Vibrational Coherence in Electron Donor−Acceptor Complexes

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 a ultrafast decay component. The oscillations in fluorescence are observed for the complexes with three different electron acceptors, tetracyanoethylene (TCNE), chloranil, and fluoranil, with various electron donors, hexamethylbenzene, carbazole, 1-chloronaphthalene, N-methylaniline, and some others. The similar oscillatory frequencies are observed for the complexes with different donors and a common acceptor: ∼155 cm-1 (period of ∼215 fs) for complexes with TCNE, 178 cm-1 (187 fs) for the complexes with chloranil, and 212 cm-1 (157 fs) for the complex with fluoranil. The assignment of the oscillatory component to a particular vibration is proposed. It is concluded that the out-of-plane vibrational mode of the acceptor, having b3u symmetry, is responsible for the observed oscillations in all three cases (three acceptors). The time-dependent fluorescence spectrum for the TCNE−HMB complex is reconstructed from the fluorescence dynamics at different wavelengths, and the fluorescence peak shift correlation function is 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 mostly to the intramolecular vibrational energy redistribution process in both donor and acceptor parts of the complex. The oscillatory component of the fluorescence peak position demonstrates the modulation of the transition frequency during the vibrations. The modulation of the mean transition moment is observed as well. 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.