Ultrafast charge separation and exciplex formation induced by strong interaction between electron donor and acceptor at short distances

We have measured the decay of fluorescence from the electron acceptors (A) dissolved in donor (D) solvent by femtosecond up-conversion spectroscopy wi th 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) an d 280 fs (TCA-DMA). Observation of the decay of acceptor fluorescence and t he concurrent rise of fluorescence from the exciplex state indicates that t he excited acceptor (A*) state directly relaxes to the exciplex state. Char ge separation rates (k(CS)) determined from the decay of the acceptor fluor escence lie between 3.6x10(12)-6.7x10(12)/s for six combinations of donors and acceptors. Very weak energy gap (Delta E) dependence of k(CS) was obser ved in the region of Delta E of 0.36-1.47 eV. This relation between k(CS) a nd Delta E is quite different from the charge recombination rate (k(CR)) vs Delta E relation in the charge transfer (CT) complexes, which has been rep orted by Mataga The present result can be explained by a model which takes into account the interaction between the zeroth-order D . A* and D+. A(-) s tates. Ultrafast charge separation would occur from the D . A(*) to exciple x state on an adiabatic potential curve without barrier along the coordinat e of the intermolecular stretching vibration. This is considered to be indu ced by the strong interaction between the donor and acceptor at short dista nces. (C) 2000 American Institute of Physics. [S0021-9606(00)50216-5].