Photochemical reactions of triplet benzophenone and anthraquinone molecules with amines in the gas phase

Abstract

The intermolecular photoinduced reactions between triplet ketone molecules and aliphatic amines and pyridine are studied by the quenching of delayed fluorescence of anthraquinone and benzophenone vapors by diethylamine, dibutylamine, cyclohexylamine, triethylamine, and pyridine. In the temperature range 423–573 K, the delayed fluorescence quenching rate constants k _q are estimated from changes in the decay rate constant and the intensity of delayed fluorescence upon increasing pressure of bath gases. It is ascertained that, in the gas phase, the mixtures under study exhibit both a negative and a positive dependence of k _q on temperature, which indicates that some photoinduced reactions do not have activation barriers. The rate constant k _q is shown to increase with decreasing ionization potential of the electron donors. This points to the importance of interactions with charge transfer in the photoreaction of triplet ketone molecules with aliphatic amines and pyridine in the gas phase. The relationship between k _q and the change in the free energy ΔG upon the photoinduced intermolecular electron transfer, which is the primary stage of the photochemical reaction, is studied. It is shown that the dependence k _q (ΔG) for the donor-acceptor pairs under study is described well by the Marcus equation, in which the average vibrational energies of the donor and acceptor are taken into account for the estimate of ΔG.