Oscillatory Fluorescence Decay of an Electron Donor−Acceptor Complex

We report an oscillatory fluorescence decay of an electron donor−acceptor complex of hexamethylbenzene and tetracyanoethylene in CCl4 at room temperature. A fast spontaneous fluorescence decay with a characteristic time of 105 ± 20 fs is observed. An underdamped oscillation of fluorescence is overlapped with this fast decay. The period of the oscillation is 209 ± 4 fs, which corresponds to a frequency of 159 ± 3 cm-1 and is tentatively assigned to the intermolecular vibration in the excited state. The damping time of the oscillation is 280 ± 30 fs.