Observation of coherent recurrence motion of excitons in anthracene dimers

Intramolecular electronic excitation transfer in anthracene dimers in solution at room temperature has been investigated by probing the fluorescence anisotropy decay with a femtosecond up-conversion method. Two types of anthracene dimers were used: anthracenophane (DTA) in which two anthracene rings are stacked parallel, but with nearly orthogonal orientation, and two dianthrylbenzenes (m- and o-DAB) in which two anthracene rings are linked to a benzene ring at meta- and ortho-positions. There appeared damped oscillations of apparent periods of 0.6-1.0 ps and damping time constants of 0.7-1.2 ps. The ordinary fluorescence decay (excited-state population decay) obtained with the magic angle excitation exhibits no oscillation in all the cases. It has been found from a theoretical analysis that the oscillatory behaviors on fluorescence anisotropy are consistent with the coherent recurrence motion of an exciton between two anthracene moieties. The magnitudes of the dipole-dipole energy transfer interaction are estimated to be 10-80 cm(-1), which are considerably smaller than the experimental value (30-100 cm(-1)) deduced from the angular frequencies of oscillation. This means that some interchromophore interaction other than the dipole-dipole resonance interaction is involved in the energy transfer interaction in strongly coupled dimers. The longer dephasing time can be explained as a consequence of rigid dimeric conformations in anthracene dimers.