External heavy-atom effect on fluorescence kinetics

Fluorescence quenching in fluid solution by the external heavy-atom effect usually follows simple Stern-Volmer kinetics, and the quenching effect is gauged by the magnitude of the bimolecular quenching rate constant. However, it is the increased unimolecular S-1-->T-n intersystem crossing rate constant of the perturbed fluorophore in the perturber-fluorophore complex that can be directly compared with that of the unperturbed fluorophore. From a simple model for external heavy atom quenching in fluid solution, the decay law is predicted to be singly exponential for all quencher concentrations, and a new expression for the unimolecular S-1-->T-n intersystem crossing rate constant of the perturbed fluorophore is obtained. The same problem, but in rigid solution, is also discussed for the first time. The model now predicts a nonexponential fluorescence decay law, from which the unimolecular S-1-->T-n intersystem crossing rate constant of the perturbed fluorophore can be directly determined.