Understanding the mechanism of catalytic hydrogenation at the local environment requires chemical and topographic information involving catalytic sites, active hydrogen species and their spatial distribution. Here, tip-enhanced Raman spectroscopy (TERS) was employed to study the catalytic hydrogenation of chloro-nitrobenzenethiol on a welldefined Pd(sub-monolayer)/Au(111) bimetallic catalyst (p H2 =1.5 bar, 298 K), where the surface topography and chemical fingerprint information were simultaneously mapped with nanoscale resolution (~10 nm). TERS imaging of the surface after catalytic hydrogenation confirms that the reaction occurs beyond the location of Pd sites. The results demonstrate that hydrogen spillover accelerates hydrogenation at the Au sites as far as 20 nm from the bimetallic Pd/Au boundary. Density functional theory was used to elucidate the thermodynamics of interfacial hydrogen transfers. We demonstrate TERS to be a powerful analytical tool that provides a unique approach to spatially investigate the local structure-reactivity relationship in catalysis.

An understanding of the photoisomerization mechanism of molecules bound to a metal surface at the molecular scale is required for designing photoswitches at surfaces. It has remained a challenge to correlate the surface structure and isomerization of photoswitches at ambient conditions. Herein, the photoisomerization of a self-assembled monolayer of azobenzene-thiol molecules on a Au surface was investigated using scanning tunneling microscopy and tip-enhanced Raman spectroscopy. The unique signature of the cis isomer at 1525 cm observed in tip-enhanced Raman spectra was clearly distinct from the trans isomer. Furthermore, tip-enhanced Raman images of azobenzene thiols after ultraviolet and blue light irradiation are shown with nanoscale spatial resolution, demonstrating a reversible conformational change. Interestingly, the cis isomers of azobenzene-thiol molecules were preferentially observed at Au grain edges, which is confirmed by density functional theory.

Photoswitches can be employed for various purposes, with the half-life being a crucial parameter to optimize for the desired application. The switching of a photochromic hydrazone functionalized with a C6 alkyl thiolate spacer (C6 HAT) was characterized on a number of metal surfaces. C6 HAT exhibits a half-life of 789 years in solution. Tip-enhanced Raman spectroscopy (TERS) was used to study the photoisomerization of the C6 HAT self-assembled monolayers (SAM) on Au, Ag and Cu surfaces. The unique spectroscopic signature of the E isomer at 1580 and 1730 cm-1 in TER spectra allowed for its discrimination from the Z isomer. It was found that C6 HAT switches on Au and Cu surfaces when irradiated with 415 nm, however it cannot isomerize on Ag surfaces, unless higher energy light is used. Based on this finding, and supported by density functional theory calculations, we propose a substrate-mediated photoisomerization mechanism to explain the behavior of C6 HAT on these different metal surfaces. This insight into the hydrazone's switching mechanism on metal surfaces will contribute to the further exploitation of this new family photochromic compounds on metal surfaces. Finally, although we found that the thermal isomerization rate of C6 HAT drastically increases on metal surfaces, the thermal half-life is still 6.9 days on gold, which is longer than the majority of azobenzene-based systems. Figure 1. A. Light induced Z/E isomerization of C6 HAT; B. UV-vis spectra of C6 HAT in toluene (1x 10-5 M) before (solid line) and after irradiation with 410 nm light (dashed line), followed by irradiation with 340 nm light (dotted line).

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Metal–organic coordination structures at interfaces play an essential role in many biological and chemical systems. Understanding the molecular specificity, orientation, and spatial distribution of the coordination complexes at the nanometer scale is of great importance for effective molecular engineering of nanostructures and fabrication of functional devices with controllable properties. However, fundamental properties of such coordination systems are still rarely studied directly. In this work, we present a spectroscopic approach on the basis of tip-enhanced Raman spectroscopy (TERS) to investigate cobalt­(II) tetraphenyl-porphyrine coordination species on the scale of a single molecule under ambient conditions. Coordination species anchored on gold surfaces modified with pyridine thiol self-assembled monolayers can be spectroscopically distinguished and mapped with ca. 2 nm resolution. In addition, in combination with density functional theory simulations, the adsorption configuration and molecular orientation of the coordination complexes are also revealed using TERS imaging.