pH-Activated Single Molecule Conductance and Binding Mechanism of Imidazole on Gold

We identify imidazole as a pH-activated linker for forming stable single molecule-gold junctions with several distinct configurations and reproducible electrical characteristics. Using a scanning tunneling microscope break junction (STMBJ) technique, we find multiple robust conductance signatures at integer multiples of 1.9 × 10^-2G₀ and 1.2 × 10^-4G₀ and determine that this molecule bridges the electrodes in its deprotonated form through the nitrogen atoms in basic conditions only, with several molecules able to bind in parallel and in series. The elongation these junctions can sustain is longer than the length of the molecule, suggesting that plastic deformation of gold electrodes occurs during stretching. Density functional theory calculations confirm that the imidazolate-linked junctions exhibit bond strengths of ∼2 eV, which can allow for plastic extraction of gold atoms. On the basis of these results, we hypothesize that lower conductance peaks correspond to chains of repeating molecule-gold units that we form and measure in situ.