Oligofluorene Molecular Wires: Synthesis and Single-Molecule Conductance

We report the synthesis and molecular junction conductance for a series of oligofluorenes to establish clear structure-property relationships for this electronically important material. We use a scanning tunneling microscopy based break-junction method (STM-BJ) to measure single-molecule conductance in oligofluorenes that vary in (a) the number of fluorene repeat units (n = 1-3), (b) bridge carbon substitution (dihydrogen, dimethyl, dihexyl, and didodecyl), and (c) linker-group termination (methyl sulfide versus primary amine). Conductance in oligofluorene molecular junctions is found to occur via tunneling, with a tunneling decay constant, β, of 0.31 per Å, or equivalently, 2.6 per fluorene unit, consistent with other π-conjugated molecular wires. Simple tunnel coupling calculations for model Au₂-oligofluorene molecular clusters are reported to validate experimental conductance measurements. Finally, molecular conductance distributions for methyl sulfide terminated oligofluorenes are observed to be extremely broad due to the relatively flat torsional potential energy surface for rotation about the Au-S bond and the strong orientation effect of the conductance through a π-coupled state.