![]() 2c) reveals that the molecule rotated clockwise by the azimuthal angle α = 60° (Fig. A sudden increase in the tunneling current (Fig. To induce this motion in a controlled fashion we applied a voltage pulse of 1.3 V at the location indicated in Fig. Very low tunneling currents (<1 pA) are required for stable imaging as the molecules otherwise move during scanning. Controlled rotation around a fixed pivot point One can identify that the adamantane groups reflect the two large lobes of the STM image while the dimethylamine is responsible for the asymmetric appearance, indicated by the arrow in Fig. 1g) that correlates well to the experimental result. From this calculated structure, an STM image was simulated (Fig. 1f) reveals that the axis between the two adamantane groups lies ~18° rotated with respect to the \(\) direction of the surface. Further, the computationally determined geometry (Fig. As adatoms adsorb in hollow 20 and CO in on-top sites 21 on Ag(111), we can superimpose the silver lattice onto an STM image, thus deriving the position of a single DDNB molecule with respect to the surface (Fig. 1e) from their characteristic size (diameter of ~0.8 nm) 19. ![]() Additionally, CO molecules are identified as depressions (blue in Fig. The adsorption site of the molecule was determined by placing single silver adatoms in the vicinity of a molecule 18. Only one of the two enantiomers is considered in the following (see Supplementary Fig. Taking into account the dimethylamine-nitro axis with respect to the adamantane axis, the surface imposes a chirality on the molecule 17 and two enantiomers exist on the surface, identified via a slight protrusion on the convex side of the molecule (arrow in Fig. The peanut-shape asymmetry of the molecule comes from the polar −N(CH 3) 2 and −NO 2 substituents of the central benzene ring. 1d), which we assign to the adamantane groups. They appear with two lobes at a distance of 13.0 ± 0.1 Å (Fig. 2) allows individual isolated molecules to be investigated. 1).ĭeconstructing the islands molecule by molecule (Supplementary Fig. 1c) with ordering determined by the dipolar interaction between molecules (Supplementary Fig. Depositing these molecules onto a Ag(111) surface at room temperature, results in highly ordered honeycomb assemblies (Fig. To protect the polar groups from strong adsorption with the surface, DDNB contains two peripheral adamantane groups which allows for facile diffusion and interaction with the electric field 16. 1b) with a resulting net dipole moment of 6.78 Debye in the gas phase (see Supplementary Table 1). This arises from the electron-donating character of the dimethylamine (−N(CH 3) 2) and the electron-withdrawing properties of the nitro (−NO 2) group attached to the central benzene ring (Fig. We have developed the 2,5-di(ethynyladamantanyl)−4-(dimethylamino)nitrobenzene molecule (DDNB) 16 which, at its core, has a strong electric dipole. From these precision manipulation experiments, the electric field-induced motion can be mapped and used to determine the internal dipole moment of a single molecule. ![]() Here, we manipulate single dipolar molecules and obtain unidirectional rotation on demand, and thus deterministic behavior. In contrast to non-deterministic processes, we show that the electric field in an STM junction can be used to manipulate polar molecules with absolute precision (Fig. Molecular rotors in double-decker complexes have been rotated in large assemblies, albeit without control over the direction of rotation 15. While rotation is often combined with translation, e.g., along the edge of a molecular island 12, it can also be constrained to a fixed rotational axis 13, 14. Moreover, the STM tip has been used to induce rotation of various molecules 9, 10, 11. Scanning tunneling microscopy (STM) is attractive in this regard as it provides the means to image and manipulate matter at the single-molecule scale and track thermally induced processes like rotation and translation on metallic surfaces 8. Deterministically controlling the direction of molecular rotation and translation remains a challenge because numerous degrees of freedom exist and thermally activated processes offer little selectivity 7. Specifically, the orientation of a molecule or a functional group can affect many processes, for instance, molecular diffusion on a surface 1, 2, the efficiency of a chemical reaction 3, 4, and heterogeneous catalysis 5 or biochemical reactions with enzymes as catalyst 6. Controlling the motion and orientation of single molecules is key to the understanding of heterogeneous catalysis, growth and assembly processes as well as to the operation of surface-adsorbed molecular machines. ![]()
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