Andreas Pöppl

Structural Complexity in Metal–Organic Frameworks: Simultaneous Modification of Open Metal Sites and Hierarchical Porosity by Systematic Doping with Defective Linkers

A series of defect-engineered metal-organic frameworks (DEMOFs) derived from parent microporous MOFs was obtained by systematic doping with defective linkers during synthesis, leading to the simultaneous and controllable modification of coordinatively unsaturated metal sites (CUS) and introduction of functionalized mesopores. These materials were investigated via temperature-dependent adsorption/desorption of CO monitored by FTIR spectroscopy under ultra-high-vacuum conditions. Accurate structural models for the generated point defects at CUS were deduced by matching experimental data with theoretical simulation. The results reveal multivariate diversity of electronic and steric properties at CUS, demonstrating the MOF defect structure modulation at two length scales in a single step to overcome restricted active site specificity and confined coordination space at CUS. Moreover, the DEMOFs exhibit promising modified physical properties, including band gap, magnetism, and porosity, with hierarchical micro/mesopore structures correlated with the nature and the degree of defective linker incorporation into the framework.

Reduction of a Metal–Organic Framework by an Organometallic Complex: Magnetic Properties and Structure of the Inclusion Compound [(η5‐C5H5)2Co]0.5@MIL‐47(V)

Probing and fine tuning the physical and chemical properties of metal–organic frameworks (MOFs) by post-synthetic functionalization of the organic linkers is a challenging topic. For example, redox-inactive MIL-53(Al) can be made redox-active by functionalization of the bridging OH groups of the AlO6-type secondary building units (SBUs) of the framework with 1,1’-ferrocenediyldimethylsilane. The host–guest chemistry of MOFs (including crystalline porous coordination polymers, PCPs), which exhibit a redox-active framework and in particular involving charge-transfer between the framework and the adsorbed molecules, is an underdeveloped, but very promising area of research. The few existing reports include the framework oxidation of a nickel(II)-based MOF with I2, silver(I), and gold(III) salts. [4] Furthermore, the partial reduction of MOFs with lithium, which also involves the aromatic linkers, was discussed to increase the hydrogen uptake. Herein, we present the first case of a stoichiometric reduction of the inorganic backbone of a neutral framework in the course of gas-phase loading with an organometallic reducing agent (OMR), and the elucidation of the novel adsorbate structure of the type [OMR]@[MOF ]. The adsorption of the volatile OMR cobaltocene, [(h-C5H5)2Co], in channels of [VO(bdc)] (MIL-47(V); bdc = 1,4-benzenedicarboxylic acid; MIL = materials of the Institute Lavoisier) leads selectively to the inclusion compound of the formula [(h-C5H5)2Co]0.5@MIL-47(V) (1; Figure 1). The MIL-47(V) network is the isostructural analogue of the well-investigated, redox-inactive phase MIL-53(Al). The preparation of 1 was carried out according to previously published procedures for solvent-free loading of MOFs with organometallic compounds. The elemental analysis of 1 reveals a V/Co ratio of exactly 2:1. The FT-IR spectrum of 1 has a strong vibrational band at 860 cm , which is attributed to the formation of cobaltocenium species (Figure 2). Figure 1. Structure of 1 in the [010] projection. The cobaltocenium occupancy is 50%; that is, only every second position shown is occupied. C black, O red, Co purple; {VO6} fragments are displayed as green octahedra.

Electrochemically oxidised graphite.: Characterisation and some ion exchange properties

An evaluation of some of the properties of electrochemically oxidised graphite has been carried out. These studies include textural characterisation, magic angle spinning NMR, ESR and ion exchange properties. A study of the surface morphology has also been carried out using high-resolution transmission electron microscopy and identification of surface groups confirmed by FTIR spectroscopy. The electrochemical method of preparation is shown to confer, to the porous graphite oxide obtained, different surface chemical groups that can be used for ion exchange purposes. ESR shows that Cu(II) is coordinated to the oxidised graphite.

Broadband dielectric spectroscopy of water confined in MCM-41 molecular sieve materials : low-temperature freezing phenomena

Dielectric properties of water adsorbed in pure siliceous and aluminium containing mesoporous MCM-41 materials have been investigated in the frequency range from 20 Hz to 1 MHz. The dielectric spectra revealed three dispersion regions, liquid-like free water in the centre of the mesopores, an intermediate water layer with reduced mobility, and an interfacial water layer at the inner surface of the mesopores. The analysis of the relaxation time distribution by means of a double-well potential indicates a strong dependence of the barrier height of reorienting water molecules dipoles in the interfacial layer on the Si/Al ratio in the framework.

The effects of hyaluronan and its fragments on lipid models exposed to UV irradiation

The effects of hyaluronan and its degradation products on irradiation-induced lipid peroxidation were investigated. Liposomal skin lipid models with increasing complexity were used. Hyaluronan and its fragments were able to reduce the amount of lipid peroxidation secondary products quantified by the thiobarbituric acid (TBA) assay. The qualitative changes were studied by mass spectrometry. To elucidate the nature of free radical involvement electron paramagnetic resonance (EPR) studies were carried out. The influence of hyaluronan and its fragments on the concentration of hydroxyl radicals generated by the Fenton system was examined using the spin trapping technique. Moreover, the mucopolysaccharides ability to react with stable radicals was checked. The quantification assay of 2,2-diphenyl-1-picrylhydrazyl hydrate (DPPH) showed no concentration changes of the stable radical caused by hyaluronan. Hyaluronan was found to exhibit prooxidative effects in the Fenton assay in a concentration dependent manner. A transition metal chelation was proposed as a mechanism of this behavior. Considering human skin and its constant exposure to UV light and oxygen and an increased pool of iron in irradiated skin the administration of hyaluronan or its fragments in cosmetic formulations or sunscreens could be helpful for the protection of the human skin.

Role of Ascorbic Acid in Stratum Corneum Lipid Models Exposed to UV Irradiation

AbstractPurpose. The effects of ascorbic acid on Stratum corneum lipid models following ultraviolet irradiation were studied adding iron ions as transition metal catalysts. Methods. Lipid peroxidation was quantified by the thiobarbituric acid assay. The qualitative changes were studied on a molecular level by mass spectrometry. To elucidate the nature of free radical involvement we carried out electron paramagnetic resonance studies. The influence of ascorbic acid on the concentration of hydroxyl radicals was examined using the spin trapping technique. Moreover, we checked the vitamins ability to react with stable radicals. Results. Ascorbic acid was found to have prooxidative effects in all lipid systems in a concentration dependent manner. The degradation products of ascorbic acid after its prooxidative action were detected. The concentration of the hydroxyl radicals in the Fenton assay was decreased by ascorbic acid. The quantification assay of 2,2-diphenyl-1-picrylhydrazyl hydrate showed reduced concentration levels of the stable radical caused by ascorbic acid. Conclusions. Considering human skin and its constant exposure to UV light and oxygen, an increased pool of iron ions in irradiated skin and the depletion of co-antioxidants, the administration of ascorbic acid in cosmetic formulations or in sunscreens could unfold adverse effects among the Stratum corneum lipids.

X, Q and W band electron paramagnetic resonance study of the sorption of NO in Na-A and Na-ZSM-5 zeolites

EPR measurements at temperatures T<40 K using nitric monoxide as probe molecules are very suitable for characterizing Lewis acid centers on nanoporous materials such as A-type and ZSM-5-type zeolites. These studies are especially useful if different EPR resonance frequencies are applied. In particular, it was possible to perform W band EPR measurements on adsorbed NO molecules for the first time, which require a special sample preparation under high vacuum conditions. The continuous wave (c.w.) EPR spectra show a 14N hyperfine (hf) coupling, an orthorhombic g tensor, and a pronounced inhomogeneous line broadening. In particular, the use of W band frequencies allows the separation of hyperfine and g tensor components in the powder spectrum. This can be used to calculate the AN tensor principal axes values for the 14N hfs and the deviation of the g tensor from axial symmetry of NO probe molecules in Na-A and Na-ZSM-5 zeolites. Furthermore, linewidth contributions arising from a distribution of the g tensor principal values can be separated from other inhomogeneous line broadening contributions by the multi-frequency EPR approach. On the basis of the g tensor principal values and the analysis of the line broadening effects, the mean splitting between the energy levels of the adsorbed NO molecules and their distribution widths were obtained for Na-A and Na-ZSM-5 zeolites. A small distribution of the g values of the adsorption complexes with similar energy splitting values Δ and E reveals a nearly uniform type of Na adsorption sites for Na-A/NO zeolites. In contrast, the relatively wide distribution of the energy splitting Δ for the system Na-ZSM-5/NO shows a large variety of those adsorption sites in Na-ZSM-5 zeolites.

Structural phase transitions in partially deuterated betaine phosphite crystals studied by dielectric and electron paramagnetic resonance methods

Abstract Partially deuterated betaine phosphite (DBPI) crystals were investigated by measurements of the dielectric constant and by means of electron paramagnetic resonance (EPR). Two phase transitions could be detected in the temperature range from 100 K to 380 K: a high-temperature phase transition at Tc1 = (355 ± 3) K and a ferroelectric phase transition at Tc2 = (297 ± 3) K. The EPR studies show that the ferroelectric phase transition is related to the simultaneous ordering of the protons and deuterons in the O-H/D… O bonds.

Multi-approach Electron Paramagnetic Resonance Investigations of UV-Photoinduced Ti3+ in Titanium Oxide-Based Gels

EPR investigations of the photoreduction of Ti(4+) into Ti(3+) under UV irradiation were carried out on three titanium-based materials for which the initial concentration of Ti(4+) was defined in the ternary phase diagram (TiOCl(2), H(2)O, DMF). The kinetics of this photoreduction was monitored at 200 K and related to the initial concentration of Ti(4+) in the solution. This study was complemented by a multi-approach EPR method (pulsed electron paramagnetic resonance (EPR), pulsed electron nuclear double resonance, and hyperfine sublevel correlation spectroscopy (HYSCORE)) with the aim of probing the proton environment of the Ti(3+) ions. Indeed, many species such as H(2)O, OH(-), HCOO(-) are located in the immediate vicinity of Ti(3+). Although we found that a distribution of g tensors was involved, for simplicity, two types of g tensor were used to describe the main features of the EPR signal related to the paramagnetic ions. Additionally, we have evidenced that two kinds of protons are identified next to Ti(3+) species, with specific distances determined from the hyperfine coupling parameters obtained by the HYSCORE method.

EPR study of NO adsorption–desorption behaviour on Lewis acid sites in NaA zeolites†

The Na+–NO adsorption complex in zeolite NaA was investigated in a wide temperature range from 4 to 300 K by electron paramagnetic resonance (EPR) spectroscopy. The ground state of the adsorbed NO molecules and the rotational ground state of the 2Π3/2 state of desorbed gaseous NO could be observed. At low temperatures the NO molecules were found to be coordinated to the sodium ion in a bent complex structure. The EPR spectra are subjected to motional averaging by a two‐site jump process between two chemically identical complex structures at T > 30 K. Desorption processes of the NO molecules from the Lewis acid centres manifest themselves in homogeneous line broadening effects in the EPR spectra of the distorted 2Π1/2 state of NO in the Na+–NO adsorption complex at higher temperatures. In this model, the homogeneous linewidths are determined by the lifetime of the adsorption complex and provide a direct measure of the adsorption energy of NO at the sodium cation sites. Furthermore, desorption of the nitric oxide from the zeolite was monitored simultaneously by the EPR signal of the 2Π3/2 state of free NO molecules in the gas phase to support this approach. Copyright