LOCALIZATION OF EXCITATIONS BY ELECTRON-DONATING SIDE-GROUPS IN THE NOVEL COPOLYMER HYLPHENYL-CO-METHYL-4-(DIMETHYLAMINO)PHENYLSILANE]

The effect on the coherence length of excitations in polysilane copoly mers due to electron-donating (dimethylamino)phenyl (DMAP) side groups is investigated by time-resolved and steady-state fluorescence studie s. In the case of poly(methylphenylsilane) the lowest excitonic state has been shown to delocalize over several tens of monomer units. Stead y-state studies of the novel copolymers indicate three emitting specie s which can be assigned to three fluorescence lifetimes identified by time-resolved experiments. A fast lifetime of a few tens of picosecond s corresponds to the delocalized lowest excited state. A second deloca lized excitation with a short lifetime and a third ''self-trapped exci ton'' with a coherence length of approximately a single silicon bond a re also proposed. The size of the delocalized excitations in the copol ymers exhibits a strong dependence on the concentration of the electro n-donating DMAP side group. The coherence length of the excitations ca n be predicted by a simple statistical treatment of the distribution o f homogeneous chain segments terminated at each end by DMAP side group s. Consequently, a perturbation of the backbone states due to the elec tronic nature of the DMAP side groups is proposed to be the origin of the force which defines the size of the exciton.