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

The ultrafast spectroscopy of the electron donor–acceptor complex of tetracyanoethylene with toluene has been investigated in several polar and non-polar solvents. After ultrafast excitation of the charge transfer band, formation 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 electron transfer, absolute rate constants for the non-radiative return electron transfer, varying over three orders of magnitude, can be predicted from limited 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.