Using Surface-Enhanced Raman Spectroscopy to Unravel the Wingtip-Dependent Orientation of N-Heterocyclic Carbenes on Gold Nanoparticles

Abstract

N-Heterocyclic carbenes (NHCs) are an attractive alternative to thiol ligands when forming self-assembled monolayers on noble-metal surfaces; however, relative to the well-studied thiol monolayers, comparatively little is known about the binding, orientation, and packing of NHC monolayers. Herein, we combine surface-enhanced Raman spectroscopy (SERS) and first-principles theory to investigate how the alkyl “wingtip” groups, i.e., those attached to the nitrogens of N-heterocyclic carbenes, affect the NHC orientation on gold nanoparticles. Consistent with previous literature, smaller wingtip groups lead to stable flat configurations; surprisingly, bulkier wingtips also have stable flat configurations likely due to the presence of an adatom. Comparison of experimental SERS results with the theoretically calculated spectra for flat and vertical configurations shows that we are simultaneously detecting both NHC configurations. In addition to providing information on the adsorbate geometry, this study highlights the extreme SERS enhancement of vibrational modes perpendicular to the surface.

On the use of NMR distance measurements for assessing surface site homogeneity

“…We stress that, at least in the case of NMR, the primary challenge is in the differentiation of highly similar structures, such as conformations, or changes in podality. These differences can be far clearer when using methods such as SERS, which has been shown to be highly sensitive to molecular orientation on metal nanoparticle surfaces, 17 and titration methods, which enable the independent measurement of the average podality. When structures differ in their bonding, or when the NMR of the metal center can be done directly, large changes in chemical shifts and quadrupolar coupling do enable the resolution of multiple sites with little difficulty.…”

“…7 We later expanded on this concept by applying surface-to-atom distance measurements, which can be modeled accurately using multispin simulations. 8,9 The latter strategy has been applied to a growing number of supported metal complexes and has made use of surface 17 O, 27 Al, 29 Si, 133 Cs, and 207 Pb nuclei. [8][9][10][11][12][13][14] Recently, Jabbour et al have applied the strategy introduced by Berruyer et al to study the surface structure of an Ir NHC complex.…”

“…16 In sites supported on amorphous surfaces, however, this assumption is difficult to justify. 11,15,17 We thus sought to determine whether solid-state NMR-based distance measurements could be used to resolve distinct conformers with overlapping resonances. Indications that this may be possible originate from the concept of inverting rotational-echo doubleresonance (REDOR) 18 data and "de-Pake-ing".…”

Computational Insights into the Adsorption of Ligands on Gold Nanosurfaces

“…We focus on the parameters affecting the molecule–surface interaction, i.e., the average distance of the molecule from the surface, its orientation once it is adsorbed on it, and the nature of the interaction, as mediated by the surrounding water molecules. Such a computational protocol may help the understanding of the adsorption process at the microscopic level and the in silico design of (bio)­sensors for detection purposes. − …”

Seeking a Au–C stretch on gold nanoparticles with 13C-labeled N-heterocyclic carbenes

“…19 Surface enhanced Raman spectroscopy of NHC-functionalized gold nanoparticles has rapidly developed since the discovery that gold complexes can be employed to append the NHCs to the AuNPs. 6,16,[20][21][22][23] Indeed, SERS measurements of NHCs on AuNPs have tracked post-synthetic modification on the nanoparticle surface 20 as well as monitoring ligand degradation in biological media. 21 Yet, despite the proven utility of SERS for characterizing NHCs on AuNPs, no SERS study of the Au-C bond for NHCs on AuNPs has been conducted.…”