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

Novel switchable chiroptical characteristics of poly(alkylarylsilane) micro aggregates; controllable by the choice of good/poor solvent ratio (solvent polarity), solvent addition order, and Sample temperature are described. Th e formation of stable chiral aggregates depends critically on the polysilan e structure and stereochemistry. Poly[n-hexyl-(p-(S)-2-methylbutoxyphenyl)s ilane] (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 sy stems. The sign and magnitude of the CD signals are dependent on solvent po larity, solvent addition order, and thermal effects. The less sterically hi ndered 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 th e helicity. In contrast, although the even less sterically hindered, less p olar 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 sig nal disappears on formation of aggregates in good/poor cosolvent systems.