Solvent and temperature effects on the chiral aggregation of poly(alkylarylsilane)s bearing remote chiral groups.

Abstract

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.