Assignment of the low frequency vibrations in electron donor-acceptor complexes by ultrafast coherent spectroscopy

Abstract

Abstract Coherent oscillations in a spontaneous fluorescence of electron donor-acceptor (EDA) complexes in solution were observed with three different acceptors: tetracyanoethylene (TCNE), chloranil, and fluoranil with various electron donors. Different oscillatory frequencies were observed for the complexes with different acceptors. The similar frequencies are observed for the complexes with different donors and a common acceptor: ∼155 cm−1 (period of ∼215 fs) for the complexes with TCNE, 178 cm−1 (187 fs) for the complexes with chloranil, and 212 cm−1 (157 fs) for the complex with fluoranil. 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 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 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 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.