The decay of the energyhyphen;donor phosphorescence has been observed using a N2laser on the benzophenonendash;naphthalene andphyphen;methoxybenzaldehydendash;naphthalene systems, and the results have been examined in the light of the theory developed by Inokuti and Hirayama for energy transfer by exchange interaction. Substantial discrepancies have been found between the theory and the observed phosphorescence decay curves, in particular, for the short time region after excitation. We have introduced the concept of the nonvanishing closest donorndash;acceptor intermolecular distance into the framework of the theory. Over the whole range of time, the experimental data have been found to agree with the theory thus amended.
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