Synthesis, characterization and postsynthetic modification of a novel two-dimensional Zr-based metal-organic framework

The focus in many fields of research is the generation of materials with adjustable properties for specific applications. Among porous materials, metal-organic frameworks (MOFs) have developed over the past 20 years to a material class with enormous potential. Due to the modular design of MOFs from metal cations and organic linkers, it is possible to tailor the properties of the resulting extended frameworks by selecting individual components for the needs of specific applications. Careful selection of metal ions and organic molecules with functional groups makes it possible to adjust and influence the pore system, the type of linkage and the physical and chemical properties of the framework. In the present work, the main aspect was the preparation of a novel MOF with high adjustability of its properties, based on light-induced postsynthetic reactions on the organic linker molecules. The framework is composed of Zr4+ ion-based oxo clusters and benzophenone-4,4'-dicarboxylate anions (bzpdc2–) as linker molecules. The benzophenone unit is able to react with any molecule that contains C–H bonds after excitation with photons, resulting in a covalent bond. This reaction opens the possibility of changing the properties of the framework post-synthetically. The novel Zr-bzpdc MOF is chemically and thermally stable and has a two-dimensional structure, which opens up the possibility of obtaining nanometer-thin layers through delamination methods. Further studies have shown that the adaptation of the surface chemistry by the linkage of molecules has a huge impact on the dispersibility in polar and nonpolar solvents. Furthermore, the direct polymerization of a conductive polymer, starting from the surface of the MOF, yields electrically conductive composite materials. Interestingly, in the course of this study it was found that the unmodified MOF is also electrically conductive. Furthermore, a systematic study concerned with the postsynthetic modification of Zr-bzpdc-MOF with alkanes and alcohols of different chain lengths was undertaken. This showed that small hydrophilic molecules react with all linker molecules throughout the crystal, whereas molecules with longer chain lengths and thus more hydrophobic properties only modify the surface of the MOF crystals. The so far largely unaccessed approach to employ specific photochemical modification reactions on the linker molecule the modification of MOFs has been developed here to upgrade the novel Zr-bzpdc-MOF to an unexpectedly versatile compound which is further augmented by the possibility to obtain nanosheets of this substance. Various pathways to adapt this material to specific applications have thus been opened up

Organic gelators as growth control agents for stable and reproducible hybrid perovskite-based solar cells

Low molecular-weight organic gelators are widely used to influence the solidification of polymers, with applications ranging from packaging items, food containers to organic electronic devices, including organic photovoltaics. Here, this concept is extended to hybrid halide perovskite-based materials. In-situ time-resolved grazing incidence wide angle x-ray scattering (GIWAXS) measurements performed during spin-coating reveal that organic gelators beneficially influence the nucleation and growth of the perovskite precursor phase. This can be exploited for the fabrication of planar n-i-p heterojunction devices with MAPbI3 (MA = CH3NH3 +) that display a performance that not only is enhanced by ~25% compared to solar cells where the active layer was produced without the use of a gelator but that also feature a higher stability to moisture and a reduced hysteresis. Most importantly, the presented approach is straight-forward and simple, and it provides a general method to render the film-formation of hybrid perovskites more reliable and robust, analogous to the control that is afforded by these additives in the processing of commodity ‘plastics’

Inside/Outside : Post-Synthetic Modification of the Zr-Benzophenonedicarboxylate Metal–Organic Framework

The Zr-based metal–organic framework, Zr-bzpdc-MOF, contains the photoreactive linker molecule benzophenone-4,4'-dicarboxylate (bzpdc) which imparts the possibility for photochemical post-synthetic modification. Upon irradiation with UV light, the keto group of the benzophenone moiety will react with nearly every C-H bond-containing molecule. Within this paper, we further explore the photochemical reactivity of the Zr-bzpdc-MOF, especially with regard to which restrictions govern internal versus external reactions. We show that apart from reactions with C-H bond-containing molecules, the MOF reacts also with water. By studying the reactivity versus linear alcohols we find a clear delineation in that shorter alcohol molecules (up to butanol as a borderline case) react with photoexcited keto groups throughout the whole crystals whereas longer ones react only with surface-standing keto groups. In addition, we show that with the alkanes n-butane to n-octane, the reaction is restricted to the outer surface. We hypothesize that the reactivity of the Zr-bzpdc-MOF versus different reagents depends on the accessibility of the pore system which in turn depends mainly on the size of the reagents and on their polarity. The possibility to direct the post-synthetic modification of the Zr-bzpdc-MOF (selective modification of the whole pore system versus surface modification) gives additional degrees of freedom in the design of this metal–organic framework for shaping and for applications. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Diffusion across a gel-gel interface-molecular-scale mobility of self-assembled 'solid-like' gel nanofibres in multi-component supramolecular organogels

This paper explores macroscopic-scale diffusion of the molecular-scale building blocks of two-component self-assembled organogel nanofibres using a diffusion cell in which two different gels are in contact with one another. Both components of the 'solid-like' nanofibres (lysine peptide dendron acids and amines) can diffuse through these gels and across a gel-gel interface, although diffusion is significantly slower than that of a non-interactive additive in the 'liquid-like' phase of the gel. Amine diffusion was probed by bringing similar gels with different amines into contact. Dendron acid diffusion was tested by bringing similar gels with enantiomeric dendrons into contact. Surprisingly, dendron and amine diffusion rates were similar, even though the peptide dendron is more intimately hydrogen bonded in the self-assembled nanofibres. It is proposed that thermal disassembly of the acid-amine complex delivers both components into the liquid-like phase, allowing them to diffuse via a decomplexation/recomplexation mechanism. This is a rare observation in which molecules assembled into solid-like gel nanofibres are mobile-in dynamic equilibrium with the liquid-like phase. Gel nanofibre diffusion and reorganisation are vital in understanding dynamic materials processes such as metastability, self-healing and adaptability