Self-assembled monolayers of dithiols, diisocyanides, and isocyanothiols on gold: ‘chemically sticky’ surfaces for covalent attachment of metal clusters and studies of interfacial electron transfer

Abstract

Abstract The preparation of self-assembled monolayers (SAMs) of the double-ended dithiols 4,4−biphenyldithiol, and αα- p -xylidithiol, the double-ended diisocyanides 1,4-phenylenediisocyanide, 4,4′-biphenyldiisocyanide, 4,4′- p -terphenyldiisocyanide, 1,6-hexanediisocyanide, 1,12-dodecanediisocyanide, and 1,4-di(4-isocyano-phenylethynyl)-2-ethylbenzene, and the 4-sulfido phenyl isocyanide capped trinuclear nickel cluster 4-( μ 3 -iodo-tris(bis(diphenylphosphino)methane)-trinickel-(isocyano)phenylenesulfide by direct adsorption or by displacement of a pre-existing SAM of 1-octadecanethiol on gold is reported. The SAMs were characterized using reflection-absorption infrared spectroscopy (RAIR), optical ellipsometry, and advancing contact-angle ( θ a ) measurements. The substitution chemistry of SAMs was found to be irreversible. The dithiols and aryl diisocyanides were found to form SAMs with only one functional group attached to the surface. The SAMs of dithiols were used to covalently attach nanometer scale gold clusters to the exposed thiol surface of the SAM. Scanning tunneling microscopy (STM) was used to image these immobilized gold clusters. The diisocyanides have been used to covalently anchor trinuclear nickel clusters. The SAM of the 4-sulfido phenyl isocyanide capped trinuclear nickel cluster 4-( μ 3 -iodo-tris(bis(diphenylphosphino)methane)-trinickel-(isocyano)-phenylenesulfide, was studied by cyclic voltammetry. The electron acceptors methylviologen (MV 2+ ) and the methyl ester of cobaltocenium [MeCOOCpCp] + [PF 6 ] − were used to demonstrate rectification in the interfacial electron transfer from nickel cluster SAM modified gold electrodes to the electron acceptors.