Dipole effects on the formation of molecular junctions

Measuring the tunnelling current is a promising way of identifying individual molecules in a liquid, wherein molecular conformations in an electrode gap play a crucial role in the electron transport properties. Here we report that molecular dipole interactions with the electric field effectively restrict the configurational degrees of freedom in metal-molecule-metal systems. We utilized a mechanically tunable Au nanoelectrode gap to electrically detect diaminobenzene isomers. We found suppression of a variation in the single-molecule conductance of 1,2-benzenediamines (BDAs) in water suggesting a significant influence of the huge electric field created between the nanoprobes to align the molecular dipole along the potential gradient and concomitant formation of well-defined junction structures for the molecules through-space coupled to one side of the electrodes. On the other hand, the field effect was absent in 1,3- and 1,4-BDAs, which is attributed to their smaller dipole moments and the more rigid chemical connections to the electrodes via Au-amine bonds.