Dynamics of Functionalized Surface Molecular Monolayers Studied with Ultrafast Infrared Vibrational Spectroscopy

Abstract

The structural dynamics of thin films consisting of tricarbonyl (1,10-phenanthroline)rhenium chloride (RePhen(CO)3Cl) linked to an alkyl silane monolayer through a triazole linker synthesized on silica-on-calcium-fluoride substrates are investigated using ultrafast infrared (IR) techniques. Ultrafast 2D IR vibrational echo experiments and polarization selective heterodyne detected transient grating (HDTG) measurements, as well as polarization dependent FT-IR and AFM experiments are employed to study the samples. The vibrational echo experiments measure spectral diffusion, while the HDTG experiments measure the vibrational excited state population relaxation and investigate the vibrational transition dipole orientational anisotropy decay. To investigate the anticipated impact of vibrational excitation transfer, which can be caused by the high concentration of RePhen(CO)3Cl in the monolayer, a concentration dependence of the spectral diffusion is measured. To generate a range of concentrations, mixed monolayers consisting of both hydrogen terminated and triazole/RePhen(CO)3Cl terminated alkyl silanes are synthesized. It is found that the measured rate of spectral diffusion is independent of concentration, with all samples showing spectral diffusion of 37 ± 6 ps. To definitively test for vibrational excitation transfer, polarization selective HDTG experiments are conducted. Excitation transfer will cause anisotropy decay. Polarization resolved heterodyne detected transient grating spectroscopy is sensitive to anisotropy decay (depolarization) caused by excitation transfer and molecular reorientation. The HDTG experiments show no evidence of anisotropy decay on the appropriate time scale, demonstrating the absence of excitation transfer the RePhen(CO)3Cl. Therefore the influence of excitation transfer on spectral diffusion is inconsequential in these samples, and the vibrational echo measurements of spectral diffusion report solely on structural dynamics. A small amount of very fast (~2 ps time scale) anisotropy decay is observed. The decay is concentration independent, and is assigned to wobbling-in-a-cone orientational motions of the RePhen(CO)3Cl. Theoretical calculations reported previously for experiments on a single concentration of the same type of sample suggested the presence of some vibrational excitation transfer and excitation transfer induced spectral diffusion. Possible reasons for the experimentally observed lack of excitation transfer in these high concentration samples are discussed.

Short-Range Catalyst–Surface Interactions Revealed by Heterodyne Two-Dimensional Sum Frequency Generation Spectroscopy

“…In the 2D IR spectra of a similar Re complex measured by Fayer and co-workers, it was observed that spectral inhomogeneity is not sensitive to the chain length of linker groups between the Re complex and the solid surface. 30 Our results demonstrate that the CLSs of HD 2D spectra is an unambiguous way to reveal subtle interactions or dynamics changes when other parameters such as spectral peak positions or linewidths might not be sensitive to the changes because of spectral convolutions.…”

“…We rule out the possibility of intermolecular interactions, since it has been shown in previous studies that the coupling between molecules in a similar Re-complex monolayer is negligible. 30 Among these short range interactions, contributions to homogenous broadening that require the molecules to be chemisorbed on gold surface can be ruled out as well, because the CO modes do not directly attach to the gold surface through either chemi-or physi-adsorptions. These interactions include the non-adiabatic charge transfer interaction 31 and the coupling to low frequency modes, 32,33 such as hindered rotations and translations.…”

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

“…16,[49][50][51][52][53] In this case, one molecule is initially excited but subsequently transfers its excitation energy to another oscillator that may have a different transition frequency. Such a mechanism may be particularly relevant in ML samples, where the vibrational probes are fairly close to each other.…”

“…Comparable investigations have been performed recently for water at water-air interfaces 51 or metal-carbonyl MLs on silica. 16,50 In the first case, spectral diffusion is dominated by vibrational energy transfer. 51 Conversely, energy-transfer between different molecules in metal-carbonyl MLs was excluded by means of spectral diffusion measurements for chemically diluted samples, i.e., MLs containing a mixture of molecules, only a fraction of which containing the vibrational chromophore.…”

Surface-enhanced, multi-dimensional attenuated total reflectance spectroscopy

“…For instance, dilution of functional groups on the surface increases the mutual distance between the CN groups and thus results in less favourable condition for, e.g. excitonic coupling 31 or excitation energy transfer 6,32 (EET) between CN groups. The expectation is that the spectral diffusion gets slower in MMLs as compared to MLs if such contributions are eliminated from the underlying dynamics.…”