Electrochemically Driven Assembly of Mixed Dithiol Bilayers via Sulfur Dimers

We report on the electroformation of mixed bilayers of rigid (1,4-benzenedimethanethiol) and flexible (1,6-hexanedithiol) dithiols on Au(111) from alkaline aqueous solution. These bilayers are formed by first electrodepositing a monolayer of vertically aligned dithiols. The unreacted dithiols are then oxidatively dimerized with the other dithiols, and a bilayer is formed. ACIS (ac impedance spectroscopy) measurements indicate that the monolayer of the rigid dithiol is a better substrate for dimerization than the flexible dithiol. ACIS also reveals that the electroformation of a bilayer is more complete, and the bilayer has fewer defects than one obtained through chemical incubation. Infrared reflection−absorption spectroscopy provides proof for mixed-bilayer formation of the alkanedithiol with the benzenedimethanethiol having its methylene groups deuterated. The mechanism for bilayer formation via the oxidative dimerization of thiols is supported by high-resolution electron energy loss spectra of bilayers, which show a S−S stretch at 515 cm - 1 . Our results show that this electrochemical approach is a viable methodology for the formation of complex organic interfaces.