Molecular Wire Formation from Viologen Assemblies

The adsorption behavior of viologen α,ω-dithiols (viologen dithiols) on gold has been investigated. At short exposures, a low-coverage phase consisting of flat-lying molecules has been determined by STM and IR spectroscopy. In contrast, multilayer films are formed after long adsorption times. Single molecular wires could be formed between a gold STM tip and a surface with a low coverage of the adsorbed dithiols, and their electrical behavior was investigated. Molecular conductivity was determined either by the repeated measurement of I(s) curves or by recording I−V curves for different tip−sample separations. These methods concurred in producing a value of (0.5 ± 0.1) nS for the single-molecule conductivity of the α,ω-viologen dithiol molecule HS−6V6−SH. The high conductivity of HS−6V6−SH, as compared to that of HS−C12−SH, may be related to the low-lying LUMO, which provides a barrier indentation for electron transport in a two-step electron-transfer mechanism.