We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy with a time resolution of 75 fs. The measurements have been made for several acceptors (9-cyanoanthracene, CA; 9,10-dicyanoanthracene, DCA; and 1,2,9,10-tetracyanoanthracene, TCA) in the donor solvent (N,N-dimethylaniline, DMA, and aniline, ANL). The decay times obtained are between 150 fs (CA-ANL) and 280 fs (TCA-DMA). Observation of the decay of acceptor fluorescence and the concurrent rise of fluorescence from the exciplex state indicates that the excited acceptor (A*) state directly relaxes to the exciplex state. Charge separation rates (kcs) determined from the decay of the acceptor fluorescence lie between 3.6×1012-6.7×1012/s for six combinations of donors and acceptors. Very weak energy gap (ΔE) dependence of kcs was observed in the region of ΔE of 0.36-1.47 eV. This relation between kcs and ΔE is quite different from the charge recombination rate (kCR) vs ΔE relation in the charge transfer (CT) complexes, which has been reported by Mataga et al. The present result can be explained by a model which takes into account the interaction between the zeroth-order D·A* and D+ ·A- states. Ultrafast charge separation would occur from the D·A* to exciplex state on an adiabatic potential curve without barrier along the coordinate of the intermolecular stretching vibration. This is considered to be induced by the strong interaction between the donor and acceptor at short distances.
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