Vibrational coherence in excited state of an electron donor-acceptor complex.

The oscillatory fluorescence decay of an electron donor-acceptor complexes was observed including one with hexamethylbenzene (HMB) and tetracyanoethylene (TCNE). A fast fluorescence decay component was assigned to vibrational relaxation of intramolecular modes. The 140 fs characteristic time was obtained from reconstructed shift of the fluorescence spectrum (C(t)). The underdamped oscillation (212 +/- 4 fs) superimposed with the fast decay corresponds to a frequency of 157 +/- 3 cm(-1). It is shown that the fluorescence spectrum is oscillating in time with the same frequency as a fluorescence observed at a single wavelength. The fluorescence anisotropy data show a fast decay component of ca. 500 fs which was assigned to conversion from CT2 to CT1 - too closely located excited charge-transfer states. The angle between transition moments of CT1 and CT2 states was estimated to be ca. 30 degrees.