STRUCTURE AND CHIROOPTICAL PROPERTIES OF BIS[(S)-METHYLBUTYL]SILYLENE-DIPENTYLSILYLENE COPOLYMERS

Copolymers have been derived from poly(di-n-pentylsilylene) by incorporation of bis[(S)-2-methylbutyl]silylene units. Copolymers with 5-50% chiral comonomers were investigated with respect to their structures and chirooptical properties in solution, in the crystalline state, and in the columnar mesophase. Electron and X-ray diffraction showed that the 7/3 helical structure of PDPS was mainly retained. Only higher fractions of the branched chiral units hindered crystallization and caused disappearance of the mesomorphic transition. Circular dichroism (CD) spectroscopy revealed weak, temperature-dependent circular dichroism in cyclohexane solution correlated to the (sigma-sigma*) backbone absorption. Remarkably strong CD couplet bands with an exciton-splitting shape were observed in THF as well as for thin spin-cast films, indicating enantioselective ordering in solution as well as in the solid. Transformation of the copolymers into the columnar, conformationally disordered mesophase led to a decrease of the strong CD bands.