Characterization of SAMs Derived from Octadecyloxyphenylethanethiols by Sum Frequency Generation

Abstract

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.

Friction Modifier Additives

“…Adapted from [67] A variety of reflection-based vibrational spectroscopies including IR, Raman and sum frequency generation (SFG) have been applied to study adsorbed or reacted surfactant films on surfaces [84,85]. Most work has focused simply on exploring the structure of these films on immersed surfaces, to map features such as molecular tilt or the linearity of the alkyl chain [86], but a few studies have been applied to observe film behaviour within rubbing contacts [87][88]. Beattie and co-workers studied a Langmuir-Blodgett film of zinc arachidate in sliding sapphire/quartz contacts and found that film order was maintained during sliding but some film was transferred from the monolayer to the opposing surface [87].…”

Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic Generation, and Sum-Frequency Generation

“…Although the uncertainty in the fitting is quite large, the d + /r + ratio is always very small, even for times as short as 2 min, indicating a nearly all-trans conformation for the SAM in all cases. In addition, the molecular orientation of the terminal methyl group of the SAMs can be obtained quantitatively from the ratio of the amplitudes for the symmetric and asymmetric CH 3 stretches (r + /r − or B 2 /B 5 in our fittings) in the spectrum with PPP polarization, as described in detail more recently by Jacob et al 41 Figure 5 also shows this amplitude ratio as a function of the adsorption time (black dots). Again, the uncertainty in the fitting is too large for a quantitative interpretation, but the large value for the r + /r − ratio indicates The Journal of Physical Chemistry C that the average methyl orientation is close to the expected value of 27°6 1 and seems to be nearly constant for all adsorption times.…”

“…In contrast, SFG spectroscopy is a powerful technique to characterize the orientation and conformation of thiol molecules on SAMs, thus providing information at the molecular level, either averaged over large regions (∼milli-meters) or with micrometer resolution, in the case of SFG microscopy. 41 Nevertheless, we showed that its sensitivity to defects in the monolayer is much smaller than in STM, and it is also a more demanding technique in terms of instrumentation and data analysis. The related SHG technique has the advantage of being of simple implementation and capable of probing molecular adsorption in situ and with good temporal resolution (a few seconds), but here we showed that it probes the electronic response of the substrate to molecular adsorption, making it difficult to relate to the SAM molecular structure.…”

“…The slightly lower value for the sample prepared in 2 min is probably because the monolayer is less homogeneous in terms of chain orientation. 41 Although the quantitative analysis of these SFG spectra is useful, one important qualitative feature can be readily seen in Figure 4, with striking consequences: the amplitude of the r + mode at ∼2880 cm −1 (see inset of Figure 5) is always quite large, even for 2 min adsorption time. In fact, the SFG spectra for all samples are similar, with only minor quantitative changes, as described in the previous paragraph.…”

Assessing the Gold Standard: The Complex Vibrational Nonlinear Susceptibility of Metals

“…The nonresonant response from gold provides the background signal that interferes constructively with the ODT methyl response to provide the upwards-pointing peaks, and destructively with the response from the octadecanol methyl group resulting in downwards-pointing peaks. The versatility of this method is evidenced by the range of molecules and interfaces it has since been applied to, providing phase information from polymer-metal interfaces, 18 lipid bilayers and cell structures, [19][20][21] monolayers, [22][23][24][25][26] and immobilized proteins. 17,27 The above method for phase detection allows the relative phase of the resonant modes to be determined.…”