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The deuterium isotope effects on S1→S0 internal conversion of alkyl benzene/tetracyanobenzene EDA complex depend on the stoichiometric ratio of the donor (alkyl benzenes) to the acceptor (tetracyanobenzene) molecules. Thus, the nonradiative decay rate of the 2:1 complex is more greatly affected by ring deuteration than by methyl deuteration of the donor molecule, in sharp contrast to the very dominant methyl deuteration effect in the 1:1 complex. A theoretical consideration of the binding in the termolecular (2:1) complex suggests that the different isotope effect and the smaller S1→S0 internal conversion rate in the 2:1 complex arise primarily from the smaller cationlike character of the donor molecule in the excited singlet state of the termolecular complex.

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