Effect of Modified Thiophene Anchor on Molecule-Electrode Bonding

The anchor group of a molecule determines its binding characteristics with electrodes. It impacts the molecular conductance of the formed single molecule junctions and is of great importance to the field of molecular electronics. Thiophene unit is an emerging anchor ligand and shows the ability to bind with Au electrodes. As a building block in designing organic photoelectric materials, thiophene has a potential in expanding the variety of target molecules in single molecule electronics. In this work, we designed and synthesized three analogous pi-conjugated molecules, 1,4-di(thiophen-2-yl)benzene (BT -H), 1,4-bis(5 hexylthiophen-2-yl)benzene (BT -Hex) and 1,4-bis(5-chlorothiophen-2-yl)benzene (BT-Cl). These molecules have the same backbone, but different substituents (H, C6 and Cl atoms, respectively) at position 4 of both end-capped thiophenes. Enabled by thiophene anchors, these molecules can be readily incorporated into the nano gaps between electrodes to form molecular junctions. Charge transport properties of three types of single molecule junctions are explored using scanning tunneling microscopy based break junction (STM-BJ) technique and the influence of different substituents at thiophene on the molecule-electrode binding modes are comparatively studied. Two separate binding modes with a conductance discrepancy of more than an order of magnitude are observed for all three molecules, with a high conductance state (G(H)) corresponding to a Au-pi linked junction (Au electrode coupled with the thiophene pi orbital) and a low conductance state (G(L)) originating from a Au -S binding scheme. Interestingly, the values of the G(L) state for three molecules are greatly affected by the different substituents at thiophenes, yielding a conductance trend of G(BT-Hex) > G(BT-H) > G(BT-Cl). This can be explained by the electron affinity of different substituents, which shifts their highest occupied molecular orbital (HOMO) with respect to Au Fermi level and thus changes the energy barrier. In contrast, the G(H) state values of three molecules are not affected obviously by different substituents. We also statistically analyzed the formation rate of two binding modes for three molecules and found that the ratio between two binding modes can be changed with different addition of substituents. This work provides a useful method in modifying the binding properties of thiophene as an anchor group to gold and sheds light on a simple strategy in the design of anchor ligands.