We have reported the comparison of molecular conductance by the combination method of STM apparent height measurement in constant current mode and the 2D phase-separation technique. As STM handles tunneling current, STM apparent height of a molecule reflects the conductance of the molecule. Two porphyrin complexes with different molecular wires each having different lengths of side chains form different domains in the STM image when the mixture of two complexes is deposited at the solid-liquid interface. This 2D phase separation enables us to measure the apparent height of each molecule separately and to compare the molecular conductances of each wire unit statistically.<br/>Here we describe a comparison of single molecular conductance between two π-conjugated wires with and without radical substituent. Namely, methyl- and iminonitroxide-substituted 4-(biphenyl-4-yl)pyridine wires bound onto porphyrin template were subjected to STM apparent height measurement at the 1-octanoic acid/HOPG interface. Statistical analysis of STM images revealed that the radical-substituted wire has 3.2±1.7 times larger conductance than the methyl-substituted reference. Although DFT calculation suggests that only 17% of the SOMO is distributed on the wire moiety, the effect was significant.

Investigation on the single-molecular conductance of molecular wire by STM at the solid-liquid interface: Enhancement by a radical substituent

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