<i>In Situ</i> Electrochemical Tip-Enhanced Raman Spectroscopy with a Chemically Modified Tip

Chemically modified tips in scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to improve the imaging resolution or provide richer chemical information, mostly in ultrahigh vacuum (UHV) environments. Tip-enhanced Raman spectroscopy (TERS) is a nanoscale spectroscopic technique that already provides chemical information and can provide subnanometer spatial resolution. Chemical modification of TERS tips has mainly been focused on increasing their lifetimes for ambient and <i>in situ</i> experiments. Under UHV conditions, chemical functionalization has recently been carried out to increase the amount of chemical information provided by TERS. However, this strategy has not yet been extended to <i>in situ</i> electrochemical (EC)-TERS studies. The independent control of the tip and sample potentials offered by EC-STM allows us to prove the <i>in situ</i> functionalization of a tip in EC-STM-TERS. Additionally, the Raman response of chemically modified TERS tips can be switched on and off at will, which makes EC-STM-TERS an ideal platform for the development of <i>in situ</i> chemical probes on the nanoscale

Electrochemical STM Tip-Enhanced Raman Spectroscopy Study of Electron Transfer Reactions of Covalently Tethered Chromophores on Au(111)

The ability to study electron transfer reactions at the solid–liquid interface with nanometer resolution has the potential to critically improve our understanding of electrocatalytic processes. However, few techniques are capable of studying electrode surfaces <i>in situ</i> at the nanoscale. We study the redox reactions of Nile Blue (NB) covalently tethered to an Au(111) electrode using <i>in situ</i> tip-enhanced Raman spectroscopy (TERS) and show that TERS amplitude decreases reversibly as NB is reduced. The potential dependent TERS intensity allows us to associate an electrochemical wave with the loss of electronic resonance of NB and another with the peak of fluorescence of tethered NB, which we tentatively attribute to the disassembly of on-surface NB aggregates. The study of the electrochemical activity of immobile adsorbates at the solid–liquid interface with TERS is an essential step toward the realization of <i>in situ</i> spectroscopic mapping at the nanoscale

Isolating a Reaction Intermediate in the Hydrogenation of 2,2,2-Trifluoroacetophenone on Pt(111)

The isolation and identification of surface intermediates is of the utmost importance for the elucidation of mechanisms and selectivity patterns in heterogeneous catalysis. However, the metastable nature of reaction intermediates makes their detection and differentiation from other species challenging. This work reports a combined variable temperature scanning tunneling microscopy (VT-STM) and van der Waals-corrected density functional theory (opt88-vdW DFT) study showing that a hydroxy intermediate (hy-TFAP) formed in the hydrogenation of 2,2,2-tri­fluoro­aceto­phenone (TFAP) is trapped by parent TFAP to form a H-bonded bimolecular TFAP/hy-TFAP structure. The facile formation of the hydroxy intermediate, by residual hydrogen present in the ultrahigh vacuum chamber, was predicted based on a previous DFT study of the hydrogenation pathway for TFAP on Pt(111). The prediction is confirmed by comparison of calculated TFAP/TFAP and TFAP/hy-TFAP structures with STM images of bimolecular structures formed through TFAP adsorption and treatment at different temperatures

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111)

The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (<i>R</i>)-(+)-1-(1-naphthyl)­ethylamine, ((<i>R</i>)-NEA), as the modifier. On the nonmodified surface, introduction of H₂ at a background pressure of ∼1 × 10^–6 mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H₂ to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst

Scanning Tunneling Microscopy Measurements of the Full Cycle of a Heterogeneous Asymmetric Hydrogenation Reaction on Chirally Modified Pt(111)

The hydrogenation of a prochiral substrate, 2,2,2-trifluoroacetophenone (TFAP), on Pt(111) was studied using room-temperature scanning tunneling microscopy (STM) measurements. The experiments were carried out both on a clean surface and on a chirally modified surface, using chemisorbed (<i>R</i>)-(+)-1-(1-naphthyl)­ethylamine, ((<i>R</i>)-NEA), as the modifier. On the nonmodified surface, introduction of H₂ at a background pressure of ∼1 × 10^–6 mbar leads to the rapid break-up of TFAP dimer structures followed by the gradual removal of all TFAP-related images. During the latter step, some monomers display an extra protrusion compared to TFAP in dimer structures. They are attributed to a half-hydrogenated intermediate. The introduction of H₂ to a mixture of (R)-NEA and TFAP on Pt(111) leads to the removal of TFAP without any change in the population of the modifier, as required for an efficient chirally modified catalyst

Aminolactone Chiral Modifiers for Heterogeneous Asymmetric Hydrogenation: Corrected Structure of Pantoyl-Naphthylethylamine, In-Situ Hydrogenolysis, and Scanning Tunneling Microscopy Observation of Supramolecular Aminolactone/Substrate Assemblies on Pt(111)

As established by Baiker and co-workers, pantoyl-naphthylethylamine (PNEA) is an efficient synthetic chiral modifier for the asymmetric hydrogenation of ketopantolactone (KPL) to pantolactone on supported Pt catalysts. We report a scanning tunneling microscopy (STM) study of PNEA and PNEA-derived aminolactone species on Pt(111) and a reassignment of the relative stereochemistry of the modifier. Robust organic chemistry methods were used to establish that the structure of PNEA is <i>R</i>,<i>S</i> rather than <i>R</i>,<i>R</i>. The dissociative chemisorption of a fraction of PNEA adsorbed on Pt(111) yields two fragments that we attribute to a process involving C–N bond scission. We show that C–N bond scission occurs under hydrogenation conditions on PNEA-modified Pt/Al₂O₃ catalysts, forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF). STM measurements on (<i>S</i>)-AF and 2,2,2-trifluoroacetophenone coadsorbed on Pt(111) show the formation of isolated 1:1 complexes. In contrast, measurements on coadsorbed (<i>S</i>)-AF and KPL show fluxional supramolecular AF/KPL assemblies. The possibility that such assemblies contribute to the overall enantioselectivity observed for PNEA-modified Pt catalysts is discussed

Aminolactone Chiral Modifiers for Heterogeneous Asymmetric Hydrogenation: Corrected Structure of Pantoyl-Naphthylethylamine, In-Situ Hydrogenolysis, and Scanning Tunneling Microscopy Observation of Supramolecular Aminolactone/Substrate Assemblies on Pt(111)

As established by Baiker and co-workers, pantoyl-naphthylethylamine (PNEA) is an efficient synthetic chiral modifier for the asymmetric hydrogenation of ketopantolactone (KPL) to pantolactone on supported Pt catalysts. We report a scanning tunneling microscopy (STM) study of PNEA and PNEA-derived aminolactone species on Pt(111) and a reassignment of the relative stereochemistry of the modifier. Robust organic chemistry methods were used to establish that the structure of PNEA is <i>R</i>,<i>S</i> rather than <i>R</i>,<i>R</i>. The dissociative chemisorption of a fraction of PNEA adsorbed on Pt(111) yields two fragments that we attribute to a process involving C–N bond scission. We show that C–N bond scission occurs under hydrogenation conditions on PNEA-modified Pt/Al₂O₃ catalysts, forming the aminolactone amino-4,4-dimethyldihydrofuran-2-one (AF). STM measurements on (<i>S</i>)-AF and 2,2,2-trifluoroacetophenone coadsorbed on Pt(111) show the formation of isolated 1:1 complexes. In contrast, measurements on coadsorbed (<i>S</i>)-AF and KPL show fluxional supramolecular AF/KPL assemblies. The possibility that such assemblies contribute to the overall enantioselectivity observed for PNEA-modified Pt catalysts is discussed

Stereodirection of an α‑Ketoester at Sub-molecular Sites on Chirally Modified Pt(111): Heterogeneous Asymmetric Catalysis

Chirally modified Pt catalysts are used in the heterogeneous asymmetric hydrogenation of α-ketoesters. Stereoinduction is believed to occur through the formation of chemisorbed modifier–substrate complexes. In this study, the formation of diastereomeric complexes by coadsorbed methyl 3,3,3-trifluoropyruvate, MTFP, and (<i>R</i>)-(+)-1-(1-naphthyl)­ethylamine, (<i>R</i>)-NEA, on Pt(111) was studied using scanning tunneling microscopy and density functional theory methods. Individual complexes were imaged with sub-molecular resolution at 260 K and at room temperature. The calculations find that the most stable complex isolated in room-temperature experiments is formed by the minority rotamer of (<i>R</i>)-NEA and pro-S MTFP. The stereodirecting forces in this complex are identified as a combination of site-specific chemisorption of MTFP and multiple non-covalent attractive interactions between the carbonyl groups of MTFP and the amine and aromatic groups of (<i>R</i>)-NEA

Conformational Contrast of Surface-Mediated Molecular Switches Yields Ångstrom-Scale Spatial Resolution in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) combines the ability of scanning probe microscopy (SPM) to resolve atomic-scale surface features with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). Here, we report additional insights into the nature of the conformational dynamics of a free-base porphyrin at room temperature adsorbed on a metal surface. We have interrogated the conformational switch between two metastable surface-mediated isomers of meso-tetrakis­(3,5-ditertiarybutylphenyl)-porphyrin (H₂TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H₂TBPP-Cu­(111) system is easily overcome, and a 2.6 Å lateral resolution by simultaneous TERS and STM analysis is achieved under ultrahigh vacuum (UHV) conditions. This work demonstrates the first UHV-TERS on Cu(111) and shows TERS can unambiguously distinguish the conformational differences between neighboring molecules with Ångstrom-scale spatial resolution, thereby establishing it as a leading method for the study of metal–adsorbate interactions

Conformational Contrast of Surface-Mediated Molecular Switches Yields Ångstrom-Scale Spatial Resolution in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) combines the ability of scanning probe microscopy (SPM) to resolve atomic-scale surface features with the single-molecule chemical sensitivity of surface-enhanced Raman spectroscopy (SERS). Here, we report additional insights into the nature of the conformational dynamics of a free-base porphyrin at room temperature adsorbed on a metal surface. We have interrogated the conformational switch between two metastable surface-mediated isomers of meso-tetrakis­(3,5-ditertiarybutylphenyl)-porphyrin (H₂TBPP) on a Cu(111) surface. At room temperature, the barrier between the porphyrin ring buckled up/down conformations of the H₂TBPP-Cu­(111) system is easily overcome, and a 2.6 Å lateral resolution by simultaneous TERS and STM analysis is achieved under ultrahigh vacuum (UHV) conditions. This work demonstrates the first UHV-TERS on Cu(111) and shows TERS can unambiguously distinguish the conformational differences between neighboring molecules with Ångstrom-scale spatial resolution, thereby establishing it as a leading method for the study of metal–adsorbate interactions