Conduction Pathways of Quinoxalinyl Molecules in the STM-BJ-Fabricated Nanogap

Quinoxaline (Qx) terminated with two mercaptomethyl (-SMe) anchoring ligands demonstrated two conductance values when studied using the scanning tunneling microscope-based break-junction (STM-BJ) technique. Further research showed that the observed low and high conductances (termed <i>G</i>_L and <i>G</i>_H) resulted from two electron transfer pathways of different lengths with distinct molecular binding configurations. <i>G</i>_L arises from the two terminal -SMe groups attached to the Au electrodes, and <i>G</i>_H appears when one of the two Au-S linkages is replaced by an Au-N linkage where N of Qx is anchored to the electrode. This is one of the few instances where a single molecule can independently exhibit two different conductance states without an external stimulus, thereby offering a desired molecular prototype for developing conductance-dependent molecular electronics, such as molecular switches and other functional molecular devices.