EXCIPLEX MECHANISM OF FLUORESCENCE QUENCHING IN POLAR MEDIA

The formation of exciplexes (non-emitting or poorly emitting) is sugge sted as one of the causes for deviations of experimental data on fluor escence quenching in polar solvents from the classical model of excite d-state electron transfer yielding radical ion pairs. Several evidence s for the formation of such exciplexes were found for fluorescence que nching of aromatic compounds by weak electron donors and acceptors. Fo r cyano-substituted anthracenes exciplex emission can be observed in t he presence of quenchers even in polar solvents. In other systems, ind irect evidences of exciplex formation were observed: nonlinear depende nce of the inverse value of excited pyrene lifetime on the concentrati on of the quencher, very small and, in some cases, even negative exper imental activation energies of pyrene fluorescence quenching, which ar e much less than activation energies, calculated from the experimental values of the quenching rate constants etc. The proposed model explai ns the difference between theoretical and experimental dependencies of log kQ vs. Gibbs energy of electron transfer DELTAG(ET) and other exp erimental features known for fluorescence quenching by electron donors and acceptors This model states that the exciplex is in equilibrium w ith the encounter complex and apparent quenching rate constants are co ntrolled by two main factors - the lifetime of the exciplex and the en thalpy of its formation. Experimentally observed dependence of apparen t quenching rate constant on DELTAG(ET) is caused by the dependence of the exciplex formation enthalpy on DELTAG(ET), which is quite differe nt from the dependence of electron transfer activation energy on DELTA G(ET) predicted by the theoretical models. Simulations of the dependen cies of log k(Q) vs. DELTAG(ET) according to the exciplex formation mo del confirms its agreement with the experimental data. Electronic stru cture of the exciplex involved may be close to contact radical-ion pai r only at DELTAG(ET) < 0, when the rate of quenching is limited mainly by the diffusion, but for DELTAG(ET) > 0, the structure of the excipl ex should be much less polar.