LASER PHOTOLYSIS STUDIES ON TRIPLET-EQUILIBRIUM FORMATION IN A SMALL TRIPLET-ENERGY GAP SYSTEM

Formation of a triplet-equilibrium between benzophenone (BP) and diben zothiophene (DBT) was studied in acetonitrile by 355 nm laser flash ph otolysis. The triplet-equilibrium constant (K-eq = 1.7) was determined at 295 K as well as the absolute rate constants for triplet energy tr ansfer from BP to DBT (k(f) = 3.1 X 10(9) M(-1)s(-1)) and back triplet -energy transfer (k(b) = 2.0 X 10(9) M(-1)s(-1)) whose ratio (k(f)/k(b )) was in agreement with the K-eq value. It was revealed that the trip let-equi librium formation in the BP-DBT system was promoted by the en tropy change (Delta S = 2.0 cal mol(-1) K-1) rather than the enthalpy change (Delta H = 0.3 kcal/mol). (C) 1997 Elsevier Science B.V.