Remarks on Excitation Transfer Between Unlike Molecules

Abstract

Vibronic states of a ``dimer'' formed by two unlike molecules are studied. Criteria for strong coupling and weak coupling are examined. According to these criteria, the strong-coupling case, namely, the case where the adiabatic approximation is applicable to the entire system, is more important in a system of two unlike molecules than in a system of two identical molecules. In some systems of two unlike molecules the adiabatic approximation is applicable to the entire system even when the interaction between the two molecules is very weak. Unlike the vibronic states of a strongly coupled system of two identical molecules, in which electronic excitation is completely delocalized, the vibronic states of a strongly coupled system of two unlike molecules exhibit, to some extent, localization of electronic excitation on one of the molecules. Thus, in the latter case, the transition between the vibronic states of the entire system may result in the transfer of electronic excitation from one molecule to the other. The probability of such transition is estimated, and the relation to Forster's theory is discussed. It is shown that Forster's theory is valid only when the emission spectrum of an excitation donor molecule and the absorption spectrum of an excitation acceptor molecule overlap each other substantially.