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 bot...