Orientational Order of Phenyl Rotors on Triangular Platforms on Ag and Au(111)

TL;DR

This work addresses how phenyl rotors orient on triangular TOTA platforms supported on Ag(111) and Au(111). By combining low-temperature STM with vdW-corrected DFT, it demonstrates that van der Waals interactions dominate adsorption and drive a long-range orientational order, producing a striped arrangement with two of three phenyl orientations separated by $60^\circ$, and reveals an apparent dimerization in STM linked to a LUMO nodal plane and intramolecular hydrogen bonding. The TOTA platform constrains ligands to three symmetry-related orientations, while direct phenyl–phenyl interactions select among them, leading to surface-induced chirality and robust self-assembly patterns. The findings clarify design principles for surface confined aromatic assemblies and highlight how subtle electronic and long-range effects govern supramolecular organization at interfaces.

Abstract

We investigated trioxatriangulenium functionalized with phenyl (phenyl-TOTA) on the (111) surfaces of Ag and Au using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). On Ag(111), the molecules form hexagonal arrays, and on Au(111), honeycomb patterns are also observed. The orientations of the phenyl moieties are resolved on both substrates. On Ag(111), the orientations are parallel within a row and they differ by approximately $60^\circ$ between adjacent molecular rows, and STM images suggest dimerization of the molecules. DFT calculations for Ag(111) reveal that van der Waals interactions dominate this system. The optimized structure matches the experimental pattern, and the simulated STM images exhibit apparent dimerization. This dimerization results from an asymmetry of the phenyl wavefunction, which reflects intramolecular hydrogen bonding between the ligand and an oxygen atom within the triangulenium platform. The orientation of the phenyl moieties is explained by the interaction of each phenyl moiety with its triangulenium platform combined with the direct long-range interaction between phenyl moieties across molecules.

Figures (19)

• Figure 1: (a) Overview topograph ($V=0.15$ V) of a submonolayer amount of phenyl-TOTA on Ag(111). The inset shows a scheme of phenyl-TOTA. (b) More detailed topograph ($V=0.7$ V) from the interior of a molecular island. A unit cell is indicated. (c) Model of the molecular layer and the substrate as determined from experimental STM images.
• Figure 2: Topographs of a molecular island recorded at different sample voltages $V$. A defect (most likely a TOTA platform without phenyl ligand) near to top of the image serves as a marker. (a) At $V=0.3$ V, the molecules appear as nearly circular protrusions. (b, c) As $V$ is increased to 2.1 and 2.2 V, the molecules develop an increasingly elliptical shape. The long axes of the ellipses exhibit alternating orientations. (d) At $V=2.4$ V, the orientations of the ellipses are clearly discernible. In addition, some indication of a constriction, possibly a nodal line, is barely visible. Moreover, the dense molecular rows appear to have rearranged into double rows that are separated by dark groves.
• Figure 3: (a) Topograph ($V=2.2$ V) showing the orientation of the molecular ellipses. (b) Model of the TOTA platforms and the phenyl orientation (red lines) along with schematic $\pi$ orbitals (gray patches). Green dots indicate O atoms involved in bonding to the neighboring molecules.
• Figure 4: (a) Topograph of an island edge. The positions used for d$I/$d$V$ spectroscopy are indicated by numbers. (b) Low-bias d$I/$d$V$ spectra recorded from the four molecules marked in panel a. For clarity, the spectra have been arbitrarily shifted in the vertical direction. The current feedback was disabled at 30 pA and 0.5 V.
• Figure 5: (a) Topograph ($V=0.5$ V) of phenyl-TOTA on Au(111). (b) Topograph ($V=-2$,V) of an isolated hexagon of phenyl-TOTA molecules on Au(111). (c) Constant-height image of the same hexagon ($V=-2$ V).