Electron transfer from electron-donor to electron-acceptor molecules via a molecular 'bridge' is a feature of many biological and chemical systems. The electronic structure of the bridge component in donor-bridge-acceptor (DBA) systems is known to play a critical role in determining the ease of electron transfer. In most DBA systems, the rate at which electron transfer occurs scales exponentially with the donor-acceptor distance-effectively the length of the bridge molecule. But theory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent molecular wires. Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-defined DBA molecules that incorporate tetracene as the donor and pyromellitimide as the acceptor, linked by p-phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibits very weak distance dependence for donor-acceptor separations as large as 40 Å, with rate constants of the order of 1011 s-1. These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.
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