Reduced Conductance Attenuation in Cyanine Dye Molecular Junctions

Charge transport properties of molecular wires are determined not only by the properties of the molecular junction (i.e. contact geometry or relative level alignment of frontier molecular orbitals with the Fermi level of the metal electrodes) but can also be influenced by the external environment. Here, we study conductance trends in single-molecular junctions of polymethine dyes fabricated with the scanning tunneling microscope based break-junction technique. These junctions are shown to present very low attenuation of conductance with the molecular length, with the trend being tunable (either positively or negatively) by the choice of solvent. Supported by a Hückel model and ab-initio quantum transport calculations, we argue that the unusually low attenuation factor and solvent-induced modifications in the conductance trends can be related to reduced bond-length alternation (BLA) in the wire. This work details on the impact that BLA has on single-molecular junctions, which could lead to improved design for efficient nanoscale electron transport devices.