Near Length-Independent Conductance in Polymethine Molecular Wires

Polymethine dyes are linear pi-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor beta = 0.04 angstrom(-1)) compared to traditional polyenes (beta approximate to 0.2 angstrom(-1)). Furthermore, we show that by changing solvent we are able to shift the beta value, demonstrating a remarkable negative beta value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in beta with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Huckel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.