Enhanced hopping conductivity in low band gap donor-acceptor molecular wires Up to 20 nm in length.

We have measured the current-voltage characteristics of conjugated oligo-tetrathiafulvalene-pyromelliticdiimide-imine (OTPI) wires ranging in length from 2.5 to 20.2 nm, contacted by Au electrodes. OTPI wires were built from Au substrates using alternating donor (tetrathiafulvalene, TTF) and acceptor (pyromelliticdiimide, PMDI) building blocks linked via aryl imine groups. Metal-molecule-metal junctions consisting of approximately 100 wires in parallel were prepared by contacting the wire films with an Au-coated atomic force microscope tip. The long OTPI wires exhibit a narrow band gap (<1.5 eV) and multiple redox states, which facilitate carrier injection from the Au contacts for hopping transport. We observe the theoretically predicted change in direct current (DC) transport from tunneling to hopping as a function of systematically controlled wire length, as well as strongly enhanced wire conductivity (0.02 S/cm) in the hopping regime. Hopping conduction is confirmed by length-, temperature-, and field-dependent transport measurements. These nanoscale transport measurements illuminate the role of molecular length and bond architecture on molecular conductivity and open opportunities for greater understanding of hopping transport in conjugated polymer films.