Thermodynamic analysis of some electrochemical properties of transition metal complexes in electronically excited states

A theoretical treatment is presented of the relationship between absorption and emission maxima for (0–0) electronic transitions in transition metal complexes such as the ruthenium(II) polypyridyl chromophores used as dye sensitizers in photovoltaic cells, and the corresponding shift in standard redox (or half-wave) potential of these ionic complexes. It is shown that the potential shift is proportional to the average of absorption and emission wavenumbers for oxidized or reduced forms, a result that follows from a detailed analysis of medium relaxation effects, based on the Onsager cavity model, and attendant solvatochromic shifts. It is also shown how additional thermodynamic properties (reaction entropies and enthalpies) pertaining to the half reaction if the reactants are present in solution can be obtained from observation of the variation of spectra with temperature.For the case of reactants present in an insoluble monolayer, a general expression is derived for the Gibbs energy change that results from the promotion of all absorbing species to an excited state, and the changes in Gibbs energy resulting from the processes that return the system to ground-state equilibrium.