Exciplex mechanism of fluorescence quenching in polar media

Abstract

The formation of exciplexes (non-emitting or poorly emitting) is suggested as one of the causes for deviations of experimental data on fluorescence quenching in polar solvents from the classical model of excited-state electron transfer yielding radical ion pairs. Several evidences for the formation of such exciplexes were found for fluorescence quenching of aromatic compounds by weak electron donors and acceptors. For cyano-substituted anthracenes exciplex emission can be observed in the presence of quenchers even in polar solvents. In other systems, indirect evidences of exciplex formation were observed: nonlinear dependence of the inverse value of excited pyrene lifetime on the concentration of the quencher; very small and, in some cases, even negative experimental activation energies of pyrene fluorescence quenching, which are much less than activation energies, calculated from the experimental values of the quenching rate constants etc.

The proposed model explains the difference between theoretical and experimental dependencies of logk _Q vs. Gibbs energy of electron transfer ‡DG_ET and other experimental features known for fluorescence quenching by electron donors and acceptors. This model states that the exciplex is in equilibrium with the encounter complex and apparent quenching rate constants are controlled by two main factors — the lifetime of the exciplex and the enthalpy of its formation. Experimentally observed dependence of apparent quenching rate constant on ‡G_ET is caused by the dependence of the exciplex formation enthalpy on ‡G_ET, which is quite different from the dependence of electron transfer activation energy on ‡G_ET predicted by the theoretical models. Simulations of the dependencies of logk _Q vs. ‡G_ET according to the exciplex formation model confirms its agreement with the experimental data.

Electronic structure of the exciplex involved may be close to contact radical-ion pair only at ‡G_ET < 0, when the rate of quenching is limited mainly by the diffusion, but for ‡G_ET> 0, the structure of the exciplex should be much less polar.