A new carboxylic acid-terminated alkanethiol having bidentate character, 16-(3,5-bis(mercaptomethyl)phenoxy)hexadecanoic acid (BMPHA), was designed as an absorbate and protectant to form thermally stable carboxylic acid-terminated organic thin films on flat gold and nanoparticles, respectively. The structural features of the organic thin films derived from BMPHA were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and compared to those derived from mercaptohexadecanoic acid (MHA) and 16-(4-(mercaptomethyl)phenoxy)hexadecanoic acid (MMPHA). This study demonstrates that films derived from BMPHA are less densely packed than films derived from MHA and MMPHA. However, the results of solution-phase thermal desorption tests revealed that the carboxylic acid-terminated films generated from BMPHA exhibit an enhanced thermal stability compared to those generated from MHA and MMPHA. Furthermore, as a nanoparticle protectant, BMPHA can be used to stabilize large gold nanoparticles (~45 nm diameter) in solution, and BMPHA-protected gold nanoparticles exhibited a high thermal stability in solution thermolysis studies.

The tridentate adsorbates cyclohexane-cis,cis-1,3,5-triyltrimethanethiol (CyTSH) and (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-triyl)trimethanethiol (3C1CyTSH) were designed and synthesized. Thin films prepared by the adsorption of these molecules onto the surface of gold were characterized by ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy (XPS). CyTSH was found to generate multilayer thin films with many unbound thiol species and oxidized sulfur moieties (e.g., disulfides and sulfones). In contrast, 3C1CyTSH was found to generate monolayer films in which ~90% of the thiols were bound to the surface of gold (~10% unbound), and there were no oxidized sulfur species. In comparing CyTSH and 3C1CyTSH, the methyl groups of 3C1CyTSH impart rigidity to the structure, which significantly enhances the chemisorption of sulfur to the surface of gold. Ellipsometric measurements and analysis by XPS indicate that the thickness of the self-assembled monolayer formed from 3C1CyTSH is ~5 Å.

A series of 2-phenylpropane-1,3-dithiol derivatives with single (R1ArDT), double (R2ArDT), and triple (R3ArDT) octadecyloxy chains substituted at the 4-, 3,5-, and, 3,4,5-positions, respectively, on the aromatic ring were synthesized and used to form self-assembled monolayers (SAMs) on gold. Insight into the relationship between the surface chain and headgroup packing densities was investigated by varying the number of surface chains for the bidentate adsorbates in these monolayers. Characterization of the resulting SAMs using ellipsometry, X-ray photoelectron spectroscopy, polarization modulation infrared reflection-absorption spectroscopy, and contact angle goniometry revealed that the tailgroups become more comformationally ordered and more densely packed as the number of alkyl chains per adsorbate was increased. Conversely, the molecular packing density (i.e., number of molecules per unit area) decreased as the number of alkyl chains per adsorbate was increased. Of particular interest, the desorption profiles obtained in isooctane at 80 °C suggested that the bidentate adsorbate with the most densely packed alkyl chains, R3ArDT, was significantly more stable than the other SAMs, producing the following relative order for thermal stability for the dithiolate SAMs: R3ArDT > R2ArDT > R1ArDT.

Sum frequency generation (SFG) vibrational spectroscopy analyses were performed on self-assembled monolayers (SAMs) formed via the adsorption of 2-(4-(octadecyloxy)phenylethanethiol (R1ArMT), 2-(3,5-bis(octadecyloxy)phenylethanethiol (R2ArMT), and 2-(3,4,5-tris(octadecyloxy)phenyl-ethanethiol (R3ArMT) on gold. SFG spectra showed that the monolayers formed were closely packed and well ordered. It was determined, using orientation analysis, that as additional octadecyloxy chains were attached to the aromatic ring the apparent methyl group tilt increased and conformational order decreased. Aromatic C–H modes exhibited in the SFG spectra suggest the aromatic rings have a C2 axis along the surface normal. Model phenylethanethiols with different symmetries along the benzene ring were synthesized and used to identify vibrational modes and the surface orientation of the alkoxyphenylethanethiols. SAMs derived from R1ArMT exhibit aromatic C–H stretching modes due to the C 2v symmetry at the ring. In contrast, SAMs derived from R2ArMT and R3ArMT exhibit no aromatic C–H stretching modes due to their low Raman cross section.

This article describes the design, synthesis, and study of alkoxyphenylethanethiol-based adsorbates with one (R1ArMT), two (R2ArMT), and three (R3ArMT) pendant octadecyloxy chains substituted at the 4-, 3,5-, and 3,4,5-positions, respectively, of the phenylethanethiol group. These adsorbates are being developed for use in the preparation of compositionally versatile "mixed" self-assembled monolayer (SAM) coatings. The resultant SAMs were characterized by ellipsometry, contact angle goniometry, polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The studies revealed that R1ArMT generates a well-ordered monolayer film, while R2ArMT and R3ArMT generate monolayer films with diminished conformational order in which the degree of crystallinity decreases as follows: C18 ∼ R1ArMT > R3ArMT > R2ArMT. In addition, comparison of the molecular and chain packing densities of SAMs derived from these new adsorbates reveals that the R2ArMT and R3ArMT adsorbates give rise to SAMs with reduced chain tilt and smaller surface area per chain when compared to the SAMs derived from C18 and R1ArMT.