Ultrafast dynamics of the excited tetracyanoethylene-toluene electron donor-acceptor complex

The ultrafast spectroscopy of the electron donor-acceptor complex of tetrac yanoethylene with toluene has been investigated in several polar and non-po lar solvents. After ultrafast excitation of the charge transfer band, forma tion and then decay of the contact ion pair is observed. The decay time of the contact ion pair due to charge recombination is, in most cases, longer than the solvation time and is observed on a time scale from 1 ps to 1 ns, depending on the solvent. Using a conventional non-adiabatic theory of elec tron transfer, absolute rate constants for the non-radiative return electro n transfer, varying over three orders of magnitude, can be predicted from l imited information obtained from the analysis of the radiative processes in non-polar solvents only. The model fails in cases where solvent relaxation is slower or comparable to the rate of the return electron transfer. (C) 1 999 Elsevier Science B.V. All rights reserved.