Oxygen quenching of aromatic triplet states in solution. Part 1

The rate constants for oxygen quenching of aromatic hydrocarbon triplets have been measured by the laser flash photolysis technique. The quenching of high triplet energy compounds is characterised by rates which are inversely proportional to triplet energy. The reaction probabilities, which are as small as 10–2 in hexane, have been found to increase in polar or viscous solvents. Molecules with low triplet energies are quenched at one-ninth the measured diffusion controlled encounter rate.These data are analysed in terms of the non-radiative transitions of a collision complex of the aromatic triplet and ground state oxygen. The importance of restrictive Franck–Condon factors which are determined by the hydrocarbon is discussed. The data are shown to be consistent with electronic matrix elements for the energy transfer quenching processes that are dependent on orbital symmetry matching and charge transfer interactions.