Rodlike helical polymers with an excess of one-handedness arising from an optically active component incorporated into the main chain or at the pendants often show chiral liquidcrystalline (LC) phases in concentrated solutions or in a melt.[1] Since the 1980s, such LC helical polymers have been extensively studied with much interest. Typical biological macromolecules, such as DNA, [2] polysaccharides, [3] and polypeptides, [4] which adopt an ordered structure such as a helical structure, with a controlled helix sense stabilized by intra-and/or intermolecular hydrogen-bonding networks, also exhibit chiral LC phases resulting from the rigid-rod characteristics of the polymer main chains. Intramolecular hydrogen bonding has been used to construct synthetic helical polymers, such as polyisocyanopeptides, [5] and amino acid bound polyacetylenes.[6] The former helical polymers showed a clear cholesteric LC phase.Recently, we reported the first helical poly(phenylacetylene)s bearing l-or d-alanine pendants with a long alkyl chain (poly-l-1 and poly-d-1, respectively) that showed cholesteric LC phases in organic solvents owing to their main-chain stiffness assisted by intramolecular hydrogen bonds; their persistence lengths were determined to be approximately 40 nm in chloroform, [7] whereas the previously prepared monosubstituted polyacetylenes appear to be too flexible to exhibit LC phases.[6a-c, 8] We also found inversion of the helicity of poly-l-1 and poly-d-1 in response to the solvent polarity; the Cotton effect signs corresponding to the helix sense of poly-1 in benzene were inverted to the opposite signs in polar solvents such as THF and chloroform.[9] Furthermore, the macromolecular helicity inversion process could be directly visualized by atomic force microscopy (AFM), which revealed their diastereomeric helical conformations and enabled the determination of the helical sense. [9,10] We now show a dramatic change in the main-chain stiffness of poly-l-1 [11] accompanied with inversion of the helical sense of the polymer (Figure 1), resulting from the "on and off" fashion of the intramolecular hydrogen-bonding networks in polar and nonpolar solvents as revealed by the changes in their circular dichroism (CD) and IR spectra, persistence lengths, and rheological properties.Figure 2 a shows the CD spectra of poly-l-1 in polar and nonpolar solvents. Poly-l-1 exhibited split-type intense induced circular dichroisms (ICDs) in the conjugated polyene chromophore region. The ICD patterns measured in nonpolar Figure 1. Illustration of the helix-sense inversion of poly-l-1 regulated by solvents with different polarities, leading to diastereomeric helical poly-l-1s with extremely different main-chain stiffnesses. The helical senses of the diastereomeric poly-l-1s were determined by AFM.

The design and synthesis of helical polymers with a controlled helix sense [1] have been attracting great interest because of the wide variety of possible applications in materials science, chemical sensing, separation of enantiomers, and enantioselective catalysis.[2] The detailed structural characterizations of the helical polymers are essential to explore the relationships between their structures and properties and to further develop more sophisticated chiral materials. However, the determination of helical structures including helical pitch and handedness still remains very difficult. Conventional spectroscopic methods in dilute solution are not straightforward, and X-ray analysis in the solid state is a laborious task; these methods may not provide unambiguous helical structural information, and in particular, the helical sense. The direct observation of the helical polymers by atomic force microscopy (AFM) on solid substrates should be one of the most promising methods to settle this problem and has been extensively studied.[3] Recently, we reported that rigid rodlike helical poly(phenylacetylene)s bearing l-or d-alanine residues with a long n-decyl chain as the pendants (poly-l-Ala in Figure 1 a) hierarchically self-assembled on highly oriented pyrolytic graphite (HOPG) upon exposure to organic solvent vapors, such as benzene; flat poly(phenylacetylene) monolayers losing helical structures epitaxially formed on the basal plane of the graphite, on which helical poly(phenylacetylene)s further self-assembled into chiral two-dimensional (2D) helix-bundles with a controlled helicity.[4] High-resolution AFM revealed their helical conformations in the 2D crystals and enabled us to determine the molecular packing, helical pitch, and handedness.[5] Herein we show that an optically inactive but dynamically racemic helical poly(phenylacetylene) bearing achiral a-aminoisobutyric acid (Aib) residues with the same n-decyl chain as the pendants (poly-Aib in Figure 1 a) [6] also crystallizes under an organic solvent vapor atmosphere on HOPG, resulting in 2D helix-bundle formation (Figure 1 c). Poly-Aib no longer has any stereogenic centers, but likely consists of an equal mixture of interconvertible right-and left-handed helical segments separated by rarely occurring helical reversals in solution (Figure 1 c, left), as do polyisocyanates and polysilanes, and therefore, polyAib in dynamic equilibrium is a chiral or dynamically racemic helical polymer. [1a,h,j, 2e, 8] We could successfully visualize such enantiomeric right-and left-handed helical segments and the helical reversals of poly-Aib on HOPG by high-resolution AFM, and the results were quantified by X-ray diffraction of the oriented liquid crystalline (LC) poly-Aib film.Poly-Aib formed a lyotropic nematic LC phase in a concentrated benzene solution (above 10 wt %) as seen in its polarized optical micrograph (Figure 1 b), indicating that poly-Aib is a rigid-rod helical polymer. This assumption is supported by its extremely long persistence length of 89.6 nm in to...

Three chiral N-methylfulleropyrrolidine bisadducts were prepared, isolated, and completely resolved into each enantiomer using a chiral HPLC column, which were then converted to the corresponding optically active, cationic C(60)-bisadducts to investigate if they could act as a macromolecular helicity inducer in a poly(phenylacetylene) bearing an anionic monoethyl phosphonate pendant (poly-1) in aqueous solution. Upon complexation with the chiral C(60)-bisadducts, only the trans-3 bisadduct exhibited the characteristic induced circular dichroism (ICD) in the UV-visible region of the polymer backbone in dimethyl sulfoxide-water mixtures due to the predominantly one-handed helix formation of poly-1, while the trans-2 and cis-3 bisadducts induced almost no apparent CD in the same region. These results indicate that the helicity induction on poly-1 is highly sensitive to the structure and geometry of the cationic C(60)-bisadducts with a different distance between the separated charges.

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.