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 O-2(a(1)Delta(g)) -> O-2(X-3 Sigma(-)(g)) radiative transition at similar to 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 O-2(a(1)Delta(g)) radiative deactivation, k(r), also responds to changes in temperature. This result can have practical ramifications in experiments that use O-2(a(1)Delta(g)) phosphorescence to quantify yields of photosensitized O-2(a(1)Delta(g)) 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.