Improving single-molecule conductance by the quinoid structure of furan derivatives

Quinoid structures are considered to be conducive to the charge transport of organic molecules, but this hypothesis is rarely proven at single-molecule level. Herein, as a proof of concept, the single-molecule conductance of two furan-based isomers, 3,3’-bis(4-(methylthio)phenyl)-2,2’-bifuran (2,2’-SMPBF) and 4,4’-bis(4-(methylthio)phenyl)-3,3’-bifuran (3,3’-SMPBF), is investigated by the scanning tunneling microscopy break junction (STM-BJ) technique and theoretical simulation. 2,2’-SMPBF prefers to adopt a nearly planar conformation with intact alternating single and double bonds extended via 2,2’-bifuran moiety and therefore exhibits good π-conjugation and a prominent quinoid structure. However, the π-conjugation of 3,3’-SMPBF is interrupted due to ineffective cross-conjugation in the 3,3’-bifuran moiety, leading to the absence of a quinoid structure. 2,2’-SMPBF displays switchable multiple conductances induced by the interconversion between folded and unfolded conformations and an abnormal rebound of conductance along with the increases of electrode displacement, which is demonstrated to be caused by the quinoid structure in a nearly planar conformation during the stretching process. However, 3,3’-SMPBF without a quinoid structure in unfolded conformation exhibits extremely low conductance that cannot be captured in STM-BJ measurements. These results reveal the significant contribution of quinoid structure to molecular charge transport and provide valuable information on the structure-transport relationship for the design of efficient organic semiconductors.

Graphical abstract