Pressure tuning of solvent viscosity for the formation of twisted intramolecular charge-transfer state in 4,4′-diaminodiphenyl sulfone in alcohol solution

Abstract

The effect of solvent shear viscosity on the formation of twisted intramolecular charge-transfer (TICT) state in the excited state for 4,4′-diaminodiphenyl sulfone (DAPS) in three linear alcohol solvents has been investigated by measuring the picosecond fluorescence lifetime as a function of pressure. At lower pressures the kinetics of the TICT-state formation is controlled by solvent relaxation. While with increasing pressure, the reaction path shifts toward the ‘‘high viscosity regime’’ in which the reaction proceeds through the nonrelaxed path independent of solvent coordinate on the free energy surface of the S1 excited state. This behavior could be called as ‘‘pressure tuning effect’’ of solvent viscosity. The power law parameter (α) is used as a measure of the viscosity dependence. For the high viscosity regime, α=0.2 can be attributed to the intrinsic viscosity dependence for barrier crossing.