Intermolecular Hydrogen Transfer Reactions in the Excited States of EDA-Complexes of Benzophenone with Aromatic Amines at Low Temperature

Abstract

Abstract Electronic absorption and emission spectra of the mixtures of benzophenone and aromatic amines have been studied in rigid solutions. From absorption spectral measurements, it has been concluded that benzophenone as an acceptor forms the molecular complexes with various aromatic amines as donors. Emission spectra different from those of the component molecules were observed at longer wavelengths in the same systems. The maxima of the anomalous emission spectra of these systems were found to shift slightly to lower frequencies with the decrease in the ionization potentials of donor molecules. On account of their decay times, the emissions may be ascribed to the phosphorescence of benzophenone-aromatic amine complexes. Photolysis of benzophenone-aromatic amine complexes in rigid matrices always resulted in the appearance of the visible absorption spectrum of benzoketyl radical, together with ESR spectra, the analysis of which indicated that there is an interacting pair of radicals which is in the combined triplet (S=1) state. From the evaluation of the zero field splitting constants, the average distances between the interacting radicals were estimated. These results suggest that intermolecular hydrogen atom transfer from aromatic amines to benzophenone occurs, yielding the ketyl and aromatic amine radicals which lie close to each other. The reaction was found to proceed by way of the lowest excited triplet state of benzophenone.