The prediction of dye sensitization from irreversible electrochemical potentials

Abstract

This study was undertaken to ascertain if the redox potentials of sensitizing dyes lose their meaning in the context of dye sensitization when the dye electroreactions in solution are irreversible. Only the first reduction and oxidation potentials are considered here and in the theory of sensitization. It is shown that reversibility of the electrode process involving dissolved dye in fact has little significance for most sensitizing dyes. The reversibility of the redox reactions of adsorbed dye, on the other hand, is quite important for sustained photocurrents and photovoltages, and the great majority of good sensitizing dyes can apparently act reversibly in the solid state regardless of their irreversibility in solution. The prediction of dye sensitization from a mixed group of reversible, semi-reversible and irreversible potentials utilizing cut-off potential limits for action remains valid as long as the potentials are reproducible and are obtained with the same electrochemical system, solvent and electrolyte included; albeit the correlation must stand as a somewhat weakened semi-empirical one. One electron per molecule of dye appears to be transferred in sensitization of the silver halides, as well as in the first electrochemical reactions in solution, but not without exception. The potentials and reversibility for several dyes are documented for the first time in ACN/TBAP. Some excursions beyond the first waves were observed and discussed for the rhodamine dye family and for some related dyes.