VARIABLE-TEMPERATURE STUDY OF AROMATIC HYDROCARBON TRIPLET-STATE QUENCHING BY MOLECULAR-OXYGEN IN SOLUTION

The temperature dependence of the bimolecular rate constant for O2(3SI GMA(g)) quenching of a series of aromatic hydrocarbon triplet states, k(T)ox, has been determined in toluene. Highly curved Arrhenius plots were obtained, indicating a change in rate-determining step over the a vailable temperature range. At low temperatures, k(T)ox of all hydroca rbons approached a common limiting slope that was consistent with 4/9k (d). At higher temperatures, low positive or negative activation energ ies were observed, indicating the involvement of exciplex intermediacy in the overall quenching process. In each case, the room temperature value of k(T)ox reflects the reversible formation of an exciplex where product formation is the rate-determining step. The temperature depen dence of naphthalene k(T)ox was determined in an additional four solve nts exhibiting a wide range of polarity. Again strong curvature was ap parent in the Arrhenius plots, with k(T)ox consistently exceeding 1/9k (d) at low temperatures. In all solvents the preequilibrium preexponen tial factors were identical within experimental error; variations in r oom temperature rate constants solely reflect solvent-dependent preequ ilibrium activation energies. These results lead to direct predictions of the temperature dependence of energy-transfer efficiencies when in terpreted on the basis of the well-known spin-statistically derived sc heme.