Subpicosecond fluorescence up-conversion and transient absorption spectroscopy is applied to study the excited-state dynamics of auramine, a diphenylmethane dye, in liquid solutions. The fluorescence decays, on a time
scale of a few picoseconds to a few tens of picoseconds, are found to be nonexponential and solvent viscosity
dependent. They can be fitted as a sum of two exponentials in ethanol and three exponentials in decanol
with a larger average lifetime in the more viscous solvent. The decays exhibit wavelength-dependent time
constants, whereas the fluorescence rise time is instrument limited (150 fs) at all wavelengths. The average
decay time increases with the wavelength across the steady-state emission spectrum. The spectral reconstruction
indicates a few hundred wavenumbers dynamic Stokes shift accompanied by a drop in the intensity in both
solvents. From transient absorption experiments, the fluorescent state population is shown to decay to an
intermediate dark state and then to the ground state, with a viscosity-dependent rate. A barrierless or quasi-barrierless photoreaction involving the rotational diffusion of the phenyl rings, with a change in the radiative
transition rate along the reaction path, is proposed to explain the wavelength-dependent nonexponential
fluorescence decays. Both fluorescence and transient absorption data are discussed in support of an adiabatic
photoreaction involving internal twisting and charge shift.
The photoinduced processes in three dimethylamino derivatives of the triphenylphosphine oxide (OMAP, ODAP, and OTAP) are studied in solution at room temperature by time-resolved fluorescence spectroscopy with a streak camera and a 500 fs UV laser excitation source. These compounds exhibit a dual fluorescence in polar solvents explained by the fast formation of an emissive charge-transfer state as in the model compound (dimethylamino)benzonitrile (DMABN). Fluorescence decays are also measured for solutions of DMABN under the same conditions. For both compounds, the intramolecular charge-transfer time is shown to vary from a few picoseconds to a few tens of picoseconds depending on the polarity of the solvent and, for the triphenylphosphine derivatives, on the number of dimethylamino substituents. The charge-transfer process is described as a barrier-activated process with a solvent polarity dependent height. The solvent dynamics and solvent viscosity effects on the charge-transfer rate are examined for both the (dimethylaminophenyl)diphenylphosphine oxide (OMAP) and DMABN. In protic solvents, the charge-transfer time is found to be shorter than the average solvation time for both compounds, suggesting that the charge-transfer mechanism involves an intramolecular coordinate in addition to the solvent coordinate. The charge-transfer times found for DMABN are in good agreement with those recently calculated by Kim and Hynes (J. Photochem. Photobiol. A 1997, 105, 337-343), who derived a two-dimensional model using the initially proposed twisting motion of the dimethylamino group as the intramolecular coordinate. The twisting motion of the whole aniline moiety is discussed as the possible intramolecular motion for OMAP on the basis of the solvent viscosity effects, which are found to differentiate this compound from DMABN.
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