Assignments of near-ultraviolet absorption and intramolecular charge-transfer emission in phenylpentamethyldisilane

Abstract

Assignments of the u.v. absorption and intramolecular charge-transfer (c.t.) emission in phenylpentamethyldisilane have been studied by measurements of m.c.d. spectra and fluorescence polarization spectra with the aid of CNDO/S calculations. From the m.c.d. spectra the first absorption band at 38.5 × 10³ cm^–1(ε= 300 mol^–1 dm³ cm^–1) is assigned to the ¹B₂→¹A₁(or ¹L_b→¹A) transition, which corresponds to that of ¹B_2u→¹A_1g in benzene. Using m.c.d. measurements and molecular-orbital calculations, the mechanism of charge migration from the phenyl ring to the disilanyl group in the first excited state is established. On the basis of fluorescence polarization, in addition to the m.c.d. results, the c.t. emission of phenyldisilane is ascribed to that from the ¹(2pπ, 3dπ) c.t. state produced by the 2pπ* orbital (the phenyl ring) to the vacant 3dπ orbital (the Si—Si bond) intramolecular charge-transfer transition.