Temperature effect on radiative lifetimes: the case of singlet oxygen in liquid solvents.

A change in solvent can have an appreciable effect on the rate constant for the O2(a(1)Δg) → O2(X(3)Σg(-)) radiative transition at ~1275 nm. The data thus obtained have played an important role in understanding mechanisms by which environment-dependent perturbations can influence forbidden electronic transitions. We now report that the rate constant for O2(a(1)Δg) radiative deactivation, kr, also responds to changes in temperature. This result can have practical ramifications in experiments that use O2(a(1)Δg) phosphorescence to quantify yields of photosensitized O2(a(1)Δg) production. From a fundamental perspective, this result is significant, partly because there is little precedence for temperature-dependent changes in radiative rate constants. The data also require a re-evaluation of the current model by which oxygen is perturbed by solvent. Specifically, the evidence indicates that it is not appropriate to evaluate the interaction as a 1:1 complex between a given solvent molecule M and oxygen. Rather, one must consider an ensemble of solvent molecules surrounding oxygen.