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.

Observation of Collective Photoswitching in Free-Standing TATA-Based Azobenzenes on Au(111).

Light-induced transitions between the trans and cis isomer of triazatriangulenium-based azobenzene derivatives on Au(111) surfaces were observed directly by scanning tunneling microscopy, allowing atomic-scale studies of the photoisomerization kinetics. Although the azobenzene units in these adlayers are free-standing and spaced at uniform distances of 1.26 nm, their photoswitching depends on the isomeric state of the surrounding molecules and, specifically, is accelerated by neighboring cis isomers.

Norbornadiene-functionalized triazatriangulenium and trioxatriangulenium platforms.

Triazatriangulenium (TATA) and trioxatriangulenium (TOTA) ions are particularly suited systems to mount functional molecules onto atomically flat surfaces such as Au(111). The TATA and TOTA units serve as platforms that absorb onto the surface and form ordered monolayers, while the functional groups are protruding upright and freestanding from the central carbon atoms. Azobenzene derivatized TATA's are known to exhibit extremely fast → isomerization on metal surfaces, via a peculiar non-adiabatic singlet→triplet→singlet mechanism.

Diazocine-functionalized TATA platforms.

Recently, it has been shown that the thermochemical → isomerization of azobenzenes is accelerated by a factor of more than 1000 by electronic coupling to a gold surface via a conjugated system with 11 bonds and a distance of 14 Å. The corresponding molecular architecture consists of a platform (triazatriangulenium (TATA)) which adsorbs on the gold surface, with an acetylene spacer standing upright, like a post in the middle of the platform and the azobenzene unit mounted on top. The rate acceleration is due to a very peculiar thermal singlet-triplet-singlet mechanism mediated by bulk gold.

Molecular platforms as versatile building blocks for multifunctional photoswitchable surfaces.

Controlled attachment of photoswitchable molecules to solid surfaces is a promising route for the realization of complex machine-like molecular functions. A central next step here is the preparation of adlayers with multiple chemical functions that have defined intermolecular spacings and orientations and interact with each other in a controlled way, resulting in novel advanced system properties. We demonstrate that this is possible using molecular platforms with vertical functional units.

Long-Distance Rate Acceleration by Bulk Gold.

We report on a very unusual case of surface catalysis involving azobenzenes in contact with a Au(111) surface. A rate acceleration of the cis-trans isomerization on gold up to a factor of 1300 compared to solution is observed. By using carefully designed molecular frameworks, the electronic coupling to the surface can be systematically tuned.

Influence of a Metal Substrate on Small-Molecule Activation Mediated by a Surface-Adsorbed Complex.

Activating small molecules with transition metal complexes adsorbed on metal surfaces is a novel approach combining aspects of homogeneous and heterogeneous catalysis. In order to study the influence of an Au(111) substrate on the activation of the small-molecule ligand carbon monoxide, a molybdenum tricarbonyl complex containing a PN P pincer ligand was synthesized and investigated in the bulk, in solution, and adsorbed on an Au(111) surface. By means of a platform approach, a perpendicular orientation of the molybdenum complex was achieved and confirmed by IRRAS and NEXAFS.

Imine-Functionalized Triazatriangulenium Platforms: Towards an Artificial Ciliated Epithelium.

Triazatriangulenium (TATA) platforms have been used to prepare highly ordered, self-assembled monolayers of free- and vertically standing imines on Au(111) surfaces. Upon irradiation, the imines undergo trans-cis isomerization and a fast thermal reaction (t1/2 =0.58 s at 20 °C) back to the more stable trans form.

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.