ABSTRACT:In the family of optically active synthetic polymers, optically active polysilanes, which comprise a helical main chain of silicon-silicon single bonds and chiral and/or achiral side groups, exhibit unique absorption, circular dichroism, and fluorescence spectra around 300-400 nm due to σ-conjugation. Since the first brief report of optically active polysilane synthesis in 1992, the field has now widened to include various homo-and copolymers of optically active poly(dialkylsilane)s, poly(dialkoxysilane)s, poly[alkyl(aryl)silane]s, and poly(diarylsilane)s. This review comprehensively covers work on (i) the relationship between side chain structure, (ii) local structure-global shape relationship, (iii) (chir)optical properties, (iv) (semi)quantitative population analysis of right-and left-handed helices based on Kuhn's dissymmetry ratio, (v) several helical cooperativity effects, (vi) molecular imaging, (vii) inversion of screw-sense, (viii) chiroptical switch and memory, (ix) transfer and amplification of molecular chirality to aggregates, (x) cholesteric liquid crystallinity, (xi) helical supramolecular structures, and (xii) latent helicity, as consequences of side group internal interactions and other external stimuli. Such knowledge and understanding may stimulate optically active polymer research in the realm of nanomaterial science and nanotechnology at the sub-nm level as well as traditional polymer science, and may advance these polymers to new functional nanomaterials and thence to the realization of nanodevices in the future.

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Novel switchable chiroptical characteristics of poly(alkylarylsilane) microaggregates, controllable by the choice of good/poor solvent ratio (solvent polarity), solvent addition order, and sample temperature are described. The formation of stable chiral aggregates depends critically on the polysilane structure and stereochemistry. Poly[n-hexyl-(p-(S)-2-methylbutoxyphenyl)silane] (1), optically inactive in molecularly dispersed THF solution due to the existence of dynamically equivalent amounts of right (P)- and left (M)-handed screw sense helical main chain domains, shows a marked bisignate CD signal due to the formation of chiral aggregates in good/poor cosolvent systems. The sign and magnitude of the CD signals are dependent on solvent polarity, solvent addition order, and thermal effects. The less sterically hindered poly[methyl-(p-(S)-2-methylbutoxyphenyl)silane] (2) exhibits a weak, bisignate, nonswitchable CD signal in only the toluene/acetonitrile system, and no CD signals are evident in pure toluene or THF due to masking of the helicity. In contrast, although the even less sterically hindered, less polar poly[methyl-(m-(S)-2-methylbutoxyphenyl)silane] (3) does show optical activity in pure THF or toluene (negative CD signal at 310 nm), the CD signal disappears on formation of aggregates in good/poor cosolvent systems.

To elucidate the factors involved in the chiroptical properties of polymer aggregates composed of helical building blocks, a series of rigid rod helical poly[alkyl-(S)-2-methylbutylsilane]s (achiral alkyl side chains = ethyl, n-propyl, n-butyl, n-pentyl, n-hexyl) have been investigated. It was found that the chiroptical sign in the circular dichroism (CD) spectra of the polysilane aggregates depends on the achiral side chain length and cosolvent fraction. Concerning the achiral side chains, the n-propyl group was of a critical length for solvent-dependent chiroptical inversion on aggregation. This unique side chain length-dependent chiroptical inversion was theoretically predictable by using the novel approach of combining the cholesteric hard-core model and exciton chirality method. The latter was also investigated theoretically by Gaussian 03 (TD-DFT, B3LYP, 6-31G(d) basis set) calculations applied to two spatially arranged helical Si-Si bonded decamer models.