Charge transport through organic molecules over long distances is important in many biological systems, in organic photovoltaics, and in molecular electronics. One of the goals of molecular electronics is the design of robust molecular wires which can transport charge efficiently over extended distances, thereby enabling molecules to act as device interconnects. An important parameter in defining charge transport along molecular bridges is the decrease in transmission as a function of distance. For long-range charge transport it is important that this factor is low and that there is strong electronic coupling between the bridge and the terminal contacts. The decrease in transmission as a function of bridge length has commonly been determined through photophysical measurements of charge transfer kinetics between donor and acceptor moieties at the bridge termini, by measurement of rates of electrochemical electron transfer across organic monolayers on electrodes, or by measuring current through organic monolayers between metal electrodes. Recently it has been possible to directly measure the conductance of single molecules between pairs of gold electrodes, including π-conjugatedoligomers. The decrease in transmission as a function of bridge length is commonly observed to follow an exponential distance dependence in single molecule measurements, which is taken to indicate a superex-change (or tunneling) mechanism:
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