Cooperative Helical Order in Optically Active Poly(diarylsilylenes)

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Abstract

Microscopic helical order in poly(diarylsilylene) copolymers containing enantiopure chiral (S)-2-methylbutylphenyl and achiral n-butylphenyl side chains results in macroscopically observable optical activity. These polymers are shown to adopt helical backbone conformations with a prevailing screw sense in solution through cooperative side chain interactions. Ultraviolet (UV) and circular dichroism (CD) spectroscopic studies over the temperature range −70 to 80 °C indicate temperature dependence of the dissymmetric ratio, gabs. For copolymers of the type (Ar*2Si)x(Ar2Si)(1-x) (where Ar* = p-(S)-2-methylbutylphenyl, Ar = p-n-butylphenyl, and x = 0.2, 0.5, or 0.8), positive Cotton effects in the CD spectra indicate optical activity due to helical polymer backbones, optimally for the case where x = 0.5. This is in contrast to the copolymers with Ar* = m-(S)-2-methylbutylphenyl and x = 0.2, 0.5, or 0.8, for which the Cotton effects are of smaller magnitude and negative, except in the case of x = 0.2, for which a temperature-dependent inversion of Cotton effect sign is observed, as we communicated recently.1 There is no optical activity, as expected, in the special case where x = 0, although from spectroscopic data and force field calculations, it is also concluded that even optically inactive poly(diarylsilylenes), including the above case where x = 0, adopt helical forms, but in an internal racemate. For the most highly chirally substituted bis-para copolymer, poly[bis(p-(S)-2-methylbutylphenyl)0.8-co-bis(p-n-butylphenyl)0.2silylene], an unusual (hypsochromic) thermochromic transition is observed at 320 nm, the origin of which is postulated in a helical backbone conformation with a concomitant phenyl ring twist to inhibit phenyl ring/Si−Si conjugation. Viscometric studies indicate that incorporation of branched side chains on the aryl rings results in polymers with stiffer, more extended structures, and this is related to the observed increase in UV absorption intensities.

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