Photochemical Reactions of p-Benzoquinone Complexes with Aromatic Molecules

Abstract

Abstract Electronic absorption spectra and ESR spectra have been measured for the photolyzed charge transfer complexes between p-benzoquinone and aromatic donors in low temperature matrices. When the p-benzoquinone complex with toluene or p-xylene was irradiated at 77°K, an electronic absorption spectrum attributable to the semiquinone radical was obtained together with that of the benzyl-type radical. The spectral shape and position of the former are somewhat different from those of the semiquinone radicals obtained by the photolysis of p-benzoquinone or hydroquinone. These differences indicate the presence of interaction between semiquinone and benzyl-type radicals. The ESR spectra obtained from these systems show that these radicals form pairs in the triplet (S=1) state. It is concluded that the radical pairs are produced as a result of intermolecular hydrogen atom transfer from the aromatic donor molecule to p-benzoquinone in the excited states of the CT complexes formed between them. The irradiations of various p-benzoquinone complexes were carried out in the wavelength regions corresponding to the lowest CT absorption bands at 77°K, and the reactivities were discussed in relation to the electronic structures and geometrical configurations of the CT complexes in the excited states. From the analysis of ESR spectra, the fine structure constant was evaluated for each of the radical pairs, from which the average distance between interacting unpaired electrons was calculated.