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The dynamics of unimolecular photo-triggered reactions can be stronglyaffected by the surrounding medium. An accurate description of these reactionsrequires knowing the free energy surface (FES) and the friction felt by thereactants. Most of theories start from the Langevin equation to derive thedynamics, but there are few examples comparing it with experiments. Here weexplore the applicability of a Generalized Langevin Equation (GLE) with anarbitrary potential and a non-markovian friction. To this end we have performedbroadband fluorescence measurements with sub-picosecond time resolution of acovalently linked organic electron donor-acceptor system in solvents ofchanging viscosity and dielectric permittivity. In order to establish the FESof the reaction we resort to stationary electronic spectroscopy. On the otherhand, the dynamics of a non-reacting substance, Coumarin 153, provide thecalibrating tool for the friction over the FES, which is assumed to be soluteindependent. A simpler and computationally faster approach uses the GeneralizedSmoluchowski Equation (GSE), which can be derived from the GLE for pureharmonic potentials. Both approaches reproduce the measurements in most of thesolvents reasonably well. At long times, some differences arise from the errorsinherited from the analysis of the stationary solvatochromism and at shorttimes from the excess excitation energy. However, whenever the dynamics becomeslow the GSE shows larger deviations than the GLE, the results of which alwaysagree qualitatively with the measured dynamics, regardless of the solventviscosity or dielectric properties. The here applied method can be used topredict the dynamics of any other reacting system, given the FES parameters andsolvent dynamics are provided. Thus no fitting parameters enter the GLEsimulations, within the applicability limits found for the model in this work.

How good is the generalized Langevin equation to describe the dynamics of photo-induced electron transfer in fluid solution?