ELECTRONIC-STRUCTURES OF EXCIPLEXES AND EXCITED CHARGE-TRANSFER COMPLEXES

For the exciplexes and excited charge-transfer (CT) complexes formed between 9,10-dicyanoanthracene and 2,6,9,10-tetracyanoanthracene as electron accepters and alkylbenzenes as donors, the radiative rate constants (k(f)) increase with increasing emission energy. The increase in k(f) is attributed to a corresponding decrease in the charge-transfer character of the emitting species. This is explained in terms of the relative contributions of pure ion-pair and locally excited states to the emitting state. With decreasing solvent polarity and with increasing redox energy of the acceptor/donor pair (E(D)(ox) - E(A)(red)), the energy of the pure ion-pair state is raised and mixing with the locally excited states increases. The dependence of k(f) on emission energy is analyzed quantitatively using a three-state model in which mixing among the first locally excited singlet state of the cyanoanthracenes, the pure ion-pair state, and the neutral state is taken into account. Simplified methods for data analysis are also discussed. From the analyses, the relationship between the electronic structures of the exciplex/excited CT complexes and the emission frequency is obtained. For these acceptor/donor systems, the emitting species can be considered to be essentially pure contact radical-ion pairs (>90% CT character) when their emission maxima are lower in energy than the 0,0 transition of the acceptor excited singlet states by ca. 5000 cm(-1). Values of ca. 1300-1350 cm(-1) are obtained for the electronic matrix elements coupling the locally excited and ion-pair states. The corresponding matrix elements for coupling the ion-pair and the neutral states are ca. 750-900 cm(-1), which are similar to those estimated previously from studies of the rates of nonradiative electron transfer in closely related species.