Verification and Temperature-Dependent Rectification by HBQ, the Smallest Unimolecular Donor–Acceptor Rectifier

[Image: see text] Five years ago, rectification of electrical current was found in 4′-bromo-3,4-dicyano-2′,5′-dimethoxy-[1,1′-biphenyl]-2,5-dione (1), a hemibiquinone (which we will call either 1 or HBQ) that has a very small working length (1.1 nm). Monolayers of HBQ on Au(TS) were detected by “nanodozing” atomic force microscopy (AFM) and were contacted with two types of top electrodes: either cold Au or eutectic Ga–In. Here, we describe cyclic voltammetry of a self-assembled monolayer (SAM) of HBQ and its orientation on a gold substrate with angle-resolved X-ray photoelectron spectroscopy. New measurements of its rectification as a monolayer as a function of bias range and temperature confirm and prove that HBQ is truly the smallest donor–acceptor rectifier and provide some insight into the mechanism of rectification.