Dirk Volkmer

A thin-film electrochromic device based on a polyoxometalate cluster

The realization of molecular-based switching and display devices faces two entirely different challenges that require a scientific remedy. First, components possessing addressable states with distinct physical properties have to be identified and synthesized. The components must operate reversibly with long-term stability and suitable response times. The stimulus threshold and the power consumption for switching between states ought to be low. Second, one or several active components have to be positioned in a predefined way into the actual device’s structure. An efficient and rational approach towards this goal is to use strategies from supramolecular chemistry. The future fabrication of such materials may, therefore, rely on principles of molecular self-organization. [1] Most likely, thin-film technologies will play a major role in future applications. [2] In terms of possible components for electrochromic devices, polyoxometalates (POMs) are promising candidates due to their ability to act as an electron reservoir, thereby giving rise to colored mixed-valence state species while retaining their structural integrity. [3‐7] In contrast to many semiconductor nanoparticles and quantum dots, POMs are discrete, molecularly defined metal‐oxide clusters with an extensive range of structures and properties. For the realization of such devices it will, therefore, be necessary to identify existing and to synthesize novel POMs with sizeable electrochromic response and stable redox states. [8] Despite the potential of POMs, their implementation in advanced materials has remained elusive, mainly due to the fact that they are obtained as crystalline solids, which are difficult to process. [9] Traditionally, thin films of POMs are made by spin coating, the Langmuir‐Blodgett technique, electrodeposition, or simply by compressing POM solids against an indium tin oxide (ITO)-coated glass slide. [10] More recent approaches involve the use of surfactant-encapsulated POM clusters. [11] A simple

Surfactant -Encapsulated Clusters (SECs): (DODA)20(NH4)[H3Mo57V6(NO)6O183(H2O)18], a Case Study

We present a comprehensive study of the partially reduced polyoxomolybdate [H3-Mo57V6(NO)6O183(H2O)18]21-encapsulated in a shell of dimethyldioctadecylammonium (DODA) surfacmolecules. Treatment of an aqueous solution of (NH4)21[H3Mo57V6-(NO)6O183(H2O)18] . 65H2O (1a) with a trichloromethane solution of the surfactant leads to instant transfer of the encapsulated complex anion into the organic phase. Results from vibrational spectroscopy. analytical ultracentrifugation, small-angle X-ray scattering, transmission electron microscopy, elemental analysis, and Langmuir compression isotherms are consistent with a single polyoxometalate core encapsulated within a shell of 20 DODA molecules. The molar mass of the supramolecular assembly is 20249 gmol(-1) and the diameter is 3.5 nm. A material with the empirical formula (DODA)20(NH4)[H3-Mo57V6NO)6O183(H2O)18] (2) was isolated as a dark violet solid, which readily dissolves in organic solvents. Slow evaporation of solutions of 2 on solid substrates forces the hydrophobic particles to aggregate into a cubic lattice. Annealing these so-formed films at elevated temperature causes de-wetting with terrace formation similar to liquid crystals and block copolymers. Compound 2 forms a stable Langmuir monolayer at the air-water interface; Langmuir-Blodgett multilayers are readily prepared by repeated transfer of monolayers on solid substrates. The films were characterized by optical ellipsometry, Brewster angle microscopy, transmission electron microscopy, and X-ray reflectance.

Heterogeneous Catalytic Oxidation by MFU‐1: A Cobalt(II)‐Containing Metal–Organic Framework

Porous metal–organic frameworks (MOFs) are a rapidly emerging class of multifunctional hybrid materials that might be useful for diverse technical applications, such as gas or liquid adsorption and separation, molecular recognition, or catalysis. Combining polycarboxylate ligands and (transition) metal ions, moderately robust MOFs can be prepared; 1,4-benzenedicarboxylate (bdc, terephthalic acid) and 4,4’biphenyldicarboxylate (bpdc) are often used as linkers. Highly porous non-interpenetrated frameworks, such as the well-known MOF-5 ([Zn4O(bdc)3]) [2] or IRMOF-9 ([Zn4O(bpdc)3]) [3] then form. These microporous MOFs generally show good thermal stabilities (decomposition occurs at T> 350 8C). A fundamental disadvantage, however, is their low hydrolytic stability: Decomposition of the framework occurs rapidly when the gas or liquid phase contains a small amount of water, which imposes severe limitations on their usage in catalytic oxygenation reactions, in which water is a major reaction product. Preliminary attempts to use MOF-5 as a photocatalyst have been reported recently; however, the fact that these frameworks contain Lewis acidic zinc(II) ions only imposes severe limitations on their use in redox catalytic applications in general. Conceptually different approaches have been reported to circumvent the intrinsic disadvantages of MOF-5-type frameworks. Fischer et al. reported the gas-phase deposition of volatile organometallic complexes in the open cavities of MOF-5. Subsequent photolytic or reductive cleavage of the precursors led to catalytically active metal clusters (Cu, Pd, Au) that are finely dispersed in the MOF-5 framework. Nguyen, Hupp et al. were among the first to present oxidations using a MOF catalyst. They used an enantiomerically pure manganese complex of a modified salen ligand as a building block to construct a three-dimensional porous framework. A distinct approach towards heterogeneous asymmetric catalysis based on a homochiral metal–organic framework was recently proposed by Lin et al. However, industrial oxidation or oxygenation reactions typically require very high turn-over numbers (TONs) and frequencies (TOFs), which have not been achieved to date by current MOF catalysts. To produce thermally and hydrolytically stable redoxactive MOFs, our initial efforts focused on the isostructural replacement of a single zinc ion by an open-shell transition metal ion M within the tetranuclear {Zn4O} coordination unit of MOF-5. However, all attempts in this direction led to heteronuclear MOFs containing trinuclear coordination units (for example, [MZn2(bpdc)3(dmf)2], M = Co , Ni, Cd), which are structurally different from MOF-5. A search of the CSD database, however, led to the tetranuclear complex [Co4O(3,5-dmpz)6] (3,5-dmpz = 3,5-dimethylpyrazolate), [10]

Pyrazolate-Based Cobalt(II)-Containing Metal–Organic Frameworks in Heterogeneous Catalytic Oxidation Reactions: Elucidating the Role of Entatic States for Biomimetic Oxidation Processes

Crystal structures of two metal-organic frameworks (MFU-1 and MFU-2) are presented, both of which contain redox-active Co(II) centres coordinated by linear 1,4-bis[(3,5-dimethyl)pyrazol-4-yl] ligands. In contrast to many MOFs reported previously, these compounds show excellent stability against hydrolytic decomposition. Catalytic turnover is achieved in oxidation reactions by employing tert-butyl hydroperoxide and the solid catalysts are easily recovered from the reaction mixture. Whereas heterogeneous catalysis is unambiguously demonstrated for MFU-1, MFU-2 shows catalytic activity due to slow metal leaching, emphasising the need for a deeper understanding of structure-reactivity relationships in the future design of redox-active metal-organic frameworks. Mechanistic details for oxidation reactions employing tert-butyl hydroperoxide are studied by UV/Vis and IR spectroscopy and XRPD measurements. The catalytic process accompanying changes of redox states and structural changes were investigated by means of cobalt K-edge X-ray absorption spectroscopy. To probe the putative binding modes of molecular oxygen, the isosteric heats of adsorption of O(2) were determined and compared with models from DFT calculations. The stabilities of the frameworks in an oxygen atmosphere as a reactive gas were examined by temperature-programmed oxidation (TPO). Solution impregnation of MFU-1 with a co-catalyst (N-hydroxyphthalimide) led to NHPI@MFU-1, which oxidised a range of organic substrates under ambient conditions by employing molecular oxygen from air. The catalytic reaction involved a biomimetic reaction cascade based on free radicals. The concept of an entatic state of the cobalt centres is proposed and its relevance for sustained catalytic activity is briefly discussed.

Enhanced selectivity of CO2 over CH4 in sulphonate-, carboxylate- and iodo-functionalized UiO-66 frameworks

Three new functionalized UiO-66-X (X = -SO(3)H, 1; -CO(2)H, 2; -I; 3) frameworks incorporating BDC-X (BDC: 1,4-benzenedicarboxylate) linkers have been synthesized by a solvothermal method using conventional electric heating. The as-synthesized (AS) as well as the thermally activated compounds were characterized by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, thermogravimetric (TG), and elemental analysis. The occluded H(2)BDC-X molecules can be removed by exchange with polar solvent molecules followed by thermal treatment under vacuum leading to the empty-pore forms of the title compounds. Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that 1, 2 and 3 are stable up to 260, 340 and 360 °C, respectively. The compounds maintain their structural integrity in water, acetic acid and 1 M HCl, as verified by XRPD analysis of the samples recovered after suspending them in the respective liquids. As confirmed by N(2), CO(2) and CH(4) sorption analyses, all of the thermally activated compounds exhibit significant microporosity (S(Langmuir): 769-842 m(2) g(-1)), which are comparable to that of the parent UiO-66 compound. Compared to the unfunctionalized UiO-66 compound, all the three functionalized solids possess higher ideal selectivity in adsorption of CO(2) over CH(4) at 33 °C.

Functional Polyoxometalate Thin Films via Electrostatic Layer-by-Layer Self-Assembly

Polyoxometalates (POMs) comprise a structurally diverse class of inorganic transition metal oxygen clusters which—owing to their unique electronic properties—hold promise for a host of technological applications such as electrochromic windows, sensors, or heterogeneous catalysts, prototypic examples of which will be briefly exemplified. The integration of POMs into functional architectures and devices, however, necessitates the development of general methods that allow positioning these clusters in well-defined supramolecular architectures, thin films, or mesophases. This short review highlights recent advances in the preparation of composite multilayers fabricated by electrostatic layer-by-layer self-assembly (ELSA) of POMs and a variety of water-soluble cationic species, including transition metal complexes, cationic surfactants, polycations and bipolar pyridine.

Interfacial electrostatics guiding the crystallization of CaCO3 underneath monolayers of calixarenes and resorcarenes

The amphiphilic octaacids rccc-4,6,10,12,16,18,22,24-octakis-O-(carboxymethyl)-2,8,14,20-tetra(n-undecyl)resorc[4]arene (1) and 5,11,17,23,29,35,41,47-octakis(1,1,3,3-tetramethylbutyl)-49,50,51,52,53,54,55,56-octa(carboxymethoxy)calix[8]arene (2) form stable monolayers at the air–water interface which induce growth of CaCO3 crystals at the monolayer–solution interface. Uniformly (012) oriented rhombohedral calcite single crystals grow underneath a monolayer of 2, whereas crystallization under a monolayer of 1 preferentially leads to formation of acicular aggregates of aragonite crystals. While polymorph selection and orientations of the CaCO3 crystals critically depend on the average charge density of the monolayer, the molecular shape and the particular Ca coordination behaviour of the amphiphiles that form the monolayer are of minor importance. CaCO3 crystal growth underneath monolayers of macrocyclic amphiphiles is briefly reviewed and the present experimental observations are compared to previous related investigations on “template-induced” biomimetic mineralization.

MFU‐4 – A Metal‐Organic Framework for Highly Effective H2/D2 Separation

The metal-organic framework, MFU-4, possessing small cavities and apertures, is exploited for quantum sieving of hydrogen isotopes. Quantum mechanically, a molecule confined in a small cavity shows an increase in effective size depending on the particle mass, which leads to a faster deuterium adsorption from a H(2)/D(2) isotope mixture.

Crystallization of Calcium Carbonate Beneath Insoluble Monolayers: Suitable Models of Mineral–Matrix Interactions in Biomineralization?

The growth of inorganic materials below negatively charged monolayers is frequently considered to be a suitable model system for biomineralization processes. The fact that some monolayers give rise to oriented overgrowth of calcium carbonate crystals has been interpreted in terms of a geometrical and stereochemical complementarity between the arrangement of headgroups in the monolayer and the position of Ca ions in the crystal plane that attaches to the monolayer. However, comparative investigations of the growth of calcium carbonate beneath monolayers of macrocyclic polyacids have demonstrated that non-directional electrostatic parameters, such as the average charge density or the mean dipole moment of the monolayer, determine the orientation and the polymorph of the overgrowing crystals. The results show that it is possible to control the surface charge densities in monolayers by the appropriate design of amphiphilic molecules. A switch in polymorph occurs above a critical monolayer charge density at which aragonite or vaterite nucleation takes place, presumably via a kinetically controlled precipitation process.

Partially fluorinated MIL-47 and Al-MIL-53 frameworks: influence of functionalization on sorption and breathing properties

Two perfluorinated metal hydroxo terephthalates [M(III)(OH)(BDC-F)]·n(guests) (M(III) = V, MIL-47-F-AS or 1-AS; Al, Al-MIL-53-F-AS or 2-AS) (BDC-F = 2-fluoro-1,4-benzenedicarboxylate; AS = as-synthesized) have been synthesized by a hydrothermal method using microwave irradiation (1-AS) or conventional electric heating (2-AS), respectively. The unreacted or occluded H(2)BDC-F molecules can be removed under vacuum by direct thermal activation or exchange of guest molecules followed by thermal treatment leading to the empty-pore forms of the title compounds [V(IV)(O)(BDC-F)] (MIL-47-F, 1) and [Al(III)(OH)(BDC-F)] (Al-MIL-53-F, 2). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that the compounds are stable up to 385 and 480 °C, respectively. Both of the thermally activated compounds exhibit significant microporosity, as verified by N(2), CO(2), n-hexane, o- and p-xylene sorption analyses. The structural changes of 2 upon adsorption of CO(2), n-hexane, o- and p-xylene were highly influenced due to functionalization by -F groups, as compared to parent Al-MIL-53. The -F groups also introduce a certain degree of hydrophobicity into the frameworks, as demonstrated by the H(2)O sorption analyses.