Supramolecular Helices: Chirality Transfer from Conjugated Molecules to Structures

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Supramolecular Helices: Chirality Transfer from Conjugated Molecules to Structures

Yang Yang , Yajie Zhang , and Zhixiang Wei *

Y. Yang, Dr. Y. J. Zhang, Prof. Z. X. WeiNational Center for Nanoscience and TechnologyBeiyitiao 11, Zhongguancun , Beijing , 100190 , ChinaE-mail: [email protected]

DOI: 10.1002/adma.201302448

1 . Introduction

Chirality represents an important characteristic of a chemical compound or a structure in which its structure is incongruent to its mirror image. [ 1 ] Molecular chirality plays an important role in the areas of chemistry and biology, and many chiral molecules are known to display enantioselective effects in bio-logical systems. [ 2 ] Chiral molecules with asymmetric arrange-ments of atoms exist widely in nature, such as in amino acids, sugars, and nucleotides, [ 3 ] while synthetic chiral molecules play an important role in pharmaceutical applications. [ 4 ] To express the function of molecules, molecular chirality is commonly present in the structures on both natural and artifi cial matter, such as helical structures of DNA, proteins, and artifi cial supra-molecular systems. [ 5 ] Constructing and regulating different scales of chirality in helical structures using chiral molecules are of great importance for exploring the new properties of functional materials.

Self-assembly is one of the most powerful methods for pre-paring different scales of functional materials. [ 6 ] Chiral infor-mation can also pass from single molecules to supramolecular assemblies and then to macroscopic superstructures through a self-assembly process. [ 7 ] Non-covalent molecular interactions and the assembly mechanisms of biological molecules have been widely studied and used as guidelines for designing func-tional materials with single-handed chirality. Many fascinating

structures of different scales and with different hierarchies have been success-fully prepared using DNA and peptides as building blocks via self-assembly pro-cesses. [ 8 ] Hydrogen-bonding interactions play a key role in the self-assembly of bio-logical molecules. π -Conjugated molecules are widely used in organic electronic mate-rials and devices, and their self-assem-bled structures have a great infl uence on their conducting and semiconducting properties. [ 5a ] Different from biological molecules, π – π stacking interactions are often used as a preliminary driving force for the self-assembly of various struc-tures based on conjugated molecules. [ 5a ] A number of similarities have been observed

between hydrogen-bonding and π – π stacking interactions. For instance, the strength and distance over which these interac-tions act are quite similar and both types of interactions have directionality. [ 9 ] Therefore, using conjugated molecules with one-handed chirality as building blocks to prepare hierarchical superstructures by mimicking the self-assembly behavior of biomolecules is a state-of-the-art strategy for designing novel functional nanomaterials. [ 9 ]

To prepare chiral conjugated structures, achiral molecules can fi rst covalently bond with chiral groups or form complexes by non-covalent interactions with chiral molecules. The chiral molecules can then be used as building blocks to construct various helical supramolecular structures. Several excellent review papers on this topic have recently been published. [ 5a, 7 , 10 ] Meijer and cowrkers reviewed the supramolecular assemblies of π -conjugated systems, focusing on mesoscopic structure control, and their applications in organic electronic devices. [ 5a ] Yashima et al. reviewed the synthesis, structures, and functions of helical polymers, [ 10a ] in which various important effects of helical polymers are summarized, such as induced helicity, hel-ical inversion, chiral amplifi cation, memory effect and etc. [ 10a ] A recent review paper by Aida et al. emphasized the importance of chirality in functional supramolecular polymers. [ 7 ]

In this review, the chirality transfer from molecules to struc-tures in π -conjugated system is reviewed, mainly focusing on the principles that guide the formation and adjustment of supramolecular helices at different scales. We take conducting polyaniline (PANI) and other conjugated molecules as exam-ples to illustrate the chirality transfer from chiral molecules to structures. We also describe how to tune the helical sense of the structures and how to get helical heterojunctions. [ 9b, 11 ] Poten-tial applications and directions for the future development of supramolecular helices are also highlighted.

Different scales of chirality endow a material with many excellent proper-ties and potential applications. In this review, using π -conjugated molecules as functional building blocks, recent progress on supramolecular helices inspired by biological helicity is summarized. First, induced chirality on conju-gated polymers and small molecules is introduced. Molecular chirality can be amplifi ed to nanostructures, superstructures, and even macroscopic struc-tures by a self-assembly process. Then, the principles for tuning the helicity of supramolecular chirality, as well as formation of helical heterojunctions, are summarized. Finally, the potential applications of chiral structures in chiral sensing and organic electronic devices are critically reviewed. Due to recent progress in chiral structures, an interdisciplinary area called “chiral electron-ics” is expected to gain wide popularity in the near future.

Adv. Mater. 2013, DOI: 10.1002/adma.201302448

http://doi.wiley.com/10.1002/adma.201302448

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Under the induction of a chiral substance or template, the expression of chirality of achiral molecules on optical activities and structures is known as induced chirality. [ 12 ] The emerging circular dichroism (CD) signal of an achiral molecule induced by a covalent or non-covalent bonded chiral unit is also known as induced circular dichroism (ICD). [ 13 ] In this section, the induced chirality of conjugated molecules and chirality transfer from molecules to nanostructures, superstructures, and macro-scopic structures are critically reviewed.

2.1 . Induced Helical Nanostructures

In this section, the principles that guide chirality transfer from molecules to various nanostructures are reviewed using PANI as an example. [ 9 , 11b ] Other conjugated molecules, such as polyacetylenes and perylene diimide, are also briefl y introduced. [ 14 ]

As a typical conducting polymer, PANI was extensively investigated by MacDiarmid in last century. [ 15 ] PANI has dif-ferent oxidation states (i.e., leucoemeraldine, emeraldine, and pernigraniline) ( Scheme 1 ). In the emeraldine state, PANI can change from base to salt by protonic acid doping, which is one of its most important features, and both the conductivity and other functions of PANI can be facilely adjusted by counter ions (A − in the emeraldine state in Scheme 1 ). Although various acids may be used, camphorsulfonic acid (CSA) has attracted most attention because it can guarantee the process-ability and high conductivity of PANI. [ 16 ] When chiral CSA is used as dopant, the helical conformation of PANI is induced by ionic interactions with a chiral molecule. This phenom-enon was independently reported in 1994 by Wallace and co-workers [ 17 ] and Havinga et al. [ 18 ] using electropolymerization and chemical polymerization, respectively. Afterward, Wallace, Kane-Maguire and co-workers carried out a comprehensive study on the induced chirality of PANI by chemical or electro-chemical polymerization. [ 19 ]

PANI can also self-assemble into one-dimensional nano-structures by an in situ polymerization procedure. [ 20 ] The nano-structures of PANI can be obtained by a “soft-template” method or by preventing the overgrowth of PANI during polymeri-zation. [ 20 ] Li and Wang found that adding a small amount of oligoaniline can produce PANI nanofi bers with good optical properties in the presence of enantiomeric CSA as a dopant. [ 21 ] However, single-handed helical nanostructures of conducting polymers are diffi cult to visualize because of the rigid struc-ture of the PANI molecules. [ 21,22 ] To visually observe the helical morphologies of PANI nanostructures, the synthetic conditions of PANI nanostructures are optimized in the presence of D - or L -CSA as a dopant. In the presence of high concentrations of enantiomeric dopants, we successfully obtained nanostruc-tures of conducting PANI with predominately single-handed helicity. [ 11b ]

At optimized conditions, right- and left-handed helical nanofi bers may respectively be obtained using D -CSA and L -CSA as dopants ( Figure 1 a,b), with their CD spectra exhib-iting mirror image-induced CD signals for both dopants

Yang Yang is a graduate stu-dent under the supervision of Prof. Z. X. Wei at National Center for Nanoscience and Technology (NCNST) of China. He received his B.S. degree in Polymer Materials and Engineering from Taiyuan University of Technology in China. His current research interests focus on the self-assembly of

organic optoelectronic materials.

Yajie Zhang has been an assistant professor at NCNST since 2010. She obtained her B.S. and M.S. degrees from the Northeast Normal University, China. She recieved her Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences (CAS) in 2010. Her research focuses on conducting and photoelectric materials and devices.

Zhixiang Wei has been a professor at NCNST since 2006. He graduated with a B.S. degree in 1997 and an M.S. degree in 2000 from Xi'an Jiaotong University. He obtained his Ph.D. degree in 2003 from the Institute of Chemistry, CAS. From 2003 to 2005, he took postdoc-toral research at the Max-Planck-Institute of Colloid

and Interfaces and at the University of Toronto. He was awarded with the “Hundred Talents Program” in 2006 and the “National Science Fund for Distinguished Young Scholars” in 2011. His research focuses on self-assembled organic functional nanomaterials and related fl exible devices.

(Figure 1 c). The CD spectra prove that a predominantly one-handed helical polymer conformation is induced by enantio-meric dopants. [ 19d ] Helical PANI molecules are self-assembled into helical nanofi bers and nanofi brillar bundles due to their rigid molecular structure and π – π stacking interactions. Left- and right-handed PANIs show exactly the same UV-vis spectra (Figure 1 d), proving that they possess the same molecular structure.


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dopant as an example, nanostructures with a left-handed helical sense could be clearly observed by scanning electron micro-scope (SEM) and transmission electron microscope (TEM) (Figure 2 b–d). PANI with single-handed twisted conformation is fi rst induced by polymerization in its good solvent using enantiomeric CSA as a dopant. Since PANI dissolves well in its good solvent, the molecules are mainly arranged in an extended conformation and only a slight twist may be found in its CD

Although a clear single-handed morphology is observed, the arrangement of PANI molecules into its nanostructures is diffi cult to determine because of the poor crystallinity of the nanofi bers. [ 11b ] As such, we further focus on the preparation PANI nanofi bers with high crystallinity.

To obtain a well-defi ned helical structure, a two-step method is applied to produce chiral PANI nanostructures with high crystallinity (see schematic in Figure 2 a). [ 9a ] Taking D -CSA as a

Scheme 1. Molecular structures of PANI at different states.

Figure 1. Induced helical PANI nanofi bers. SEM images of nanofi bers obtained using the dopants: a) D -CSA and b) L -CSA. The insets show their helical directions, and the dark lines represent individual helical PANI chains in the helical nanofi bers. c,d) Corresponding CD and UV–vis spectra. The curves in the UV–vis plot are shifted vertically for clarity. Reproduced with permission. [ 11b ] Copyright 2007, WILEY-VCH Verlag GmbH & Co. KGaA.


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indicated by the (040) direction in the selected area electron dif-fraction (SAED) pattern (Figure 2 f). [ 9a ]

The molecular arrangement of PANI in its nanostructures is quite similar to that of β -pleated sheets of proteins in biological systems. [ 8a, 23 ] In a β -pleated sheet, the chiral peptide chains are normal to the direction of their fi brillar aggregates. The bioin-spired preparation of helical functional nanostructures not only promotes a deeper understanding of the self-assembly behavior

spectra (Figure 2 e). After formation of nanostructures, the CD spectra increased signifi cantly due to a helical arrangement of the PANI molecules. TEM images at high magnifi cation clearly show dark helical strips of the nanofi bers (Figure 2 d). When L -CSA is used as a dopant, an opposite helical sense could be obtained. PANI molecules have long-range order in its nanostructures, and π – π stacking interactions are observed along the direction of the long axis of the nanofi bers, as

Figure 2. Crystalline PANI helical nanostructures. a) Schematic of the formation process of crystalline helical nanostructures. b) SEM image of helical nanofi bers. c) TEM image of helical nanofi bers. d) Magnifi ed image showing left-handed helical screws. e) CD spectra with varying good-to-poor solvent ratios. f ) SAED pattern of helical nanofi bers. Reproduced with permission. [ 9a ] Copyright 2010, Wiley-VCH Verlag GmbH & Co. KGaA.


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has the superior ability of preparing the materials with dif-ferent-levels of hierarchy. Using different scales of building blocks (i.e., from oligomers to polymers to colloids), Ikkala and Brinke prepared artifi cial materials with different levels of hier-archy by self-assembly. [ 32 ] Hierarchical structures with homo-chirality are of great interest because of their multifunctions. Stupp and co-workers reported that peptide amphiphiles can self-assemble into hierarchical structures at different con-ditions, from helical nanofi bers to giant nanobelts, and to macro scopic sacs and membranes. [ 33 ] Using conjugated PANI as building blocks, ordered hierarchical superstructures with single handed chirality can be prepared in a well-controlled self-assembly process ( Figure 3 ). [ 9b ]

The preparation of highly ordered superstructures fi rst requires the production of monodispersed nanostructures with a predominantly single-handed polymer helical confor-mation as building blocks. [ 9b ] A slow self-assembly process of PANI by tuning the good to poor solvent ratio can produce monodispersed nanorices as shown in Figure 3 b. When the self-assembly process occurs slightly faster, the PANI nanorices can self-assemble into superstructured microplates (Figure 3 c).

of biological systems, but also provides self-assembled struc-tures with novel functions. [ 9a ]

Aside from PANI, many molecules with extended π -conjugated ring systems form helical supramolecular struc-tures. [ 24 ] Some of these molecules include phthalocyanines, [ 25 ] hexabenzocoronenes, [ 26 ] p -Phenylenevinylenes [ 27 ] and perylene diimides (PTCDI), [ 10c, 28 ] among others. [ 29 ] For instance, the supramolecular chirality of PTCDI can be induced by compl-exation with an anionic chiral phosphate surfactant (BDP). [ 14,30 ] Using a cationic perylene diimide dye (PTCDI) as a functional unit, a chiral supramolecular liquid-crystalline material can be induced by an anionic chiral phosphate surfactant (BDP), in which PTCDI-BDP molecules are organized in a single-handed helical sense during aggregation because of the steric hin-drance of chiral alkyl groups. [ 14 ]

2.2 . Hierarchical Helical Superstructures

Inspired by the helicity of biological materials, interest in pro-ducing hierarchical synthetic materials has grown. [ 31 ] Nature

Figure 3. Formation of PANI superstructures. a) Schematic of the hierachical self-assembly of well-defi ned nanostrucures and superstructures. b) SEM image of chiral PANI nanorices. c–d) SEM images of two dimensional superstructured microplates and one magnifi ed hexagonal microplate. d) Side view of the microplate. Reproduced with permission. [ 9b ] Copyright 2010, American Chemical Society.


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2.3 . Induced Macroscopic Helical Structures

Various macroscopic helical structures can be found in nature, such as spiral sea shells, cucumber tendril coils, and chiral seed pods. [ 37 ] Many objects with macroscopic helical structures are utilized in daily life, including screws, springs, and ropes. Although aligned helical nanostructures have been success-fully prepared by several groups, [ 38 ] direct construction of macroscopic helical structures by self-assembly remains a chal-lenging task.

Sharon and co-workers studied the mechanical process of seed-pod opening and proposed a chirality-creating mechanism to explain how fl at pods twist into a helix. [ 37b ] Interestingly, the authors further constructed a two-sided strip to mimic the pod geometry. Two identical thin latex sheets were stretched uniaxi-ally by the same elongation factor. When the two sheets were glued together at an angle of 45 ° or 135 ° , the two layers shrank uniaxially in perpendicular directions, leading to left-handed and right-handed helical confi gurations, respectively. Detailed experiments and theoretical analysis prove that different con-fi gurations of helical structures can be produced when two stretched sheets are glued together at different angles. [ 37b ] Trans-formation of the molecule chirality to macroscopic chirality has also been realized recently by Urayama and co-workers ( Figure 5 ). [ 39 ] The origin of chirality in twist-nematic-elastomer fi lms has been investigated in detail, [ 39 ] and the chiral arrange-ment of mesogens has been found to induce different helical states of macroscopic ribbons because of the coupling between the liquid crystalline order and the elasticity of the fi lm. Interest-ingly, by changing the temperature, the pitch of the helical rib-bons can be tuned by changes in the interactions between liquid crystals. [ 39 ]

As discussed above, chirality transfer from molecules to nanostructures, superstructures, and macroscopic structures can be realized by the self-assembly method. In such cases, the chirality of structures originates from molecular chirality but is amplifi ed to a larger scale by the cooperative effect of non-covalent interactions.

Figure 4. Superhelical structures. (a) Molecular structure and TEM images of PTCDI-HAG. Reproduced with permission. [ 34 ] Copyright 2011, The Royal Society of Chemistry. b) Molecular structure and TEM image of the quadruple helix of an amphiphilic peptide. Reproduced with permission. [ 36 ] Copyright 2008, American Chemical Society.

The top view of the microplate shows that some of the nanorices form well-defi ned hexagonal shapes (Figure 3 d). The side view of the microplate proves that nanorices are sub-units of the microplate and that they are arranged in a shoulder-to-shoulder manner (Figure 3 e).

The formation of superstructured micro-plates is ascribed to the delicate control of the non-covalent interactions, including polymer-polymer interactions and the polymer-solvent interaction. Although the various interac-tions are diffi cult to separate from each other, polymer-polymer interactions, especially π – π stacking interactions, play a dominant role in the formation of the crystalline nanorices, in which the twisted PANI molecules are stacked along the axial-direction nanorice in a helical manner. [ 9 ] On the other hand, polymer-solvent interactions induce the aggregation of the nanorices during their self-assembly, thereby resulting in a shoulder-to-shoulder arrangement. Helical stacking changes the rectangular column phase of conventional PANI [ 28b ] to a hexagonal phase, such that a hexagonal shape may be observed for the superstructured microplate.

If the building blocks are nanofi bers instead of short nanorices, obtaining superhelices of conjugated molecules is possible. Inspired by the supercoiled structures of collagen, Nolte and co-workers reported that disk-shaped molecules can self-assemble into coiled-coil aggregates with a tunable helicity. [ 25a ] Controlled superhelical structures can be real-ized by designing new molecules. PTCDI-HAG has a unique amphiphilic structure ( Figure 4 a) [ 34 ] consisting of a perylene diimide scaffold, a hydrophobic 1-hexylheptyl group, and a hydrophilic galactosyl residue. The supramolecular chirality of PTCDI-HAG can be tuned through simple solvent interactions. When dissolved in THF and then adding water to initiate self-assembly, the galactosyl groups induce the right-handed helical arrangement of perylene diimide scaffold. Multiple O–H···O hydrogen bonds between the galactosyl groups induce fur-ther aggregation of the preliminary nanofi bers, consequently leading to the formation of double or multiple super-helical structures (Figure 4 a). [ 34 ]

Huang and co-workers reported an amphiphilic molecule containing three segments: a butyl group, an azobenzene group, and a sugar moiety. [ 35 ] The molecule can self-assemble into double helical nanofi bers in water and further form vis-coelastic hydrogels. Stupp and co-workers reported a well defi ned quadruple helical fi ber of amphiphilic peptides by controlling interactions among molecules in neighboring nanofi bers, including hydrogen bonds and hydrohobic interac-tions (Figure 4 b). [ 36 ] Control over the non-covalent interactions proved that quadruple fi berscan convert into single fi bers upon photochemical cleavage of the 2-nitrobenzyl group. [ 36 ]

Scientists have shown the ability to obtain hierarchical superstructures with single-handed helicity by the delicate con-trol of non-covalent interactions. However, a rational design for self-assembly requires further experimental and theoretical studies.


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right-handed helicity is very small. Thus, external stimuli or kinetic controlled growth may lead to differences in chirality at the thermodynamic state.

To understand how thermodynamic and kinetic factors affect the chirality of supramolecular structures, we synthe-sized a sugar-based perylenediimide derivative PTCDI-BAG ( Figure 6 ). [ 24 ] Theoretical simulations showed that the dimer is unstable when two molecules are stacked in a totally eclipsed fashion (the rotation angle = 0 ° ). The structure with a right-handed rotation angle of 28.5 ° has the lowest binding energy. Right-handed helicity is preferred by the structures that form at thermodynamically controlled conditions. In contrast, struc-tures with a left-handed rotation angle of –23.3 ° have a less-stable local minimum, which means left-handed structures can be obtained under some kinetically controlled conditions. Therefore, the supramolecular chirality of PTCDI-BAG can be tuned by changing the self-assembly conditions. In fact, helical inversion is a fairly common phenomenon in induced chirality that has been observed in various polymers, foldamers, and supramolecular systems stimulated by light, solvent, tempera-ture, etc. [ 10a, 41 ]

Since the energy difference between left-handed and right-handed supramolecular structures is quite small, a small differ-ence in molecular structures may lead to differences in supra-molecular helicity. Induced by D -CSA and L -CSA dopants, right- and left-handed helical nanofi bers of PANI can respectively be prepared by in situ polymerization of the aniline monomer. [ 11b ] The copolymer obtained by copolymerization of aniline with m- toluidine (PMANI), however, is completely reversed with respect to the helicity of PANI, whereas the copolymer with aniline and o- toluidine (POANI) features the same helicity as PANI. Theoretical simulations on the oligomers reveal that the steric hindrance of the methyl group at the m -position of the phenyl ring induces a helical inversion of the nitrogen atoms at the early stages of polymerization, while the methyl groups at the o -position have no infl uence on the nitrogen atoms ( Figure 7 ). [ 11a ] Therefore, single-handed helicity can be induced by chiral dopants, and changes in a subunit can further tune the helical sense of the molecular and supramolecular structures. These fi ndings are of great importance for controlling the heli-city of functional helical structures for future applications.

3.2 . Helical Heterojuction Structures

Heterojunctions, such as p-n junctions and metal-semi-conductor junctions, are extremely important in solid-state electronic devices ranging from LEDs to photovoltaics to transistors. [ 42 ] Since the helical sense of supramolecular struc-tures can be tuned by various factors, one may expect to obtain helical heterojunctions consisting of a left-helical segment and right-helical segment in one structure.

Although most helical biological structures prefer to form single-handed helicity, “helical junctions” exist in some plants, such as cucumber tendril coils. [ 37a ] Obtaining high-yield helical junctions by helical inversion is diffi cult. Two general principles for chiral amplifi cation, called “majority rule” and the “sergeant and soldiers effect,” govern the formation of single-handed supramolecular helicity in the case of an excess or presence

3 . Tuning the Helicity of Structures

Induced by the chirality of the composing unit or dopant, single-handed helical structures may be obtained from bio-logical or synthetic macromolecules. [ 10a ] The helicity of the structures can normally be tuned by changing the molecular chirality. Some helical supramolecular structures, however, may undergo variations in helicity between right-handedness and left-handedness, despite featuring the same molecular chirality. This phenomenon is called the helix-helix transition, which is often observed in solution and induced by an external stimulus, such as light, temperature, solvents, and chiral additives. [ 40 ] In this section, tuning the helical sense of structures at different scales will be reviewed. Helical heterojunctions (i.e., one struc-ture consisting of left-handed and right-handed segments) will be introduced.

3.1 . Tuning the Helical Sense of Nanostructures

Induced single-handed helicity is mainly attributed to ener-getic differences between left- and right-handed helicity brought about by the steric hindrance of chiral groups. In some cases, the energy difference between left-handed helicity and

Figure 5. Chirality transfer for molecules to macrocopic scales. a) Mono-acrylate mesogenic monomer and chiral dopant for constructing twist-nematic-elastomer ribbons. b) Schematic of the direct confi guration of liquid crystal mesogens in the ribbon. c) Helicoids formed by narrow twist-nematic-elastomer fi lms with left-handed and right-handed shapes. a–c) Reproduced with permission. [ 39 ] Copyright 2011, National Academy of Sciences, USA.


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driving force of steric hindrance in the polymerization process (Figure 8 b). [ 45 ] Helical heterojunctions may be formed by strong steric hindrance to reverse the helical sense of nanofi bers via the delicate control of reaction conditions.

A more delicate control of supramolecular heterojunctions was proposed by Aida and co-workers (Figure 8 c), [ 46 ] who used morphologically stabilized hexa- perihexabenzocoronene (HBC) nanotubes as seeds. In their study, a second HBC mon-omer was further assembled from the extremely thin facets of the seed nanotube termini. Because no chiral group was used, the chirality of different nanotubes may differ. However, the arrangement of HBC molecules in the same nanotube with supramolecular heterojunctions should follow the same chi-rality due to the “sergeant and soldiers” rule.

4 . Potential Application of Helical Structures

Because of the ordered chiral arrangement of molecules, chiral supramolecular structures are regarded as a powerful tool for the recognition and separation of target chiral molecules. [ 10a , 47 ] Moreover, chiral supramolecular structures may play an impor-tant role in supramolecular electronics because of the ability of the molecules to be arranged in a highly controlled manner. [ 5a, 48 ]

of particular enantiomeric molecules. [ 43 ] In the helical reversal process of polyacetylene conformation, however, some helical heterojunctions in molecules have been observed by Yashima and co-workers; these heterojunctions are believed to be due to dynamically interconvertible helical segments ( Figure 8 a). [ 44 ] In PANI nanofi bers, “helical heterojunctions” composed of right- and left-handed helical segments in one nanofi ber have been successfully obtained by delicately controlling the inversion

Figure 6. Simulations to investigate the dependence of energy on the rotation of the upper molecules of PTCDI-BAG in a stacked dimer. Reproduced with permission. [ 24 ] Copyright 2012, Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 7. Theoretical model for the stable forms of dimers when aniline: a) self-polymerizes, b) copolymerizes with m -toluidine, and c) copoly-merizes with o -toluidine. Reproduced with permission. [ 11a ] Copyright 2009, Wiley-VCH Verlag GmbH & Co. KGaA.


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result provides a general approach for the detection of chiral molecules by the electrical method.

The chiral arrangement of molecules provides better control of the resulting structures; thus, chiral structures have been used to tune the properties of organic electronic devices. Meijer and co-workers produced a complex of n-type perylene bisimide derivatives with p-type oligo(p-phenylene vinylene) and further self-assembled a p-n junction into one helical nanofi ber. [ 48a ] Aida used gemini-shaped HBC molecules to construct a p-n junction in one coaxial nanotube, which allows quick photocon-ductive responses with a large on/off ratio. [ 48b ] Feng, Muellen and co-workers reported that stacking at certain twisting angles of discotic liquid crystals causes electronic coupling and reduces structural defects, thereby increasing charge carrier mobility in fi eld-effect transistor devices. [ 51 ] These results suggest an effi -cient way of increasing the performance of organic electronic materials by controlling their molecular arrangement.

5 . Conclusions and Perspectives

Conjugated molecules can be successfully induced into pre-dominately single-handed structures by chiral dopants or sub-stituted groups. For example, achiral PANI may be induced into a predominately one-handed chiral conformation. These

The recognition and separation of chiral molecules are cru-cial in biotechnology, especially in pharmaceutics. Kaner and co-workers reported that chiral PANI thin fi lms can be used for the enantioselective discrimination of D - and L -phenylalanine. [ 47b, 49 ] Although the chiral recognition effect is quite similar to “molecular imprinting”, PANI could potentially serve as a more-general host matrix for various chiral molecules via a doping/dedoping mechanism. [ 47b ] Chiral nanostructures with larger surface areas are expected to possess better chiral recog-nition and separation properties. Polypyrrole (PPy) nanowires doped with chiral CSA molecules acting as both the dopant and pseudo-template can be used as an enantioselective matrix for chiral phenylalanine ( Figure 9 ). [ 47a ] CSA molecules com-bine with PPy through multiple non-covalent interactions: i) electrostatic interactions between the –SO 3 − group of the CSA and positively charged groups in the PPy molecules; and ii) hydrogen-bonding interactions between the carbonyl oxygen of CSA and the nitrogen atom of the PPy backbone. These multi ple interactions supply multiple anchoring positions during the enantioselective recognition of chiral phenylala-nines. Chiral recognition can be observed by detecting conduc-tivity changes or CD spectra. Recently, a thin enantioselective layer on the surface of organic thin-fi lm transistors was found to show fi eld-effect-amplifi ed sensitivity for the differential detection of optical isomers with high sensitivity. [ 50 ] Such a

Figure 8. Helical heterojunctions. a) Polyaceytylene interconvertible helical segments observed by AFM phase imaging. Reproduced with permission. [ 44 ] Copyright 2007, Wiley-VCH Verlag GmbH & Co. KGaA. b) SEM and TEM images of helical PANI heterojuctions. Reproduced with permission. [ 45 ] Copyright 2011, The Royal Society of Chemistry. c) Supramolecular heterojunctions of HBC. Reproduced with permission. [ 46 ] Copyright 2011, American Association for the Advancement of Science (AAAS).


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in resistance or a fi eld-effect mechanism. Chiral-conjugated polymers have recently been used to construct a photovoltaic cell, the responses of which depend on the circular polariza-tion of the incoming light. [ 52 ] Moreover, chiral stacking of dis-cotic liquid crystals causes electronic coupling and reduces structural defects, thereby increasing charge carrier mobility in fi eld-effect-transistor devices. [ 51 ] Recent progress on the chiral structures of nanoparticles, such as gold nanoparticles and quantum dots, has also indicated that chiral optical effects can lead to the fabrication of other functional structures with unique properties. [ 53 ] Future studies will focus on developing functional structures with unique chiral electronic, optical, and optoelectronic properties.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 91027031 and 21125420), the Ministry of Science and Technology of China (Grant Nos. 2009CB930400, 2010DFB63530, and 2011CB932300), and the Chinese Academy of Sciences.

Received: May 29, 2013 Published online:

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building blocks then further self-assemble into single-handed helical nanofi bers and nanofi brillar bundles. Chirality can be further amplifi ed to superstructures and macroscopic struc-tures by controlling the molecular arrangement. Furthermore, tuning the helical sense of supramolecular structures is pos-sible due to the small energy difference between right-handed and left-handed arrangements. Helical reversal of the structures can be further used to construct novel helical heterojunctions composed of right- and left-handed segments in one structure. The various helical nanostructures obtained thus far provide a platform by which to study the interaction between chiral struc-tures and molecules, which may have potential applications in chiral recognition and separation.

With the rapid development of chiral structures, the fi eld of “chiral electronics” has recently emerged. Conjugated mole-cules possess excellent electronic, optical, and optoelectronic properties. Integrating chirality with these areas may result in new interdisciplinary areas, including chiral electronics, chiral optics, and chiral optoelectronics ( Figure 10 ). Chiral structures

Figure 9. Schematic of chiral amino acid recognition in which the chiral CSA molecule acts as both the dopant and pseudo-template. Reproduced with permission. [ 47a ] Copyright 2008, Elsevier B.V.

Figure 10. Interdisciplinary areas of chirality with electronics, optics, and optoelectronics.


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Supramolecular Helices: Chirality Transfer from Conjugated Molecules to Structures