Unconventional Conduction

Single-molecule charge transport experiments indicate that electrical current does not necessarily take the shortest route through a molecule, as is commonly assumed (J. Am. Chem. Soc., DOI: 10.1021/ja208600v). The study deepens understanding of electron transport through individual molecules and shows that the microscopy method used to probe this process can distinguish between isomers with subtle structural differences. Zhihai Li and Eric Borguet of Temple University repeatedly brought a scanning tunneling microscope tip in and out of contact with an electrode treated with a porphyrin known as TPyP while recording current-distance data. Conventional wisdom suggests that because of TPyP’s structure, with four symmetrically positioned pyridyl groups, two modes of conduction—via “ortho” and “para” substituents—should be observed, and the shorter ortho route should predominate. Yet the team observed only a single signal. They showed that it arose from the longer para pathway (shown) by repeating the measurements with the ortho and para disubstituted porphyrins.