Studies of the Conformation and Packing of Polysilanes

Abstract

A series of symmetric and asymmetric poly(di-n-alkylsilanes) have been investigated to assess their minimum-energy conformations, crystal packing, and, in the case of poly(di-n-tetradecylsilane) (PdnTDS), the conformation of the n-alkyl side chains. All of the polymers exhibit a thermochromic transition. X-ray diffraction measurements were made above and below the transition temperature to determine unit cell parameters for both the ordered and disordered phases. A TGTG′ silicon backbone conformation has been identified and found to occur routinely in symmetric poly (di-n-alkylsilanes) having side chain lengths between 9 and 14 carbon atoms. This unique conformation has previously been shown to be energetically feasible by molecular modeling calculations. Spectroscopic measurements on PdnTDS indicate that the side chains have predominantly an all-trans conformation. In all the polysilanes studied above their transition temperatures, it has been found that the backbone conformations become somewhat disordered, but the chains maintain a columnar arrangement on a hexagonal lattice. X-ray diffraction patterns from the disordered phases feature one strong and one or two weaker equatorial reflections. The d-spacing of the strongest reflection, which relates directly to the radius of the cylindrical molecular envelope, increases as the lengths of the n-alkyl side chains increase, apparently to accommodate the extra side chain length. The side chains themselves are disordered above the transition. Based on molecular models and the X-ray values of the cylindrical radii, the side chains are not extended but on average are shortened, presumably by the presence of one or more gauche bonds. © 1993, American Chemical Society. All rights reserved.