Ultrafast vibrational dynamics and spectroscopy of a siloxane self-assembled monolayer

Abstract

Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (∼1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.

Vibrational Dynamics of Interfacial Water by Free Induction Decay Sum Frequency Generation (FID-SFG) at the Al2O3(1120)/H2O Interface

“…To fit the normalized Vis-SFG spectrum at the Al 2 O 3 /H 2 O interface, the well-known equation that describes the SFG process by a broad-band infrared pulse and a visible pulse is used. 17…”

“…17,18 This limitation may be overcome if vSFG experiments are carried out in the time domain, i.e., free induction decay vSFG (FID-SFG), with the condition that time domain measurements have higher temporal resolution compared to the vibrational dynamics of the interface. Additionally, time domain vSFG is more sensitive to inhomogeneous broadenings simultaneously present in frequency domain vSFG spectra.…”

2D attenuated total reflectance infrared spectroscopy reveals ultrafast vibrational dynamics of organic monolayers at metal-liquid interfaces

“…These studies demonstrated the usefulness of multi-dimensional spectroscopy regarding the resolution of intermolecular interactions, 17,24 vibrational dephasing and relaxation, as well as spectral diffusion from samples at interfaces. 16,19,49 For instance, variations in spectral diffusion and vibrational relaxation dynamics have been discussed between octahedral rheniumcarbonyl photocatalysts at silica interfaces and in bulk solution, 16,49 revealing that spectral diffusion at surfaces is slower than in bulk solution. This effect was attributed to interactions of molecules within the monolayer and variations in structural evolution.…”

“…Regarding spectroscopic investigations of MLs at interfaces, central points of interest are represented by details of binding configurations or influences of the environmental interactions on molecular properties as compared to the case of isolated molecules in bulk solution. [16][17][18][19] This is because the structure of molecules in MLs is often influenced by intermolecular interactions within the densely packed aggregate. 20,21 In addition, solvent molecules directly at the interface are likely to exhibit differences in their properties such as hydrogen-bonding networks and dipolar orientation as compared to the bulk solvent.…”

“…be a useful tool to unravel changes in molecular structure and dynamics, which occur down to the femtosecond time scale. [16][17][18][19] In particular, multi-dimensional techniques 17,[24][25][26][27] have been developed in recent years as powerful methods to tackle the issues mentioned above. These techniques are sensitive to, e.g., structural heterogeneity and flexibility, energy transfer, or intermolecular interactions, signatures of which are challenging to extract unambiguously from conventional (1D) vibrational spectra.…”