Effect of Alkyl-\nvs Alkoxy-Arene Substituents on the\nDeactivation Processes and Fluorescence Quantum Yields of Exciplexes

Decay processes of exciplexes of cyanoanthracenes with\nalkylbenzene\ndonors were compared to those with alkoxybenzenes. For the three decay\nprocesses of exciplexes, the radiative rate constant (<i>k</i><sub>f</sub>) of alkoxy derivatives is slightly lower than those\nof alkylbenzenes at the same average exciplex energy. However, the\ncorresponding deactivation rate constants, intersystem crossing (<i>k</i><sub>isc</sub>) and nonradiative decay (<i>k</i><sub>nr</sub>), are considerably higher. Consequently, the fluorescence\nquantum yields of the alkoxybenzene exciplexes are one-half to one-fifth\nof the corresponding alkylbenzenes at comparable emission energies.\nThis trend is solvent-independent. The results can be rationalized\nin terms of the differing electronic character of the donor radical\ncation moieties in the alkoxy- vs alkylbenzene exciplexes and differences\nin their reorganization energies. The impact of these results for\nthe design of exciplexes that emit more efficiently is discussed.