Unraveling the dynamics and structure of functionalized self-assembled monolayers on gold using 2D IR spectroscopy and MD simulations

Abstract

Functionalized self-assembled monolayers (SAMs) are the focus of ongoing investigations because they can be chemically tuned to control their structure and dynamics for a wide variety of applications, including electrochemistry, catalysis, and as models of biological interfaces. Here we combine reflection 2D infrared vibrational echo spectroscopy (R-2D IR) and molecular dynamics simulations to determine the relationship between the structures of functionalized alkanethiol SAMs on gold surfaces and their underlying molecular motions on timescales of tens to hundreds of picoseconds. We find that at higher head group density, the monolayers have more disorder in the alkyl chain packing and faster dynamics. The dynamics of alkanethiol SAMs on gold are much slower than the dynamics of alkylsiloxane SAMs on silica. Using the simulations, we assess how the different molecular motions of the alkyl chain monolayers give rise to the dynamics observed in the experiments.self-assembled monolayer | dynamics | 2D IR spectroscopy | MD simulation S elf-assembled monolayers (SAMs) on planar metal surfaces enable the tailoring of interfacial properties by functionalization of the alkyl chains. SAMs formed by alkanethiol chains on gold surfaces are of particular interest due to the ordered packing of the chains, chemical stability, and facile methods of preparation, as well as the diverse array of chemical functionalization that can be added (1). The properties of SAMs on gold have led to applications including electrochemical devices (2), surface patterning (3), model biological surfaces (4), and heterogeneous catalysis (5). In many of these applications, the interfacial properties of the monolayer are determined largely by the particular head group linked at the terminal site of the alkyl chain.The structure of SAMs on gold has been well characterized by scanning probe microscopy (6), helium diffraction (7), X-ray photoelectron spectroscopy (8), sum-frequency generation spectroscopy (SFG) (9), and linear infrared spectroscopy (10). However, to determine how the physical and chemical properties of SAMs are related to their microscopic dynamics and structure and the influence of head groups present in most applications, fast time-resolved experimental techniques, with sufficient selectivity and sensitivity, are required to measure the structural dynamics of a monolayer of molecules on the appropriate picosecond (ps) timescale.Two-dimensional infrared vibrational echo spectroscopy (2D IR) provides the necessary observables by measuring spectral diffusion, i.e., the time-dependent evolution of the probe vibrational frequency in response to structural fluctuations of the chemical environment (11-13). To use 2D IR to investigate monolayer dynamics requires selectivity for the interfacial region. One method to achieve this is to combine 2D IR spectroscopy with SFG (14, 15), which requires a vibrational mode that has both a large IR transition dipole and a large Raman cross-section. However, for a monolayer functionalized with the vibra...

Plasmonic Substrates Do Not Promote Vibrational Energy Transfer at Solid–Liquid Interfaces

“…49,[67][68][69][70] In this context, particularly purposely tailored nanostructures such as nano-antennae or nano-slits with plasmon resonances located directly in the mid-IR spectral range have been shown to dramatically enhance IR absorbances. 67,[69][70][71][72][73][74] In that case, the quasi-static approximation, which we used in the derivation of Among the previous 2D IR investigations of Re-carbonyl complexes on various types of surfaces, 18,20,24,25,[38][39][40][41][42][43][44][45] the studies by Xiong et al 42,43 are the most closely related to our present report. That works has specifically looked at substrate-adsorbate interactions between an immobilized Re-complex and an underlying Au surface, suggesting that dipole/image-dipole interactions cause severe differences in lineshapes on the same molecule.…”

Watching Proteins Wiggle: Mapping Structures with Two-Dimensional Infrared Spectroscopy

“…288–291 The sensitivity of 2D IR spectroscopy is also improving via technological and scientific advances. 292,293 Surface-enhanced 2D IR has been recently demonstrated using an ATR optical geometry. 294–296 Additionally, the combination of 2D IR with plasmonic nanostructures 297,298 promises to expand the technique’s impact by probing even smaller sample volumes and opening the door to vibrational modes with weaker oscillator strengths.…”

Ultrafast Vibrational Energy Transfer in Catalytic Monolayers at Solid–Liquid Interfaces

“…20 Since we however do not observe any direct cross peak, we assume that Eq. 3 overestimates the coupling, which is counter-balanced by the fact that cross-relaxation can proceed to more than one coupling partner on the surface (the model Despite significant efforts reported in literature 30,[47][48][49] , intermolecular couplings of surface-bound molecules have only rarely been observed 38 , and the estimates given above help to understand why that is so. In the present case, we can observe cross peaks with good signal-to-noise ratio, but this is since we have the very strong transition dipole of the symmetric stretch vibration of a metal-carbonyl together with (presumably) a close to optimal packing on the surface.…”

“…Particularly for potentially catalytically-active systems, the ultrafast intramolecular vibrational dynamics of Re-carbonyl complexes in electronic ground and excited states have been investigated in detail before, mainly for bulk solution samples, but recently also for immobilized systems under a variety of experimental conditions. [37][38][39]45,47,48,50,52,[61][62][63][64][65] However, intermolecular dynamics are largely unexplored to date, since femtosecond multi-dimensional vibrational spectroscopy on surfaces became available only recently. 17,47,48,66 Nonetheless, it has been proposed that if the vibrational states of two molecules are coupled, the electronic states will be as well, which in turn may have a decisive impact on the performance of working devices.…”