Effect of electrostatic interactions and dynamic disorder on the distance dependence of charge transfer in donor-bridge-acceptor systems.

Using a tight-binding model of charge transport in systems with static and dynamic disorder, we present a theoretical study of the positive charge transfer in molecular assemblies that involve a hole donor and an acceptor connected by fluorene and phenyl bridges. Two parameters that determine the rate of charge transfer within the proposed model are the charge transfer integral between neighboring units and the site energies. Fluctuations in the values of the charge transfer integral and the energy landscape for hole transport were calculated by taking into account variations of the dihedral angle between neighboring units and electrostatic interaction of positive charge moving along the bridge and the negative charge that remains on the hole donor. Analysis of the dynamics of hole transfer and the distribution of the positive charge during this process allows the conclusion that the rapid fall of the hole transfer rate coefficient observed in experiments with short bridges (three and four structural units for systems with fluorene and phenyl bridges, respectively) can be attributed to the electrostatic interaction. This interaction is responsible for the formation of the effective barrier between donor and acceptor with the height that increases as the number of structural bridge units remains less than 3 (fluorene bridge) or 4 (phenyl bridge). For longer bridges, however, the effective barrier changes only weakly and now the charge transport is mostly dominated by the fluctuation-assisted incoherent hole migration along the bridge. The latter mechanism exhibits much weaker dependence of the rate coefficient on the bridge length in agreement with the available experimental results.