Generalised Forster Cycle: Thermodynamic and Extrathermodynamic Relationships Between Proton Transfer, Electron Transfer and Electronic Excitation

Abstract

The thermodynamic quantities characterizing one electron reduction, protonation and electronic excitation, are mutually related in the ground and excited states by 6 thermodynamic (or approximate thermodynamic) cycles. The system of cycles is used to predict unknown values, and its validity may be extended to other compounds by means of extra thermodynamic (e.g., Hammett-type) relations. Examples of known data concerning pK and pK* values of protolytic equilibria for the oxidized (Ox) and reduced (R) species are evaluated, tabulated and discussed in a search for a correlation between the changes of pK on excitation (ΔpK*) and on reduction (ΔpKe). Both values are thermodynamically independent but a general common trend is empirically observed in several groups of systems. Some rules are derived for the excited state redox potentials and their dependence on pH, which may be useful for photochemistry.