ULTRAVIOLET ABSORPTION SPECTRA OF DERIVATIVES OF POLYSILANES . A PROBE OF (P-]D)PI BONDING IN ORGANOSILICON COMPOUNDS

The electronic absorption spectra of permethylated polysilanes, Me(SiMe2)nMe, are discussed, together with the possibility that the perturbation of the spectra by various substituents may provide an estimate of (p → d)π, bonding. The various possible assignments of the ground and excited states of the long-wavelength absorption band are considered in the light of ionization potential measurements, and the spectral properties of phenyl derivatives and the other group IV catenates. The ultraviolet absorption spectra of 1- and 2-substituted derivatives of heptamethyltrisilane, Me3SiMe2SiMe2SiX and (Me3Si)2SiMeX, where X includes hydrogen, halogen, oxygen, sulfur, and nitrogen groups, have been measured. The observed perturbations of the 216-nm absorption band of the parent trisilane are interpreted in terms of σsi-px and dsi-px interactions. Some of the 2-substituted derivatives exhibit a second, low-intensity, band to the red of the 216-nm absorption. Based on the spectra of model compounds and simple energy considerations, this partially forbidden band is assigned to a charge-transfer transition from silicon to the substituent. The spectra of some di- and tnsubstituted compounds are also reported. The relevance of the spectra to the question of (p → d)π bonding is discussed. © 1969, American Chemical Society. All rights reserved.