ULTRAFAST CHARGE-TRANSFER IN AN ELECTRON DONOR-ACCEPTOR COMPLEX

Ultrafast pump-probe measurements on the electron donor-acceptor complex of tetracyanoethylene with hexamethylbenzene in polar and nonpolar solvents are reported. Ground state coherence in the complex stretching mode at 165 cm-1 excited by impulsive stimulated Raman scattering is observed as well as decay of the ground state bleaching signal due to return electron transfer to the ground state. The experimental electron-transfer rates are compared with nonadiabatic and adiabatic electron-transfer theories using a previously published analysis of all the vibrational modes active in the reaction. It is shown that a breakdown of the Born-Oppenheimer approximation can give rise to a coupling that leads to the observed electron-transfer reaction. The non-Born-Oppenheimer matrix element is estimated using information obtained from the absorption and Raman spectra. Using this coupling, good agreement is found between the experimentally observed and theoretically predicted rates. Caveats of the various theories, the reliability of the normal mode analysis, and aspects of electron transfer that theory should address are discussed.