  The dynamics of unimolecular photo-triggered reactions can be strongly
affected by the surrounding medium. An accurate description of these reactions
requires knowing the free energy surface (FES) and the friction felt by the
reactants. Most of theories start from the Langevin equation to derive the
dynamics, but there are few examples comparing it with experiments. Here we
explore the applicability of a Generalized Langevin Equation (GLE) with an
arbitrary potential and a non-markovian friction. To this end we have performed
broadband fluorescence measurements with sub-picosecond time resolution of a
covalently linked organic electron donor-acceptor system in solvents of
changing viscosity and dielectric permittivity. In order to establish the FES
of the reaction we resort to stationary electronic spectroscopy. On the other
hand, the dynamics of a non-reacting substance, Coumarin 153, provide the
calibrating tool for the friction over the FES, which is assumed to be solute
independent. A simpler and computationally faster approach uses the Generalized
Smoluchowski Equation (GSE), which can be derived from the GLE for pure
harmonic potentials. Both approaches reproduce the measurements in most of the
solvents reasonably well. At long times, some differences arise from the errors
inherited from the analysis of the stationary solvatochromism and at short
times from the excess excitation energy. However, whenever the dynamics become
slow the GSE shows larger deviations than the GLE, the results of which always
agree qualitatively with the measured dynamics, regardless of the solvent
viscosity or dielectric properties. The here applied method can be used to
predict the dynamics of any other reacting system, given the FES parameters and
solvent dynamics are provided. Thus no fitting parameters enter the GLE
simulations, within the applicability limits found for the model in this work.
