X-ray absorption spectra of poly-p-phenylenes and polyacenes: localization of π orbitals

Abstract

Carbon K-edge X-ray absorption near edge structure (XANES) spectra of evaporated films of poly-p-phenylenes and polyacenes were measured by use of synchrotron radiation. Only one dominant resonance associated with a Cls-to-π* transition was observed for each poly-p-phenylene, although there should exist several π* unoccupied orbitals which are energetically well separated from each other. These features were characterized by means of semi-empirical molecular orbital (MO) calculations based on the equivalent core approximation, which leads to a satisfactory agreement between the observed and calculated spectra. The calculations show that the π* orbitals, which are delocalized in the ground state, become localized by the creation of a Cls core hole. Spectral features of polyacenes, which are somewhat different from those of poly-p-phenylenes in their π* regions, indicate a lesser degree of π* localization.