No abstract

Employing high-throughput methods, the synthesis conditions for a series of six new MOFs based on aluminium ions and the V-shaped linker molecule 1,3-benzene dicarboxylic acid, denoted as CAU-10-X (CAU = Christian-Albrechts-University) with the sum formula [Al(OH)(C 8 H 3 O 4 X)]•solvent, were established (X = functional group in 5-position of the aromatic ring; X = H (1), CH 3 (2), OCH 3 (3) NO 2 (4), NH 2 (5), or OH (6)). Due to the absence of macroscopic crystals, the obtained compounds were structurally characterized employing XRPD-methods. The crystal structures of 1, 2 and 3 were refined using Rietveld methods. Although the described MOFs are isoreticular, they crystallize in several, sometimes non-centrosymmetric space groups (1, 4, 6), due to slight structural changes induced by the functionalization. These space groups were confirmed with second-harmonic generation measurements. All compounds are highly stable as confirmed by temperature-dependent XRPD-and IR-experiments and decompose at temperatures above 350 °C. The stabilities of all compounds in aqueous solutions of varying pH were confirmed by XRPD-measurements and their sorption properties towards nitrogen, hydrogen, carbon dioxide and water vapor at low pressures are reported. A drastic influence of the functional group on affinity, capacity and accessibility of the pores for these gases is observed. These properties depend on the polarity and size of the functional group as well as on subtle structural differences between the CAU-10-X compounds.

The use of metal organic frameworks as photocatalysts is critically reviewed and their main advantages and challenges are evaluated.

Metal-organic frameworks (MOFs) containing d0 metals such as NH2-MIL-125(Ti), NH2-UiO-66(Zr) and NH2-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH2-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.

Aluminium isophthalate CAU-10-H is a promising stable adsorbent for application in heat-exchange processes.

ABSTRACT:The metal−organic framework NH 2 -MIL-53(Al) is the first solid-state material displaying nonlinear optical switching due to a conformational change upon breathing. A switching contrast of at least 38 was observed. This transition originates in the restrained linker mobility in the very narrow pore configuration.T he field of nonlinear optics has experienced an everincreasing interest due to multiple applications in information processing, electro-optical switching, and telecommunications.1,2 While commercial nonlinear optical (NLO) materials are still largely inorganic, organic compounds and metal−organic complexes have attracted much attention.3 As a result, during the past decade, the possibility of changing the quadratic or second-order NLO response by an external stimulus has been increasingly addressed. A molecule or solid able to change its NLO response reversibly is called an "NLO switch". Several families of molecules and metal−organic complexes display this property in the liquid phase.4−9 NLO switches in the solid state, however, are much more scarce. A necessary requirement for a quadratic NLO material is that it be noncentrosymmetric. While it is easy to synthesize individual noncentrosymmetric molecules and metal−organic complexes, these typically dipolar entities often organize in an antiparallel fashion into centrosymmetric crystals. A common strategy to obtain polar order on the macroscopic level is via electric field poling of polymers containing dipolar chromophores. The change of centrosymmetric to noncentrosymmetric order is associated with a large change in quadratic NLO response, but the change is not readily reversible.10 As a consequence, hardly any reversible solid-state second-order NLO switches have been reported to date: only anil crystals (Schiff bases, based on photoswitching)11−14 and thin films of ruthenium complexes (based on redox switching) 15 have been shown to display a certain degree of reversible switching. For these materials, the NLO contrast, defined as the ratio of the second harmonic generation (SHG) intensities (see below) before and after the external stimulus, varies by a factor between 1.3 and 10. This limited contrast is due to the fact that all reported NLO switches essentially retain their noncentrosymmetric order upon switching. Herein we report that the metal−organic framework (MOF) NH 2 -MIL-53(Al), which contains Al 3+ and 2-aminoterephthalate, is a novel solid reversible NLO switch. The switching capacity is due to a reversible conformational change that greatly diminishes the polar ordering of the material.MOFs have also attracted a lot of scientific attention in the field of nonlinear optics, where the design of several noncentrosymmetric frameworks has been reported. 16−19 In a single case, the SHG intensity of a MOF could be modulated by cation exchange, with a contrast of 1.75. 20 However, the effects of organic guest molecules on the SHG intensity have not been reported to date.A special class of MOFs are those that can reversibly alter the...

Interpenetration, the entwining of multiple lattices, is a common phenomenon in metal-organic frameworks (MOFs). Typically, in interpenetrated MOFs the sub-lattices are fully occupied. Here we report a family of MOFs in which one sub-lattice is fully occupied and the occupancy level of the other can be controlled during synthesis to produce frameworks with variable levels of partial interpenetration. We also report an 'autocatenation' process, a transformation of non-interpenetrated lattices into doubly interpenetrated frameworks via progressively higher degrees of interpenetration that involves no external reagents. Autocatenation maintains crystallinity and can be triggered either thermally or by shear forces. The ligand used to construct these MOFs is chiral, and both racemic and enantiopure partially interpenetrated frameworks can be accessed. X-ray diffraction, nonlinear optical microscopy and theoretical calculations offer insights into the structures and dynamic behaviour of these materials and the growth mechanisms of interpenetrated MOFs.

Three metal-organic frameworks (MOFs) with similar pore window diameters, [Cu(3)(BTC)(2)], MIL-47 and MIL-53(Al), are tested for adsorption of olefins, alkylnaphthalenes and dichlorobenzenes in the liquid phase. Selective adsorption of olefins is possible only on [Cu(3)(BTC)(2)] viapi-complexation on its open metal sites. This material shows a remarkable preference for cis-olefins over trans-olefins. All three MOFs have high affinities for alkylnaphthalene and dichlorobenzene isomers. Separation of 1,4-dimethylnaphthalene from other alkylnaphthalene isomers and of p- and m-dichlorobenzene can be carried out on both MIL-47 and MIL-53(Al), as shown with breakthrough experiments. For the alkylnaphthalenes, column experiments at different concentrations point to enthalpic interactions as important factors determining selectivity, and the occurrence of steric effects during the adsorption of 1,4-dimethylnaphthalene shows that its kinetic diameter approaches the pore diameter of the adsorbents. For the dichlorobenzenes, packing effects dominate the adsorption selectivity.