Dynamics of Geminate Recombination in Excited State Proton Transfer Reactions

Abstract

Ion pairs are produced when aqueous solutions of certain hydroxy aromatic compounds are irradiated by light. The excited hydrated molecules dissociate in their first excited singlet state according to Scheme 1.(1–3).

where R*OH is the electronically excited molecule in its acidic form, R*0-is the excited anion. k₁ and k₂ are the dissociation and recombination rate constants in the excited state and k₃, k₄ are the recombination and dissociation rate constants in the ground state. k_f and k_f’ are the natural radiative rate constants of the excited molecule and the excited anion, respectively. Since k₄ is very small compared to all other constants the decay of the anion to its ground state is followed by a fast neutralization reaction. Thermodynamic calculations, known as the “Forster Cycle” (3), can be applied to determine the pK* values of these molecules. Another method to obtain the pK* values is by steady state titration of the excited state (3).