Synthesis, Structure, and Surface Adsorption Characteristics of a Polynuclear MnII,IV-YbIII Complex.

The controlled adsorption of polynuclear coordination compounds with specific structural and electronic characteristics on surfaces is crucial for the prospective implementation of molecule-surface interfaces into practical electronic devices. From this perspective, a neutral 3d,4f-coordination cluster [MnII3MnIVYb3O3(OH)(L·SMe)3(OOCMe)9]·2MeCN·3EtOH (1·2MeCN·3EtOH), where L·SMe- is a Schiff base, has been synthesized and fully characterized and its adsorption on two different solid substrates, gold and graphite, has been studied. The mixed-valence compound with a bilayered metal core structure and the structurally exposed thioether groups exhibits a substantially different surface bonding to metallic gold and semimetallic graphite substrates. While on graphite the adsorption takes place only on distinguished attraction points with a locally increased number of potential bonding sites such as terrace edges and other surface defects, on gold the molecules were found to adsorb rather weakly on randomly distributed adsorption sites of the surface terraces. This entirely different behavior provides important information for the development of advanced surface materials that may enable well-distributed ordered molecular assemblies.

Insertion of VIV Ions into the Polyoxotungstate Archetype {As4W40}.

A new polyoxometalate compound consisting of the 39-tungsto-4-arsenate(III) unit with four incorporated VIV ions, isolated as (NH4)22[(VIVO)2(VIVO(H2O))(AsIIIWVI9O33)2(AsIIIWVI8.5VIV0.5(OH)O32)2(WVIO2)4]·48H2O (NH4-As4W39(V4)), was synthesized and fully characterized. SQUID magnetometry shows three weakly coupled VIV centers with an antiferromagnetic exchange interaction and one isolated VIV ion as a spin-1/2 Curie paramagnet. UV-vis spectroscopy indicates that the As4W39(V4) structure remains intact in aqueous solution for at least 24 h. To enable the deposition of As4W39(V4) from solution on gold surfaces, its trihexyltetradecylphosphonium salt, THTDP-As4W39(V4), was prepared. The IR spectra of both congeners reveal the structural identity of As4W39(V4) independent of the countercations. The X-ray absorption near-edge structure data confirm the presence of VIV centers in a distorted square-pyramidal coordination geometry in NH4-As4W39(V4) and THTDP-As4W39(V4). X-ray photoelectron spectroscopy of the latter, deposited on Au(111), shows that the 4 V and 35 W centers preserve their IV+ and VI+ oxidation states, while the remaining 4 W ions are reduced to IV+.

Tuning electrochemically driven surface transformation in atomically flat LaNiO3 thin films for enhanced water electrolysis.

Structure-activity relationships built on descriptors of bulk and bulk-terminated surfaces are the basis for the rational design of electrocatalysts. However, electrochemically driven surface transformations complicate the identification of such descriptors. Here we demonstrate how the as-prepared surface composition of (001)-terminated LaNiO3 epitaxial thin films dictates the surface transformation and the electrocatalytic activity for the oxygen evolution reaction. Specifically, the Ni termination (in the as-prepared state) is considerably more active than the La termination, with overpotential differences of up to 150 mV. A combined electrochemical, spectroscopic and density-functional theory investigation suggests that this activity trend originates from a thermodynamically stable, disordered NiO2 surface layer that forms during the operation of Ni-terminated surfaces, which is kinetically inaccessible when starting with a La termination. Our work thus demonstrates the tunability of surface transformation pathways by modifying a single atomic layer at the surface and that active surface phases only develop for select as-synthesized surface terminations.

Conductive Self-Assembled Monolayers of Paramagnetic {CoII Co 4 III } and { Co 4 II Co 2 III } Coordination Clusters on Gold Surfaces.

Two polynuclear cobalt(II,III) complexes, [Co5(N3)4(N-n-bda)4(bza·SMe)2] (1) and [Co6(N3)4(N-n-bda)2(bza·SMe)5(MeOH)4]Cl (2), where Hbza·SMe = 4-(methylthio)benzoic acid and N-n-H2bda = N-n-butyldiethanolamine, were synthesized and fully characterized by various techniques. Compound 1 exhibits an unusual, approximately C 2-symmetric {CoII Co 4 III } core of two isosceles Co3 triangles with perpendicularly oriented planes, sharing a central, high-spin CoII ion residing in a distorted tetrahedral coordination environment. This central CoII ion is connected to four outer, octahedrally coordinated low-spin CoIII ions via oxo bridges. Compound 2 comprises a semi-circular { Co 4 II Co 2 III } motif of four non-interacting high-spin CoII and two low-spin CoIII centers in octahedral coordination environments. Self-assembled monolayers (SAMs) of 1 and 2 were physisorbed on template-stripped gold surfaces contacted by an eutectic gallium-indium (EGaIn) tip. The acquired current density-voltage (I-V) data revealed that the cobalt-based SAMs are more electrically robust than those of the previously reported dinuclear {CuIILnIII} complexes with Ln = Gd, Tb, Dy, or Y (Schmitz et al., 2018a). In addition, between 170 and 220°C, the neutral, mixed-valence compound 1 undergoes a redox modification, yielding a {Co5}-based coordination cluster (1-A) with five non-interacting, high-spin octahedral CoII centers as indicated by SQUID magnetometry analysis in combination with X-ray photoelectron spectroscopy and infrared spectroscopy. Solvothermal treatment of 1 results in a high-nuclearity coordination cluster, [Co10(N3)2(N-n-bda)6(bza·SMe)6] (3), containing 10 virtually non-interacting high-spin CoII centers.

Interplay between the amphipathic polyoxometalate interactions in solution and at solid-liquid interfaces: a toolbox for the technical application.

The far-reaching interplay between the speciation of polyoxometalates (POMs) in the liquid phase and the POM adsorption characteristics on substrate surfaces yet remains to be understood. The significance of this interplay is however paramount because it indicates the degree of technical applicability of solvent-processable POM molecules. Herein, we target this fundamentally important issue, shedding light on the "POM-counterion-solvent" and "POM-counterion-solvent-substrate" processes. We effectively combine the results from small-angle X-ray scattering in solution with surface sensitive scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements and present on this basis a semi-quantitative analysis which provides an excellent correlation between both approaches. The MeCN-solution speciation of a tris(alkoxo)-ligated Wells-Dawson-type polyoxoanion - explored as a representative of commonly negatively charged POM-based inorganic-organic nanostructures - is strikingly connected with the growth of porous two-dimensional molecular layers on highly oriented pyrolytic graphite (HOPG). Low water amounts dramatically transform intermolecular relationships toward hierarchical agglomeration that inhibits the layer formation on HOPG. The obtained findings lay the groundwork for a mechanistic study of controlled nucleation and growth of POM nanostructures on weakly interacting surfaces.

Triplet Harvesting with a Simple Aromatic Carbonyl.

The efficiency of organic light-emitting diodes crucially depends on triplet harvesters. These accept energy from triplet correlated electron hole pairs and convert it into light. Here, experimental evidence is given that simple aromatic carbonyls, such as thioxanthone, could serve this purpose. In these compounds, the emissive 1 ππ* excitation may rapidly equilibrate with an upper triplet state (3 nπ*). This equilibrium may persist for nanoseconds. Population of the 3 nπ* state via energy transfer from an electron hole pair should result in fluorescence emission and thereby triplet harvesting. To demonstrate the effect, solutions of 1,4-dichlorobenzene (triplet sensitizer) and thioxanthone (harvester) were excited at 266 nm with a nanosecond laser. The emission decay reveals a 100 ns decay absent in the thioxanthone only sample. This matches predictions for an energy transfer limited by diffusion and gives clear evidence that thioxanthone can convert triplet excitations into light.