Dynamics

Dimethylamino-substituted triphenylphosphines exhibit dual fluorescence in polar solvents. The long-wavelength band solvatochromism indicates emission from a highly polar state with a 20 to 25 D dipole moment. Fluorescence decay measurements show that photoinduced intramolecular charge transfer occurs in a few picoseconds even in weakly polar solvents and suggest that the charge transfer reaction is reversible leading to an equilibrium between the locally excited state and charge transfer state populations during the excited state lifetime. A transient absorption band with a profile similar to that of the dimethylaniline cation radical is observed in polar solvents by time-resolved picosecond absorption spectroscopy. The reaction rate is found to decrease in viscous solvents bringing support to a mechanism involving solute geometrical relaxation. On the other hand, the compounds are shown to form clusters and the rôle of clusters in the observed transient phenomena is discussed.