Role of intramolecular torsion and solvent dynamics in the charge-transfer kinetics in triphenylphosphine oxide derivatives and DMABN

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.