Electronic structure of polycarbosilane studied by UV photoelectron spectroscopy

An ultraviolet photoelectron spectroscopic study was performed to examine t he electronic structure of poly-(1,1,2,2-tetramethyldisilylenemethylene), [ (SiMe2)(2)CH](n), which consists of repeating Si-Si-C units. We compared th e observed spectra to reported gas-phase spectra of Si2Me6 and Si4Me10 and theoretical molecular orbital calculations. Theoretical calculations by the ab initio and PM3 methods agree well with the observed spectra, which enab led assignment of the observed spectral features. The broad peak derived fr om Si-Si bond orbitals in polycarbosilane suggests that Si-Si units, which are separated by carbon atoms, still interact with each other, to form sigm a -conjugation through the backbone. The observed ionization threshold ener gy of the polycarbosilane[(SiMe2)(2)CH2](n), is 6.4 eV. The dispersion of t he highest valence band of polycarbosilane is less than that of polysilane, indicating that the degree of sigma -conjugation of polycarbosilane is sma ller than that of polysilane.