Preparation, Characterization, and Heck Reaction of Multidentate Thiolate Films on Gold Surfaces

Abstract

Multidentate thiolate films were prepared by solution phase deposition of dendritic oligothiols on gold surfaces. The dendritic molecules contain three (G0) and nine (G1) thiol groups at the periphery and a functional group (bromophenyl) at the corn. The films were characterized by contact-angle goniometry, ellipsometry, polarization-modulation infrared reflection−absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). Results of the XPS measurement indicate the presence of ∼20% of unbound S in the G0 films while ∼28% in the G1 films. Some of the unreacted thiol groups may be located at the upper part of the films and some of the bromophenyl groups at the lower part of the films. Three G0 molecules (containing three SH groups) cover about the same area on the substrate as one G1 molecule (containing nine SH groups), and both of the films exhibit similar thicknesses, indicating the flattening of the dendrons to maximize the bonding of SH groups to the gold surface. The G1 films exhibit higher stability than the G0 films, and both of them are much more stable than octadecanethiolate monolayers on gold in hot solvents, attributed to the formation of multiple S−Au bonds per dendron unit. The high stability of the films allows for the palladium-catalyzed Heck reaction on thiolate films. Treatment of both the G0 and G1 films with 4-fluorostyrene under Heck reaction conditions led to the disappearance of about 76 and 68% of the Br atoms among which about 60−70% were replaced by the 4-fluorostyryl groups, as shown by XPS. Prolonging the reaction time did not consume the remaining Br atoms, probably due to unfavorable orientation of the bromophenyl groups and/or the presence of adjacent thiol groups that may deactivate the catalyst in the film assembly.