Vibronic relaxation of polyatomic molecule in nonpolar solvent:: Femtosecond anisotropy intensity measurements of the Sn and S1 fluorescence of tetracene

The electronic and vibrational relaxation of tetracene have been studied in solution by femtosecond time-resolved fluorescence spectroscopy. Tetracene was initially photoexcited to the highly excited singlet (S-n) state, B-1(b), and the dynamics of the fluorescence from the B-1(b) State and the L-1(a) state (S-1) were investigated by fluorescence up-conversion. The fluorescence from the 1Bb State was observed in the ultraviolet region, and its lifetime was determined as similar to 120 fs. The anisotropy of the B-1(b) fluorescence was close to 0.4, which assured that the fluorescence is emitted from the excited state that was prepared by photoexcitation. The visible fluorescence from the L-1(a) state showed a finite rise that agreed well with the decay of the B-1(b) fluorescence. Negative anisotropy was observed for the L-1(a) fluorescence, reflecting that the L-1(a) transition moment is parallel to the short axis of the molecule and hence perpendicular to the B-1(b) transition moment. The anisotropy of the L-1(a) fluorescence, however, showed a very characteristic temporal behavior in the femtosecond time region: it exhibited a very rapid change and reached a certain value that is deviated from -0.2. The anisotropy data indicate that the L-1(a) fluorescence contains not only a short-axis polarized component but also a long-axis polarized component and that the ratio between the two components depends on both time and wavelength. The long-axis polarized component in the L-1(a) fluorescence was assigned to the B-1(b)-type fluorescence that appears as the result of the vibronic coupling between the L-1(a) state and the B-1(b) state. The observed initial rapid change of the anisotropy suggests that the highly excited vibrational states in the L-1(a) state which are strongly coupled with the B-1(b) state are first populated preferentially when the molecule is relaxed from the B-1(b) State to the L-1(a) state. The visible fluorescence anisotropy vanishes gradually because of the rotational diffusion in a few tens of picoseconds. In the picosecond region, we also observed additional dynamics in the fluorescence intensity whose time constant was about 12 ps. This dynamics was assigned to the vibrational relaxation (cooling) in the L-1(a) state. A series of relaxation processes taking place after photoexcitation of the molecule in solution are discussed.