Oscillatory excitation transfer in dithiaanthracenophane: Quantum beat in a coherent photochemical process in solution

The intramolecular electronic excitation transfer in dithiaanthracenophane (DTA) in THF solution has been investigated by probing the fluorescence anisotropy decay with a femtosecond up-conversion method. In DTA, two anthracene rings are known to be stacked parallel, but with nearly orthogonal orientation. There appeared a damped oscillation of an apparent period of 1.2 +/- 0.2 ps and a damping time constant of 1.0 +/- 0.1 ps. It has been found that the oscillatory behavior is consistent with the recurrence motion of an excitation between two anthracene moieties, from a theoretical analysis following a manner of Wynne and Hochstrasser (Wynne, K.; Hochstrasser, R. M. J. Raman Spectrosc. 1995, 26, 561). The magnitude of the dipole-dipole energy transfer interaction is estimated to be 40 cm(-1) which is in an acceptable agreement with the experimental value (29 cm-1) deduced from the oscillation period. In comparison with the similar case of 2,2'-binaphthyl (BN) reported by Zhu et al. (Zhu, F.; Galli, C.; Hochstrasser, R. M. J. Chem. Phys. 1993, 98, 1042), the electronic dephasing time T-2' is significantly longer in DTA (1.0 ps) than in BN (0.2 ps). The longer dephasing time in DTA can be explained as arising from a much more fixed and rigid dimeric conformation than in BN.