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(1,2). 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(3-6). 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 Angstrom, with rate constants of the order of 10(11) s(-1). These findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.