Letter pubs.acs.org/JPCL
Sum Frequency Generation Spectrum of a Self-Assembled Monolayer Containing Two Different Methyl Group Orientations P. J. N. Kett,† M. T. L. Casford, and P. B. Davies* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. ABSTRACT: The theory of sum frequency generation (SFG) has recently been extended to monolayers where the SFG active functional group can adopt two different orientations [Kett, P. J. N.; Casford, M. T. L.; Davies, P. B. Mol. Phys. 2012, DOI: 10.1080/00268976.2012.711492]. This Letter compares the SFG spectra for the C−H stretching vibrations of the methyl group in a model test molecule, 2,4-dimethylbenzenethiol (DMBT), self-assembled on a gold substrate, with theoretical predictions. The spectrum in the PPP polarization combination and counterpropagating beam geometry is comprised of peaks for the two symmetric stretching bands (r+ resonances) and a dip for the asymmetric stretching mode (r− resonance). This spectrum agrees with theory for the case where the methyl groups are assumed to orientate in opposite directions with respect to the surface normal. Calculated topographical plots show that the r resonances appear with the same phases as the experimental spectrum when the tilt angle of the DMBT lies between 0 and 48° and the twist angle between −22 and 30°. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis
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spectra corresponding to the methyl stretching modes were peaks or dips depended on the particular combination of the two methyl group tilt angles. Furthermore, it was demonstrated that certain combinations of the tilt angles can result in one of the resonances being a peak while the other is a dip, a phenomenon that has previously only been observed for much thicker films.9,14 In this Letter, we describe the experimental spectra, recorded on a gold substrate, of a model monolayer molecule containing two methyl groups with a fixed angular divergence of 120° due to the molecule’s structure and compare this experimental spectrum with the above-mentioned theoretical predictions. To examine the validity of the predictions presented earlier,1 the molecule 2,4-dimethylbenzenethiol (DMBT), which has two methyl groups with different orientations, was selected (Figure 1). The SFG spectrum of a monolayer of DMBT on gold, formed after immersion in a 1 mM methanolic DMBT solution for 48 h, recorded with counter-propagating infrared
he technique of sum frequency generation (SFG) vibrational spectroscopy2−4 has been widely used to determine the orientation and conformation of molecules located at interfaces. It has proved to be particularly useful for studying Langmuir−Blodgett (LB) films of surfactants at the air/liquid interface,5−9 and monolayers and multilayers deposited by both the LB technique and solvent casting on dielectric or noble metal substrates.10−12 In many cases, these films have consisted of amphipathic molecules with one or two long aliphatic chains. SFG spectroscopy of these films in the C−H stretching region has provided quantitative information on the conformational structure of the hydrocarbon chain. Specifically, on metal surfaces, the symmetric and asymmetric C−H stretching modes of the chain-terminating methyl groups provide information on the polar orientation of the molecule with respect to the interface.13 The theory required to interpret the spectra arising from the terminal methyl groups when they all have a single unique orientation is well-established. However, the theory needs to be extended to molecules that have methyl groups with more than one orientation such as cholesterol, in which there are five methyl groups that have inherently different orientations from each other relative to the molecule as a whole. The development of a theoretical model to account for multiple methyl group orientations is required not only for understanding the spectra of molecules like cholesterol but more generally when employing SFG for studying biologically relevant films. Recently, we have extended the theory of SFG to consider the effect that multiple methyl group orientations have on SFG spectra. Specifically, by simulating spectra of monolayer films in which there were methyl groups present in two different orientations, it was shown that whether the features in the © 2012 American Chemical Society
Figure 1. Structure of 2,4-dimethylbenzenethiol (DMBT). Received: September 7, 2012 Accepted: October 22, 2012 Published: October 25, 2012 3276
dx.doi.org/10.1021/jz301363k | J. Phys. Chem. Lett. 2012, 3, 3276−3280
The Journal of Physical Chemistry Letters
Letter
Table 1. Parameters Derived from the Least-Squares Fit to the SFG Equation for the Spectrum Presented in Figure 2 (top spectrum)a peak center/cm−1
intensity
width/cm−1
phase
2852 (2854) 2924 (2825) 2947 (2951)
8 16 27
16 35 42
89° 89° −97°
a
Phase refers to the observed phase relative to the nonresonant background signal of the gold surface. Peak centers and values relate to Figure 2 (top spectrum), with bracketed values giving the average of all experimental runs. Peak centers are ±5 cm−1 from the average value.
monolayer on gold. As a result, the phases of the r+ and r− signals, and specifically the fact that one is a peak and the other a dip, cannot be attributed to film thickness interference effects (to achieve the phases of the r resonances described above would require a minimum film thickness of ∼50 nm,14,16 which is significantly thicker than a monolayer film). The phase difference between the r+ and r− resonances in the observed spectra can therefore be attributed to the presence of methyl groups in the film that have different orientations. For methyl groups with a single defined orientation, both r resonances on a gold substrate would either be peaks (methyl group orientated away from the substrate), as shown for ODT in Figure 2, or both dips (orientated toward the substrate). Using the theory developed earlier1 and noting the observed phases of the features in the SFG spectrum in Figure 2 (i.e., whether they were peaks or dips), possible combinations of the tilt and twist angles of DMBT giving rise to the phase of the observed spectrum can be deduced. The general expression to predict whether a feature in a SFG spectrum is a peak or a dip is given by1 [χR(2) (θa) + χR(2) (θ b)] sin ε > 0 0
0
0 0 0 90°, the r+ resonance will be a dip, unless the twist angle is 0
