Cooperative helical order in optically active poly(diarylsilylenes)

Microscopic helical order in poly(diarylsilylene) copolymers containing ena ntiopure chiral (S)-2-methylbutylphenyl and achiral n-butylphenyl side chai ns results in macroscopically observable optical activity. These polymers a re shown to adopt helical backbone conformations with a prevailing screw se nse in solution through cooperative side chain interactions. Ultraviolet (U V) and circular dichroism (CD) spectroscopic studies over the temperature r ange -70 to 80 degreesC indicate temperature dependence of the dissymmetric ratio, g(abs) For copolymers of the type (Ar*Si-2)(x)(Ar2Si)((1-x)) (where Ar* = p-(S)-2-methylbutylphenyl, Ar = p-n-butylphenyl, and x = 0.2, 0.5, o r 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. Th is is in contrast to the copolymers with Ar* = m-(S)-2-methylbutylphenyl an d x = 0.2, 0.5, or 0.8, for which the Cotton effects are of smaller magnitu de and negative, except in the case of x = 0.2, for which a temperature-dep endent inversion of Cotton effect sign is observed, as we communicated rece ntly.(1) There is no optical activity, as expected, in the special case whe re x = 0, although from spectroscopic data and force field calculations, it is also concluded that even optically inactive poly(diarylsilylenes), incl uding the above case where x = 0, adopt helical forms, but in an internal r acemate. For the most highly chirally substituted bis-para copolymer poly[b is(p-(S)-2-methylbutylphenyl)(0.8)-co-bis(p-n-butylphenyl)(0.2)silylene], a n unusual (hypsochromic) thermochromic transition is observed at 320 nm, th e origin of which is postulated in a helical backbone conformation with a c oncomitant phenyl ring twist to inhibit phenyl ring/Si-Si conjugation. Visc ometric 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.