Laser study of the protonation equilibrium of triplet benzophenone

Abstract

Transients observed in laser flash photolysis experiments on 2 × 10^–4 mol dm^–3 benzophenone in aqueous solution are similar to those previously reported in conventional flash experiments and assigned to the triplet B and BH⁺ species. Measurements of the initial intensities and lifetimes of the transients over a wide range of acidity permit pK(T₁) to be determined as 1.5 ± 0.1. Coupled with the widely different values found for the characteristic lifetimes of B and BH⁺(10^–4 s and 6.2 × 10^–8 s respectively) this figure for pK(T₁) satisfactorily explains the drop in phosphorescence intensity of benzophenone as the acidity increases through pH = 5. It is also in reasonable accord with Förster cycle calculations based on recent revisions of the ground state pK of benzophenone. Similar measurements are reported for the 20 % ethanol + 80 % water system, in which pK(T₁) is 0.3 ± 0.2.