Self-Assembled Monolayers As Models For Silica
The reaction of hydroquinone and 1,12-dibromododecane affords 4-(12-bromo-dodecyloxy)phenol (4, 7% yield). The alkyl bromide (4) was converted to the corresponding thiol with thiourea to afford 4-(12-mercaptododecyloxy)phenol (1) in 52% yield. The reaction of t-butyllithium with 4-bromoanisole followed by reaction with 1-12-dibromododecane affords a mixture of 4-bromoanisole, 1,12-dibromododecane ,and 4-(12-bromododecyl)-anisole (6). Silica gel chromatography resulted in an inseperable mixture of 4-bromoanisole and (6). Reaction of the mixture with BBr3 afforded 4-(12-bromododecyl)phenol (7) in 34% yield. The alkyl bromide (7) was converted to the corresponding thiol with thiourea to afford 4-(12-mercaptododecyl)phenol (2) in 9% yield. Reduction of 16-mercaptohexadecanoic acid with BH3âhTHF afforded 16-mercaptohexadecanol (3) in 53% yield. All new compounds were characterized by 1H NMR, 13C NMR, transmission IR, HRMS, and, where possible, elemental microanalysis.
Self-assembled monolayers (SAMs) on gold were prepared using thiols 1, 2, and 3. SAMs were characterized using reflectance-absorbance infrared spectroscopy (RAIRS). Diagnostic vibrational modes were assigned by comparing RAIRS spectra to normal mode frequencies and intensities calculated using DFT methods at the 6-31G* level using commercial software. Water droplet goiniometry found contact angles of 52o, 53o, and 64o for SAMs prepared from 1, 2, and 3, respectively. SAMs of 1 and 2 were found to be hydrophilic. When SAMs prepared from 1, 2, and 3 were silylated with phenyldimethylchlorosilane, the resulting contact angles were 78o, 74o, and 75o respectively. A significant increase in contact angles for silylated SAMs of 1 and 2 indicated facile silanization of the surface hydroxides. RAIRS spectra were also obtained for the functionalized SAMs. Silylated SAMs prepared from 1, 2, and 3 are currently under investigation as models for silica-immobilized metallocene olefin polymerization catalysts.