High-pressure studies on the excited-state intramolecular charge transfer of 4-(N,N-dimethylamino)triphenylphosphine in alcohols

The influence of solvent viscosity on the intramolecular charge-transfer (CT)-state formation in the excited S-1 state for 4-(N,N-dimethylamino) triphenylphosphine (DMATP) in alcohol solvents has been investigated by measuring the steady-state and time-resolved fluorescence spectra at high pressures. The kinetic mechanism of the intramolecular CT reaction has been examined as a function of solvent shear viscosity. In the lower viscosity region the reaction is controlled by the solvent relaxation. With increasing pressure, the reaction path shifts toward the ''high-viscosity regime'' in which the molecule moves along the nonrelaxed path on the free energy surface. The viscosity dependence of a congruent to 0.33, where a is the power law parameter, can be interpreted as the extreme value in which the reaction is controlled by the dynamic solvent effect due to intrinsic collisional interaction of barrier crossing. The coupling between the intramolecular CT-state formation dynamics of DMATP and the solvent relaxation dynamics is discussed.