On the Förster model: Computational and ultrafast studies of electronic energy transport Original Research Article

The Förster model was used to study the transport of electronic energy in condensed, spatially disordered systems. Several analytical theories were compared to a computation benchmark over an expansive domain of the model parameters (reduced concentration C, Förster distance R0, and chromophore diameter rmin) for systems consisting of up to 5000 chromophores. The first-order cumulant approximation (FCA) is found to be the most consistent in predicting the survival probability P0(t), the observable for which the theories differ most. Its range of applicability is usually P0(t) > 0.05, yet never better than P0(t) > 0.01. In addition, the energy transport is found to be nondiffusive in the range P0(t) > 0.001. The computations were combined with experiments to further test the Förster model. Femtosecond polarization grating methods were used to determine P0(t) and high spatial resolution population gratings determined the long-time diffusion coefficient D in concentrated dye solutions. The FCA is found to satisfactorily describe P0(t) on the femtosecond time scale for C ⩽ 41 within its range of accuracy; whereas the long-time limit is in good agreement with the theory of Loring, Franchi and Mukamel (LFM). For C ⩽ 10, D scales as C43 with a prefactor, 0.20 ± 0.03, that agreess with LFM theory (0.212).At higher concentrations, there is a decrease from C43 behavior sooner than LFM theory predicts which is discussed in the context of long-range correlations in the chromophore distribution due to electrostatic or aggregation effects. The experiments, in conjunction with the computations, identify the most accurate theoretical models for energy transport in the different concentration and time regimes, and illustrate that the details are understood at a quantitative level up to relatively high concentrations.