Using the First Steps of Hydration for the Determination of Molecular Conformation of a Single Molecule

Abstract

Determination of the exact structure of individual molecules is the ultimate goal of high-resolution microscopy. However, the resolution of scanning tunneling microscopy (STM) is intrinsically limited to the extent of molecular orbitals, which frequently do not differ for small changes in the molecular conformation. Here we use the position of water molecules during the first hydration steps of an azobenzene derivative on Au(111) to determine not only the orientation of the end groups with respect to the phenyl rings but also the orientation of the two phenyl rings with respect to the azo group. We investigate the co-adsorption of 4,4'-hydroxy-azobenzene and water molecules on Au(111) by low-temperature STM. The water molecules are attached exclusively to the hydroxyl end groups of the azobenzene derivatives. Predominantly the trans-azobenzene molecule with the two hydroxyl groups pointing into opposite directions is adsorbed. As corroborated by the attachment of a single water molecule to 4-anilino-4'-nitro azobenzene on the same inert surface, the method is generally applicable for structure determination of molecules with appropriate end groups. Our study thus gives unprecedented information about the intramolecular orientation based on the first real space observation of the hydration of a functional molecule.

Hydrophilicity and Microsolvation of an Organic Molecule Resolved on the Sub‐molecular Level by Scanning Tunneling Microscopy

“…While water structures as well as organic molecules were characterized separately,athorough investigation of structures resulting from the combination of the two is still lacking.Inarecent study,weco-adsorbed 4,4'dihydroxy-azobenzene with small amounts of water. [21] The water molecules were found to attach exclusively to the hydroxy groups,but as they are identical no conclusion about relative hydrophilicity of the endgroups could be drawn. Clusters of water [19] and organic molecules [22][23][24] are often dominated by intermolecular bonds and it was shown that these interactions are only marginally influenced by the presence of the Au(111) surface.T herefore,o ur approach towards am olecular-scale picture of solvation is to adsorb solute (an azobenzene derivate) and solvent (water) onto Au(111) at low temperature and to image the formed structures using low-temperature scanning tunneling microscopy.H erein, the functionalized azo dye 5-(4-Nitrophenylazo)salicylic acid (hereafter designated NPAS) with groups of different water affinity (carboxyl, hydroxy,a nd nitro group) offers ad iversified binding environment to water molecules.…”

Imaging the Solvation of a One‐Dimensional Solid on the Molecular Scale

“…[10][11][12][13] However,a st hese experiments are conducted mostly under ambient conditions,t he interactions of individual solvent molecules with the solid are not traceable owing to intrinsic thermal fluctuations.Low-temperature scanning tunneling microscopy (STM) eliminates the thermal motion of molecules and offers the required spatial resolution. [14,15] Here,weused this technique to follow the solvation of ah ydrogen-bonded solid in real space.T he hydrogenbond-assisted formation of supramolecular networks on surfaces has been thoroughly investigated by scanning probe microscopic techniques in recent years. [16][17][18][19][20][21] These networks may be understood as low-dimensional crystals,which makes them excellent test subjects for the investigation of solvation processes by STM.…”

“…[27] Indeed, for functionalized azobenzenes, which possess similar aromatic hydrogen atoms,w ater molecules did not attach at these positions under comparable experimental conditions. [14,15] In contrast to the single hydrogen atoms available for binding in Refs. [14] and [15],MDAF and the chains offer positions where aw ater molecule might attractively interact with more than one hydrogen atom;these are indeed the adsorption positions revealed by as uperposition of the STM image and gas-phase structures of MDAF molecules (Figure 2c).…”

“…[14,15] In contrast to the single hydrogen atoms available for binding in Refs. [14] and [15],MDAF and the chains offer positions where aw ater molecule might attractively interact with more than one hydrogen atom;these are indeed the adsorption positions revealed by as uperposition of the STM image and gas-phase structures of MDAF molecules (Figure 2c). Theo bserved attachment sites are either located at the hydrogen atoms of the C4 and C5 carbon atoms or close to the C1 carbon atom and the diazo group.…”

Imaging the Solvation of a One‐Dimensional Solid on the Molecular Scale

“…However, as these experiments are conducted mostly under ambient conditions, the interactions of individual solvent molecules with the solid are not traceable owing to intrinsic thermal fluctuations. Low‐temperature scanning tunneling microscopy (STM) eliminates the thermal motion of molecules and offers the required spatial resolution . Here, we used this technique to follow the solvation of a hydrogen‐bonded solid in real space.…”