Vibronic relaxation of polyatomic molecule in nonpolar solvent: Femtosecond anisotropy intensity measurements of the S-n and S-1 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 fluores cence from the 1Bb State was observed in the ultraviolet region, and its li fetime was determined as similar to 120 fs. The anisotropy of the B-1(b) fl uorescence was close to 0.4, which assured that the fluorescence is emitted from the excited state that was prepared by photoexcitation. The visible f luorescence from the L-1(a) state showed a finite rise that agreed well wit h the decay of the B-1(b) fluorescence. Negative anisotropy was observed fo r 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 re gion: 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) fluorescen ce contains not only a short-axis polarized component but also a long-axis polarized component and that the ratio between the two components depends o n 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 t he 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 diffus ion in a few tens of picoseconds. In the picosecond region, we also observe d additional dynamics in the fluorescence intensity whose time constant was about 12 ps. This dynamics was assigned to the vibrational relaxation (coo ling) in the L-1(a) state. A series of relaxation processes taking place af ter photoexcitation of the molecule in solution are discussed.