Quenching of excited triplets of aromatic hydrocarbons by molecular oxygen

The quenching by molecular oxygen of photoexcited triplets of a number of aromatic hydrocarbons dissolved in a solid polystyrene matrix at 298°K was studied. The compounds studied are anthracene, pyrene, a, h-dibenzanthracene, chrysene, coronene, naphthalene, benz(a)anthracene, and eight of the latter's monomethyl derivatives. The quenching constant γ decreases with increasing triplet energy of the aromatic molecules and varies by a factor of seven, coronene yielding the smallest value and anthracene the highest. This result is in good agreement with the quenching studies in fluid solutions by Porter and co-workers and is due mainly to a decrease in the Franck-Condon factors with increasing triplet energy. Within the methyl benz(a)anthracene family, variations in the Franck-Condon factors are minor but differences in γ of up to 45% are observed depending on the position of the methyl group. With the methyl groups at the 7 or 12 positions of benz(a)anthracene, which are sites of high electron densities, the quenching constant is the smallest. These results are attributed to variations in the electronic matrix elements β coupling the initial and final states within the intermediate bimolecular complex. Relative values of β are calculated using INDO molecular orbital coefficients and are compared to the experimental results.