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Vibronic Relaxation of Polyatomic Molecule in Nonpolar Solvent: Femtosecond Anisotropy/Intensity Measurements of the S n and S 1 Fluorescence of Tetracene

1999; American Chemical Society; Volume: 103; Issue: 25 Linguagem: Inglês

10.1021/jp9906785

1520-5215

Nilmoni Sarkar, Satoshi Takeuchi, Tahei Tahara,

Free Radicals and Antioxidants

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 (Sn) state, 1Bb, and the dynamics of the fluorescence from the 1Bb state and the 1La state (S1) were investigated by fluorescence up-conversion. The fluorescence from the 1Bb state was observed in the ultraviolet region, and its lifetime was determined as ∼120 fs. The anisotropy of the 1Bb 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 1La state showed a finite rise that agreed well with the decay of the 1Bb fluorescence. Negative anisotropy was observed for the 1La fluorescence, reflecting that the 1La transition moment is parallel to the short axis of the molecule and hence perpendicular to the 1Bb transition moment. The anisotropy of the 1La 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 1La 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 1La fluorescence was assigned to the 1Bb-type fluorescence that appears as the result of the vibronic coupling between the 1La state and the 1Bb state. The observed initial rapid change of the anisotropy suggests that the highly excited vibrational states in the 1La state which are strongly coupled with the 1Bb state are first populated preferentially when the molecule is relaxed from the 1Bb state to the 1La 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 1La state. A series of relaxation processes taking place after photoexcitation of the molecule in solution are discussed.