Probing the deformation of [12]cycloparaphenylene molecular nanohoops adsorbed on metal surfaces by tip-enhanced Raman spectroscopy

Abstract

[n]Cycloparaphenylene ([n]CPP) molecules have attracted broad interests due to their unique properties resulting from the distorted and strained aromatic hoop structures. In this work, we apply sub-nanometer resolved tip-enhanced Raman spectroscopy (TERS) to investigate the adsorption configurations and structural deformations of [12]CPP molecules on metal substrates with different crystallographic orientations. The TERS spectra for a [12]CPP molecule adsorbed on the isotropic Cu(100) surface are found to be essentially the same over the whole nanohoop, indicating an alternately twisted structure that is similar to the [12]CPP molecule in free space. However, when the [12]CPP molecules are adsorbed on the anisotropic Ag(110) surface, the molecular shape is found to be severely deformed into two types of adsorption configurations: one showing an interesting “Möbius-like” feature and the other showing a symmetric bending structure. Their TERS spectral features are found to be site-dependent over the hoop and even show peak splitting for the out-of-plane C–H bending vibrations. The deformed structural models gain strong support from the spatial distribution of “symmetric” TERS spectra at different positions on the hoop. Further TERS imaging, with a spatial resolution down to ∼2 Å, provides a panoramic view on the local structural deformations caused by different tilting of the benzene units in real space, which offers insights into the subtle changes in the aromatic properties over the deformed hoop owing to inhomogeneous molecule−substrate interactions. The ability of TERS to probe the molecular structure and local deformation at the sub-molecular level, as demonstrated here, is important for understanding surface science as well as molecular electronics and optoelectronics at the nanoscale.

Toward a New Era of SERS and TERS at the Nanometer Scale: From Fundamentals to Innovative Applications

“…323 For the [12] cycloparaphenylene nanohoop molecule, its adsorption configuration and structural deformation on different metal substrates can be directly identified through different features in the spatial mapping images, revealing the local structural deformation associated with different tilting of the benzene units in real space caused by inhomogeneous molecule−substrate interactions. 324 As a brief summary to the SRP methodology, the various experimental studies described above have demonstrated its strong structural characterization ability, regardless of whether a molecule is planar or three-dimensional. Such a Lego-like building process can be generalized with the aid of artificial intelligence (AI), including imaging recognition and machine learning, and could become even more powerful when combined with noncontact AFM or inelastic tunneling probe techniques that can reveal molecular skeletons.…”

“…The SRP technique can also be applied to more complex molecular systems, e.g., for the H 2 TBPP porphyrin molecule on Ag(100), the orientation of the N–H bonds around the porphyrin core can be directly identified through the spatial mapping . For the [12] cycloparaphenylene nanohoop molecule, its adsorption configuration and structural deformation on different metal substrates can be directly identified through different features in the spatial mapping images, revealing the local structural deformation associated with different tilting of the benzene units in real space caused by inhomogeneous molecule–substrate interactions …”

“… 323 For the [12] cycloparaphenylene nanohoop molecule, its adsorption configuration and structural deformation on different metal substrates can be directly identified through different features in the spatial mapping images, revealing the local structural deformation associated with different tilting of the benzene units in real space caused by inhomogeneous molecule–substrate interactions. 324 …”

Planar π-extended all-armchair edge topological cycloparaphenylenes

“…29,30 The two-dimensional restriction effect on the adsorption energy of CPP molecules on the metal surface cannot overcome the resulting planarization energy. 31 Instead, surface synthesis has been proven to be a versatile bottom-up strategy that has attracted a lot of attention from researchers as soon as it was proposed. 32 Xiang et al 33 experimentally synthesized planar p-extended rings with full armchair edge structures on the surface of Au(111) in this approach.…”

Chemically imaging nanostructures formed by the covalent assembly of molecular building blocks on a surface with ultrahigh vacuum tip-enhanced Raman spectroscopy

“…By bringing Raman spectroscopy into the near-field, tip-enhanced Raman spectroscopy (TERS) enables the full characterization of molecules and low-dimensional materials at the subnanoscale [13][14][15]. It is especially sensitive to intermolecular interactions [16][17][18], binding conformations [19][20][21][22], molecule-substrate interactions [23,24], and even chemistry at the single-bond level [25][26][27], resulting in an ideal tool to consider the interactions and reactions that drive the formation of two-dimensional (2D) covalent networks on surfaces [28].…”