzene by X-ray diffraction method,

Figure 4. ORTEP view of isomeric form I, i.e., 8, and form II, i.e., 9, of hexakis(1-pyrazolyl)benzene. Reproduced with permission from ref 9. Copyright 1996 Wiley-VCH.

which showed the existence of two isomers 8 and 9, respectively (Figure 4), which were obtained in acetic acid and ethanol/ dichloromethane, respectively. These isomers differ in the position of the nitrogen atom of the peripheral pyrazole group with respect to the central benzene. Out of these two, 8, in which pyrazole nitrogen alternates between both sides of the phenyl plane, exhibited the dihedral angle of ∼80° between planes of pyrazole rings and benzene ring. This isomer 8 (with 1.6−2.0 kcal mol−1 lesser energy) showed more stability than 9, in which pyrazole nitrogens are up or down with a dihedral angle of 64.3− 69.1°, which is almost similar to that in HPB. Thus, we can say that the position of heteroatom in HAB plays a pivotal role in structural parameters that can further affect the HOMO−LUMO gap of molecules. This interesting molecular geometry intrigued many researchers to develop new methods for the synthesis of different HABs 10−12 carrying different heteroaryl groups (Figure 5) like pyridine, thiophene, pyrrole, etc.44−51 Despite these reports, synthesis of new and tedious HAB derivatives using a more facile method and their structural analyses are still a major topic of research. For instance, Suga and co-workers52 reported HABbased derivatives bearing furan groups in skeletal 13a−k (Figure 5). The cyclotrimerization of respective diarylacetylene was optimized using RhCl3·3H2O as catalyst and diisopropylethylamine as base. The single-crystal X-ray diffraction study and 9568


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Figure 5. HAB derivatives 10−13 with heteroaryl groups.

Figure 6. HAB derivatives 14−21 with 5−6 different aryl groups.

energy-minimized structure of derivative 13b showed C2 molecular symmetry with two chemically nonequivalent furan rings making a dihedral angle of 82° with central benzene. The furan ring was further appended with a methyl group and a bispinacolatoboryl group, and the yields of HAB derivatives 13a−c were compared. The reason for using furan here was its tendency as a key building block for synthesizing π-conjugated compounds, which can be turned into semiconducting and photovoltaic devices. The HAB derivative 13c was further used as a substrate to synthesize various hexakis(5-arylfuran-2yl)benzenes 13d−j using Suzuki−Miyaura coupling. The yields of all these derivatives 13a−k were compared with each other, and keeping in mind the good to excellent yields (60−92%) in these reactions, derivative 13c can apparently be considered as the best starting material for synthesizing various dendrimeric HAB derivatives. These moderate to highly soluble systems were further modified by varying the functional units at their periphery that were proved advantageous for constructing highly ordered functional arrays. Despite the vast history of synthetic optimization of HAB derivatives, most of the known derivatives are either completely

symmetric or less symmetric with limited variation of substitution patterns. The synthesis of HAB derivatives with five or six different aryl groups around the central benzene is still a challenge as no synthetic protocol has been fully accomplished in the desired regioselective installation of an aromatic group independent of the stereoelectronics of the other existing radial substituents. Yamaguchi and co-workers53 recently reported the first programmed synthesis of HAB derivatives with five or six different radial aryl groups. This multistep synthesis of HABs mainly involved stepwise cross-coupling of 3-methoxythiophene with different aryl groups following C−H activation of the thiophene group to give tetrasubstituted thiophene. The [4 + 2] cycloaddition of tetraaryl thiophene S-oxide with desired unsymmetric diarylethyne in mesitylene at 160 °C produced HAB derivatives 14−21 (Figure 6) whose structures were confirmed by X-ray crystallography. Although the yields of the final products were reportedly low (10−29%), this programmed regioselective radial substitution will allow development of new functional molecules and the study of their structure−property relationships. 9569


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Figure 7. HAB derivatives 22−25 with uncommon symmetries.

Figure 8. Dithienylbenzo[2,1,3]thiadiazole-based HPB donor−acceptor array 26a−c and 27a−c and benzo[1,2,5]thiadiazole derivative 28.

harvesting molecules based on multicomponent systems consisting of several chromophores as absorbents that transfer the energy to the single acceptor core have a great role in OLEDs, especially those with red dopants.66−68 The organic materials to be integerated as OLEDs should exhibit high self-luminescent efficiency; high thermal, electric, and optical stability; and high quantum yield. Although most of the OLED materials show high quantum yield in the solution phase, many of these undergo concentration quenching, a phenomenon in which these materials exhibit strong emission in dilute solution phase but very weak or no emission at high concentration or solid state due to the formation of excimer/exciplexes to follow the nonradiating decay path.66−69 However, the star-shaped HPB derivatives, due to their amorphous nature, rarely undergo molecular aggregations of any level and hence exhibit high emission properties in solid state with high glass-transition temperatures and thermal stability.70,71 The OLEDs having an emission of three primary colors, red, green, and blue, with full-color displays have been the biggest challenge so far. Herein, we will discuss some HAB-based OLEDs reported so far with a full range of spectral emissions.

In the league of development of new synthetic methods for unconventionally dissymmetric and uncommonly symmetric HAB derivatives, Lungerich et al.54 very recently reported a completely different approach toward the synthesis of HAB derivatives 22−25 (Figure 7) with five different radial aryl groups. Using 4-nitroaniline as a starting material, this approach involved a cascade of electrophilic halogenations, followed by Suzuki cross-coupling reactions. Because of the high-yielding synthetic protocols, this approach allows the bulk production of multiaryl HAB derivatives. In the next sections, we will discuss applications and properties of different HAB derivatives reported so far, as well as the future prospects of this scaffold in the development of advanced functional materials.

3. HAB DERIVATIVES FOR ORGANIC LIGHT-EMITTING DIODES Organic light-emitting diodes (OLEDs) are electronic devices fabricated by depositing a thin layer of a luminescent organic material between two conducting layers with different work functions.55−58 On applying an electrical voltage, the electrons and holes are generated into the electroluminescent material, which then recombines to form excitons near the emissive layer to emit light in the visible region.59,60 The organic molecules with extended π-conjugation generally consist of a hole-injection layer and a hole-transport layer along with an emissive layer and an electron-transport layer, which makes them potential candidates for the development of emitting devices. The characteristics of these OLEDs like low-weight and flexible display, high efficiency, low thickness, and wider view angle lend to it several advantages over previously known technologies. During the last few decades, the development of organic electroluminescent materials as lightemitting diodes with intense luminescent properties has attracted the attention of many scientists worldwide.61−65 The energy-

3.1. Red Light-Emitting Diodes and Color Tuning in HAB Derivatives

During the past decade, the development of deep-red and nearinfrared (NIR) emitting polymeric materials with a much larger π-conjugated system has attracted a lot of attention due to their applications in information processing, bioimaging, and telecommunication.72−76 Because red emitting organic molecules are much more susceptible toward concentration quenching, the most challenging part in fabricating deep-red OLEDs from these materials is to retain their high quantum yields in solid state, which can be easily and dramatially reduced due to π−π stacking and dipole−dipole alignments.77 HAB derivatives with strong πconjugation and weak tendency to undergo molecular aggregation are apparently one of the best chromophores for 9570


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these OLEDs. Lin and co-workers18 reported the first bright-red OLED from star-shaped HAB-appended dithienylbenzo[2,1,3]thiadiazole-based donor−acceptor arrays 26a−c and 27a−c (Figure 8), which possessed high glass-transition temperatures Tg (123−202 °C) and high decomposition temperatures (402− 574 °C). The comparison of derivatives 26a−c with 27a−c showed that molecules with higher molecular weight, 27a−c, exhibited higher Tg and decomposition temperatures. In addition to this, derivatives 26a−c emitted in the purple to blue region whereas derivatives 27a−c emitted in the red region, and among derivatives 27a−c, derivative 27a took the minimum time (τET = 2.60 ns) for energy transfer to take place in comparison to 27b and 27c (5.65 and 2.83 ns, respectively). Thus, an OLED was integrated using derivative 27a as a hole-transporting as well as emitting layer with configuration as ITO/27a (40 nm)/TPBI (40 nm)/Mg/Ag; ITO/27a(40 nm)/Alq3 (40 nm)/Mg/Ag; (III) ITO/NPB (40 nm)/27a (10 nm)/TPBI (40 nm)/Mg/Ag [ITO = indium tin oxide; TPBI = 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene; Alq3 = tris(8hydroxyquinolinolato)aluminum(III); NPB = 1,4-bis(1naphthylphenylamino)biphenyl]. Thus, molecule 27a exhibited highly efficient intramolecular energy transfer and bipolar character, which is used as either hole-transporting or both hole-transporting and emitting layer in fabricating orange emitting electroluminescent OLED devices. Thus, strongly suppressed self-quenching tendencies of star-shaped HAB derivatives make them an ideal candidate for energy-harvesting and light-emitting materials. As an alternative to avoid concentration quenching, the doping approach is invented to fabricate red emitting OLEDs, although nondoped devices are preferred due to more simplified device formation. Thus, Liu and co-workers69 reported a benzo[1,2,5]thiadiazole-appended HAB derivative 28 (Figure 8), which exhibited very weak solvatochromism in different solvents and stable photoluminescence in different concentrations as well as in solid state so as to confirm the absence of intermolecular dipole− dipole and π−π interactions. The benzo[1,2,5]thiadiazole showed 95% quantum yield in solution state, 78 which dramatically decreased in solid state due to aggregation, which Liu and co-workers improved by appending it with a hyperbranched scaffold. Being a function of emitter thickness, the electroluminescence quantum efficiency of the devices was used to determine the width of the carrier-recombination zone.79 With an increase in thickness of the emitter up to a limit, the width of the carrier-recombination zone was enlarged so as to increase the quantum efficiency of the device. The electroluminescent properties of this thermally stable (Td ≈ 340 °C) red emitting molecule 28 showed that, with an increase in thickness of the thin film of this nondoped material, the intensity of the emission band in photoluminescent spectra increases along with a red shift in emission wavelength and color purity. The OLEDs were integrated with the following configuration ITO/NPB (30 nm)/28 (20−50 nm)/TPBI (30 nm)/LiF (1 nm)/Al using sequential thermal-deposition technique. With an increase in the thickness of the film of derivative 28 from 20 to 50 nm, the brightness of 835 cd m−2 and current density of 200 mA cm−2 increased to 1572 cd m−2 with current density of 200 mA cm−2 and more than double quantum efficiency. In another very interesting report on red light-emitting metal− organic charge-transfer complex, Yam and co-workers80 reported on how the emission can be finely regulated toward lower energy upon the introduction of HPB units to the amine groups. In this work, the intense emission from intraligand CT excited state has

Figure 9. Rhenium complex of HAB-substituted amino derivative 29.

been observed due to a donor-π−acceptor-π type structural hybrid of rhenium(I) and HPB (Figure 9). The emission wavelengths of Re complex of amines appended with HAB unit were observed to be more red-shifted than those in the case of pentaphenylbenzene, carbazole, and phenylacetylene. This is because larger π-conjugation would increase the electrondonating abilities of HAB-functionalized amine, resulting in a much narrower HOMO−LUMO energy gap in comparison to triphenylamine. On the basis of the computational studies performed, HPB-based rhenium(I) complex 29 was selected for fabrication of organic light-emitting diodes (OLEDs) with an emission extending up to 800 nm (Figure 9) and achieved a maximum luminance of 920 cd m−2 by doping only 5% of complex 29. 3.2. Blue Light-Emitting Diodes and Color Tuning Based on HAB Derivatives

Among various OLEDs, development of blue emitting organic diodes is still a big challenge. Because spectra-stable blue lightemitting materials can be easily converted to green or red light emitters with proper doping, these OLEDs are also most stipulated as well. Most promising blue emitting polyfluorene derivatives are highly prone to intermolecular aggregation to reduce the quantum yield of materials. On the other hand, blue emitting bulky dendrimers that require tedious synthetic procedures are also not considered as hopeful by synthetic chemists. However, natively blue emitting HAB has been thoroughly studied as an organic chromophore as the substitute of dendritic polyphenylenes and conjugated polymers. Yang and co-workers20 reported two hexakis(9,9-dihexyl-9H-fluoren-2yl)benzene-based HAB derivatives 30a−b and 31a−b (Figure 10) appended with carbazole (Cz) and N,N-diphenylamino (DPA) groups, respectively, using the Buchwald−Hartwig coupling protocol. These derivatives have suitable alkyl chains so as to provide solubility to the molecules. In addition to this, these star-shaped molecules, due to their highly amorphous nature, were capable of forming impeccably even film and show very high thermal stability with high Tg values. The carbazole and diphenylamino groups introduced the good hole-transporting properties in these molecules. The X-ray crystallographic structures of both derivatives 30−Cz and 31−DPA belonged to P1 triclinic space group in which fluorene arms attached to C2/C2A were nearly perpendicular to the central benzene with dihedral angles of 85.0° and 86.6°, respectively (Figure 11), while the smaller dihedral angles of 78.3° (C1/C1A) and 72.4° (C3/ C3A) for derivative 30 and 73.4° (C1/C1A) and 61.7° (C3/ C3A) for derivative 31 were observed. Due to more rigidity of the peripheral carbazole units, the derivative 30 exhibited lesser rotation of freedom and thus larger dihedral angles than in derivative 31. The solution-processed green OLED devices using 30 and 31 as hole-transporting materials exhibited maximum current efficiency up to 6.2 cd A−1 and maximum power efficiency of 3.1 lm W−1 and 6.2 lm W−1, 9571


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Figure 10. Fluorenyl group appended HAB derivatives 30a−b and 31a−b.

Figure 11. Single-crystal structures of 30−Cz and 31−DPA (hydrogen atoms and n-hexyl chains were omitted for clarity). Reproduced with permission from ref 20. Copyright 2012 Royal Society of Chemistry. Fluorine-based, star-shaped HAB derivatives 32 a−c.

respectively. These efficiencies were drastically higher than the widely used 1,4-bis((1- naphthylphenyl)amino)biphenyl NPBbased contemporary control device with similar hole-transporting materials.81−83 Thus, we can conclude that the more rigid substituents and more twisted molecular geometry can help to reduce hole mobility to support better electron−hole fluxes to enhance the native LED properties of the HAB core. In a continuation of this work, recently Yang and co-workers84 reported three fluorene-based, star-shaped HAB derivatives 32 a−c (Figure 11) bearing very good electron-transporting pyridyl

groups at the periphery. All three derivatives showed good thermal stability and high electron affinity. However, the position of the pyridyl groups significantly affects the HOMO and LUMO of the respective molecules, which results in differences in the electron-transfer properties of the derivatives. As an electrontransport material, 32b exhibited better device performance (current efficiency of 5.6 cd A−1 and quantum efficiency of 4.68%) in multilayer blue phosphorescent OLEDs than did 32a and 32c, which is reportedly because of easier electron transport due to the lower LUMO level of 32b. 9572


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Figure 12. Scheme for synthesis of HPB-based polycondensation products 36−39.

Figure 13. HAB-based hole-transporting molecules 40−42.

maximum luminescence of 251 cd/m2, with the maximum luminescence efficiency of 0.46. This signifies that a hyperbranched HPB derivative consequently results in suppression of intermolecular excimers, although if not controlled, the synthesis of hyperbranched HAB derivatives can lead to unwanted gelated product. Tao and co-workers86 further broadened the applications of HPB derivatives by decorating it with hole-transporting pyrene amine moieties for integration of OLED devices. Because the charge transfer (CT) from amine to pyrene in these derivatives was a polar interaction and was severely affected by the polarity of media, the effect of polarity on the emission wavelength was studied, which showed that an increase in polarity leads to the bathochromic shift. The OLEDs based on derivatives 40a−f and 41 (Figure 13) as hole transporter were combined with two different electron transporters 1,3,5-tris(N-phenylbenzimidazol2-yl)benzene (TPBI) and Alq3, and it was observed that a brightblue emission (maximum luminescence 21 110 cd/m2 at 14 V for 40a) was observed in the case of TPBI whereas green emission was observed in the case of Alq3 (maximum luminescence 16 210 cd/m2 at 14 V for 40c). It is believed that the LUMO barrier of the Alq3/HTL (hole-transporting layer) interface is greater than that in the case of TBPI/HTL, due to which the mobile electron

In another study that exhibits the role of the covalently attached hole-transporting unit to HAB, Li et al.85 synthesized a new series of hyperbranched polymers 36−39 by reaction of HPB and oxadiazole by the A3+B2+C2 approach using the onepot Suzuki polycondensation reaction between 33 and 34 using boronic ester 35 (Figure 12) in the presence of phenylboronic acid and bromobenzene. The molar ratios of precursors 34 and 35 were determined from relative integration of phenyl and methylene protons in 1H NMR spectra of polymers. The role of oxadiazole moiety was to balance the charge transport by good electron-transporting properties with high thermal and chemical stabilities and high photoluminescence efficiency. However, the extent of restriction of the anticipated excimer formation in these thermally stable (Td > 310 °C and Tg > 155 °C) hyperbranched molecules can be estimated from the solid-state emission spectra of these derivatives, which showed the absence of the excimer emission band at longer wavelengths (500−600 nm). Because of the good electron-transporting tendency of the introduced oxadiazole groups, the electroluminescent properties of 38 were greatly improved to give the maximum luminescence of ∼550 cd/m2 and the maximum luminescence efficiency of 0.72 cd/A at an input voltage of 16 V in comparison to their least-efficient counterpart without oxadiazole moiety, i.e., 36, which gave a 9573


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Figure 14. Fluorene- and/or triarylamine-substituted HPB derivatives 43−47.

of molecules became the main reason for the excellent behavior of the OLED, which exhibited bright-blue emission of 400 Cd/ m2 at a driving voltage of ∼10 V and a maximum efficiency of 0.2 Cd/A at a driving voltage of 7.5 V with a current density of 22 mA/cm2. In addition, polymer 44 showed a decomposition temperature of 426 °C, thus promising very high thermal stability. In a very recent report on HAB-based polymers as emissive layers in OLEDs, Wang et al.95 synthesized polymer 45 (Figure 14), which shows contrasting solvatochromic and electrochromic color-changing properties. It exhibited a solvatochromic shift in fluorescence emission from green to orange with an increase in solvent polarity, whereas its conjugated polymeric film on ITO or graphene−poly(ethylene terephthalate) (PET) surfaces showed a color change from yellow to deep blue with an increase in applied voltage. At 0 V potential, the flexible film of the polymer on ITO shows a significant absorption band in the NIR region (at 1231 nm), which increases significantly, accompanied by the formation of new bands at 510 and 1030 nm with an increase in applied potential leading to color change from pale yellow to blue. Similar behavior was observed in the case of polymer-coated graphene−PTE sheets. This electrochromic behavior has been attributed to the intervalence charge transfer due to delocalization of radicals generated from triarylamine over the HPB core, which makes the incorporation of the HPB unit necessary to enhance NIR absorption. Another new blue emitting compound based on HPB group was reported by Park and co-workers96 in which an aromatic amine moiety was introduced as a side group to improve hole mobility by preventing intermolecular interactions. In addition to this, the effect of various side groups on the device efficiency and emission wavelengths was studied. The fabricated OLED device structures were ITO/2-TNATA 60 nm/NPB 15 nm/EML 35 nm/TPBI 20 nm/LiF 1 nm/Al 200 nm, where 2-TNATA (4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)triphenylamine) was used as a hole-injection layer, NPB (N,N′-bis(naphthalene1-ly)-N,N′-bis(phenyl)benzidine) was used as a hole-transporting layer, and the synthesized materials 46a−c were used

easily jumps off the HOMO of the HTL, resulting in leaking into the Alq3 layer to raise the number of holes in the recombination zone. To restrict the recombination zone within the HTL layer, the hole-blocking layer (HBL) was used. Because TBPI is capable to block the hole transport while encouraging electron transport into the hole-transporting layer,87−90 its thin film was used as HBL in the device, which results in pure deep-blue emission instead of green-colored emission. These blue lightemitting properties with high thermal robustness of these organic materials subsequently raised the scope of HPB derivatives for electroluminescent applications. In another recent report, Park and co-workers91 compared the performances of newly synthesized HPB derivatives 42a−c (Figure 13) as emissive layers in blue−green emitting OLEDs depending upon their side groups (triphenylamine, pyrene, and anthracene). The higher band gap in the case of 42a resulted in a high energy barrier for electron transport and in high operating voltage and low current efficiency (2.1 cd A−1). Derivative 42c, having pyrene side groups, exhibited a maximum current efficiency of 4.80 cd A−1. For development of blue OLED devices, it is mandatory to avoid the formation of aggregates emitting at longer wavelengths. The bulky polymeric molecules bearing side chains have advantages like enhanced solubility and low degree of aggregation, which results in high HOMO−LUMO gap to favor stable organic blue emission.92,93 Müllen et al.94 prepared pentaphenylbenzene-based bulky polymer 44 incorporating fluorene moieties from 43a under Ni(0)-mediated Yamamoto conditions and compared its thermal and photophysical properties with monomeric HPB derivative 43b (Figure 14). However, the polymerization of HPB derivative 43b was not successful under these conditions. The steric effects of dendrimeric side chains in the solid state were studied by the crystal structure of HPB monomer 43b, which exhibited cocrystals with chloroform. This study suggested that the HPB units are situated perpendicular to the fluorene unit, resulting in shielding of the polymer backbone to prevent the π−π stacking between the adjacent molecules. This suppression of aggregation 9574


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substituents and preferably star-shaped HPB with more twisted molecular geometry can dramatically enhance the quantum yield and hence the performance of OLED device. The HABs with different functional groups or HAB itself as a nonplanar substituent to conjugated polymers can alter its band gap so as to help in tuning the color of emitting light over a broad range of wavelengths. Thus, using different combinations or doping of suitable fluorophores with HAB derivatives can result in highly efficient light-emitting devices in the future that may prove to be tough contenders for conventional light-emitting diodes

as an emitting layer (EML) in the corresponding OLED (Figure 14). TPBI as an electron-transporting layer (ETL) and HBL, lithium fluoride (LiF) as an electron-injection layer, ITO as anode, and Al as cathode were used. The external quantum efficiencies and Commission Internationale de l’Eclairage (CIE) values of devices corresponding to emitting materials 46a, b, and c were 1.89%, 3.59%, 3.34%, and (0.154, 0.196), (0.150, 0.076), (0.148, 0.120), respectively, with an emitting area of 4 mm2. Among all, 46b and 46c showed better thermal stability with high Td of 448 and 449 °C, respectively, and superior blue emission. The derivative 46b showed pure color coordinates in turn to be used in colorful displays, thus delivering a practical application of HPB derivative in the field of electronics. Recently, Jana and Ghorai97 reported the synthesis of three HAB end-capped tri(phenylenevinylenes) 47a−c (Figure 14) by the Horner−Wadsworth−Emmons reaction. All three derivatives 47a−c exhibited high quantum efficiencies of 51−72% in the blue emitting range of 445−492 nm. Good thermal stability (at 335−420 °C, 5% weight loss in nitrogen) and higher HOMO level (−5.42 eV) were observed, which suggests that these HPB derivatives can be employed in light-emitting applications.

4. MOLECULAR INCLUSION AND RECEPTOR PROPERTIES OF HAB The study of host−guest chemistry has mainly two-dimensional significance, i.e., molecular recognition and supramolecular catalysis. The incorporation of metal ions into the aromatic systems via weak noncovalent interactions has shown remarkable influence on the chemical properties of the basic scaffold, which renders potential applications in electrical conductors and photosensitive devices.98−102 These weak noncovalent forces between host and guest play a vital role both in supramolecular chemistry and biological sciences.103,104 Conventionally, the macrocyclic derivatives based on crown ethers, cyclophanes, cyclodextrins, calixarenes, and cryptands were the most used host molecules.105−112 Lately, a significant consideration has been given to architecture and engineering of multitopic ligands to yield metallasupramolecular structures with predetermined shapes, geometries, and symmetries. Many such inclusion compounds have been reported, forming discrete nanoscopic supramolecular species.113−119 Transition metal ions are ubiquitous constituents of materials with intriguing electronic, magnetic, and catalytic properties. At the atomic level, metal inclusion chemistry has been well-explored to develop new chemical tools for metal organic arrays. However, there is a paucity of HAB derivatives that possess these chemical tools. A few HAB analogous hetero-oligopolyphenylene derivatives doped with nitrogen or sulfur atoms have been synthesized to act as receptors for metal ions. However, for bulky and heavily substituted derivatives like HAB, the metal incorporation can be achieved by utilizing both metal−π interactions and anionassisted intermolecular bridging to construct a two-dimensional network with a nonplanar architecture. One of the first reports based on chemical interactions between metal and HAB was given by Ning et al.,120 which accounted for cationic tetra-silver complex with HPB and its exceptional structural parameters with potential applications as functional materials. Later on, Rathore and co-workers did a significant work to explore the range of supramolecular interactions between HAB derivatives and different alkali/alkaline/transition metals or organic molecules. The molecular inclusion chemistry of HAB investigated by this group considerably raised the scope of HAB derivatives for many applications like electrochemical or fluorescence molecular detection, metal−HAB ensemble-based host molecule, metal/ molecular assisted self-assembly of nonplanar molecules, etc.

3.3. HAB Derivative As Hole-Blocking Layer for White Emitting Diodes

White OLEDs are the most promising devices for the production of large-area light sources with high efficiency to fight the world energy crisis. In the only report on HAB-based high-performance white OLEDs, Qing and co-workers19 fabricated an OLED using 48 as a hole-blocking layer in a diode structure ITO/NPB(40

Figure 15. Hexafluorenylbenzene 48.

nm)/48(5 nm)/Alq3(50 nm)/Mg/Ag(200 nm), processed by thermal evaporation. To improve the performance of the OLED device, the derivative 48 (Figure 15) was used as an HBL. The performance of this device was compared with an OLED having barium-2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a conventional HBL. At a forward bias of 15 V, the maximum luminance of the device based on 48 (8523 cd/m2) was ∼1000 cd/m2 greater in comparison to the device based on BCP. Unlike in the case of BCP, the luminescence of the OLED device was not affected by the thickness of the amorphous film of 48. Therefore, this work signifies the thermal and morphological stabilities of these star-shaped molecules over other amorphous materials. From the above discussion, we may conclude that a nonplanar scaffold of π-conjugated HAB derivatives strongly contributes toward restricting intermolecular aggregation and high quantum yields. These properties of HAB derivatives place these molecules among the best electroluminescent chromophores as functioning light-emitting devices. Also, the use of rigid

4.1. Role of Cation−π Interactions in Molecular Inclusion and Receptor Properties

Because cation−π interactions mainly depend on electrostatic interaction of metal ions, cations with higher charge density are most likely to bind with the π-e− cloud of HAB. Initially in 1989, HPB coordinated compound (η6-C6Ph6)Co(η2-C2Ph2) was reported by Biagini et al.121 where HPB complex (η6-C6Ph6)Co(η2-C2Ph) was formed as a result of a fast irreversible 9575


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Figure 16. ORTEP diagram and labeling of Ag4(HPB)(ClO4)4 49, Ag2(HPB)(CF3SO3)2(toluene) 50, and Ag2(HPB)(ClO4)2(THF)2 51. Reproduced with permission from ref 122. Copyright 1999 American Chemical Society.

derivatives, Gross et al.124 used an approach in which organic derivative was interacted with metal surface to intercalate atoms of the same metal. The hexa-tert-butylhexaphenylbenzene derivative 53 was designed and synthesized that was made to bind with a Cu(111) surface (Figure 17) through metal−π interaction instead of cation−π interaction. When pushed with the scanning tunneling microscope (STM) tip toward a Cu adatom, the HPB derivative oriented itself to locate the adatom between two adjacent arms to behave as a pair of forceps. The intercalation of Cu adatom within HPB 53 decreased the Cu-tosurface bond strength owing to direct interaction between the HPB’s π orbitals and the 4s orbital of the Cu adatom. Due to metallic cohesion between the atoms in the cluster with an increasing number of Cu atoms under the molecule, the individual Cu−molecule bond energy reduced, whereas the binding between Cu adatoms was increased. This increased the stability of a small flat metal cluster under the molecule and decreased the mobility of the Cu−HPB complex. This work would facilitate the interconnection of a molecular device to welldefined atomic-scale metallic electrodes on the surface of an insulator. Unlike in HPB derivatives specifically, heteroatoms in HAB derivatives play an important role in cation−π interactions. When a lone pair of electrons is not involved in the aromaticity of the ring, it weakens the cation−π bonding because of its strong

disproportionation when 0.03 M solution of [Co(py)6]+ was reacted with diphenylacetylene in the presence of pyridine. In this complex, the central benzene interacted with the cobalt ion in a symmetric η6 fashion. However, in a report by Ning et al.,120 Ag(1) involved two types of bondings with HPB, i.e., η1 and η2, with Ag−C distances lying between 2.420 and 2.75 Å, respectively. In a continuation of this work, Ning et al. prepared three different coordination complexes 49−51 by interaction of Ag+ ions and propeller-shaped HPB by varying the anions and solvents.122 The polymeric networks of 49 and 50 were formed by the insertion of ClO4− and CF3SO3− anions between two silver adducts using toluene as a solvent, whereas the dimer of 51 was formed in the presence of tetrahydrofuran (THF) and ClO4− ion. This study explains the role of the unique aromatic geometry of the ligand and its host−guest compatibility with sterically

Figure 17. Alkali metal complex 52, formed from interaction between HPB and Li+ ions, and hexakis(4-tert-butylphenyl)benzene 53.

demanding and nonspherical anions, as well as the effect of different solvents as shown in Figure 16.122 However, Bock et al.123 (Figure 17) reported an ultrasonically activated reaction of HPB in 1,2-dimethoxyethane (DME) with lithium metal powder to enforce 2-fold dehydrogenation and formation of two additional C−C bonds to give unexpected metal complex [(9,10-diphenyltetrabenz[a,c,h,j]anthracene)2Li+(DME)][Li+(DME)3] 52. According to the proposed mechanism, which was also supported by experimental and quantum chemical calculation, a Li-assisted skeletal deformation of HPB produced a sterically overcrowded molecular dianion, the dynamics of which led to a 2-fold H2 split-off and C−C bond formation. Two-electron reduction to the dianion as well as its Li+ contact ion pair formation was likely to result in skeletal distortion of HPB followed by disrotatory rotation dynamics enforced by spatial overcrowding. This disrotatory rotation dynamics turned the two ortho-phenyl substituents into each other, resulting in much closer neighboring phenyl hydrogen atoms. It resulted in the easier H2 split-off and C−C bond formation simultaneously. In another report presenting the radical applications of metal-inclusion abilities of HAB

Figure 18. Isomeric HPB derivatives 54 and 55.

electronegative character. Rathore and co-workers prepared HPB derivative 54125 in which the ethereal oxygens were covalently linked through polymethylene bridges present on a single face (Figure 18) along with its unsymmetrical isomer 55 as a side product. The bipolar receptor site of compound 54 made irreversible binding of a single K+ ion possible due to interaction of metal ions with the central benzene ring via cation−π interactions. These cation−π interactions were well-known to exist in gas phase126 and solid state,127 and they play a major biological role by keeping tertiary structures of various proteins stable toward denaturation.128 The interactions and the stoichiometry of potassium ions with respect to the HAB unit were studied by 1H NMR spectroscopy, which confirmed the 9576


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high binding constant for 1:1 stoichiometric complex. This strategy to generate this rotamer bearing ether groups and mimicking famous ethereal crown structure, as well as selective binding with K+ ion to the central benzene ring through synergistic interactions, was a step forward for using HAB derivatives as a molecular sensor. Later, in extension to the receptor properties of these HPB-based derivatives, Rathore and co-workers129 reported a complementary experimental and theoretical approach toward complexation of HPB-based receptor 54 with the protonated tyramine (HL+). In this report, investigation of solvent extraction of protonated tyramine into nitrobenzene by means of a synergistic mixture of cesium dicarbollylcobaltate and derivative 54 was carried out. Interestingly, a high stability constant of log βnb = 4.7 ± 0.2 of the 54− HL+ complex in nitrobenzene saturated with water was determined at room temperature, which was almost the same as that for the 54−Cs+ complex. Therefore, like K+ ions, HL+ cation can also bind with three ethereal oxygens through hydrogen bonding with synergestic cation−π interaction with the central benzene. Recently, Rathore and co-workers130 used the same approach to study complexation of molecule 54 with the pyridinium cation (HP+) through cation−π interactions in nitrobenzene with a high stability constant of 4.6 ± 0.2.

Figure 20. HPB- and pentaphenylbenzene-based dendrites 58 and 59.

and 59 contained internal voids because of their rigid framework and allowed the selective detection of polar aromatic analytes with high sensitivity and accuracy depending upon the binding site. This selectivity of host molecules positively depended upon the relative π-electron density of host and guests which means that the guest molecules are required to possess a π-e− density, nonidentical to host molecule. In addition to being highly selective, these derivatives were very stable both chemically and temperature-wise and were said to give high reproducibility. Uniquely, the other hyperbranched monodisperse macromolecules also exhibited the same sensitivity as shown by dendrimers 58 and 59. Therefore, these macromolecules offer very good gravimetric sensors based on quartz microbalance (QMB) to monitor the concentration of volatile organic compounds. Rathore and co-workers12 studied the binding behavior of previously reported HAB derivatives 54 and 55 for acetonitrile. The symmetrical derivatives 54/55 (Figure 18) were crystallized by slow evaporation of their solution in 9:1 dichloromethane/ acetonitrile, which showed the formation of two symmetrically independent but identical adducts 54·CH3CN and 55·2CH3CN (Figure 21) due to C−H···O and C−H···π interactions. The cavity with a depth of 2.1 Å and a radius of 3.15 Å was big enough to accommodate the CH3 group to facilitate binding of the acetonitrile molecule. However, the C−H···O and C−H···π interactions were more favorable for the 55·2CH3CN adduct than for the 54·CH3CN adduct depending upon their geometrical parameters. Thus, for designing molecular receptors for more effective binding, the synergic effect of different weak forces like C−H···O and C−H···π is needed to be taken under consideration. Recently Peng et al.133 studied the encapsulation behavior of brominated HPB derivatives 60−62 (Figure 22) toward solvent molecules like CH2Cl2, diethyl ether, and toluene in the solid state. Although derivative 60 encapsulated CH2Cl2 through C− H···π interactions effectively, it chose diethyl ether over CH2Cl2 in the diethyl ether/CH2Cl2 system through C−Br···O interactions. Dibrominated HPB derivatives 61 did not encapsulate any solvent in any solvent system and preferably underwent C−Br···π interactions with the same molecules to give a one-dimensional chain, which further underwent π−π interaction to form two-dimensional chains. Interestingly, monosubstituted derivative 62 selectively binds the toluene molecule in the diethyl ether/CH2Cl2/toluene system through C−Br···π bonding.

Figure 19. HAB-based receptors 56 and 57.

Rathore and co-workers131 reported another similar HPB derivative 56 and its unsymmetrical isomer 57, which contained six peripheral 2,5-dialkoxytolyl groups that behaved as electroactive aryl groups, which was a prerequisite for the study of toroidal electronic coupling in HAB derivatives (Figure 19). The unsymmetrical isomer 57 underwent 6 reversible 1-e− oxidations in the range of ∼600 mV, which indicated that the circularly arranged aryl units were electronically coupled. In comparison to 56, the unsymmetrical derivative 57 utilized the closer position of the ethereal oxygens to make a complex with K+ ion, which, however, can be decomplexed using electrochemical oxidation to achieve reversibility, which is the prerequisite for materialization of an electrochemical sensor. 4.2. Molecular Inclusion in HAB Derivatives Due to π−π and C−H···π Interactions

The affinity of HABs toward metal ions discussed in the last subsection led the researchers to another perspective of the metal−HAB relationship, which is host−guest chemistry. Müllen and co-workers132 reported suitability of HPB-based derivative 58 and 59 (Figure 20) as sensor layers over the surface of a quartz microbalance for detecting volatile organic compounds like acetophenone, aniline, benzaldehyde, benzonitrile, fluorobenzene, nitrobenzene, and 2-methylbenzonitrile. These hosts 58 9577


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Figure 21. Partial space-filling representation of the [54·CH3CN] complex and the [55·2CH3CN] complex. Reproduced with permission from ref 12. Copyright 2007 Royal Society of Chemistry.

(Figure 24a). On the other hand, AMP bound to Zn2+ ions was accompanied by additional hydrogen bonding between the hydrogen of the phosphate group of AMP and the nitrogen of the quinoline moiety without breaking the existing 63−Zn2+ bonds (Figure 24b). Using the fluorescence response of the zinc ensemble of derivative 63 toward H2PO4− ions and AMP, a multichannel keypad system was constructed that depends not only on the inputs themselves but also on the order of the inputs (Figure 25). Keeping in mind the versatile performance of derivative 63, the highly amorphous nature and electroluminescent properties of the HPB core can be well considered to enhance the scope for materializing molecular sensory devices. Such an extraordinary sensitive detection of intracellular zinc by 63 in living cells enhanced the scope of HAB derivatives in biological applications. However, at the same time, high energy emitting HAB derivatives with small emission Stokes shifts are apparently not a suitable choice for practical biosensing purpose. To overcome this drawback of the HAB central core, we tried to introduce resonance energy transfer in HPB derivatives to push the final emission wavelength toward longer wavelength range so as to increase the native fluorescence Stokes shift of HAB. We135 synthesized rhodamine B-appended HPB-based Hg2+ sensors 64 and 65 (Figure 26). These disubstituted 64 and hexasubstituted 65 HPB derivatives undergo competition between fluorescence resonance energy transfer (FRET) and through-bond energy transfer (TBET) from donor HPB to acceptor RhB on addition of Hg2+ ions depending upon the solvent polarity (Figure 27). Disubstituted HPB derivative 64 displayed quite high TBET efficiency in the presence of Hg2+ ions in polar protic solvents, whereas derivative 65 exhibited better FRET efficiency in the presence of Hg2+ ions in nonpolar aprotic solvents than derivative 64. Figure 27 shows the schematic representation of FRET and TBET depending upon the solvent polarity. However, no such resonance energy transfer was observed in model compound 66 in which acceptor RhB is linked with donor HPB unit without any spacer (Figure 26), thus confirming the role of the spacer in FRET and TBET. Therefore, the limitation of HAB or particularly HPB derivatives of exhibiting low wavelength emission can be resolved by using FRET/TBET, which can effectively raise the possibility of these molecules in biological applications without exhibiting very stringent effects toward living cells. In a continuation of this work, recently we reported an HPB derivative 66 (Figure 26)136 that exhibited throughbond energy transfer (Figure 27B) in the presence of picric acid in methanol and serves as a “turn-on” sensor for picric acid (PA). In addition, derivative 66-solution-coated test strips were prepared that can optically detect PA with a detection limit of 104.9 pg cm−2. Wang and co-workers137 synthesized HPB-based, star-shaped molecules 67 and 68 based on the HPB core in good yield

Figure 22. Brominated HPB derivatives 60−62.

Figure 23. Quinoline-appended HPB derivative 63.

4.3. Fluorogenic Receptors Based on HAB Using Coordination Interactions

The inspiring work reported in the past decade led us to take this field of research a few steps forward. During the last 5 years, we have reported different HAB derivatives appended with various binding units to study the change in supramolecular interactions of HAB with respect to guest molecules. First, we reported a quinoline-appended HPB derivative 63 (Figure 23), which selectively served as a fluorescent chemosensor for Zn2+ ions through metal−quinoline coordination bonds among various other metal ions tested, even in the presence of bovine serum albumin and human blood serum.134 Derivative 63 was also used for in vitro bioimaging of highly accumulated Zn2+ ions in malignant prostate cancer cells without adding extracellular Zn2+ ions. Further, the zinc ensemble of derivative 63 served as a potential chemosensor for H2PO4− ions and adenosine monophosphate (AMP) (Figure 24a,b). The zinc ensemble of 63 exhibited strong binding ability toward H2PO4− ions and AMP but with different emission outputs. It was concluded from the fluorescence results that addition of H2PO4− ions to the solution of Zn ensemble of 63 resulted in weakening of existing 63−Zn2+ bonds, which led to the disintegration of the metal ensemble 9578


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Figure 24. Schematic representation of interaction of (a) H2PO4− ions and (b) AMP with the zinc ensemble of derivative 63.

(Figure 28) using Ullmann condensation or Suzuki coupling methods. The small star-shaped derivative 67 formed a complex with Ag+ ions (1:6) to possess crystallographically C3 symmetry and acquired bowl-shaped geometry due to intramolecular ionic interactions. On the other hand, π−π interactions prevailed in R3 symmetric Cu2+ complex of large star-shaped molecule 68. The effects of these metal ions on the emission properties of these molecules were quantified. The addition of 1 equiv of Ag+ ions to the solutions of 67 resulted in 55% emission quenching, which further decreased by 4% on addition of another equivalent of Ag+ ions. However, complete quenching was observed when 2 equiv of paramagnetic Cu2+ ions were added to 68. In addition to this, on subsequent addition of a few equivalents of protons, both ligands 67 and 68 showed quenching in emission intensity with a red-shift of 60 and 22 nm in emission band, respectively, with color change visible to naked eye. Therefore, owing to 7azaindolyl- and 2,2-dipyridylamino-functionalization in 67 and

Figure 25. Multichannel keypad system. Reproduced with permission from ref 134. Copyright 2012 American Chemical Society.

Figure 26. Di-, hexa-, and monosubstituted HPB-RhB derivatives 64−66. 9579


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Figure 27. Schematic presentation of (A) FRET and (B) TBET phenomenon in derivative 64 depending upon the polarity of solvents. Reproduced with permission from ref 135. Copyright 2013 Royal Society of Chemistry.

Figure 28. Star-shaped, HPB-based receptors 67 and 68 and HPB-appended phthalocyanines 69a−b.

which phthalocyanine complexed with Co2+ ion was surrounded by pentaphenylbenzene derivatives so as to avoid the intermolecular interactions (Figure 28). The binding pockets around the phthalocyanine core can undergo selective ligation control depending upon the steric recognition of the molecule. The binding of small molecules like 3-methylpyridine with

68, these molecules served as potential sensing materials for fluorescence and optical sensing of metal ions and pH. Besides behaving as a binding host itself, HAB can also be used to manipulate other covalently linked molecules to generate much more prominent binding sites. In such a context, Kimura et al.138 reported HPB-based metalophthalocyanines 69a−b in 9580


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derivative 69a−b resulted in a change in their absorption and emission spectra, which entitles these derivatives to serve as receptors. The central role of merged HPB was (a) to create nanometer-ordered pockets around phthalocyanine, which accommodated eight tert-butyl groups in 69a and enhanced the selectivity and sensitivity of binding sites of Cophthalocyanines, and (b) to prevent intermolecular aggregation of phthalocyanines in polar solvents. Keeping in view the above-mentioned reports, we may conclude that a strong π-e− system in HAB derivatives is the major reason for its superior molecular inclusion abilities. Owing to this highly delocalized π-e cloud but a nonplanar structure, the skeletal system itself has strong capacity to accommodate some metal ions and small aromatic molecules through metal−π and π−π interactions. HAB scaffold can be engineered by introducing specific binding sites in molecules to bind metal ions depending upon the geometry and electronic structure of those binding site. These abilities of HAB derivatives have many future applications such as metal-assisted self-assembled nanomaterials, molecular receptors and biosensors, etc.

Figure 29. HAB derivatives 70a−c and 71a−c.

5. SELF-ASSEMBLY, AGGREGATION, AND SUPRAMOLECULAR INTERACTIONS Self-assembly of molecules into nanostructures offers many potential applications in nanotechnology, biologically active materials, and nanoscale devices.139−143 Besides, the selfassembly can economically yield a thermodynamically stable product in high yield. The self-assembly depends upon many parameters like size and geometry of molecule, as well as extent of inter-/intramolecular supramolecular interactions like Hbonding, π−π, electrostatic, and hydrophobic interactions. The intermolecular self-assembly can be brought into effect by external electric field,144 magnetic field,145 or flow field,146 solvent processing, thermolysis/pyrolysis,147 templating, and insertion of foreign matter.148 The synthetic chemists have utilized many noncovalent interactions to direct the spontaneous self-assembly of supramolecular systems in a manner similar to what has been followed in natural materials. The self-assembled molecular membranes with a few nanometers range thickness are very much likely to help in the separation of materials on the basis of relative rate of passage of the selected gas or liquid molecules than any conventional filter.149 Additionally, these self-assembled nanostructures are widely used in stretchable electronics, as mechanical resonators in nanoelectromechanical systems, and as biomimetic interfaces for molecular recognition and sensing.150−152 HAB-based, star-shaped derivatives are generally reluctant to undergo π−π interfacial self-assembly due to their nonplanar scaffold. However, supramolecular assembly between HAB units can be encouraged by other intermolecular noncovalent interactions between its peripheral functionalities.153,154 Such supramolecular assemblies of π-conjugated HAB systems can offer to construct semiconducting wires in the 5−100 nm length scale using bottom-up strategy.155−159 These isolated or coagulated nanowires can be sandwiched between standardized electrodes in electronic devices to produce current owing to their conductive nature.

Figure 30. Polarized optical micrographs of (a) 70a at 64 °C on cooling between crossed polarizers; (b) 70b after annealing at room temperature for 2 days; and (c) 70c at 100 °C upon cooling between crossed polarizers. Reproduced with permission from ref 1. Copyright 2001 Royal Society of Chemistry.

compositions and length of aryl arms. Out of three molecules, namely, hexakis(4-dodecylphenyl)benzene 70a, hexakis(4′dodecylbiphenylyl)benzene 70b, and hexakis[4-(5′-dodecyl-2′thienyl)phenyl]benzene 70c, that possessed ordered columnar mesophase at temperatures accordingly (Figure 30a−c), derivative 70c showed a much higher clearing point because of the stronger intermolecular interactions induced by the presence of a dipole resulting from the lone electron pairs on the sulfur atom (Figure 30c). It shows that the introduction of thiophenes

5.1. Star-Shaped HAB Derivatives Undergoing Columnar Self-assembly

Among the very first reports about HAB-based, self-assembled liquid crystalline materials, Müllen et al.1 reported star-shaped HAB derivatives 70a−c (Figure 29) and studied the mesomorphic behavior of these molecules depending upon the 9581


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around HPB can result in high-order self-assembly. On the other hand, the replacement of phenyl groups with thienyl or bithienyl

Figure 32. (a) STM images of monolayers of derivative 72 physiosorbed at the graphite−1-phenyloctane interface. (b, c) Zoomed-in STM images of star-shaped molecules. Reproduced with permission from ref 162. Copyright 2009 American Chemical Society.

Figure 31. HPB derivatives 72. Images from polarized optical microscopy of the star-shaped OPV 72 by cooling from the isotropic phase at rates of (a) 10 °C/min, (b) 2 °C/min, and (c, d) 0.2 °C/min. Reproduced with permission from ref 160. Copyright 2007 American Chemical Society.

of those monolayers at the interface of graphite and 1phenyloctane (Figure 32) showed OPV units appearing bright, which characteristically reflected the star-shaped molecules. However, the orientation of flexible long alkyl chains could not be seen in STM images. The study revealed that both parallel, adjacent OPV units (at intersection of disks shown in Figure 32a, inset) can lead alkyl chains to undergo van der Waals interactions with each other and get desorbed. The zoomed-in images, i.e., Figure 32b, c, showed star-shaped morphology along with the molecules bearing four or five peripheral legs. The desorption of an OPV leg considerably decreased stability of the adjacent OPV leg in the neighboring hexapod and desorbed it too. Strikingly, the extent of OPV desorption was much greater in disordered domains (i.e., 80%) than in ordered 2D crystalline structure (4%). Further, different dynamics and the multidimensional characteristics of interactions between OPV and substrate were also studied well using time-dependent STM experiments and molecular dynamics simulations. Aida and co-workers163 demonstrated the formation of the first inorganic/organic polypseudorotaxane by the templateassisted cofacial assembly of a ring-shaped molybdenum cluster (MC) with a rigid-rod molecule having a high affinity toward the MC surface. An experiment was based on the expectation that HPB 73 (Figure 33) could bind MC more strongly than tetraaminoporphyrin (TAP) and even p-phenylenebutadiynylene polymer (PB). In fact, mixing MC/TAP complex with HPB resulted in the recovery of the fluorescence of TAP and quenching of the HPB fluorescence, which indicates the elimination of TAP by HPB (Figure 33). On the other hand, when MC/HPB was titrated with PB14, the fluorescence of 73 remained quenched, which indicates that HPB was bound with

groups (71a−c) (Figure 29) resulted in a less-ordered mesophase and low transition temperature due to distortion in conventional packing resulting from the 1,4-substituted phenylene unit. Later Schenning and co-workers160 reported a star-shaped oligo(p-phenylenevinylene) (OPV)-substituted HAB 72 (Figure 31) in which π-conjugation increased without any noncovalent interactions to stabilize the self-assembly even at high temperature, and that too at low concentration. This star-shaped HAB derivative formed highly stable and ordered columnar superstructures at room temperature to give a plastic crystalline material. Polarized optical microscope (POM) images of 72 on annealing showed fan-shaped birefringence, which revealed the pronounced tendency of 72 to undergo self-assembly to yield highly organized macroscopic superstructures.161 Interestingly, the size of this birefringence increased by several hundred micrometers with a decrease in the cooling rate (Figure 31a−d). Besides, derivative 72 forms ordered monolayers at the liquid− solid interface with a chiral hexagonal lattice, which made these molecules attractive for application in supramolecular electronic devices. Schenning and co-workers used the system 72 for an intricate study of molecular dynamics of discrete molecular units in these self-arranged monolayers, which were physisorbed at a liquid− solid interface.162 The STM analysis of these monolayers showed the molecules following both disordered domains and twodimensional ordered crystals. Large- and small-scale STM images 9582


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Figure 33. HPB derivative 73 and schematic illustration of possible liberation of TAP from MC/TAP complex. Reproduced with permission from ref 163. Copyright 2008 Wiley VCH.

Figure 34. Phosphonic acid−HPB molecule 74 and 2D wide-angle X-ray scattering (WAXS) pattern and representation of the columnar structure. Reproduced with permission from ref 153. Copyright 2009 Wiley-VCH.

Figure 35. HPB derivatives 75 and 76a−c.

architecture at high concentration through phosphonic groups were the reasons for good proton conductivity through this crystalline material. Besides, columnar stacking of molecules provided a channel for proton transport. In addition, constrained water molecules, immobilized in the crystal hydrate, provided assistance to proton transport. The temperature-independent high proton conductivity of 74 satisfied one of the qualifying conditions for development of new separator materials in fuel cell systems that had outgrown the state-of-the-art polymeric and inorganic materials. Sebastiani and co-workers164 quantitatively determined proton diffusion in derivative 74 by ab initio MD simulations. The temperature showed a significant effect on the displacement of protons. With an increase in temperature from 400 to 600 K, the number of proton displacements above 3 Å

MC much more strongly than with PB14. Because the MC is a mixed-valent inorganic cluster with chromophoric characteristics, its optoelectronic properties have attracted the attention of many researchers. Being highly reluctant to undergo intermolecular cohesion, HAB derivatives are known to possess a strong dependence on terminal substituents to undergo self-assembly. For example, Müllen and co-workers153 reported a crystalline organic material 74 (Figure 34) in which proton conductivity, unlike in common amorphous polymer electrolytes, was not supported by waterbased diffusion. The columnar structure of 74 (Figure 34) contains proton-conducting peripheral groups and an insulating core due to which it possessed inverse proton conduction. The amphoteric nature of the molecule and the self-assembled 9583


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uting to the C3 symmetry of planar derivative 77 and the adjacent hydrophobic tert-butyl groups, the synthon (H2O)2(COOH)3(L) served trigonal planar node, where ligand L is acetone [(77)(H2O)2(acetone)3]. On the other hand, trigonal pyramidal nodes were formed when bipyridine, azopyridine, trans-1,2-bis(4-pyridyl)ethene, and 1,2-bis(4pyridyl)ethane were used as ligands in [(77)(bipy)0.5(H2O)2], [(77)(azopy)0.5(H2O)2], [(77)(bipy-ete)0.5(H2O)2], and [(77)(bipy-eta)0.5(H2O)2], respectively. Thus, parallel polycatenated frameworks were formed using four different bis(pyridyl) ligands. The weaker hydrophobic interactions between tert-butyl groups were compensated by dimeric H-bonds between neighboring carboxyl groups to accomplish a closepacked structure. The hydrogen-bond-assisted self-assembly of 77 in a nonprotic solvent was justified by a model synthon (H2O)2(COOH)3 that possessed ordered geometry due to modest H-bonding strength in nonpolar solvent. However, in protic solvent (acetone/methanol), [(77)(MeOH)3], solvent serves as H-bond donor to form one-dimensional H-bonding chains with H-bond acceptors. This work illustrates the significance of the intrinsic structure of organic tectons in the formation of solid-state crystal. Besides, it helps in predicting the type of interactions involved in the crystal depending upon dimeric and one-dimensional chain structure. Molecular Landers (78) are a class of compounds that adsorb the molecule on a surface of substrate and take the aromatic board off the surface due to its bulky side groups and aromatic core. Various applications of these Landers have extensively been explored in the field of nanotechnology and as “molecular molds”, which involves boarding metal atoms underneath their aromatic cores. Keeping these applications in mind, Yu et al.168 reported the adsorption of a 1,4-bis(4-(2,4-diaminotriazine)phenyl)-2,3,5,6-tetrakis(4-tert-butylphenyl)benzene 78 (Figure 36) on Cu(110) and Au(111) surfaces using ultrahigh-vacuum (UHV) conditions. The diaminotriazine groups were introduced to invoke the intermolecular H-bond-assisted molecular organization through intermolecular H-bonding. Thus, molecular Landers based on derivative 78 are likely to help in a future study of H-bond-assisted molecular self-assembly. Further, a high-resolution STM manipulation study of Lander molecule 78 on the Cu(110) and Au(111) substrates showed that the energy surface for the close-packed and inert Au(111) surface is smaller than that of the more open, anisotropic Cu(110) surface. These results are useful for the future development of new suitable substrates to assemble one-dimensional molecular chains. These reports clearly demonstrate the capability of substituted HAB derivatives to undergo intermolecular self-assembly. An HAB skeletal system, which itself is highly reluctant toward intermolecular aggregation, can be structurally molded by varying its peripheral functionalities so as to render cohesive tendencies in molecules. The C6-symmetric, star-shaped HAB derivatives with interactive functional groups are reportedly prone to undergo columnar stacking, which can be switched to another topological structure by changing the molecular symmetry. We can say there are lot of unexplored possibilities of structural alterations that can enhance the practical utility of HAB derivatives based on their self-assembly behavior.

increased from 4 to >20. Thus, highly protic functional groups like phosphonic acid groups can be incorporated into a suitable aromatic core to develop new materials for “dry” fuel cell membrane. Floudas and co-workers165 reported an amphiphilic biphasic perfluorocarbon attached to HPB cores 75 at adequate distance to explore dipolar abilities in CF3 surroundings in consideration of self-aggregation. The fluoride dipoles of fluorinated alkyl chains (C12F23H2) in 75 offered great analogy with the vicinity of CF3 functional groups. The dynamics study of hexa(3,5substituted-phenyl)benzene 75 (Figure 35) using dielectric spectroscopy by using varying temperature and pressure exhibited four possible phases related to CF3 environment, i.e., isotropic, liquid-like lamellar, solid lamellar, and glassy state. These bulky perfluoroalkyl chains (C12F23H2) are rigid and hydrophobic/oloeophobic in nature, and their incompatibility toward hydrocarbons (C12H25) encouraged bilayered lamellar aggregation through fluorophobic effects. The introduction of a chiral substituent at the periphery of HPBs can introduce propeller chirality in the molecule to provoke the Cotton effect in the circular dichroism spectrum. As reported by Kosaka et al.,166 the Cotton effect dominated (76c > 76b > 76a) with the increase in the number of chiral groups on the periphery of HPB derivatives 76a−c (Figure 35). The induced propeller chirality of the molecule depends upon the whizzing toroids, which are affected by the solvent polarity and the temperature. Nonpolar solvents and low temperature favor a high Cotton effect, whereas polar solvents and high temperature enhance whizzing toroids in molecules. Therefore, high temperature and polar solvents encouraged interfacial π−π interactions, which were confirmed by the blue-shift in absorption spectra of HABs. On the other hand, low temperature and nonpolar solvents led to a red-shift in the absorption band of HABs, which corroborates the formation of partially πoverlapped J-aggregates. Thus, introduction of propeller chirality in HAB derivatives can stimulate the possibility of regulating their conventional self-assembly protocols.

Figure 36. C3- and C2-symmetric HAB derivatives 77 and 78.

5.2. Self-assembly in Low-Symmetric HAB Derivatives

These star-shaped, C6-symmetric, unique conformations have proved to exhibit sextuple growth of π···π interactions due to terminal substituents. However, to develop and study the different intriguing topological structures in crystal engineering of the HAB family, one of the best ideas is to vary the peripheral substituent, in a way to vary the molecular symmetry. Keeping this perspective in mind, Zheng and co-workers167 developed intriguing topological geometries using HPB 77 and the (H2O)2(COOH)3 supramolecular synthon (Figure 36). Attrib-

5.3. Hydrophobic Aggregation in HAB Derivatives and Their Applications

Because of the presence of a free rotatable group present at the periphery of central benzene ring, the HAB derivatives are known to exhibit aggregation-induced emission enhancement in the 9584


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Figure 37. Mechanism of aggregation-induced emission enhancement (AIEE).

Figure 38. Pyridyl-appended HAB derivative 79 and two-input, three-output combinatorial molecular logic circuits using (a) biomolecules BSA and cytochrome as inputs A and B and (b) Pb2+ and Pd2+ ions as inputs A and B, based on Truth tables 1 and 2, respectively.The emission wavelengths have been signified as corresponding outputs. Reproduced with permission from ref 174. Copyright 2012 Wiley-VCH.

by scanning electron microscopic (SEM) images. These fluorogenic aggregates further served as efficient chemosensors for discrimination of biomolecules like bovine serum albumin (BSA), lysozyme, cytochrome c, pepsin, Papain, and Trypsin from each other. On the other hand, these aggregates were able to selectively detect single-stranded DNA over structurally analogous RNA. On the basis of the fluorogenic results of these aggregates toward different analytes, two combinatorial logic gates were constructed (Figure 38a, b) using biomolecules (BSA and cytochrome c) and metal ions (Pd2+ and Pb2+ ions) as inputs and respective emission wavelengths as outputs. In bioelectronic logic gate (Figure 38a), we designated biomolecules BSA and cytochrome c as inputs A and B, whereas the emission wavelengths at 340, 365, and 432 nm were designated as outputs 1, 2, and 3, respectively. To follow the Boolean arithmetic, emission intensity ≤ 25 was assigned logical value “0”, which

presence of bad solvents. The aggregation-induced emissionenhancement phenomenon is ascribed to the restriction of rotation and torsion of the intramolecular bond by greatly blocking the nonradiative channel, which effectively suppresses the self-quenching and enhances the fluorescence. The fluorescence enhancement is attributed to the following two reasons (i) restriction in rotation of rotors, thus increasing intramolecular π-conjugation and resulting in high luminescence and (ii) energy of photons absorbed by the aggregates being less dissipated (Figure 37).169−173 From our lab, we174 reported first HAB derivative 79 (Figure 38) appended with pyridyl groups, which exhibited the AIEE phenomenon in H2O/THF (8:2). The spherical aggregates formed due to hydrophobic interactions between amphiphilic molecules of derivative 79 resulted in higher fluorescence quantum efficiency. The formation of aggregates was confirmed 9585


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Figure 39. Proposed compartmentalization mechanism of reduction of Pd(II) to Pd(0) by innerpool of aggregates of HAB derivative 79 and HAB derivative 80.

represented the OFF state, while emission intensity ≥ 25 was assigned logical value “1”, which denoted the ON state (Truth table 1, Figure 38). Similarly, in combinatorial chemionic logic gate (Figure 38b), Pb2+ and Pd2+ ions were designated as inputs A and B, whereas emission wavelengths 365, 328, and 472 nm represented outputs A, B, and C, respectively (Truth table 2, Figure 38). Further, these aggregates showed a selective response toward Pb2+ and Pd2+ ions among various other metal ions tested. Being nonpolar in nature, the micelle-like spherical aggregates of derivative 79 resulted in reduction of Pd(II) to Pd(0) to form palladium nanoparticles (NPs) by compartmentalization mechanism (Figure 39).175 These palladium NPs are well-known to serve as a potent catalyst for various chemical reactions.176 Very recently, we reported a similar HAB derivative 80177 (Figure 39) that underwent maximum AIEE in 60% water in acetonitrile. The center of these hydrophobic aggregates behaves as a microreactor for Au(III) ions to give gold NPs with diameters of 10−26 nm. These NPs were in situ adsorbed on the surface of oxidized multiwalled carbon nanotubes (MWCNTs) using ultrasonication. The resulting Au-MWCNT materials were used as potential

recyclable photocatalysts for carrying out degradation (98% in 45 min) of rhodamine dye in aqueous media with a reaction rate constant of 0.066 min−1. In a continuation of this work, carbazole was then incorporated instead of pyridine to enhance the degree of AIEE in derivative 81 (Figure 40) due to high electroluminescent properties of carbazole.178 To study the effect of rotors on AIEE, a pentaarylbenzene (PAB) 82 was also synthesized and its AIEE properties were compared with that of the HAB analogue (Figure 40). It was found that the degree of AIEE was greater in derivative 82 than in hexasubstituted derivative 81. Lower quantum yield of 82 as compared to that of 81 was because derivative 81 had one additional rotor in comparison to 82, which sterically hindered the rotation of rotors around their axis in 81. This means that the extent of rotation of rotors in compound 81 was already low as compared to that in compound 82. Therefore, the relative degree of restriction in rotation of rotors, and the resulting AIEE, was greater in the case of compound 82 in comparison to that in compound 81. The aggregates of derivative 81 were found to be efficient chemosensors for the detection of trinitrotoluene (TNT) in solution as well as in vapor phase. Addition of TNT to the solution of aggregates of 81 resulted in quenching of emission due to CT from photoexcited HAB to electron-deficient TNT. Besides, fluorescent test strips and thin films were prepared for the convenient and economical optical detection of TNT (Figure 41) in vapor phase (Figure 41A). These test strips detected TNT up to picomolar concentration, which is low enough for its trace detection in war-affected areas (Figure 41C). Besides, the thin films of these compounds on glass plate exhibited quenching of emission on addition of solution of TNT. We179 also reported an HPB derivative 83 appended with hydroxynaphthalene through imine linkages (Figure 42). This molecule behaved as a selective fluorescence turn-on sensor for cyanide ions, and the CN− ensemble of this molecule served as a

Figure 40. Carbazole-based HAB derivative 81 and PAB derivative 82. 9586


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The detection of nitroderivatives using fluorescent materials generally involves the quenching of emission due to electron transfer (ET) or/and energy transfer between fluorescent substrate (donor) and nitroaromatics (acceptor). In the case of HPB derivatives, due to large HOMO−LUMO gaps, the ET mechanism often reduces the chance of selective detection of nitroderivatives. To overcome this problem, we reported recently a HAB derivative that showed a no-quenching approach toward the detection of picric acid (PA). Groups of 2-pyridyl and 3-pyridyl containing HAB derivative 84 (Figure 43) were synthesized that showed maximum AIEE in 60% water in ethanol.180 These aggregates served as very selective and sensitive chemosensors for PA in solution and in solid state. The local emission band at 339 nm corresponding to fluorescent aggregates of derivative 84 got quenched on addition of PA accompanied by the formation of a new band at 446 nm. The spectral studies indicated that the quenching of native band was mainly due to energy transfer, whereas the formation of a new band at 446 nm was possibly due to excited-state intermolecular CT (Figure 43). After the reports on the detection of nitroexplosive analytes using HAB-based ensemble aggregates, an amide group containing HPB derivative 85 (Figure 44) was synthesized for detection of cyanide ions in the aggregated state.181 The molecule showed maximum AIEE in 80:20 water/ethanol, and the resulting emitting aggregates served as a potential chemosensor for the detection of CN− ions. The mechanism of detection of CN− ions and its reversible behavior in the presence of trifluoroacetic acid is shown in Figure 44. In addition to that, 2.6 ng/cm2 detection limit with high sensitivity was achieved by using disposable filter paper-based test strips, which provides a simple and low-cost protocol for the on-site instant detection of cyanide ions in aqueous phase. From our lab, we also reported an AIEE-active HPB derivative 86a182 having azaindole groups (Figure 45). The aggregates of 86a showed high affinity (detection limit = 64 nM) toward Fe3+ ions in the nanomolar range. Interestingly, these aggregates of 86a served as reactor and stabilizer for the preparation of ferromagnetic α-Fe2O3 NPs at room temperature within 30 min. Furthermore, these α-Fe2O3 NPs exhibited excellent catalytic activity for carrying out Pd, Cu, and amine-free Sonogashira cross-coupling reactions under mild conditions. These NPs also showed catalytic activity in photocatalytic degradation of RhB dye in the presence of hydrogen peroxide with the rate constant of 1.79 × 10−3 s−1. Interestingly, these ferromagnetic α-Fe2O3

Figure 41. Paper strip test. (A) Vapor mode detection of TNT, before (a, c) and after (b, d) placing the test strips of 82 and 81 over the glass vial containing TNT for 5 min. (B) Before (a, c) and after (b, d) dipping the test strips of 82 and 81, respectively, in the TNT solution (10−3 M in THF). (C) Application of a small spot of different concentrations of TNT ((i) 10−3 M, (ii) 10−5 M, (iii) 10−7 M, (iv) 10−9 M, and (v) 10−12 M) on test strips of 82 (I) and 81 (II). (D) Change in the fluorescence of 82 and 81 in the solid state in the presence of TNT. (a, c) Thin films of 82 and 81, respectively, (b, d) after adding one drop of TNT solution (10−4 M in THF) on thin film of 81. All images are taken under 365 nm UV illumination. Reproduced with permission from ref 178. Copyright 2012 American Chemical Society.

colorimetric and fluorogenic receptor for TNT. Interestingly, the fluorescent aggregates of 83 also undergo modulation on addition of CN− ions (Figure 42a−d). The fluorescent test strips allowed visible detection of traces of TNT by the naked eye with a detection limit of 23 ag cm−2.

Figure 42. 2-Hydroxynaphthalene-appended HPB derivative 83 and TEM images (a, b) before and (c, d) after the addition of 200 equiv of CN− ions in aggregates of 83 in H2O−EtOH (6:4, v/v) solution. Reproduced with permission from ref 179. Copyright 2013 Royal Society of Chemistry. 9587


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Figure 43. Mechanism of the recognition behavior of aggregates of 84 toward picric acid in H2O/EtOH (6/4). Reproduced with permission from ref 180. Copyright 2014 Royal Society of Chemistry.

Figure 44. Mechanism of detection of CN− ions using HPB derivative 85.

Figure 45. HAB derivatives 86−88.

NPs could be easily separated from the reaction mixture by a magnet, and the separated catalysts could be recycled 5 times without significant loss in their activities. Recently we utilized these α-Fe2O3 magnetic NPs as an excellent catalyst for Sonogashira and hetero-Diels−Alder reactions of activated as well as inactivated substrates. These NPs can be reused after their convenient extraction from the reaction system. The Sonogashira coupling and Diels−Alder reactions using these recyclable NPs gave moderate to very good yield (50−86% and 40−94%, respectively).183

By changing the side groups of the HPB core, the binding ability of the molecule can be tuned toward different metal ions. To tune the binding behavior of HPB derivative, we replaced the azaindole group in 86a with the thiophenolic group in 86b (Figure 45). The aggregates of AIEE-active derivative 86b selectively bind Ag+ ions to produce blue luminescent silver nanoclusters at room temperature without using any external reducing agents. These nanoclusters behave as excellent catalysts for cocyclization of isocyanides with terminal alkynes to produce corresponding pyrrole derivatives.184 In another recent report 9588


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Figure 46. Tetraphenylethene−HPB (TPE−HPB) adducts 89−93.

nm, respectively, which were ∼10 nm blue-shifted from those of 89 and 93, which confirmed that the electronic transitions of the luminogens largely depends upon their electronic structure of molecules (Figure 46). Recently, Chang et al.188 studied the effect of extension in bulky radial arms of HAB derivatives on their AIEE properties by changing the number of bridging groups between the central HPB core and six peripheral tetraphenylethene groups. With more extended conformation, derivative 94b experiences less steric repulsion due to less crowded periphery in comparison to 94a. Therefore, derivative 94b exhibited a higher degree of AIEE and higher quantum yield than 94a. However, only 94a revealed unique reversible piezofluorochromic effect, due to which the sky-blue emission of pristine crystal of 94a is changed to green− blue emission after grinding with pestle, accompanied by a redshift of 30 nm in the emission band. Because this piezofluorochromic effect occurs due to the transformation from the crystalline state of 94a to its compact planar amorphous state after grinding, this effect can be reversed by reviving the crystalline state through heating or solvent treatment. Shionoya and co-workers16 prepared self-assembled organic capsule 956 formed by induced-fit model due to hydrophobic interactions between amphiphilic molecule 95 (Figure 47). Earlier, the same molecule reportedly formed hexameric boxshaped aggregates 956 due to coagulation of six monomeric units by hydrophobic interaction in aqueous methanol.189 This hexameric capsule allowed the inclusion of biomolecular guest like 2,4,6-tribromomesitylene. Further, the authors used this molecule for the inclusion of adamantane ada-956, which not only served as guest molecule but also worked as a template for the formation of tetrahedron-shaped capsule, thus resulting in switching into aggregated form. Interestingly, this tetrahedron capsule performed a unique function of reversible encapsulation

from our lab, an AIEE-active HAB derivative 87185 has been synthesized (Figure 45) that form aggregates due to hydrophobic interactions. These aggregates served as reactors and stabilizers for the preparation of copper NPs in aqueous medium. These Cu NPs served as a reusable catalytic system for the alkyl−azide “click” reaction to synthesize 1,2,3-triazoles in excellent yields under solvent-free conditions. These reports establish the role of various amphiphilic HAB derivatives as generators as well as stabilizers of different metal NPs that can be further utilized for various catalytic, sensing, and biological applications. Besides metal ions, the interactions of molecular aggregates of HPB derivatives with nonmetallic guest molecules were also studied in our lab recently.186 The spherical stable aggregates of HPB derivative 88 (Figure 45) transformed into flower-shaped aggregates in the presence of H2S. These highly photostable, nontoxic aggregates exhibited great permeability in living HeLa cells to selectively detect H2S in two-photon microscopy, resulting in emission of bright-green light. Tang and co-workers,187 who pioneered the AIEE phenomenon, reported AIEE-active tetraphenylethene−HPB (TPE− HPB) adducts 89−93 (Figure 46) in which twisting amplitude and steric hindrance of the TPE and HPB units were found to play a significant role in their fluorescence behavior in the aggregated state. Because four phenyl rings in TPE can undergo better free rotation with large twisting amplitude in comparison to the sterically hindered phenyl rings in HPB, a weak photoluminescent (PL) peak at 334 nm in THF was observed that corresponds to HPB and no emission corresponding to TPE was observed in all derivatives 89−93. In 80% H2O in THF, the emission of HPB was 12-fold that in THF accompanied by a 5 nm blue-shift in the emission maximum, thus confirming the phenomenon of AIEE. Compounds 91 and 92 carry two HPB units in one molecule, and these molecules absorb at 310 and 324 9589


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Figure 47. HPB derivatives 94a−b and 95. Hexameric self-assembled 956 and reversible encapsulation and release of adamantane by regulating acidity and basicity. Reproduced with permission from ref 16. Copyright 2009 Wiley-VCH.

5.4. Metal-Induced Self-assembly in HAB Derivatives

and release of adamantane by regulating acidity and basicity as shown in Figure 47. Thus, this moderately emitting scaffold can be made to undergo aggregation to enhance the fluorescence quantum yield using hydrophobic interactions, metal binding, etc. Completely defying the established idea of aggregation-caused quenching, AIEE is an excellent approach to generate highly emitting materials that can be used for sensing of biomolecules, nitro explosives, metal ions, anions, amino acids, etc. These reports suggest that HAB derivatives can be topologically engineered to increase the rotatable groups with high electroluminescence properties so as to increase the percentage enhancement due to AIEE. The sensing properties of HAB aggregates are reportedly superior to that by a single HAB molecule. Therefore, these molecules can be envisioned as next-generation self-assembled carbon-based materials for molecular-detection purposes.

The HAB-based ligands containing three monodentate ligands alternatively attached to the benzene ring were envisaged to conveniently accommodate appropriate metal ions to favor linear coordination geometry. Keeping this in view, Shionoya and coworkers190 reported tris(thiazolyl) 96 and hexa(thiazolyl) 97 (Figure 48) using trimerization of corresponding acetylene precursors. The sandwich-shaped heterotopic trinuclear Ag+ complex Ag−96/97 was formed from two different disk-shaped ligands and three Ag+ ions. The metal−ligand exchange between the two neighboring thiazolyl nitrogen donors of 97 reportedly took place at the three Ag+ centers, which was proved by variabletemperature 1H NMR studies. The ΔH and ΔS for the exchange process were calculated to be 50.5 kJ mol−1 and −26.7 J mol−1 K−1, respectively. The metal−ligand exchange during complexation was accompanied by intramolecular 60°-rotational motion between the two disk-shaped ligands. Such counter rotational motion of adjacent counterparts of a molecule can help to design novel metal-mediated molecular devices for application in multicomponent molecular machinery. The supramolecular interactions between transition metal ions and HPB, a trismonodentate ligand 98 (Figure 49), were reported by Shionoya and co-workers;17 these interactions formed structurally equivalent coordination capsules [M6L8]12+ with different divalent transition metal ions, M2+ (M = Mn, Fe, Co, Ni, Pd, Pt, Cu, Zn, Cd, and Hg), through self-assembly. The resulting complexes had an octahedron-shaped structure with six metal ions lying on the apexes and each of the eight sides

Figure 48. HAB derivatives 96 and 97. 9590


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Figure 49. Pyridine-appended HAB derivatives 98 and metal-exchangeable capsule of derivative 95. Figure adapted from ref 39. Reproduced with permission from ref 17. Copyright 2006 Wiley-VCH.

Figure 50. HAB derivative 99 and schematic representation of hierarchical arrangement of Ag+ and Hg2+ ions between two disk-shaped hexamonodentate ligands 99. Reproduced with permission from ref 193. Copyright 2006 American Chemical Society.

affinity of oxazolyl ring toward both Hg2+ and Ag+, it preferably binds with Ag+ due to higher electrostatic repulsion between adjacent Hg2+ ions. This approach indicates that minimizing the electrostatic repulsion between two metal ions by synthetic strategies can easily regulate the binding affinities of different sites for different metal ions in one molecule (Figure 50). In a continuation of this work, we194 reported a HPB derivative 100a (Figure 51) in which the HPB unit is linked with N,Ndimethylphenyl groups through imine linkage. The sp 2 hybridized nitrogens of the imine group were incorporated to create a binding site for soft Hg2+ ions. AIEE-active derivative 100a formed spherical fluorescent aggregates in H2O/THF (4:6), which on addition of Hg2+ ions resulted in metalmodulated rod-shaped aggregates. These metal-ion-modulated aggregates were further used for the selective and sensitive detection of picric acid (PA) among other analogous nitroaromatic explosives tested. The addition of Hg2+ ions to the solution of aggregates of metal complex of derivative 100a resulted in quenching in fluorescence due to the proton transfer from PA to N,N-dimethyl groups followed by electron transfer from photoexcited HPB to electron-deficient PA (Figure 51a). Another HPB derivative 100b appended with two pyrene units (Figure 51)195 generated fluorescent aggregates in aqueous media (Figure 51b, c). Interestingly, on addition of Hg2+ ions to these aggregates, the formation of a network of nanofibers (Figure 51d, e) was observed that resulted in enhancement of

occupied by a ligand. These metal-exchangeable capsules were capable of site-specific replacement of internal axial ligands of the six-metal center and were provided with an isolated cavity for the molecular recognition and catalysis depending upon the size of the molecule. Shionoya and co-workers189 further explained structurally and electrostatically controlled molecular capturing191 and demonstrated the self-aggregation of a positively charged mercury cage complex of trismonodentate ligand 95 (Figure 47) and its interconversion to neutral mercury capsule complex in response to 95/Hg2+ (Figure 49). This dynamic interconversion resulted in molecular fluorescence switching192 between fluorescent cage complex and nonfluorescent capsule counterpart. Further, hexagram-shaped molecule 95 was transformed into a self-assembled organic capsule driven by hydrophobic, van der Waals, and CH−π interactions in aqueous methanol. This work introduced a new league for more complex and well-defined discrete self-assembled structures of large molecules, as seen in many biological events by means of not only directional but also directionless binding forces. In a very interesting example of selective binding of two different metal ions to a single host molecule carrying two chemically different binding sites, Shionoya and co-workers193 reported advanced guest-inclusion chemistry in which two different metal ions, i.e., Ag+ and Hg2+, bound hierarchically to two different binding sites between the two disks of hexamonodentate ligand 99 (Figure 50). Despite the high 9591


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Figure 51. HPB derivative 100a−b. (a) Proposed fluorescence-quenching mechanism by picric acid for 100a. Reproduced with permission from ref 194. Copyright 2013 American Chemical Society. SEM images (b, c) before and (d, e) after the addition of 20 equiv of Hg2+ ions in aggregates of 100b in H2O/EtOH (1:1, v/v) solution. Reproduced with permission from ref 195. Copyright 2013 Elsevier.


detection limit was achieved with fluorescent test strips prepared from aggregates of 100b, which offered an economical method for on-site detection of traces of PA in affected areas. To confirm the role of pyrene unit in formation of nanofibrils, a model HPB

intensity of the emission band at 456 nm corresponding to the excimer of pyrene along with a red-shift of 12 nm. This Hg2+− 100b hybrid was further used as an efficient chemosensor for PA among various other nitro-derivatives tested. The femtogram 9592


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Figure 53. HPB-based polymers 107−110.

Figure 54. HPB-based intrinsic microporous polymers 111 and 112.

hydrogen bonding, metal-binding abilities, and self-assembly properties. Similar to the previous report,197 isomers 105 and 106 (Figure 52) were produced from cyclotrimerization of respective unsymmetrical 1,2-disubstituted acetylene. The flocculant nanostructures of derivatives 105 turned into beltlike structures after interaction with Ag+ ions, which led to a red-shift in the emission band, whereas no such switching in aggregates or shift in emission band were observed in the case of derivative 106. However, addition of In3+ ions to aggregates of both derivatives 105 and 106 resulted in spherical aggregates with rough and smooth surfaces, respectively. We can conclude here that the proper radial functionalization of HPB derivatives can regulate metal-binding affinities and the resulting self-assembly properties, which play a key role in steering different morphologies.

derivative bearing naphthalene units was also synthesized that, however, did not show formation of any such fibrous structures in the presence of Hg2+ ions. This report from our research group established the commercial possiblilty of anion and metal ensembles of HAB derivatives for successful fluorogenic detection of nitroexplosives like picric acid and TNT. In another report from our lab, we controlled the hydrophobic aggregates by regulating the amount of water followed by reformation of aggregates induced by metal binding that resulted in ratiometric response to finally emit at longer wavelength. In this report,196 the larger aggregates of derivative 101 (Figure 52) were treated with Zn2+ ions to reform smaller metal-based aggregates. These aggregates exhibited “ON−ON” response toward threonine and detected it in the nanomolar range. On the basis of the fluorogenic response of aggregates of 101 toward Zn2+ ions and threonine, a 2-input, 3-output sequential logic circuit was constructed at the molecular level. Recently, Zhang et al.197 reported synthesis of pyrimidinecontaining HPB derivatives 102−104 (Figure 52) through cyclotrimerization of appropriate disubstituted acetylenes. The symmetric 1,2-disubstituted acetylene resulted in formation of a single major product 102, whereas unsymmetrical 1,2-disubstituted counterparts produced two isomers, 103 and 104, under similar conditions. Derivative 102 showed very contrasting binding behavior toward Ag+, Cu2+, and Zn2+ ions in fluorescence spectra. It exhibited fluorescence quenching in the presence of Cu2+ ions due to transformation of nanospherical aggregates into nanofibers. The addition of Ag+ ions resulted in deposition of Ag nanoparticles over nanospheres of 102, which led to a blue-shift in the native emission band. However, the red-shift in the emission band of 102 on addition of Zn2+ ions has been attributed to the formation of belt-shaped nanostructures. Very recently, the same group tuned the aggregation properties of an HPB derivative containing pyrimidine groups by introducing carboxylic side-arm groups.198 The purpose of the introduction of carboxylic groups is to enhance the degree of

6. MOLECULAR NETWORK AND INTRINSIC MICROPOROSITY The intrinsic properties of conjuagated microporous molecules with large specific area, high chemical and thermal stabilities, and low framework density has attracted great interest of researchers in recent years.199−204 These molecules have shown great results in heterogeneous catalysis and gas storage. These conjugated microporous materials can be obtained by selecting rigid and contorted building units, which are known to offer high flexibility in molecular engineering. In addition to commonly used planar π-conjugated system molecules, the tetrahedral carbon-based and analogous silicon-based building blocks can also be used in the synthesis of organic materials with exceptional hydrogenstorage ability.205 The nonplanar HAB derivatives usually end up in imperfect packing in aggregated and solid state, which has already been illustrated by several examples of crystal-structure analysis of its derivatives.13,14,206−209 This property of HAB generates high internal molecular free volume and results in 9593


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Figure 55. Synthetic scheme for synthesis of polymers 113 and 114.

Figure 56. HPB derivative 115 and resulting cycloparaphenylene 116 and polyphenylene 117.

subunits under mild reaction conditions. This methodology gave new and easy access to generate carbon-rich polymers at low temperatures. McKeown and co-workers211 reported a cyclic oligomer 111 and a linear polymer 112 from HAB monomeric units linked through dicyanocatechol (Figure 54). The permeabilities of various gaseous molecules in microporous sites of robust thin films of these derivatives were measured by the time-lag method. It was found that derivative 112 exhibits greater microporosity and better permeability of gas molecules with larger Brunauer− Emmett−Teller (BET) surface area of 527 m2 g−1 in comparison to derivative 111 (162 m2 g−1) on the basis of N2 adsorption isotherm. However, the order of permeability is CO2 > H2 > He >

formation of inefficiently packed macromolecules in the solid state. 6.1. One-Dimensional Network of HAB Derivatives

Müllen and co-workers210 first reported new HPB-based soluble polymers and copolymers 107−110 via a straightforward approach (Figure 53). These carbon-rich polymers had been cyclodehydrogenated using Kovacic conditions, which is a successful method for quantitative intramolecular cyclodehydrogenation resulting in polycyclic aromatic hydrocarbons (PAHs). Various analytical methods employed were said to be indicating that the polymer-analogous cyclodehydrogenation resulted in polymers containing a large extent of large graphitic 9594


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Figure 57. Acetylene-group-substituted HPB derivatives 118a−b and crystal structures 119 and 120 for HPB and PhCCH and 118b grown from toluene/hexanes, with guest molecules of toluene omitted for clarity. Reproduced with permission from ref 215. Copyright 2010 American Chemical Society.

Figure 58. HPB derivative 121 and molecular structures of derivative 121 in (a) 121·12DMSO, (b) 121·6n-PrOH, (c) 121a·H2O, and (d) 121b·H2O. Carbon, nitrogen, oxygen, and hydrogen atoms are shown in gray, blue, red, and light blue, respectively. Reproduced with permission from ref 206. Copyright 2003 American Chemical Society.

interesting surface properties of these polymers can render gas adsorption in chemically improved HAB derivatives. Recently, Golling et al.214 used two different synthetic approaches to produce HPB-based cycloparaphenylene and polyphenylene cyclinder from precursor 115. Reduction of derivative 115 using low-valent titanium (in situ) at 80 °C resulted in formation of cyclohexamer 116, whereas mild reduction using sodium naphthalenide at low temperature (−78 °C) produced oval-shaped macrocycle 117 with an additional four hydrogens (Figure 56). This is reportedly due to a temperature-dependent shift of equilibrium structure of the tetraanionic intermediate. Both 116 and 117 were subjected to oxidative cyclodehydrogenation conditions using FeCl3 at room temperature for bottom-up synthesis of a carbon nanotube. Because of the less-strained structure, 117 undergoes cyclodehydrogenation to give ribbon-like side walls of the tubular structure. However, 116 does not undergo cyclodehydrogenation completely and loses only 20 out of possible 96 hydrogens due to its inherently congested structure.

O2 > CH4 > N2, which reflected the contribution of both solubility and diffusivity to the gas permeability.212 In this report, the HAB derivative 112 possessed high permeability and polymeric rigidity, which helped in gas-separation membranes and resulted in much less degradation of material. However, further chemical modifications in these derivatives may also improve gas-separation properties. Besides, their fluorogenic behavior also suggests the utility of such HAB derivatives as active materials in optical-sensing devices. Han and co-workers213 also utilized the nonplanar structure of HPB derivatives for designing porous materials with special properties like gas adsorption. Two HPB-based porous organic polymers 113 and 114 were synthesized using palladiumcatalyzed C−C coupling reactions (Figure 55). The study of the nitrogen physisorption isotherm revealed that the BET specific surface areas for these polymers fall in the range of 742−1148 m2 g−1. Gravimetric hydrogen adsorption isotherms gave 1.5 wt % adsorption capacity for hydrogen at applied pressure and temperature of 1.13 bar and 77 K, respectively. Because of the kinetically controlled synthesis of HPB derivatives 113 and 114, all nanoporous polymers exhibited lack of ordered and amorphous structure in X-ray diffraction pattern. Such 9595


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Figure 59. HPB derivatives 122a−e, 123a−f, and 124 and MOF topologies (a) kgd, (b) yav, (c) htp, and (d) hhp of derivative 124 with different metal ions. Reproduced with permission from ref 218. Copyright 2015 American Chemical Society.

6.2. Two- and Three-Dimensional Networks of HAB Derivatives

molecule hexakis(4-carbamoylphenyl)benzene 121, which formed different porous 3-D hydrogen-bonded networks with dimethylsulfoxide (DMSO) (121·12DMSO), propanol (121· 6n-PrOH), and water (121·H2O) (Figure 58). The stability and composition of these complexes partially relied upon the binding forces involved between 121 and solvents (i.e., hydrophobic interactions and aromatic π−π interactions), depending upon the polarity of the corresponding solvent. Besides, the coordination between H-bond donor primary amide group in 121 and the H-bond acceptor oxygen atoms of 121 and guest solvents played a major role in the formation of these networks. Thus, porous networks in organic host lattice morphology were affected by the properties of the crystallizing solvents. The 3-D H-bonded network found in crystals of 121·12DMSO (orthorhombic, Pca21), 121·6n-PrOH (monoclinic, P21/c), and 121·H2O (monoclinic, P21/c) gave different conformational structures using the solvent-diffusion method, as shown in Figure 58. From this work, we can say that the type of proton (syn or anti) in a proton donor group, involved in H-bonding, plays a pivotal role in topological isomerism of 3-D H-bonded networks, which will presumably possess different chemical and physical properties. The same group reported the synthesis of various types of hexakis(4-functionalized-phenyl)benzenes 122a−d and hexakis[4-(4′-functionalized-phenylethynyl)phenyl]benzenes 123a−f by the cobalt-catalyzed cyclotrimerization of diarylacetylenes and by the Sonogashira coupling reaction of 122d with

The intrinsic porosity generated by the nonplanar structure of the HPB core can easily invoke the acetylene binding due to extraordinary capacity of the central ring to favor C−H···π interactions. Such interactions were studied by Wuest and coworkers215 using X-ray crystallography and DFT studies. Cooling a hot solution of HPB in PhCCH gave crystals with monoclinic space group C/c and composition HPB·0.5PhC CH 119 (Figure 57). The multiple C−H···π interactions (Figure 57) and reinforced C(sp)−H···π bonds between PhCCH and the central benzene of HPB maintained this monoclinic structure.216 Although both 118a and 118b carried one and two acetylene groups, respectively, the latter one exhibited better controlled supramolecular organization in the crystal of composition 118b·toluene i.e., 120 (Figure 57) with short C(sp)−H···π interactions (2.47 Å/135° and 2.46 Å/138°) and additional C(sp2)−H···π interactions where ethynyl groups behaved as acceptors (Figure 57). These interactions introduced a new supramolecular synthon that can possibly engineer new molecular network crystals. These supramolecular networks can be applied in the formation of molecular sponges for the selective sorption of acetylene.217 Unlike C−H···π interactions, which are less directional, hydrogen bonding in symmetric HAB derivatives leads to more directional two-dimensional or three-dimensional networks. Yamaguchi and co-workers206 synthesized a new host 9596


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Figure 60. HPB- and HBC-based covalent organic framework 125−127.

arylacetylenes, respectively (Figure 59).207 Further, X-ray crystallographic analysis showed that hosts 122d and 123f form a 2-D network by unique I···I and CH···OC interactions, respectively. Thus, the formation of metal-coordinated 2-D and hydrogen-bonded 2D/3D networks with guest-inclusion ability based on hosts 122 and 123 can provide a good scope in the future. Kobayashi et al., in 2000, utilized derivative 124 (Figure 59) for creating triangular pores with a cavity size of 15.2 Å in Hbond-assisted two-dimensional network.15 However, recently, Nguyen et al.218 used this hexatopic HAB core 124 bearing six carboxylate linkers as a precursor for the synthesis of four new metal organic frameworks (MOFs) with metals like Ni, Mg, and Cu. The nickel-based MOF, with monoclinic C2/c space group, consisted of triangular single-metal Ni units bound to three hexagonal cores 124 through bidentate coordinated carboxylate groups to result in “kgd” topology (Figure 59a). The Mg-based MOF, with monoclinic P21/n space group, adopts “yav” topology due to interaction between binodal rod-shaped clusters of [Mg2(CO2)4(CO2H)2−(EtOH)2]∞ with hexagonal 124 linker with only four protonated carboxylate groups (Figure 59b). In the case of Cu 3 (CO 2 ) 6 , MOF crystals with orthorhombic P212121 space group were obtained with resulting “htp” topology in which three carboxylate units link with two Cu atoms in bridging fashion and one carboxylate unit links with two

Cu atoms to bridge it to third Cu atom, which was further coordinated to the final two monodentate carboxylate units (Figure 59c). The Cu-based MOF in the presence of H2O ligand results in Pi ̅ space group due to involvement of hydrogen bonding. The resulting “hhp” topology resulted from the linking of infinite rod-shaped clusters of trigonal-prismatic [Cu3(CO2)6(H2O)2]∞ and hexagonal 124 (Figure 59d). Although all four MOFs are capable of uptaking a moderately good amount of CO2 gas, Cu-based MOF with “htp” topology exhibited selective uptake of CO2 from the mixture containing N2 and CH4. Like MOFs, covalent organic frameworks (COFs) are also crystalline porous polymers with periodic ordering of organic building blocks without any extrinsic metal units. Being nonplanar and bulky in nature, HAB scaffold has exhibited good prospects in the formation of COFs, due to their envisioned applications in gas storage, photoconductive devices, energy conversion, and heterogeneous catalysis. Recently, Dalapati et al.219 reported a new class of triangular COFs by reaction of C6symmetric HPB or its fused analogue hexa-peri-hexabenzocoronene (HBC) bearing amine groups with terephthalaldehyde. Due to propeller-shaped vertices, the interlayer distance in HPBbased COF 125 (5.17 Å) is larger than that in HBC-based COF 126 (3.54 Å) (Figure 60). Besides, the crystal energy of 125 was calculated to be one-third of 126, which indicates tighter layering 9597


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Figure 61. Tetra-, penta-, and hexasubstituted polyphenylene derivatives 128−132.

of the graphitic cores of the framework of 126. Owing to low monomeric weight and loose interlayer packing of twodimensional sheets in 125, the BET surface area of 125 was greater than that of 126. However, the lower HOMO−LUMO gap in 126 in comparison to 125 makes HBC-based COF promise better charge carrier/transport properties than HPBbased COF. The small triangular pore size of 12 Å and high πcolumn density (0.25 nm−2) of 125 makes HPB a promising candidate for the development of COFs. Alahakoon et al.220 rather used aldehyde-functionalized HPB and hydrazine to produce the desired COF 127 (Figure 60) with triangular small pores of ∼11 Å and high BET surface area of 1214 m2 g−1. This microporous framework is capable of excellent sorption of carbon dioxide (20 wt %) and methane (2.3 wt %) at 273 K and 1 atm. Wuest and co-workers208 reported an HPB-based derivative, namely, hexakis[4-(2,4-diamino-1,3,5-triazin-6-yl)phenyl]benzene 128 (Figure 61). The diaminotriazine group was incorporated because of its reliable self-associating tendency through H-bonding motifs.221−223 The molecules of derivative 128 formed a sheetlike structure by intermolecular H-bonding in a way that each molecule was surrounded by six neighboring molecules. Further, these sheets were potentially involved in secondary H-bonding interactions to give a three-dimensional network. The H-bonds created space between the sheets, thus generating the high volume accessible to the guest molecule. Very interestingly, the network and the cavities within the network were modified by changing the number and position of H-bonding sites, for example, in the case of tetrasubstituted (131 and 132), pentasubstituted (130), and hexasubstituted (129) derivatives (Figures 61 and 62). Because these networks shared many common properties despite different conditions of crystallization, it was concluded that alteration in basic molecular structures of the HAB core can have much more significant effect on architecture of the resulting network than change in conditions of crystallization (Figure 62). Because of the lack of effective methods for the reliable prediction of structures and properties of new molecules, it was required to develop a dynamic scheme for the development of novel engineered crystals based on molecular architecture composed by supramolecular forces. Keeping this in mind, Wuest and co-workers224 reported the synthesis of hexakis(4nitrophenyl)benzene 133 (Figure 63), which undergoes crystallization to give a layered structure under various conditions and was chemically intervened by guest molecules in between those layers. A single-crystal (developed in

Figure 62. Networks corresponding to (a) derivative 128; (b) derivative 130; and (c) derivative 132. Reproduced with permission from ref 208. Copyright 2007 American Chemical Society.

dimethylformamide (DMF)/ethanol) study of derivative 133 confirmed that this type of intermolecular N−O interaction between NO2 groups of adjacent molecules can help crystal engineers to predefine geometry of molecules. The role of nitro groups was confirmed by studying the hexakis(4-cyanophenyl)benzene 122a (Figure 59), a similar molecule that was unable to engage in N···O interactions, crystallized as eight pseudopoly9598


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Figure 63. HPB derivatives 133 and 134.

Figure 64. Polymeric HPB derivatives 135−137 and monomeric HPB derivatives 138a−c.

morphs with a few shared structural features.225 Thus, this work claims the role of the NO2 group in crystal engineering of the HPB core in terms of network morphology. Kojima and Hiraoka226 reported thermodynamically controlled, selective alternate functionalization of the HPB core. Using this approach, synthesis of C3-symmetric and lowersymmetric HPB derivatives, bearing more than one kind of substituent around the periphery, will be more viable and convenient. The selective lithiation of hexakis(4-bromophenyl)benzene using n-butyllithium in moderate yield was followed by electrophilic trapping of lithiated derivatives 134a−e with a trimethylsilyl group to give derivatives 134f−g. The authors studied the stabilizing properties of trilithiated derivatives bearing different substituents. The destabilization due to the electronic repulsion between the two adjacent lithiated phenyl groups resulted in the formation of “1,3,5-type” alternately trilithiated species. Similarly, the alternately trilithiated species

acquired high stability when substituents were electron-withdrawing, 134f and 134g (Figure 63). This synthetic approach for the formation of unsymmetrically substituted HPB derivatives can enable synthetic chemists to achieve HPB derivatives with a desired substitution pattern, which was not possible by conventional synthetic approaches. The intrinsic porosity of HAB polymers can be enhanced by increasing the dimensions of their polymeric structures by introducing three-dimensional building blocks. Zhang et al.227 reported synthesis of brominated HPB-based triptycene monomer, which underwent polymerization using Ni(0)catalyzed C−C coupling and dehalogenation to give the novel organic microporous polymer 135 (Figure 64) with a high BET surface area of 1151 m2 g−1. The high microporosity of the polymer makes it reversibly adsorb 12.5 wt % CO2 at 1.0 bar/273 K with high thermal stability. The higher porosity, and therefore better gas-adsorbing properties, of polymer 135 than HPB 9599


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Figure 65. Stepwise role of HPB derivative 139 in template synthesis of macrocycle 142: (i) 2,6-bis(hex-5-enyloxy)pyridine, AgBF4; (ii) [PhCH RuCl2(Cy3P)2], CH2Cl2; (iii) NaI; (iv) H2, Pd/C.

polymer 136 (Figure 64) (BET surface area = 675 m2 g−1) indicates that the building block triptycene in 135 enhances the surface area and pore-size distribution by hampering the polymeric chains from coagulating into a dense and less-porous secondary polymeric structure. In a continuation of this work, the same research group recently reported synthesis of triptycene− HPB polymer 135 and another randomly polymerized derivative 137 (Figure 64) by another approach using nonbrominated precursor monomer and cheap catalysts.228 The polymer 135 synthesized by this method has significantly lower BET surface area (569 m2 g−1) than the previous method. However, polymer 137 exhibited moderately good surface area (914 m2 g−1) and better CO2 uptake capacity (10.3 wt % at 1.0 bar and 273 K) than 135 (5.0 wt % at 1.0 bar and 273 K), which has been attributed to the perfect compatibility of microporous sites and size of CO2. Besides, 137 can reversibly absorb 1.09 wt % H2 at 1.0 bar and 77 K. Thompson et al.229 performed Sonogashira cross-coupling between building block tetrakis(4-ethynyl)−tetraphenylmethane (TPM) and various dibrominated HPBs 138a−c as well as their fused HBC derivatives to produce respective porous polymers (Figure 64). As in COFs,219 the BET surface areas of HPB polymers were also greater than those of their HBC counterparts due to the higher degree of interlayer π−π

interactions between graphitic units of HBC polymers. However, the bulky side groups on monomeric HPBs and HBCs affect the surface areas of corresponding polymers differently. The addition of larger side groups on HBC monomers decreases the surface area of resulting polymers, whereas the surface area of HPB polymers increases with the addition of larger side groups (from 138a to 138c). The HPB polymer corresponding to 138c, due to the highest surface area, exhibited maximum storage capacity for CO2 (18 wt %) at 273 K, whereas HBC polymers show maximum selectivity for CO2 and N2 at 298 K as large pores of HPB polymers have bad gas-sieving properties. 6.3. HAB Derivatives as Templates for Generating Porous Macrocyclic Molecules

Ko and co-workers33 reported the synthesis of a large macrocycle 142 by metathesis of olefin-substituted pyridine on [Pt(PEt3)2]hexatemplate 139, which was easily synthesized by a sixoxidative-addition reaction. The coordination of 2,6-bis(hex-5enyloxy)pyridine to 139 led to the hexacationic coordination complex 140 followed by metathesis of olefin-substituted pyridine with Grubbs catalyst to form 141 (Figure 65). The detachment of olefinic macrocycle from HPB template using sodium iodide in macrocycle 141 was followed by reduction of target alkene macrocycle by using palladium on charcoal and hydrogen gas to produce a huge macrocyclic alkane 142. Thus, 9600


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Figure 66. Macrocycle 144 was synthesized using HPB derivative as template 143.

pyrolytic graphite (HOPG) at ambient conditions. The STM study of adsorbed 147 on HOPG revealed the coexistence of snowflake and unstable honeycomb structures. On introduction of coronene into the 147−HOPG assembly, a drastic transformation from the snowflake structure of monolayered 147 to its honeycomb structure took place, which resulted in much larger pores. The heptameric aggregates of coronenes are captured inside these large pores of honeycomb structural templates of 147. Thus, these HPB templates on the HOPG surface served as a host porous material to immobilize functional coronene molecules to fabricate their monolayer under ambient conditions. This approach can give new vision to generate complex functional supramolecular surface assemblies and study their electronic properties. Keeping these reports in mind, we may conclude that the nonplanar but π-conjugated HAB scaffold is most likely to undergo disordered aggregation, which is a prerequisite for the formation of molecular networks with high intrinsic porosity. This porous nature of these molecular aggregates can provide high specific surface area, which can help in facilitating various organic reactions as well as various physical processes like oxygen-reduction reaction for fuel cells, hydrogen-elimination reaction, oxygen-elimination reaction, sensory device fabrication, formation of molecular sponges, etc.

the access of the distinct HAB-based template widened the potential subsequent transformations toward the synthesis of these kinds of macrocycles (Figure 65). O’Sullivan et al.34 also used HAB derivative to synthesize a butadiyne-linked, p-conjugated 12-porphyrin nanoring by Vernier templating. Very interestingly, if the number of binding sites available at the Vernier template (HPB in the present report), i.e., na, is not a multiple of the number of binding sites available at molecular building blocks (zinc porphyrin in the present report), nb, then the number of binding sites nc in the final product will be the lowest common multiple of na and nb.230,231 The authors used hexapyridyl template to give complex 143 in 39% isolated yield, which, on treatment with competing ligand pyridine, gave desired product 144 in 96% yield (Figure 66). Although the classical template route also resulted in the same respective products 145 (35%) and 146 (quantitative) in almost similar yield, it is difficult to isolate the product using the classical template technique (Figure 67). Besides, the synthesis of the classical HAB template was allegedly much more tedious than that of the Vernier template. Therefore, this Vernier template strategy is likely to be a helpful approach to the synthesis of monodisperse macromolecules of otherwise unattainable size. Chang et al.232 recently used HPB derivative 147 (Figure 68) as a template to deposit functional molecular clusters of coronenes to form an ordered monolayer over highly oriented 9601


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7. INTER-/INTRAMOLECULAR ELECTRON/CHARGE-TRANSFER AND REDOX PROPERTIES OF HAB DERIVATIVES 7.1. Donor−Acceptor Properties of HAB Derivatives

In consideration of their remarkable electron-transfer properties, HAB derivatives have also gained considerable interest in material science.233,32 HAB is quite a rigid frame due to steric hindrance to rotation about the single bonds joining the central and peripheral rings, which makes the electronic interactions between attached chromophores easier, which leads to rapid energy and electron transfer. Keeping that in view, various multichromophoric donor−acceptor systems with C3h and D3h symmetries have been explored in which the HAB core has been used as a channel for electron and energy transfer. Figueira-Duarte et al.234 synthesized HPB derivatives 148 and 149 (Figure 69) substituted with two aldehyde groups by 2+2+2 cycloaddition reaction of bisaldehyde with bisarylalkynes, respectively, using a catalytic amount of Co2(CO)8. These precursors were used to produce the corresponding bismalonates, which on direct macrocyclization lead to the formation of equatorial fullerene bisadducts 148 and 149 with the C60 sphere. The synthesis of targeted products was confirmed by UV/vis, elemental analysis, 1H NMR, 13C NMR,and mass spectroscopic techniques. The combination of C60 with a multiphoton absorption chromophore led to improved optical limiting properties,and owing to the intercomponent photoinduced energy transfer, the excited-state deactivation dynamics was dominated by the fullerene chromophore. These results might be interesting for the design of a new singlet oxygen sensitizer for photodynamic-therapy applications by multiple derivatizations of HPB bridging units. The easy access to functionalized HPB derivatives can find multiple applications in supramolecular devices depending upon the control of geometry. Gust and co-workers235 reported an HPB-based zinc porphyrin dyad 150, which experienced complexation with fullerene-bearing pyridyl groups through coordination bond with Zn2+ ions to make 1:1 adduct with a high binding constant of 7.3 × 104 M−1 in 1,2-difluorobenzene (Figure 69). The transient absorption decay associated spectrum showed that photoinduced electron transfer took place from porphyrin first excited singlet state to the fullerene, resulting in a charge-separated state with a very short lifetime of 230 ps. It also showed concurrent growth of a characteristic broad-induced absorption of the zinc porphyrin radical cation and of absorption in the 1040 nm region characteristic of the fullerene radical anion. It can help to design a more elaborate model of photosynthetic antenna-reaction center systems. Later, the same group also reported another artificial antenna based on molecular hexad 151,236,237 which consists of bis(phenylethynyl)anthracene (BPEA), borondipyrromethene (BDPY), and zinc tetraarylporphyrin (P), which served as photon-absorbing constituents (Figure 69).238 The presence of three different types of chromophores with absorption range from blue to red region increased the potential of the molecule to harvest energy within this region. The emission and absorption spectra of the three chromophores overlapped in such a way that the molecule thermodynamically went through energy transfer from BPEA to BDPY and then from BDPY to zinc tetraarylporphyrin (P). The time-resolved study confirmed that these energy transfers occurred altogether to generate the emission corresponding to P with quantum yield approaching to unity. Hexad 151 was mixed with fullerene electron acceptor to make a coordinated complex heptad through interactions

Figure 67. Macrocycle 146 was synthesized using HPB derivative 145 as template.

Figure 68. HPB derivative 147.

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Figure 69. Fullerene adducts of HPB derivatives 148−151.

Figure 70. HAB derivative 152−154.

between pyridyl nitrogens and zinc atoms of porphyrin of 151, which is a good energy-harvesting molecule. These experimental outcomes showed a good agreement with density functional theory of this multichromophoric antenna and its charge-transfer excited states studied by Basurto et al. recently.239 In a continuation of this work, Gust and co-workers240 reported HPB derivative 152 appended with two coumarin units through ester linkage (Figure 70). Depending upon the site of

initial excitation, the energy of excited coumarin units was transferred to the porphyrin units in 1−10 ps with unit quantum yield. A pyridyl-functionalized fullerene interacted with 152 to give complex 153 (Figure 70) in which energy transfer from fullerene to porphyrins took place followed by photoinduced electron transfer from porphyrin to the fullerene with a time constant of 3 ps and net unit quantum yield. The molecular mechanics modeling suggested that, in contrast to derivative 151 9603


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Figure 71. HAB derivatives with three pyrene and three triarylamine substituents 155−160.

formed in these compounds 155 and 156 in the excited state, thus confirming the proposed hypotheses. Because the HOMO and the LUMO of pyrene groups possessed nodes along the longer axis of the molecule, it decreased the conjugation effects in 155 and 156 (Figure 71). In the symmetric chromophore 156, there are six feasible through-space interactions between triphenylamine (donor) and pyrene (acceptor) as indicated in Figure 71, whereas in the asymmetric isomer 155 only four such interactions are possible. In the model compound, only one donor−acceptor interaction was feasible. This work illustrates the plausible mechanism of through-space charge transfer and the role of the relative position of donors and acceptors in favoring charge transfer in multidimensional chromophores. In a continuation of this work, Steeger et al.246 replaced pyrene with electron-deficient triarylborane as electron acceptors due to unoccupied pz orbitals of boron. Similar to the previous report,245 the symmetrical HPB derivative 158 revealed six feasible charge-transfer interactions, whereas 159 and 160 showed four and one CT transitions, respectively, between triphenylamine (donor) and triarylborane (acceptor) (Figure 71). Although time-dependent anisotropy measurements of similar HABs suggested transition dipole−dipole interactions, poor orbital overlap between adjacent donor−acceptor units obstructs the energy-transfer mechanism. However, a recently reported time-dependent solvation study by the same research group concluded that, although there is a difference in the degree of electronic coupling in derivatives 158−160 that led to different CT rates, the localized energy within one triarylamine− triarylborane pair can be redistributed within 3 ps in all derivatives.247 The experimentally proved high affinity of boranes toward fluoride ions explained the weak interactions between neighboring triarylborane units in 158 and 159 in the presence of fluorine. With better electronic and optical properties and a higher glass-transition temperature of 151 °C, unsymmetrical isomer 159 acts as a better candidate for optoelectronic applications.

(Figure 69), no exchange-mediated singlet−singlet energy transfer happened due to ester linkage. However, energy transfer took place according to Forster dipole−dipole theory only. The highest efficiencies in converting sunlight into electric energy require consistent photoconversion arrangement with at least two photoactive components with absorption spectral region between 400 and 1000 nm.241 The applications of these molecular antennas can be combined with photoprotective and photoregulatory functions in artificial photosynthesis like in our ecosystem.242,243 Rausch and Lambert designed and synthesized pyrene-based HAB derivative 154 (Figure 70), which emitted at much longer wavelength in comparison to both locally excited pyrene and its excitones in the aggregated state.244 The red-shifted emission was characteristically attributed to the hexameric aggregate of 154. Derivative 154, unlike discrete pyrene, not only displayed a concentration-dependent excimer emission band at 24 100 cm−1 (414 nm) but also exhibited a corresponding absorption band at 21 500 cm−1 (465 nm). This simple model of star-shaped HAB compound shall allow for discovery of many photophysical aspects of such 6-fold molecular arrangements. Later Lambert et al.245 reported two HAB derivatives 155 and 156 with three pyrene and three triarylamine substituents in different positions, synthesized by Sonogashira coupling followed by cyclotrimerization using cobalt catalyst (Figure 71). The CT states of these derivatives were studied by optical spectroscopy. The CT states formed in these compounds on excitation were similar in both cases. These CT states relaxed to the ground state with a moderate quantum yield. In the case of derivative 156, the symmetry was said to be broken within a fluorescence lifetime of several tens of nanoseconds (ns); the excited photons traveled through all donor−acceptor pairs and finally fell into the emission lifetime in one donor−acceptor pair to finally emit the fluorescence. A model compound 157 with one electron donor (triarylamine) and one electron acceptor (pyrene) was also synthesized that shows similar CT states 9604


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groups in 161. These observations suggest the tremendous applications of such multivalent charge-transfer chromophoric analogues in molecular batteries. Moorthy and co-workers249 reported the regioisomeric photochromic chromenes 162−164 (Figure 73) based on HAB scaffold to study their stereoelectronic effects on spectrokinetic properties of photogenerated o-quinonoid reactive intermediates. These derivatives exhibited orange−red coloration on photoexcitation, attributed to the formation of quinonoid intermediates. However, it was found that derivative 164 showed rapid reversal of quinonoid intermediate at room temperature to destabilize visible coloration, whereas derivatives 162 and 163 exhibited good stability due to intramolecular CT from orthogonally oriented donor pentaphenylbenzene rings to the acceptor quinonoid moiety. This work signifies the inflection of the spectrokinetic properties of the colored o-quinonoid intermediates derived from 2,2-diarylpyrans-functionalized HPB for application in ophthalmic lenses.250 Akita and co-workers251 reported a series of HABs 165, 166, and 167 having two peripheral thienyliron substituents in which two metal centers communicated with each other through toroidal delocalization among the peripheral aromatic groups (Figure 74). The communication between two iron centers of derivatives 165−167 was studied using CV and IR spectral study. When two thienyl groups were ortho to each other (165 and 166), they showed two separate redox processes, while in the case of para, it showed single broadened redox waves analyzed by simulation. The ortho-substituted HAB complexes turned out to show the higher comproportionation constant (KC) of 248 than those of the reference complexes, and thus derivatives 165 and 166 showed better performance than 167. This study entitled the HAB system to serve as a promising 2D wiring device like ethylene and benzene skeleton and generate toroidal interaction for dominant communication pathway for the HAB complexes. Recently, Ramos et al.252 studied the oxidant capacities of HPB derivatives 168−176 (Figure 75) substituted with different metal sandwich side groups by density functional theory. Among monometallic systems (168−170), derivative 168 exhibited the best electron-accepting tendency due to empty d-orbitals of Sc, whereas derivative 169 behaved as the best electron donor. In bimetallic HAB systems 171−176 also, the derivatives having Sc had better electron-acceptor properties and those having Cr exhibited better electron-donor capabilities. In derivatives 171 and 172, LUMOs are bonding orbitals of bis(cyclopentadienyl)scandium and HOMOs are located at Cr and Fe, respectively. Because LUMOs of 171−172 are similar to that of 168, it indicates that the electron-accepting capability of Sc dominates over the electron-donor ability of Cr/Fe. Therefore, all derivatives bearing bis(cyclopentadienyl)scandium are the best electron acceptors. Similarly, the electron-donor ability of Cr dominates and derivatives bearing Cr are the best electron donors. Therefore, bimetallic derivatives having Cr and Sc are

Figure 72. HPB dendrimer 161.

Diederich and co-workers248 synthesized HPB dendrimer 161 appended with 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) and studied its multivalent CT properties by CV (Figure 72). It was found that dendrimer 161 was oxidized by transfer of 12 e− in a single step at +0.89 V (versus Fc+/Fc), which denoted that all 4N,N-dialkylanilino groups were taking part in the redox process independently. Unlike oxidation, dendrimer 161 experienced two-step reduction by receiving 12 electrons per step at −0.70

Figure 73. Chromenes 162−164 based on HAB scaffold.

and −1.10 V, respectively, with a remarkable reversible injection of these electrons. At a very low scan rate of 1 V s−1, peaks corresponding to reduction steps varied by 100 and 220 mV. It also indicated that electron-transfer steps were unresolved and overlapped with each other at slightly different potentials, which indicated very little interaction between the different TCBD

Figure 74. HAB derivatives 165−167. 9605


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Macromolecule 177 is said to undergo reversible oxidation to give only a single reversible CV wave at 1.15 V vs SCE at a scan rate of 25−200 mV s−1 irrespective of the numbers of aryl groups attached to the central phenyl ring. This suggested that there are minimal interactions between the donor moieties in 177. In a continuation of this work, Rathore et al.254 reported a successful synthesis and isolation of hexa- and tetratrityl cations of starting materials based on HPB substrates 178, 179, and 180 (Figure 76) and explored the cocrystallization of these polytrityl cations with a variety of non-nucleophilic anions. These redoxactive materials also have the potential to behave as catalysts in polymerization reactions. Furthermore, these polytrityl cations can be chemically transformed into corresponding radicals to possess intriguing magnetic properties. The structures of the cations have been confirmed by 1H/13C NMR and UV−vis spectroscopy. These macromolecular cation radicals and isolated polytrityl cations are very much likely to show widespread application as one-electron oxidants, activators for olefin polymerization reactions, and stereoregulators in other polymerization reactions.255−258

better materials than others. In addition the bimetallic HPB derivatives having the same metal on one side of HPB and the other metal on the opposite side behave as both electron donor and acceptor at the same time, which results in a higher dipole moment than in other isomers. Clearly, Fe-based HPB derivative 170 behaved as a poor electron donor as well as electron acceptor. This study can help to design new future prospects for the development of HPB-based novel electronic systems through interesting toroidal delocalization. 7.2. Redox-Active HAB Derivatives

The design and synthesis of organic molecules containing multiple redox-active chromophores is of particular interest because of their role in the preparation of materials with novel light-emitting and charge-transport properties and as multielectron redox catalysts. Rathore et al. reported a significant amount of work related to redox applications of HAB derivatives appended with redox-active units. In their first report on redoxactive HAB, Rathore et al.253 synthesized macromolecule 177 bearing multiple redox-active units that behaved as an electron sponge toward a number of electron donors (Figure 76). 9606


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Figure 76. Redox-active HPB derivatives 177−180.


Further, Rathore et al.259 widened this concept by studying a cyclic voltammogram, which showed multiple oxidation waves and a strong transition corresponding to charge resonance in the near-IR region in the cation radical spectrum of dendritic molecule 181 (Figure 77). It indicated the delocalization of a hole via electron transfer over six identical cofacial redox-active units in circular arrangement. This was confirmed by carrying out chemical coulometry using a hindered naphthalene cation radical salt and comparing it with the CV spectrum of monosubstituted HPB model derivative 182 (Figure 77). The cyclic voltammogram of 181 showed three oxidation waves at 1.22, 1.34, and 1.47 V (vs SCE) overlapping each other, whereas the model donor 182 showed only one reversible wave at 1.28 V (vs SCE). It implied that the removal of the first electron in the case of 181 affects the removal of the next electrons, and thus, on contrary to

previous reports, various tetraphenylethylene moieties in 182 were electronically coupled. The same group investigated the CV and square-wave voltammetry of HPB derivative 183 to confirm the ejection of six electrons (oxidation) at a constant potential, which can help in fabricating photonic devices.260 The single reversible wave at 0.47 V suggested the absence of electronic coupling between ferrocenyl groups of 183 (Figure 77). Kochi and co-workers261 functionalized a series of aromatic donor groups around the periphery of HPBs 184−186 to render intramolecular electron movement among these redox centers (Figure 78). The Mulliken−Hush (MH) quantification confirmed that substantial stabilization is allowed in the one-electron oxidized hexadonor system because the resonance energy showed the extensive intra-annular electron delocalization in hexadonor radical cation relative to the localized monodonor 9607


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radical cation. Furthermore, the MH treatment of the experimental spectral behavior correctly predicted the potential shift close to the experimental value when the entropy contribution was taken into account. The hexadonor cation radical was also thermodynamically more favorable to reduce the barrier for hole hopping between six anilinyl sites. However, studies by Lambert and Nöll32 on the trication of 1,3,5-tris[(4-dianisylamino)phenyl]-2,4,6-tris[(4-ditolylamino)phenyl]benzene stood in contrast to that of derivatives 156, 183, and 184−186.262 The authors investigated the optically/ thermally induced electron transfer in C3-symmetric HAB derivative 187a and C6-symmetric 187b (Figure 78). Interestingly, both 187a and 187b showed two reversible waves corresponding to three one-electron redox processes close in potential, which was reportedly due to the potential tuning by the methyl and methoxy substituents. Owing to the significant difference of 120−150 mV in redox potential of M2+/M3+ and M3+/M4+ in 187a and 187b, respectively, the corresponding trications of 187a were synthesized, which subsequently exhibited a strong CT band in the near-IR region. The semiempirical calculations of 187a−187b and a model compound 188 (Figure 78) were done to estimate the paracoupling between linear bis(triaryl) amine systems by using Koopman’s theorem.263,264 The calculations indictated that the positively charged triarylamine centers trade their positions with neighboring neutral triarylamines through ortho-coupling, which was the reason for the resulting CT band in the NIR region. This implies that ortho-coupling between the donors is the strongest and predominant one. These results are basic applications in symmetrical energy-transfer systems found in nature, including photosynthesis in some of the bacteria. Warman and co-workers265 reported the role of HPB in ET between neighboring carbazole chromopho res in hexakis(carbazolyl)benzene 189 (Figure 79). The carbazole units in 189 reflect much greater energy migration in their absorption and fluorescence spectra compared to that in monomeric analogue N-phenylcarbazole. According to a timeresolved microwave conductivity study, compound 189

Figure 79. Redox-active HAB derivatives 189 and 190.

exhibited a quick dispersion of the excimeric state of energy between two adjacent carbazole moieties attached to the central benzene with a relaxation time of 3 ps. This signified that the excitation energy round trips coupled N-carbazoles more than 1000 times within the 5 ns lifetime of the excited state. Because energy-transfer processes are directly correlated to electrontransfer processes, this concept promises to offer new toroidal energy- and charge-delocalized systems based on HPB molecules. In a continuation of the previously discussed work,259 Rathore and co-workers reported another dendritic hexanaphthylbenzene derivative 190 possessing a single hole mobilized via electron transfer over six identical redox-active centers (Figure 79).266 This was confirmed by multiple oxidation waves and an intense charge resonance transition (extending beyond 1600 nm). Such inferences allowed for exploration of a new class of intervalence materials in which a hole can travel over the whole molecule via its redox centers, which can be useful in designing photonic devices like solar cells, optical amplifiers, and OLEDs. Wortmann and co-workers267 reported synthesis and nonlinear optical studies of an HAB derivative, namely, 1,3,5-tris(4N,N-diethylaminophenyl)-2,4,6-tris(4-nitrophenyl)benzene 191 (Figure 80), by cyclotrimerization of 4-N,N-diethylamino-4′nitrotolane using Co2(CO)8 catalyst. Crystals of 191 were 9608


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scaffold has proved to provide a good support for such materials due to its rigid core and adjustable aryl units. Gust and co-workers268 reported a molecular “hexad” 193 in which five bis(phenylethynyl)anthracene (BPEA) fluorophores and a dithienylethene photochrome were covalently immobilized over HPB (Figure 81). The BPEA units emitted in the 515 nm region with almost unit quantum yield when the dithienylethene group was in the open form 193o. The same BPEA group gave very low emission intensity (almost zero quantum yield) in the 500−700 nm region when the dithienylethene group was closed in isomer 193c as a result of photoisomerization of 193o under modulated UV light. This strategy could be useful for the detection of fluorescence from probe molecules without interference from other emitters in biomolecular or nanotechnological applications. Gust and co-workers242 demonstrated the use of photochemical and chemical switching moieties to regulate the behavior of intramolecular subunits. Molecular pentad 194 performed as an adaptive, self-regulating, and orange carotenoid protein (OCP) photoregulator mimicking artificial antennareaction center complex (Figure 82). The pentad consisted of a base porphyrin equipped and two BPEA antennas that served as antennas for light absorption. The excitation energy was transferred to the central porphyrin, which then experienced PET to fullerene, which saved this energy as electrochemical potential. The dihydroindolizine-substituted HPB photochrome in derivative 194 played the major role of conversion of light energy into electrochemical potential. Being the thermally more stable isomer, the closed spirocyclic form of the photochrome (194c) was favored by both heat and red light and can be transformed in the open isomer 194o by absorbing UV or blue− green light. In a continuation of this work, Gust and co-workers reported another hexad 195, which behaved as a chemically activated HPB-based molecular switch using porphyrin antennas to functionally mimic photosynthetic process of converting solar light into electrochemical energy (Figure 82).269 It comprised five zinc porphyrin antennas and a pH-sensitive dye attached to a central HPB core. The role of HPB was to provide a rigid template to the molecule to limit the distance between chromophores and their relative orientations. Under neutral conditions, the pH-sensitive RhB-type dye existed in the closed spirocyclic form (195c), which was converted to the positively charged open form 195o at lower pH.243 Such self-regulation of function in response to outside factors makes these kind of synthetic molecules highly susceptible for fabricating molecular “devices” like photonic analogue of a field-effect transistor and photosynthetic photo protector. Feringa and co-workers,270 however, used a cyclotrimerization method using cobalt carbonyl catalyst to synthesize 6-fold HPB derivatives 196 and 197 bearing dithienylethene groups (Figure 83) from corresponding alkynes. The CV studies of these molecules 196 and 197 were compared with other model compounds in which the numbers of dithienylethene units were different. It was observed that the number of dithienylethene units did not affect the voltammetric results, which implied that these functional units were not electronically attached to each other but were well-separated to minimize electrostatic interactions. This lack of intercomponent communication was possibly due to the nonplanar/twisted conformation of the HPB scaffold. The open-ring derivatives 196o and 197o underwent ring closure of dithienylethene on irradiation with UV light, and the redox wave of 196c and 197c indicated the single-step

Figure 80. HPB derivative 191 and derivative 192.

obtained, which indicated the strong torsion of the peripheral rings with respect to the central ring with the dihedral angle between peripheral and central phenyl rings varying between 54° and 75°. However, nitro and amine groups were coplanar with the adjacent phenyl ring. The improved planarity of the molecule due to the introduction of the alkyne spacer units resulted in a variation in interplanar angles between the peripheral and central phenyl rings in 192 from 1° to 26° (Figure 80).

8. PHOTOCHEMICAL/CHEMICAL MOLECULAR SWITCHES BASED ON HPB DERIVATIVES The most common photochemical conversion of light energy into chemical potential energy is photosynthesis, which needs a consistent photon optimization to avoid an overdose of solar energy flux to save plant cells. The imitation of these natural processes in a synthetic system at a molecular level using external modulated light or a chemical stimulation has been a very challenging task. The photochromic molecules that can oscillate between its metastable photoisomers depending upon the type of irradiation, temperature, or chemical input are the functional units in photochemical or chemical molecular switches. HAB

Figure 81. HPB-based molecular hexad 193. 9609


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Figure 82. Photochemically active HPB derivatives 194 and 195.

Figure 83. Six-fold HPB derivatives 196 and 197 bearing dithienylethene groups.

multiple electron transfer to produce highly stable charged

Photolithography is a completely different concept from photochemical molecular switches. In photolithography, patternperfect thin organic films are modeled by UV or electron-beam

species (Figure 83). 9610


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Figure 84. HPB derivatves 198−203.

comparison to the meta analogues 201. However, higher Tg of the material was likely to play a much more prominent role in the dissolution behavior. Thus, high-resolution patterns can be obtained with high Tg materials without plasticization and crosslinking. This laser interferometry technique can thus be used in future studies to optimize dissolution, patterning, and development of high-Tg molecular glasses in scCO2. A fundamental study of hexakis(hydroxyphenyl)benzene derivatives based on different sizes and structural features showed that all-para- or all-meta-substituted compounds, i.e., 202 and 203, respectively, (Figure 84) possessed asymmetric structures and were thermally less stable with lower Tg in comparison to both meta- and para-substituted counterparts 198. This possibly could be due to extremely small potential energy barriers set by the structural asymmetry of the molecule. The study showed that the branched HPB derivatives can form stable molecular glasses despite the π−π interactions present. On the basis of this study of molecular structure and thermal properties, HHPB derivatives were identified as suitable candidates for lithography evaluation. Specifically, derivative 198 (hydroxyphenylbenzene derivatives as potential molecular photoresists for patterning below 50 nm) was considered as a suitable molecular-glasses photoresist for EUV lithography for generating sub-30 nm feature sizes.276 Considering these reports, we can state that HPB template has been widely used to support photochromes to develop molecular switches using light or chemical sources. The idea of photochemical/chemical molecular switches can be further upgraded to molecular logic gates, molecular encoders, and molecular key pad locks. In addition, appropriately substituted HPBs can perform as a molecular-glass material with high glass-transition temperature to respond well to high-resolution photolithography, which can dramatically enhance the performance of semiconductor devices.

(e-beam) irradiation to focus on small features of material, which is very useful in semiconductor and microfabrication industries. Owing to their small size, the amorphous molecular glasses have the potential to dissolve in supercritical carbon dioxide without fluorination, resulting in their increased applications in the fields of organic light-emitting diodes (OLEDs), photovoltaic devices, photorefractive devices, and photoresists.271−273 In the attempt to develop such a material, Ober and co-workers274 reported hexa(hydroxyphenyl)benzene [HHPB] 198 (Figure 84), which behaved as a molecular-glass material with high glass-transition temperature (Tg > 100 °C), which was envisioned as a good material for high-resolution lithography. Having rigid phenyl rings, it offered high resistance toward plasma-etching, which was very important to impart the photoresist pattern over silicon or oxide substrate. Interestingly, hexa(4-hydroxyphenyl)benzene 198 tended to crystallize due to π−π interactions between the HPB core moieties. Thus, the hydroxyl groups of this compound were protected with tert-butoxycarbonyl (t-Boc) units in 199 (Figure 84), which chemically favored the deprotection reaction, and got separated from the core molecule. The work basically used the rigid and less-soluble HPB core to exhibit the favorable effect of t-Boc groups toward its solubility in supercritical carbon dioxide (scCO2) without fluorination and siloxanes, thus enhancing the industrial viability of scCO2 as the processing solvent. In an extension to this work, Ober and co-workers275 compared the dissolution properties of 200 and 201 (Figure 84) on the basis of number and relative positioning of peripheral substituents. It was observed that increasing the amount of carbonyl groups included in the molecular-glass molecule did not necessarily improve the dissolution rate. In fact, the opposite trend was seen in this case, and it was the position of the protecting groups (meta or para) that affected the dissolution rate. The inclusion of para-substituted versions 199 (Figure 84) and 200 (Figure 84) slowed down the rate of dissolution, possibly due to more π−π stacking in the HPB molecular core in 9611


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Figure 85. HPB-based dendrimers 204−206.

Figure 86. Polyethersulfonic HPB dendrimers 207a−c.

Figure 87. Polyethersulfonic HPB dendrimers 208.

9. HAB-BASED DENDRIMERS AND THEIR APPLICATIONS

dendrimers in both convergent and divergent fashion. The peripheral groups of nonplanar HAB derivatives undergo interlocking to exhibit interesting structural variations.281 Müllen and co-workers282 studied the exciting structural and dynamic properties of HPB-based dendrimers resulting from interlocked and rotatable phenyl groups. All three dendrimers 204, 205, and 206 were first-generation dendrimers consisting of HPB units (Figure 85). These HPB-based dendrimers had an advantage over pentaphenylbenzene (PPB)-based dendrimers due to better solubility, as it makes the growth of single crystal easier. In addition to this, because of steric congestion, there were very few degrees of freedom left for internal rotation in HPB-based dendrimers, which helped the molecule retain its native point group symmetry. X-ray diffraction analysis indicated that the

Dendrimers are three-dimensional, shape-persistent spherical macromolecules bearing various building blocks around the central core.277 The properties of these macromolecules depend upon the number of generations, nature of building blocks, surface group, and core.278 The dendritic macromolecules are known to exhibit various applications like catalysis, molecular recognition and inclusion, surface functionalization, gene transfection, diagnosis, supramolecular assembly, energy harvesting, etc. depending upon the cavity size and functional groups of building units.279,280 The HAB unit is considered as a very suitable scaffold for the synthesis of high-molecular-weight 9612


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voids generated within the molecules were filled by solvent molecules without any specific interactions and thus were highly unstable in ambient conditions. Although, due to the ideal supramolecular framework, it was envisioned that the entrapped solvent molecules can be replaced with spherical guest molecules, no such host−guest complex was formed when 206 was cocrystallized with adamantine and tetraphenylbenzene, which was said to be because of differences in solubilities of host and guest molecules in the media. Hay and co-workers283 reported two-step synthesis of multiblock copoly(ethersulfone)s (PESs) 207a−c (Figure 86) appended with a binding unit for the construction of dendritic block. It involved synthesis of PES block with both phenoxide end-groups followed by chain extension and end-capping of the block by use of excess hexafunctional agent. Sulfonation of pendant phenyl rings gave multiblock coPES bearing clusters of sulfonic acid groups only on the dendritic blocks. The imparted thin films of presulfonated dendrimers were reportedly tough and flexible, whereas the membranes of sulfonated dendritic PES were brittle and thus not favoring good conductivity. Thus, sulphonation of these HPB-based dendrimers assisted to improve the quality of the film during device fabrication. In a continuation of this work, Hay and co-workers284 described the properties of dendritic multiblock copoly(ethersulfone)s (coPESs) 208 (Figure 87) synthesized by onepot synthesis between dendritic blocks bearing difunctional dendritic block and monofunctional dendritic end-group through aromatic nucleophilic substitution of aromatic hexafluoride and phenol, followed by polycondensation of diphenol and activated aromatic difluoride monomers. These coPESs were capable of forming strong membranes and improving the mechanical properties of the resulting sulfonated dendritic PES membranes, when the longer average blocks (n = 60−80) were

Figure 89. Poly(aryletherketone)-appended HPB polymer 210.

chains with an HPB core, which is envisaged to exhibit many applications in coatings, additives, and drug and gene delivery. The previous reports suggested that the propeller-like conformation enables π−π interaction between the peripheral aromatic rings facing each other. This results in sextuple accumulation of the π−π interaction leading to “toroidal delocalization”. Therefore, the propeller-like conformation has been regarded as one of the promising three-dimensional topologies and is expected to exhibit unique electronic features. Keeping this in mind, Kim and co-workers286 demonstrated the use of polymer membranes containing the propeller-like HPB appended with poly(aryletherketone)s 210 (Figure 89) structure for good proton conductivity and cell performance. The characteristics of the reported work were to provide high proton conductivity, excellent mechanical strength, and high thermal stability with less water-uptake phenomena. The study suggested potential applications of the synthesized polymer as protonexchange membrane fuel cell. The resulted proton conductivities were 66.58−103.73 mS/cm at 80 °C with 90% relative humidity. The highest power density of a fuel cell using HP-23 and Nafion 211 was 0.47 and 0.45 W/cm2, respectively, at 0.6 V. The role of sulfonic acid on the side-chain was to provide better clustering effect and hydrophilic/hydrophobic separation, thus improving cell performance. Nierengarten and co-workers287 reported three HPB-based dendrimers 211−213 synthesized by metal-catalyzed cyclotrimerization reactions of respective dendritic diarylacetylenes (Figure 90). Further, the electrochemical behavior of these dendrimers was studied by CV studies using a high concentration of electrolyte. The study screened the electrostatic repulsion between the adjacent reduced C60 units within the molecule, which helped each C60 moiety to retain the same standard reduction potential irrespective of the reduced status of the nearby C60 units. This synergistic effect resulted in a single reduction wave obtained for all generations of the compound. On the basis of the area under the peak in the voltammogram, the ease of adsorption of reduced dendrimer was estimated. In fact, the adorption of reduced dendrimers relied upon the surface area of the molecule, which increased with an increase in generation. Therefore, the absorption of the molecular bilayer was more facilitated in the case of dendrimer 213 in comparison to 212, whereas dendrimer 211 was not observed to be adsorbed at all.288 The inherent rigidity of nonplanar HPB dendrimers offers unique spatial orientation of epitopes, whereas their water solubilities introduce advanced functions like host−guest molecular recognition and autoaggregation. On account of this, Roy and co-workers289 synthesized a series of multivalent glycodendrimers 214−216 (Figures 91) using Cu(I)-catalyzed convergent as well as divergent reactions in high yields. The dendrimers were so designed that the hydroxylated sugars were incorporated to enhance the solubility of otherwise insoluble

Figure 88. Thiophene-appended HPB dendrimer 209.

incorporated into the dendritic PESs. At a level of ion-exchange capacity similar to Nafion, PES membranes showed proton conductivities as good as Nafion to justify better mechanical properties of dendritic multiblock coPESs with larger dendritic ionic clusters. The advantage of these dendrimers over Nafion was the relatively very high proton conductivity, better mechanical stability, lower cost, and high ion-exchange capacity in fuel cells. Kiriy et al.285 developed a new method for the synthesis of head-to-tail regioregular poly(3-hexylthiophene) 209 (Figure 88) using Kumada polycondensation of 2-bromo-5-chloromagnesio-3-hexylthiophene. It established an example of the “core-first” synthesis of 6-arm star-like poly(3-hexylthiophene) 9613


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confirmed by time-resolved fluorescence spectroscopy in which the decay time did not depend on excitation power when using a 200 fs laser pulse (50−180 mW, excitation at 488 nm). The possible annihilation of the excited singlet excitons might be due to the faster energy migration than the time resolution of the present experimental conditions. More than a decade back, Housecroft and co-workers291 reported a synthetic conversion of star-shaped dendritic polyalkynes 220 bearing an HPB core to another metallodendrimer 221 containing 24−60 Co atoms, which were appended with straight or branched alkyne-containing building blocks (Figure 93). Such metallodendrimers containing multiple polymetallic units can have the potential to be polyfunctional components for catalytic applications. Utilization of the supramolecular chirality resulting from the translation of chiral information for asymmetric catalysis using HAB metallodendrimers was one of the subjects worthy of elaborated investigations. Aida and co-workers292 used the

hydrophobic HPB core. These dendrimers 214−216 also differed by the numbers of triazole moieties that affected the inner space of the molecule, which might be important for studying the effect of resulting dual rigidity and compaction on epitope orientation. Interestingly, the number of clicked moieties in these dendrimeric structures did not affect the yield. Later Morisue et al.290 reported synthesis and photophysical studies of light-harvesting dendrimer based on HPB scaffold appended with different numbers of subphthalocyanine as substituents, i.e., 217−219 (Figure 92). It was observed that only a marginal change in emission and absorption spectra was observed with a change in the number of subphthalocyanine groups. Stationary fluorescence anisotropy experiments revealed the occurrence of energy migration between subphthalocyanine groups, which was confirmed from the fact that the intrinsic redistribution in the model compound phenylsubphthalocyanine was more significant in comparison to that in 219 due to slower rotational diffusion in HPB core. The energy transfer was also 9614


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Figure 91. Triazole-appended HPB dendrimers 214−216.

(CD) in the visible region. It was found that 223 and 225 bearing 2PZn units on the dendritic scaffolds exhibited a strong induced circular dichroism (ICD) response toward the enantiomers of Py2. For example, when dendrimer 224 was treated with R,R-Py2 and S,S-Py2, the circular dichroism spectra in the two cases were exactly a mirror images of each other, originating from a twisted geometry of the guest-binding zinc porphyrin dyads on the dendrimer scaffold. The role of porphyrin chromophores for this

porphyrin groups attached at the periphery of dendrimeric molecules 222, 223, 224, 225, and 226 for such translational purposes (Figures 94 and 95). The reason for incorporating the peripheral Zn-porphyrin group was because of its known tendency for chiroptical sensing of asymmetrical molecules. Clockwise- or counterclockwise-twisted geometry of the zinc porphyrin dimers was generated by ligation with chiral guests, which was monitored as exciton-coupled circular dichroism 9615


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Figure 93. HPB dendrimeric precursor 220 and metallodendrimer 221.

cooperativity effect was much greater in 223 than in 225 due to small steric hindrance in dendrimer 223. The H-aggregates (formed due to interaction between adjacent porphyrin units) were flexible enough to adjust their geometry to accommodate the chiral guest molecules, resulting in large induced circular dichroism response. On the other hand, the dendrimers 224 and 226 with three porphyrins did not show any cooperativity in chiral recognition. In a continuation of this work, Kim and co-workers70 studied the energy migration in HPB-based dendrimers 222−226 consisting of terminal porphyrin groups complexed with Zn2+ ions and the effect of number of generations of dendrimer on the ease of such energy migration (Figures 94 and 95). Therefore, six different generation dendrimers with one, two, and three branches were synthesized. Time-resolved fluorescence studies indicated that two-branched-type dendrimers exhibited more efficient excitation energy transfer in comparison to one- or three-branched analogues. Because two-branched-type dendrimers 223 and 225 exhibited tightly packed geometries with staggered multilayer hexagonal-wheel structures, three-dimensional excitation energy transfer (EET) in hexagonal wheels of 223 and 225 is much faster in comparison to that in singlebranched dendrimer 222. On the other hand, three-branchedtype dendrimers 224 and 226 revealed that the 3PZn constituent unit acts as a cluster in the EET processes because the porphyrin units in 3PZn were already in close contact. Thus, congested, well-ordered, and tightly packed two-branched-type dendrimers

could be the most promising systems for mimicking the oxygenic light harvesting among all dendrimers.

10. SUMMARY AND FUTURE PROSPECTS 10.1. Summary

Nonplanar HAB renders a very low degree of well-ordered selfassembled structure in its aggregated state. This low degree of crystallinity of HAB derivatives offers very high amorphous character, which is extremely in demand to cater uniform, nonbrittle, and flexible thin films for the integration of optical and electronic molecular devices. Due to delocalization of high πelectron clouds, electron-rich HAB core behaves as a donor that can be engineered with a suitable acceptor moiety to create a donor−acceptor push−pull system that has many applications as electronic materials like organic light-emitting diodes and fieldeffect transistors. HAB derivatives can be used as anchoring templates for designing electronic materials like solar cells in which solar energy is harvested to be converted into electrical energy. Although the presence of rotatable aryl groups on the periphery efficiently reduces π−π interactions in HPB cores for steric reasons, incorporation of interacting pheripheral groups can induce the columnar self-assembly with high stability and high transition temperature. HAB cores substituted with protoncontaining acidic functional groups can defy their tendency not to undergo columnar stacking, which can provide a transport 9616


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Figure 94. HPB-based dendrimers 222−224 bearing one and two porphyrin groups per arm, respectively.

channel for proton conduction that can be utilized as a protonexchange membrane in fuel cells. Although rotatable side-groups of HAB derivatives sterically hinder cofacial self-assembly, the presence of rotors around the benzene core in HAB becomes the reason for the aggregationinduced emission enhancement driven by hydrophobic forces. These fluorogenic self-aggregated nanostructures can be used for molecular recognition for various analytes and generation of metal-based NPs, which are useful for catalysis in several chemical reactions like Suzuki−Miyaura coupling and Sonogashira coupling. These metal-inclusion phenomena can also assist in self-assembly of monomeric units into oligomeric molecular capsular and sandwich structures. The hexameric structure of HAB derivatives allows for integrating different heteroaromatic binding units around the central benzene to invite more than one metal ion to bind in a single host molecule, which again promotes their tendency toward metal-assisted self-assemblies. HAB molecules bearing charged species or a combination of donor and acceptor species substituted alternatively on central benzene have been concluded to have very good redox properties. These redox-active HAB derivatives possess great intramolecular charge separation, which is very advantageous for electrochemical applications. High π-conjugation in HAB molecules results in good fluorescence emission, which increases the potential of HAB derivatives as fluorogenic chemosensors through noncovalent interactions like π−π, metal−π, coordination, van der Waals, and H-bond interactions. The electron-rich HAB donor molecule can be attached with an energy-acceptor unit to introduce resonance energy transfer

properties in a donor−acceptor adduct. The HAB core covers a large emission wavelength range of 350−500 nm depending upon the substituents attached to it, which increases the possibility of spectral overlap between the emission spectrum of HAB with the absorption spectrum of dyes with NIR emission so as to favor resonance energy transfer to subsequently increase Stokes shifts. On the other hand, the HAB core favors nonplanarity in the donor−spacer−acceptor system to favor through-bond energy transfer. The sterically strained scaffold of HAB has been widely chosen to design and synthesize large dendrimeric structures, which results in formation of big voids within the molecule so as to use these dendrimers for adsorption of gaseous as well as solvent molecules. The high degree of π-e− conjugation in dendrimeric molecules is also favorable for the integration of organic lightemitting diodes, nonlinear optical devices, and energy-harvesting and energy-migrating devices. 10.2. Future Prospects

Although it has been more than 80 years since the first HAB was first reportedly synthesized, its symmetrical but not so predictable structure gained attention many decades after its invention. A lot of research on geometry and stereochemistry of the HAB scaffold has been carried out by various research groups. However, in terms of applications, this moiety was overshadowed by its planar analogues, i.e., coronenes, which are a much closer member to the highly promising graphenes. The main target of this review is to showcase the scientific journey of the HAB unit so far and to update the scientific community with the state of the art in this particular field so as to promote the possibilities and capabilities of this still underexplored scaffold. 9617


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Figure 95. HPB-based dendrimers 225−226 bearing two and three porphyrin groups per arm.

d. Metal/Covalent Organic Framework. Position-modulated introduction of metal-chelating substituents or crosscoupling susceptible terminal groups around the HAB core can be supportive in the generation of molecular and covalent organic frameworks, respectively.

Despite the fact that several reports regarding applications of HABs have been published in the last one and half decades, at this point a lot of promising and productive work is still unprecedented. Apart from upgrading the role of HAB derivatives in the discussed applications, we can positively put the following possibilities in our future prospect: a. Energy Materials. Due to intrinsically high carbon content, metal-assisted HAB polymers can be used as selfsufficient precursors for the synthesis of cathodic materials for fuel cells and metal−air batteries without any supporting carbon material like graphene and carbon nanotubes. b. Heteroatom Doping. HAB derivatives bearing heteroaromatic rings like pyrroles, thiazoles, pyridines, thiophenes, etc. can potentially replace conventional but sluggish heteroaromatic polymers for doping of carbon materials using thermal and chemical vapor deposition (CVD) approaches to increase surface polarization and, hence, the electric conductivity of the material. c. Graphene Nanoribbons. The great tendency of HPB to undergo dehydrogenation can be capitalized on by onedimensional covalent polymers of HPB to generate highly conducting graphene nanoribbons.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Varun Vij received his B.Sc. (Hons. School) in chemistry in 2006 and M.Sc. (Hons. School) in chemistry in 2008 from Department of Chemistry, Guru Nanak Dev University, Amritsar. He received his Ph.D. from Department of Chemistry, Guru Nanak Dev University, Amritsar, in 2013 under the supervision of Prof. Manoj Kumar. His area of research in Ph.D. was supramolecular chemistry, self-aggregation, and 9618


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sensing materials based on hexaarylbenzene and coronenes. He is now working as a postdoctoral researcher in Ulsan National Institute of Science and Technology, South Korea. His present research interests involve development of nonprecious and advanced functional energy materials.

TAP BSA NP

tetraaminoporphyrin bovine serum albumin nanoparticle

Vandana Bhalla received her Ph.D. from Department of Chemistry, Guru Nanak Dev University, Amritsar, and joined as a lecturer at B.B.K. D.A.V. College, Amritsar. She was JSPS postdoctoral fellow for two years at Tohoku University, Sendai, Japan, and also worked as JST researcher in Aida Nanospace project with Prof. Takuzo Aida of University of Tokyo, Tokyo, Japan. She then joined Department of Chemistry, Guru Nanak Dev University, Amritsar, India. Her area of research is functional materials. She has published 146 research papers in international journals. She was conferred with CRSI Bronze medal and Thomson Reuters Research Excellence India Citation awards in 2015. She received Bhagyatara award for contribution in organic chemistry from Panjab University Chandigarh in 2015.

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Manoj Kumar studied chemistry at Guru Nanak Dev University, Amritsar, India, and received his Ph.D. in 1988. He has worked as a postdoc fellow at Instituto de Quimica Medica, Madrid, Spain. He was a visiting scientist at Johannnes-Guttenberg University, Mainz, Germany, and at Tohoku University, Japan. Currently, he is working as a Professor at the Department of Chemistry, Guru Nanak Dev University, Amritsar. He was conferred with CRSI Bronze medal in 2011. His research interest includes supramolecular host−guest chemistry of cyclic and acyclic receptors, fluorescent probes, and liquid crystalline materials. He has published 156 research papers in international journals.

ABBREVIATIONS HAB hexaarylbenzene HPB hexaphenylbenzene BHPB bis(hexaphenylbenzene) HHPB hexakis(hydroxyphenyl)benzene OLED organic light-emitting diode Alq3 tris(8-hydroxyquinolinolato)aluminum(III) ITO indium tin oxide NPB N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine TPBI 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene BCP barium-2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline ETL electron-transporting layer HBL hole-blocking layer HOMO highest occupied molecular orbital LUMO lowest unoccupied molecular orbital FRET fluorescence resonance energy transfer TBET through-bond energy transfer AIEE aggregation-induced emission enhancement PA picric acid TNT trinitrotoluene RhB rhodamine B TCBD 1,1,4,4-tetracyanobuta-1,3-diene CV cyclic voltammogram RDV rotatory disc voltammetry CT charge transfer EUV extreme ultraviolet PES poly(ether sulfone) PZn zinc porphyrin EET excitation energy transfer DME dimethoxyethane AMP adenosinemonophosphate OPV oligo(p-phenylenevinylene) 9619


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