Protonation-Independent Charge Transport Across Diphenylamine Single-Molecule Junctions

Amines are one of the most ubiquitous functional groups in molecular junctions; however, the exact regulation of the charge transport through the protonation state of an amine group in the junction backbone remains elusive. We address this question here by designing a diphenylamine molecular backbone and experimentally investigating how protonation of the central amine group affects the charge transport. Our ultraviolet-visible spectroscopy measurements demonstrate the protonation reaction of the diphenylamine compound in the presence of either trifluoroacetic acid or HCl, and we observe a consistent trend of a modestly increased conductance for diphenylamine in the presence of acid, indicating that a protonated amine group in a diphenylamine backbone slightly enhances the electron conduction. We further investigate the charge transport across diphenylamine under a series of applied tip bias voltages between -0.9 to 0.9 V in an electrochemical environment in the absence and presence of acid for determining the frontier molecular orbital alignment with the Fermi level and the coupling coefficient between the molecule and the electrodes. Our finding shows that the highest occupied molecular orbital (HOMO) is the dominating transport channel of the diphenylamine junction, and a modest conductance increase is an outcome of the HOMO resonance energy moving closer to the Fermi level upon protonation of the amine.