Ordered self-assembled monolayers of 4,4′-biphenyldithiol on polycrystalline silver:: Suppression of multilayer formation by addition of tri-n-butylphosphine

Self-assembled monolayers (SAMs) of 4-biphenylthiol (BT) and 4,4'-biphenyldithiol (BDT) formed on polycrystalline silver substrates were characterized by surface plasmon resonance spectroscopy (SPR), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The RAMS measurements reveal that the molecules in a BT-SAM are oriented perpendicular to the surface whereas the adsorption of BDT results in the formation of multilayers due to the oxidative coupling of the terminal thiol groups forming disulfide-bridged species. The process of multilayer formation was monitored by SPR and RAIRS indicating that BDT initially forms a monolayer which exhibits a structure similar to the one found for BT and that disorder is introduced by the rapid oxidation of the exposed thiol groups resulting in the formation of a multilayer. The addition of tri-n-butylphosphine to the self-assembly solution of 4,4'-biphenyldithiol prevents the formation of multilayers and allows the generation of well-ordered monolayers of the dithiol in which the molecules adopt a standing-up orientation as observed for 4-biphenylthiol. The existence of free thiol groups at the surface of the dithiol SAM was proven by quantitative XPS measurements.