Fluorescence Quenching by Reversible Charge Separation Followed by Ions Recombination and Their Separation Suppressed by Coulomb Attraction

The Stern-Volmer constant is specified for the luminescence quenched by reversible ionization of excited molecules. The exergonic branch of the Rehm-Weller free energy dependence of this constant is known to be a plateau determined by irreversible ionization being under diffusion control. In the endergonic region the ionization is reversible and competes with the irreversible in-cage recombination of ions and their escape from the cage. At strong Coulomb attraction the latter phenomenon is shown to be negligible compared to the former that determines the shape and location of the descending branch of the Rehm-Weller curve. At weaker Coulomb attraction (at higher solvent polarity), this curve turns down at larger endergonicity. The experimental data obtained in solvents of different polarities are put in order and in full accordance with present theory. 1. Introduction When the luminescence of the excited electron acceptors are quenched in encounters with electron donors, the created ion pairs are subjected to geminate charge recombination and their separation. In the case of pulse induced luminescence at low concentration of quenchers, the bulk reactions of separated ions may be ignored, provided the time of luminescence detection is shorter than the expected delayed fluorescence. In [1] such a reaction was first considered accounting for its reversibility which is essential for endergonic ionization. The corresponding reaction scheme is the following one: Using the Integral Encounter Theory (IET), the following kinetic equations were employed [1]: where and are the concentrations of the excited acceptors (with a life time ) and charged products (ion pairs) respectively, while is a permanent concentration of quenchers (electron donors ), assumed to be present in great excess. The Laplace transforms of the kernels are where and are the forward and backward electron transfer rates, while is the rate of ion recombination to the ground state. The originals of and are the reactants and ion pairs correlation functions, and , which obey the auxiliary diffusional equations. The free energy dependence of the Stern-Volmer constant specified by solution of these equations in [1] was used for interpretation of the Rehm-Weller phenomenon [2], which appears to be qualitatively different from the classical one peculiar to irreversible photoionization [3]. Unfortunately, the authors of the pioneering work [1] did not account for solvent polarity since they neglected the Coulomb attraction between produced counter ions. Meanwhile, the experimentally studied quenching of