Pressure dependence of triplet formation in acridine and phenazine vapours

The pressure dependence of T₁ formation in acridine and phenazine vapours was studied between 0.05 torr and 40 torr of an added buffer gas, by microsecond flash photolysis. In contrast to the pressure dependence of anthracene triplet formation, in the heterocyclic compounds the triplet absorption increases slightly with pressure up to 1 torr and then decreases strongly with an increase in pressure up to 30–40 torr. These effects were confirmed by the slight increase followed by a decrease with time of the integrated areas of the T₁ acridine spectra observed at much shorter times by means of nanosecond flash photolysis. The absorption of the lowest triplet was found to have an induction period more than one order of magnitude longer than the laser pulse or the singlet lifetimes revealing that the T₁ state is not populated directly from S₁ but via other electronic triplet states. The pressure effects on triplet formation are interpreted in terms of a reversible intersystem crossing S₁ ? T₁ associated with vibrational relaxation in the triplet states.