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103 nm (26)), the thermodynamic stability of the helical conformations and their electric properties including the absorption, CD and fluorescence spectral profiles. The chiroptical properties of helical polysilanes obey the sergeants and soldiers principle and majority rule [46, 47].

11.3.1.3 Polyacetylenes

A large number of p-conjugated, dynamic helical polyacetylenes have also been prepared by the polymerization of phenylacetylenes (27-32) [48-51], propiolic esters (33) [52] and N-propargylamides (34) [53, 54] bearing optically active sub-stituents or by copolymerization with achiral acetylenes. Rhodium catalysts, such as [Rh(nbd)Cl]2 (nbd: norbornadiene), are often used to produce stereoregular (cis-transoidal) polyacetylenes [55], resulting in the formation of a twisted helical structure. The stereoregularities are essential for the induction of a helical conformation [56, 57]. In contrast to stiff rodlike helical polyisocyanates and polysilanes, helical polyacetylenes are rather flexible, and the reported q values are 8.6 and 13.5 nm for a helical poly(4-carboxyphenylacetylene) induced by chiral amines [58] and poly(N-propargyl-2-ethylhexylamide) (34a) [59], respectively. However,

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the temperature-dependent changes in the induced circular dichroism (ICD) intensities of a series of homopolymers and copolymers of phenylacetylenes and N-propargylamides showed that their DGr values (ca. 3.7 kcal mol-1) are close to or slightly greater than those for polyisocyanates and poly(dialkylsilane)s, indicating that the polyacetylenes consist of long one helical-sense domains (ca. 660 monomer units) separated by rarely occurring helical reversals [60, 61]. Therefore, a similar chiral amplification (sergeants and soldiers effect and majority rule) also takes place in polyacetylenes, although the amplification efficiency was rather low for the covalent systems [52, 61].

Tang et al. and Masuda et al. synthesized helical polyacetylenes bearing various amino acids as the pendants, and their chiroptical properties including their helical conformations and helicity inversion (see Section 11.4) were investigated [49, 53, 62]. The intramolecular hydrogen bonds between the pendant groups appear to induce and stabilize the helical structure, although the hydrogen-bonded poly(N-propargylamide) (34a) is still semiflexible judging from its short persistence length (q = 13.5 nm) in chloroform [59]. An exceptionally stiff helical poly(phenylacetylene) was obtained by the polymerization of phenylacetylenes bearing an l- or d-alanine residue with a long alkyl chain as the pendant (30). The resulting cis-transoidal poly(phenylacetylene)s form a well-defined lyotropic cholesteric liquid crystalline (LC) phase in concentrated organic solvents based on the main-chain stiffness, which was confirmed by their long persistence lengths of around 40 nm in chloroform [63]. The l- or d-30 undergoes an inversion of helicity in polar and nonpolar solvents accompanied with a dramatic change in its persistence length (q) from 135 nm in CCl4 to 19 nm in tetrahydro-furan (THF); the former value is the highest among all synthetic helical polymers to the best of our knowledge. The macromolecular helicity inversion process can be directly followed by AFM (see Section 11.4).

Aoki et al. reported that an achiral phenylacetylene bearing two hydroxy groups on the phenyl residue gave an optically active poly(phenylacetylene) (35) when polymerized with a rhodium catalyst in the presence of (S)- or (R)-1-phenylethylamine. The resulting optically active polymer showing an ICD may have an excess of the preferred helical sense stabilized by intramolecular hydrogen bonds, and was stable in chloroform at high temperatures, but the CD disappeared in the presence of dimethyl sulfoxide (DMSO) [64].

Quite recently, the right- and left-handed helical structures of 30 have been directly observed using AFM. Rigid rodlike helical polyacetylenes were found to hierarchically self-assemble on highly oriented pyrolytic graphite (HOPG) upon exposure to benzene vapors (Fig. 11.8). Flat polyacetylene monolayers epitaxially formed on the basal plane of the graphite, on which helical polyacetylenes further self-assembled into chiral two-dimensional (2-D) helix-bundles with controlled helicity [65]. These AFM observations combined with the X-ray structural analysis suggest that the helices of the l-30 and d-30 single chains are enantiomers, and both the enantiomeric left- and right-handed helical 30 showing opposite Cotton effect signs (Fig. 11.8A) provide the enantiomorphic 2D structures on graphite.

Fig. 11.8 (A) CD and absorption spectra of Land d-30 in benzene. (B) AFM phase images of L- and d-30 (scale: 20 x 40 nm) with schematic drawings of the mirror-image relationship of helical L-and d-30 2-D crystals with antipodal oblique pendant arrangements. Possible models (left and right) were

constructed on the basis of the X-ray structural analysis. (C) Schematic representation of the hierarchical structure of the self-assembled l-30 on HOPG. (Reproduced with permission from Ref. 65. Copyright 2006 Wiley-VCH.)

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