The study of photoinitiated electron transfer in donor-bridgeacceptor molecules has helped elucidate the distance dependence of electron transfer rates and behavior of various electron transfer mechanisms. In all reported cases, the energies of the bridge electronic states involved in the electron transfer change dramatically as the length of the bridge is varied. We report here, in contrast, an instance in which the length of the bridge, and therefore the distance over which the electron is transferred, can be varied without significantly changing the energies of the relevant bridge states. A series of donor-bridge-acceptor molecules having phenothiazine (PTZ) donors, 2,7-oligofluorene (FLn) bridges, and perylene-3,4:9,10-bis(dicarboximide) (PDI) acceptors was studied. Photoexcitation of PDI to its lowest excited singlet state results in oxidation of PTZ via the FLn bridge. In toluene, the rate constants for both charge separation and recombination as well as the energy levels of the relevant FL n Űâą bridge states for n â«Ű⏠electron transfer Í hopping Í superexchange E fficient long-distance electron transfer is a prerequisite for molecular materials designed to serve as active components in solar cells and nanoscale devices. Being able to consistently achieve ''wire-like'' charge transport in synthetic systems requires a thorough understanding of the mechanisms involved. Recently progress has been made toward this goal (1, 2), but the complexity of even simple molecular systems poses a formidable challenge. In particular, the distance dependence of electron transfer has been shown to be a complex function of individual parameters, including molecular geometry and energetics (3,4). While the use of rigidly bound electron donor (D), bridge (B), electron acceptor (A) compounds has simplified the investigation of the distance dependence of electron transfer by keeping the donor-acceptor distance (r DA ) well defined, it is difficult to vary r DA by changing the length of the bridge without substantially altering the energy levels of the bridge.In this report we examine electron transfer in a system in which r DA can be changed considerably, whereas the oxidation potential of the intervening bridge changes very little. Approximate matching between the donor and bridge energy levels (2, 5, 6) has been shown to be critical for promoting incoherent, or ''wire-like,'' transport over coherent, or exponentially distance-dependent, superexchange transport (7). These earlier studies have shown how the energetic convergence of the relevant donor and bridge energy levels results in a minimal injection barrier for hole transfer to the bridge, leading to a striking change in mechanism as the bridge is lengthened.The relative importance of incoherent transport at larger values of r DA has been confirmed by independent measurements of the decreasing contribution of superexchange to the overall electron transfer rate with increasing length (8, 9). The indirect electronic coupling between the electron donor and acceptor via the br...