Optical properties of silylene-biphenylene copolymer films

We have measured absorption, photoluminescence, and photoluminescence excitation spectra, and the photoluminescence time response for films of silylene-biphenylene copolymer, ((C₆H₄)₂(Si(CH₃)₂)_m)_n with m=1,2,4, and 6. The excitation spectra clearly reveal that the lowest absorption band in each copolymer consists of two bands, i.e., a band at 4.7 eV and a band of which energy depends on m. Since the latter band is absent in the copolymer with m=1, the former band is attributed to the lowest π−π* transition in biphenylene subunits. The latter band is attributed to the lowest σ−σ* transition in the silylene subunits, considering its dependence on m. In contrast to the result for solution, the peak energy of photoluminescence band is independent of m. The band has a Stokes shift of more than 1 eV and a large band width of 0.5 eV. The time responses of photoluminescence intensity consist of more than two decay components and the intensity decays more slowly at smaller energy. The large Stokes shift is explained as due to excimer formation between biphenylene subunits. In order to explain the energy dependence of time responses, energy migration is discussed.