Quantum interference dependence on molecular configurations for cross-conjugated systems in single-molecule junctions

We report a combined experimental and computational study of seven cross-conjugated enediyne derivatives functionalised with a pendant group (diphenyl, 9-fluorenyl, 9-thioxanthene or cyclohexyl) at the central alkene site, and with thiomethyl (SMe) or thioacetate, as protected thiol, (SAc) groups as anchors. Measurements of the conductance (G) and Seebeck coefficient (S) of gold|single-molecule|gold junctions were obtained using a modified scanning tunnelling microscope-break junction (STM-BJ) technique. It is shown that most of the molecules give multiple conductance plateaus ascribed to different molecular configurations inside the junction. The higher conductance plateaus are consistent with the aryl pendant units interacting with one of the gold electrodes, thereby circumventing transmission of electrons through the enediyne system; the lower conductance plateaus are consistent with anchoring of both of the terminal SMe or S units to the electrodes. Most of the compounds show a positive value of S in the range 3.7-12.7 mu V K-1 indicating electronic transport through the HOMO, while one of them presents a negative value of S (-6.2 mu V K-1) indicating a predominance of the LUMO in the electronic transport. Theoretical calculations using density functional theory show a destructive quantum interference (DQI) feature in the gap between the highest occupied and lowest unoccupied molecular orbitals (the HOMO-LUMO gap) for the lower conductance plateaus, supporting the trends observed in the experimental data.