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 t riphenylphosphine 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 compound s exhibit a dual fluorescence in polar solvents explained by the fast formation of an emissive charge-transfer state as in the model compoun d (dimethylamino)benzonitrile (DMABN). Fluorescence decays are also me asured for solutions of DMABN under the same conditions. For both comp ounds, the intramolecular charge-transfer time is shown to vary from a few picoseconds to a few tens of picoseconds depending on the polarit y of the solvent and, for the triphenylphosphine derivatives, on the n umber of dimethylamino substituents. The charge-transfer process is de scribed as a barrier-activated process with a solvent polarity depende nt height. The solvent dynamics and solvent viscosity effects on the c harge-transfer rate are examined for both the (dimethylaminophenyl)dip henylphosphine oxide (OMAP) and DMABN. In protic solvents, the charge- transfer time is found to be shorter than the average solvation time f or both compounds, suggesting that the charge-transfer mechanism invol ves 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 i nitially proposed twisting motion of the dimethylamino group as the in tramolecular coordinate. The twisting motion of the whale aniline moie ty is discussed as the possible intramolecular motion for OMAP on the basis of the solvent viscosity effects, which are found to differentia te this compound from DMABN.