Picosecond Dynamics of Proton-Anion Ion Pair Geminate Recombination

Abstract

Listen

Translate article

Charge separation is induced in solutions of many chemical and biochemical systems by light absorption. The primary step in these reactions is either an electron or a proton transfer from a parent molecule to a suitable acceptor. The solvated ion pairs which are produced can either geminately recombine or separate by diffusion. Geminate recombination was recognized to be extremely important in radiation induced electron-cation ion pair generation [1]. As for proton transfer reactions, much less attention has been paid to this phenomenon mainly because of two reasons. The first reason is that proton transfer reactions are usually being carried in aqueous solutions where the coulombic attraction is very efficiently screened. In contrast, electron transfer reactions are usually being carried in hydrocarbon solutions where the coulombic screening is much less effective. The second and less obvious reason is that geminate electron-excited cation recombination usually quenches the excited state where in many cases proton transfer to an excited anion does not quench the anion [2]. As a result, proton transfer reactions are usually bidirectional both in the ground and the excited state [3]. It means that upon recombination the excited parent molecule can undergo redissociation. Thus, the combination of effective coulombic screening by water molecules and consecutive dissociations makes geminate recombination much less apparent in proton transfer reactions than in electron transfer ones.