Reversible photoionization in liquid solutions

The Stern-Volmer constant of fluorescence quenching by reversible intermole cular charge transfer is obtained by means of integral encounter theory. Th e latter provides the first non-Markovian description of the phenomenon whi ch accounts for the reversibility of excited-state ionization. The forward and backward electron transfers (bimolecular and geminate) are specified by the position-dependent rates of ionization and recombination. Assuming tha t the conventional free energy gap law is inherent to all of them, a reason able explanation is given for the famous Rehm and Weller free energy depend ence of the Stern-Volmer constant. It requires the production of ions in ex cited states when forward electron transfer is highly exergonic and implies that the charge recombination occurs not only to the ground but also to th e excited triplet state. It is assumed that spin conversion in the radical ion pairs is faster than the geminate recombination.