Engineering the transport orbital in a molecular nanoscale junction

Metal-molecule-metal junctions are of fundamental interest for integration in electronic devices as molecules are the smallest objects whose properties can be tailored by chemical design¹. Tuning charge transport through molecules requires engineering of the molecular orbital whose energy lie closest to the Fermi level of the contacts. This can be achieved through the introduction of proper side-groups on the molecular backbone².  In this work we study the charge transport properties of a series of tolanes using the Mechanically Controllable Break Junction (MCBJ) technique, paired with Density Functional Theory (DFT) calculations. The molecules have been substituted either with electron-withdrawing nitro-groups, or electron-donating dimethylamino-groups. Effects of the anchoring were also investigated by using two different types of binding groups: thioacetate- and methylsulfanyl-, which form covalent and dative bonds to gold respectively. The investigated side groups are able to shift the energy levels and shrink the HOMO-LUMO gap as expected, as shown both by DFT calculations, Cyclic Voltammetry and UV/Vis spectroscopy. However, MCBJ measurements do not reflect these changes.  This effect can be explained with DFT calculations by considering that the Fermi energy of the contacts can take on a wide range of values.

1. Gehring, P., Thijssen, J. M. & van der Zant, H. S. J. Single-molecule quantum-transport phenomena in break junctions. Nat. Rev. Phys. 1, 381–396 (2019).

2. Venkataraman, L. et al. Electronics and chemistry: Varying single-molecule junction conductance using chemical substituents. Nano Lett. 7, 502–506 (2007).