Proton Transfer Reactions in the Triplet State of Benzophenone Studied by Nanosecond Laser Flash Photolysis

Abstract

Abstract Proton-transfer reactions in the triplet state of benzophenone in H2O–acetonitrile (4:1) at 20 °C have been studied by means of nanosecond nitrogen laser flash photolysis at 337 nm. The proton association (k1) and dissociation (k2) rates are determined to be 5.0 (±0.5)×109 mol−1 dm3 s−1 and 3.3 (±0.3)×109 s−1 respectively by a transient kinetic treatment. These rates are large enough to make an acid-base equilibrium during the lifetime of the triplet state. The acidity constants (pKa*) in the triplet state of benzophenone obtained by the transient kinetic treatment, the Ware plot, and the Tn←T1 absorbance titration curve are the same [0.18 (±0.1)]. The pKa* value determined by the titration method is appreciably dependent upon the delay time after the start of pulsing (the accurate value was obtained at the delay time 20 ns). There is no proton-induced quenching in the triplet state of benzophenone, since electrophilic protonation occurs at the oxygen atom in the carbonyl group. The reactive state for protonation in the triplet state is not 3(n,π*), but 3(π,π*), judging from the large k1 value and the pKa* value.