Vibrational Motion of Electron Donor-Acceptor Complexes Time-Resolved by Femtosecond Fluorescence Spectroscopy

Abstract

The excited-state dynamics of several electron donor-acceptor complexes were studied by a femtosecond fluorescence up-conversion technique. The spontaneous fluorescence decay of the complexes contains an oscillatory component superimposed with an ultrafast decay component. The oscillatory component was assigned to the out-of-plane vibration of the acceptors (tetracyanoethylene (TCNE), chloranil, fluoranil) having b3u symmetry. The time-dependent fluorescence spectrum for the TCNE - hexamethylbenzene (HMB) complex was reconstructed from the fluorescence dynamics at different wavelengths and the fluorescence peak-shift correlation function was obtained. The ultrafast relaxation of the fluorescence peak position is superimposed with the oscillatory component. The main component of the spectral relaxation of 115 fs is attributed mainly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory behavior of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The time dependence of the mean transition moment was constructed. It oscillates with the same frequency (155 cm−1), which is an indication of the non-Condon transition. Thus, two mechanisms are observed to be responsible for the oscillations: the modulation of the transition frequency and the modulation of the mean transition moment by the vibrational motion.