Electron localization in polysilane radical ions

The localization mechanism for the excess electron and positive hole in the polysilane radical anion and cation has been investigated by means of both molecular dynamics (MD) and extended Huckel molecular orbital (EHMO) calcu lations. A linear oligosilane (SinH2n+2) (n = 32 and 64) was chosen as the model of polysilane. The geometry optimization of the polysilane at the MM2 level gave a regular all-trans form as the most stable structure. The MD c alculations, started from the optimized structure, showed that the conforma tion of the polysilane skeleton was gradually randomized as a function of t ime by thermal activation at 300 K. The conformations at 0.0, 0.05, 1.0, 1. 5, and 2.0 ps were chosen as sampling points, and spin densities on the sil icon atoms were calculated by the EHMO calculations at each sampling point. An excess electron and a positive hole were fully delocalized along the Si chain in the regular all-trans form (time = 0.0 ps), whereas in the disord ered conformations (time not equal 0) both electron and hole were completel y localized on a few Si atoms (10-20 monomer units). The present calculatio ns suggested that a continuous disorder of the Si main chain randomized by thermal energy (i.e., disordered dihedral angle of Si skeletons) is dominan t in both electron and hole localization. The localization mechanism was di scussed on the basis of theoretical results.