ON THE FORSTER MODEL - COMPUTATIONAL AND ULTRAFAST STUDIES OF ELECTRONIC-ENERGY TRANSPORT

The Forster model was used to study the transport of electronic energy in condensed, spatially disordered systems. Several analytical theori es were compared to a computation benchmark over an expansive domain o f the model parameters (reduced concentration C, Forster distance R-0, and chromophore diameter r(min)) for systems consisting of up to 5000 chromophores. The first-order cumulant approximation (FCA) is found t o be the most consistent in predicting the survival probability P-0(t) , the observable for which the theories differ most. Its range of appl icability is usually P-0(t) > 0.05, yet never better than P-0(t) > 0.0 1, In addition, the energy transport is found to be nondiffusive in th e range P-0(t) > 0.001. The computations were combined with experiment s to further test the Forster model. Femtosecond polarization grating methods were used to determine P-0(t) and high spatial resolution popu lation gratings determined the long-time diffusion coefficient D in co ncentrated dye solutions. The FCA is found to satisfactorily describe P-0(t) on the femtosecond time scale for C less than or equal to 41 wi thin its range of accuracy; whereas the long-time limit is in good agr eement with the theory of Loring, Franchi and Mukamel (LFM). For C les s than or equal to 10, D scales as C-4/3 With a prefactor, 0.20 +/- 0. 03, that agrees with LFM theory (0.212). At higher concentrations, the re is a decrease from C-4/3 behavior sooner than LFM theory predicts w hich is discussed in the context of long-range correlations in the chr omophore 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 conc entration and time regimes, and illustrate that the details are unders tood at a quantitative level up to relatively high concentrations. (C) 1997 Published by Elsevier Science B.V.