Molybdenum tricarbonyl complex functionalised with a molecular triazatriangulene platform on Au(111): surface spectroscopic characterisation.

Transition metal complexes form the basis for small molecule activation and are relevant for electrocatalysis. To combine both approaches the attachment of homogeneous catalysts to metallic surfaces is of significant interest. Towards this goal a molybdenum tricarbonyl complex supported by a tripodal phosphine ligand was covalently bound to a triazatriangulene (TATA) platform via an acetylene unit and the resulting TATA-functionalised complex was deposited on a Au(111) surface. The corresponding self-assembled monolayer was characterised with scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS). The vibrational properties of the surface-adsorbed complexes were investigated with the help of infrared reflection absorption spectroscopy (IRRAS), and the frequency/intensity changes with respect to the bulk spectrum were analysed. A full vibrational analysis was performed with the help of DFT.

Robust and Selective Switching of an FeIII Spin-Crossover Compound on Cu2N/Cu(100) with Memristance Behavior.

The switching between two spin states makes spin-crossover molecules on surfaces very attractive for potential applications in molecular spintronics. Using scanning tunneling microscopy, the successful deposition of [Fe(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato) molecules on Cu2N/Cu(100) surface is evidenced. The deposited FeIII spin-crossover compound is controllably switched between three different states, each of them exhibiting a characteristic tunneling conductance. The conductance is therefore employed to readily read the state of the molecules. A comparison of the experimental data with the results of density functional theory calculations reveals that all Fe(pap)2 molecules are initially in their high-spin state. The two other states are compatible with the low-spin state of the molecule but differ with respect to their coupling to the substrate. As a proof of concept, the reversible and selective nature of the switching is used to build a two-molecule memory.

Deposition of a Cationic FeIII Spin-Crossover Complex on Au(111): Impact of the Counter Ion.

Spin-crossover molecules on metallic substrates have recently attracted considerable interest for their potential applications in molecular spintronics. Using scanning tunneling microscopy, we evidence the first successful deposition of a charged FeIII spin-crossover complex, [Fe(pap)2]+ (pap = N-2-pyridylmethylidene-2-hydroxyphenylaminato), on Au(111). Furthermore, the bulk form of the molecules is stabilized by a perchlorate counterion, which depending on the deposition technique may affect the quality of the deposition and the measurements. Finally, we evidence switching of the molecules on Au(111).

Monitoring the reversible photoisomerization of an azobenzene-functionalized molecular triazatriangulene platform on Au(111) by IRRAS.

Spectroscopic evidence of a reversible, photoinduced trans ↔ cis photoisomerization is provided for an azobenzene-functionalized triazatriangulene (TATA) platform on Au(111). As shown by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), these molecules form a well-ordered self-assembled monolayer (SAM) on Au(111). The surface-adsorbed azo-TATA platforms are also investigated by infrared reflection absorption spectroscopy (IRRAS); a methoxy marker group at the upper phenyl ring of the azo moiety is employed to monitor the switching state. The IRRAS data are analyzed by comparison with theoretical and transmission IR spectra as well as bulk and surface-enhanced Raman spectroscopic (SERS) data. IRRAS shows that the methoxy group is oriented perpendicular to the surface in trans- and tilted with respect to the surface normal in cis-configuration. This indicates that the photoswitching capability of the azobenzene moieties is retained on the gold surface. The lifetime of the cis-configuration is, however, reduced by a factor of ∼10(3) with respect to the homogeneous solution.

Synthesis and surface-spectroscopic characterization of photoisomerizable glyco-SAMs on Au(111).

Photoisomerizable glyco-SAMs (self-assembled monolayers), utilizing synthetic azobenzene glycoside derivatives were fabricated. The ultimate goal of this project is to assay the influence of the 3D arrangement of sugar ligands on cell adhesion, and eventually make cell adhesion photoswitchable. However, it is a prerequisite for any biological study on the spatial conditions of carbohydrate recognition, that photoisomerization of the surface molecules can be verified. Here, we employed IRRAS and XPS to spectroscopically characterize glyco-SAMs. In particular and unprecedented to date, we prove reversible E→Z→E isomerization of azobenzene glycoside-terminated SAMs.

Remotely triggered geometrical isomerization of a binuclear complex.

Binuclear organometallic molecules are model systems for investigating intramolecular spin-coupling and charge-transfer processes. Using electrospray ionization, Fe(salten) dimers linked by dipyridyl disulfide are deposited on gold for probing with a low-temperature scanning tunneling microscope. Each monomer constitutes a multistable switch owing to its geometric isomerism. Controlled and reversible remote switching within a single dimer is demonstrated. The process is attributed to intramolecular electron transfer.

Grafting of functionalized [Fe(III)(salten)] complexes to Au(111) surfaces via thiolate groups: surface spectroscopic characterization and comparison of different linker designs.

Functionalization of surfaces with spin crossover complexes is an intensively studied topic. Starting from dinuclear iron(III)-salten complexes [Fe(salten)(pyS)]2(BPh4)2 and [Fe(thiotolylsalten)(NCS)]2 with disulfide-containing bridging ligands, corresponding mononuclear complexes [Fe(salten)(pyS)](+) and [Fe(thiotolylsalten)(NCS)] are covalently attached to Au(111) surfaces (pySH, pyridinethiol; salten, bis(3-salicylidene-aminopropyl)amine). The adsorbed monolayers are investigated by infrared reflection absorption spectroscopy (IRRAS) in combination with X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Comparison of the surface vibrational spectra with bulk data allows us to draw conclusions with respect to the geometry of the adsorbed complexes. An anomaly is observed in the spectra of the surface-adsorbed monolayer of [Fe(salten)(pyS)](+), which suggests that the salten ligand is partially decoordinated from the Fe(III) center and one of its phenolate arms binds to the Au(111) surface. For complex [Fe(thiotolylsalten)(NCS)] that is bound to the Au(111) surface via a thiolate-functionalized salten ligand, this anomaly is not observed, which indicates that the coordination sphere of the complex in the bulk is retained on the surface. The implications of these results with respect to the preparation of surface-adsorbed monolayers of functional transition-metal complexes are discussed.