MOLECULAR-WIRE BEHAVIOR IN P-PHENYLENEVINYLENE OLIGOMERS

Electron transfer from electron-donor to electron-acceptor molecules v ia a molecular 'bridge' is a feature of many biological and chemical s ystems. The electronic structure of the bridge component in donor-brid ge-acceptor (DBA) systems is known to play a critical role in determin ing 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 t heory predicts that regimes exist wherein the distance dependence may be very weak, the bridge molecules essentially acting as incoherent mo lecular wires(3-6). Here we show how these regimes can be accessed by molecular design. We have synthesized a series of structurally well-de fined DBA molecules that incorporate tetracene as the donor and pyrome llitimide as the acceptor, linked by p-phenylenevinylene oligomers of various lengths. Photoinduced electron transfer in this series exhibit s very weak distance dependence for donor-acceptor separations as larg e as 40 Angstrom, with rate constants of the order of 10(11) s(-1). Th ese findings demonstrate the importance of energy matching between the donor and bridge components for achieving molecular-wire behaviour.