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 acceptors 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^ox_D - E^red_A), 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 radication 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.