In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Figure 13. Structures of poly(dialkylsilane)s with different main-chain stiffness (74s76) and schematic illustrations of stiffness-dependent polymer wrapping behavior onto SWNT by the high-speed vibration milling (HSVM) method. (Reproduced with permission from ref 133.

The helicity of biological macromolecules such as DNA and proteins is largely governed by the homochirality of their components (D-sugars and L-amino acids). In polymer and supramolecular chemistry, control of helicity is an attractive goal because of possible applications in materials science, chemical sensing and enantioselective catalysis 1-13 . We reported recently that macromolecular helicity can be induced in a polymer by an optically active amine 14 . Here we show that this helicity can be 'memorized' when the amine is replaced by various achiral amines. Although the maintenance of helicity in the polymer is not perfect, it can 'repair' itself over time. Small structural changes in the achiral amines influence the efficiency of helicity retention markedly.We recently reported the induction of helicity in a stereoregular, cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1; Fig. 1) 14 . The polymer possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amines such as (R)-2 and (S)-3 capable of interacting with the polymer's carboxy groups, a dynamic, onehanded macromolecular helicity is induced in the polymer, resulting in optical activity (Fig. 1). Analogous induction of macromolecular helicity occurs in the tobacco mosaic virus, which induces a right-handed helix in RNA on the inside of the protein shell 15 . The complexes of poly-1 with optically active amines show a characteristic, induced circular dichroism (ICD) in the ultravioletvisible region which depends on the absolute configuration of the chiral amines, and the signs corresponding to the helix-sense can be used as a probe for the chirality assignment of the amines. We now find that the macromolecular helicity of poly-1 induced by optically active amines remains even after removal and replacement of the optically active amines by achiral ones (Fig. 1). Figure 2, trace a shows the circular dichroism (CD) spectrum of poly-1 in the presence of (R)-2 (10 equiv. to monomer units of poly-1) in dimethyl sulphoxide (DMSO). The poly-1-(R)-2 complex shows a characteristic, exciton-type coupled ICD 16 . As expected, the optical activity of the poly-1-(R)-2 complex instantly disappeared when the complex was exposed to a stronger acid (such as trifluoroacetic acid), which frees the poly-1 so it reverts to the original, optically inactive (non-helical)polymer. On addition of 5 equiv. (S)-3 to the solution of poly-1-(R)-2, the ICD spontaneously changed and the signs inverted to give mirror images (Fig. 2, trace b). This clearly indicates that the induced helix of the poly-1 is dynamic in nature, and the helix-sense can be controlled with a rather small amount of the chiral amino alcohol (S)-3 ( Fig.

In the chromatographic separation of enantiomers the order of elution is determined by the strength of diasteromeric interactions between the components of the mixture and a chiral stationary phase. For analytical purposes, it is ideal to have the minor component elute first, whereas in the preparative mode a faster elution of the major component is desirable. Here we describe a stationary phase constructed from a polyacetylene that bears 2,2'-bisphenol-derived side chains in which chirality can be switched in the solid state prior to use. Both the macromolecular helicity of the polymer backbone and the axial chirality of the side chains can be switched in the solid state by interaction with a chiral alcohol, but importantly are maintained after removal of the chiral alcohol because of a memory effect. The chiral stationary phase thus prepared was used to separate the enantiomers of trans-stilbene oxide with the enantiomer elution order determined by the preseparation treatment.

A unique feature of synthetic helical polymers for the detection and amplification of chirality is briefly described in this article. In sharp contrast to host-guest and supramolecular systems that use small synthetic receptor molecules, chirality can be significantly amplified in a helical polymer, such as poly(phenylacetylene)s with functional pendants, which enable the detection of a tiny imbalance in biologically important chiral molecules through a noncovalent bonding interaction with high cooperativity. The rational design of polymeric receptors can be possible by using chromophoric helical polymers combined with functional groups as the pendants, which target particular chiral guest molecules for developing a highly efficient chirality-sensing system. The chirality sensing of other small molecular and supramolecular systems is also briefly described for comparison.

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.

Unique macromolecules that fold into a preferred-handed helical conformation induced by external chiral stimuli are mainly described in this review. In contrast to small molecular systems, the chiral information of nonracemic guest molecules transfers with a significant amplification in a dynamic helical polymer, such as stereoregular poly(phenylacetylene)s bearing functional pendant groups as an excess of a single-handed helix through noncovalent bonding interaction, which provides an efficient chirality-sensing system. The chirality sensing of other oligomers (foldamers) is also briefly described.

Cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1) is an optically inactive polymer but forms an induced one-handed helical structure upon complexation with optically active amines such as (R)-(1-(1-naphthyl)ethyl)amine ((R)-2) in DMSO. The complexes show a characteristic induced circular dichroism (ICD) in the UV-visible region of the polymer backbone. Moreover, the macromolecular helicity of poly-1 induced by (R)-2 can be "memorized" even after complete replacement of (R)-2 by various achiral amines. We now report fully detailed studies on the mechanism of the helicity induction and memory of the helical chirality of poly-1 by means of UV-visible, CD, and infrared spectroscopies. We have found that a one-handed helix is cooperatively induced on poly-1 upon the ion pair formation of the carboxy groups of poly-1 with optically active amines and that the bulkiness of the chiral amines plays a crucial role for inducing an excess of a single-handed helix. On the other hand, the free ion formation was found to be essential for the macromolecular helicity memory of poly-1 after the replacement of the chiral amine by achiral amines, since the intramolecular electrostatic repulsion between the neighboring carboxylate ions of poly-1 significantly contributes to reduce the atropisomerization process of poly-1. On the basis of the mechanism of helicity induction and the memory of the helical chirality drawn from the present studies, we succeeded in creating an almost perfect memory of the induced macromolecular helicity of poly-1 with (R)-2 by using 2-aminoethanol as an achiral chaperoning molecule to assist in maintaining the memory of helical chirality.