Adsorption of diisocyanides on gold

Four known diisocyanides, 1,6-diisocyanohexane (1), 1,12-diisocyanododecane (2), 1,4-phenylenediisocyanide (3), 4,4'-biphenyldiisocyanide (4), and two new diisocyanides, 4,4"-p-terphenyldiisocyanide (5) and 1,4-di(4-isocyanophenylacetelynyl)-2-ethylbenzene (6), were used to prepare self-assembled monolayers (SAMs) on gold. The SAMs were characterized by ellipsometry, by measurement of the advancing water droplet contact angle, and by reflection absorption infrared (RAIR) spectroscopy. SAMs prepared from the diisocyanides 1 and 2, which have a flexible alkyl chain between the two isocyanide end groups, were found to form monolayer films in which both isocyanide groups are bound to the surface. SAMs prepared from the diisocyanides 3-6, which have rigid aryl or phenylethynyl structures between the two isocyanide end groups, were found to bind to gold through one CN group. Analyses of the RAIR spectra of these SAMs are consistent with surface selection rules for adsorbates that are oriented with their long molecular axis normal to the surface. Measurements of advancing water droplet contact angles and ellipsometric film thicknesses of SAMs prepared from 3-6 further support a surface structure in which the diisocyanide molecules are oriented vertically with one CN group bound to the surface and the other remote from the surface.