Picosecond Stokes shift studies of solvent friction: Experimental measurements of time-dependent band shape and integrated intensity

The dynamic Stokes shift of the emission from a charge transfer excited state is examined in ethanol solutions over the temperature range from - 10°C to -60°C. Measurements of the time-dependent band shape and integrated intensity suggest that there is strong coupling between the electronic structure of the probe molecule and the time-dependent solvation. The time-dependent integrated intensity exhibits a biexponential decay in which the fast component is equivalent to solvation times determined from the Stokes shift correlation function, C(t). This data reflects the time-dependent contribution of the pure LE and TICT states to the excited electronic state as solvation occurs. In addition, over the temperature range studied, the time-dependent shape of the emission spectrum is found to depend solely on the motion along the solvent coordinate, suggesting that the initial decrease in integrated intensity is not the result of the presence of an inhomogeneous distribution of solvated molecules following photolysis.