Laser photolysis studies on triplet-equilibrium formation in a small triplet-energy gap system

Formation of a triplet-equilibrium between benzophenone (BP) and dibenzothiophene (DBT) was studied in acetonitrile by 355 nm laser flash photolysis. The triplet-equilibrium constant (Keq = 1.7) was determined at 295 K as well as the absolute rate constants for triplet energy transfer from BP to DBT (kf = 3.1 × 109 M−1 s−1) and back triplet-energy transfer (kb = 2.0 × 109 M−1 s−1) whose ratio (kf/kb) was in agreement with the Keq value. It was revealed that the triplet-equilibrium formation in the BP-DBT system was promoted by the entropy change (ΔS = 2.0 cal mol−1 K−1) rather than the enthalpy change (ΔH = 0.3 kcal/mol).