DETERMINATION OF THE LIFETIME OF THE 2ND EXCITED TRIPLET-STATE OF ANTHRACENES

The lifetimes of the T2 States of several substituted anthracenes have been determined using competitive quenching techniques in combination with laser flash photolysis. Thus, Stern-Volmer quenching of 1,3-octadiene yields different slopes depending on whether the triplet or the fluorescence yield are monitored. The difference, which can be treated quantitatively, is the result of T2 quenching by the diene. Typical T2 lifetimes are ca. 23, 11, and 3 ps for 9, 10-dichloroanthracene, anthracene, and 1-methylanthracene, respectively. It is shown that consideration of time-dependent diffusional quenching is essential and that discrepancies with earlier studies may largely reflect the contribution from this process, as well as S1 quenching by typical T2 quenchers. Theoretical calculations suggest that interactions of T2 with the nearby T3 state may play a key role in determining the rate of internal conversion to T1. Calculations have been carried out for the methyl derivatives; these, while consistent with experiments, tend to reflect the limitations of current theory in the interpretation of substituent effects.