New insights into solvent-induced structural changes of C-13 labelled metal-organic frameworks by solid state NMR

Selective C-13-labelling of carboxylate carbons in the linker molecules of flexible metal-organic frameworks (MOFs) makes solid-state NMR spectroscopy very powerful to investigate solvent-induced local structural changes as demonstrated by C-13 and H-1 NMR spectroscopy on the pillared layer MOF DUT-8(Ni). Selective identification of polar solvent-node interactions becomes feasible

Impact of Defects and Crystal Size on Negative Gas Adsorption in DUT-49 Analyzed by in Situ ¹²⁹Xe NMR Spectroscopy

The origin of crystal-size-dependent adsorption behavior of flexible metal-organic frameworks is increasingly studied. In this contribution, we probe the solid-fluid interactions of DUT-49 crystals of different size by in situ 129Xe NMR spectroscopy at 200 K. With decreasing size of the crystals, the average solid-fluid interactions are found to decrease reflected by a decrease in chemical shift of adsorbed xenon from 230 to 200 ppm, explaining the lack of adsorption-induced transitions for smaller crystals. However, recent studies propose that these results can also originate from the presence of lattice defects. To investigate the influence of defects on the adsorption behavior of DUT-49, we synthesized a series of samples with tailored defect concentrations and characterized them by in situ 129Xe NMR. In comparison to the results obtained for crystals with different size, we find pronounced changes of the adsorption behavior and influence of the chemical shift only for very high concentrations of defects, which further emphasizes the important role of particle size phenomena

Role of particle size and surface functionalisation on the flexibility behaviour of switchable metal-organic framework DUT-8(Ni)

Flexible MOF nanoparticles, i.e. MOF nanoparticles that change their structure upon external stimuli such as guest uptake, are promising for numerous applications including advanced gas adsorption, drug delivery and sensory devices. However, the properties of MOFs are typically characterised based on the bulk material with no consideration of how the particle size and external surface influences their performance. This combined computational and experimental contribution investigates the influence of the particle size and surface functionalisation on the flexibility of DUT-8(Ni) (Ni2 (2,6-ndc)2 dabco, ndc = naphthalene dicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane, DUT=Dresden University of Technology). DUT-8 nanoparticles remain rigid in their open pore form while microparticles, synthesised under slightly different conditions, undergo gate opening upon nitrogen adsorption suggesting that the particle size has an important role to play in the flexibility of DUT-8. While the adsorption environment at the surface capped with modulators smaller than the 2,6-ndc ligand is very different compared to the bulk of the crystal with considerably weaker guest-framework interaction, simulations reveal that the nanoparticles should close. We conclude that the size of the nanoparticles is not the major contributor for keeping DUT-8 nanoparticles open but that it is more likely that defects or nucleation barriers dominate. Moreover, our work reveals for the first time that functionalising the external surface of nanoparticles with different modulators or capping groups offers the opportunity to manipulate the gate opening / closing pressure. This principle is generally applicable and could be exploited to tune the gate openig / closing pressure for the application of interest

Cooperative light-induced breathing of soft porous crystals via azobenzene buckling

Although light is a prominent stimulus for smart materials, the application of photoswitches as light-responsive triggers for phase transitions of porous materials remains poorly explored. Here we incorporate an azobenzene photoswitch in the backbone of a metal-organic framework producing light-induced structural contraction of the porous network in parallel to gas adsorption. Light-stimulation enables non-invasive spatiotemporal control over the mechanical properties of the framework, which ultimately leads to pore contraction and subsequent guest release via negative gas adsorption. The complex mechanism of light-gated breathing is established by a series of in situ diffraction and spectroscopic experiments, supported by quantum mechanical and molecular dynamic simulations. Unexpectedly, this study identifies a novel light-induced deformation mechanism of constrained azobenzene photoswitches relevant to the future design of light-responsive materials

The impact of defects and crystal size on negative gas adsorption in DUT-49 analyzed by in situ 129Xe NMR spectroscopy.

The origin of crystal size-dependent adsorption behavior of flexible metal-organic frameworks is increasingly studied. In this contribution, we probe the solid-fluid interactions of DUT-49 crystals of different size by in situ 129Xe NMR spectroscopy at 200 K. With decreasing size of the crystals, the average solid-fluid interactions are found to decrease reflected by a decrease in chemical shift of adsorbed xenon from 230 to 200 ppm explaining the lack of adsorption-induced transitions for smaller crystals. However, recent studies propose that these results can also originate from the presence of lattice defects. To investigate the influence of defects on the adsorption behavior of DUT-49 we synthesized a series of samples with tailored defect concentrations and characterized them by in situ 129Xe NMR. In comparison to the results obtained for crystals with different size we find pronounced changes of the adsorption behavior and influence of the chemical shift only for very high concentrations of defects, which further emphasizes the important role of particle size phenomena

Monitoring Dynamics, Structure, and Magnetism of Switchable Metal-Organic Frameworks via 1 H-Detected MAS NMR

International audienceWe present a toolbox for the rapid characterisation of powdered samples of paramagnetic metal–organic frame-works at natural abundance by 1H-detected solid-state NMR. Very fast MAS rates at room and cryogenic temperatures and a set of tailored radiofrequency irradiation schemes help overcome the sensitivity and resolution limits often associated with the characterisation of MOF materials. We demonstrate the approach on DUT-8(Ni), a framework containing Ni2+ paddle-wheel units which can exist in two markedly different architectures. Resolved 1H and 13C resonances of organiclinkers are detected and assigned in few hours with only 1– 2 mg of sample at natural isotopic abundance, and used to rapidly extract information on structure and local internal dynamics of the assemblies, as well as to elucidate the metal electronic properties over an extended temperature range. The experiments disclose new possibilities for describing local and global structural changes and correlating them to electronic and magnetic properties of the assemblies

Single particle Raman spectroscopy analysis of the metal-organic framework DUT-8(Ni) switching transition under hydrostatic pressure

Experimental in situ observations of phase coexistence in switchable metal-organic frameworks are reported to provide a fundamental understanding of dynamic adsorbents that can change their pore structure in response to external stimuli. A prototypical flexible pillared layer framework DUT-8(Ni) (DUT = Dresden University of Technology) was studied under hydrostatic pressure by in situ Raman spectroscopy on single crystals. The closing transition of the open pore phase (op) containing DMF in the pores in silicon oil as a pressure transmitting fluid, as well as the closed pore phase (cp) to op transition under pressure in methanol, were studied. Phase coexistences during both transitions were observed

CCDC 1983549: Experimental Crystal Structure Determination

Related Article: Simon Krause, Florian S. Reuter, Sebastian Ehrling, Volodymyr Bon, Irena Senkovska, Stefan Kaskel, Eike Brunner|2020|Chem.Mater.|32|4641|doi:10.1021/acs.chemmater.0c0105

Tailoring Adsorption Induced Switchability of a Pillared Layer MOF by Crystal Size Engineering

The pillared layer framework DUT-8(Zn) (Zn2(2,6-ndc)2(dabco), 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, DUT = Dresden University of Technology) is a prototypical switchable MOF, showing characteristic adsorption and desorption induced open phase (op) to closed phase (cp) transformation associated with huge changes in cell volume. We demonstrate switchability strongly depends on a framework-specific critical particle size (dcrit). The solvent removal process (pore desolvation stress contracting the framework) significantly controls the cp/op ratio after desolvation and, subsequently, the adsorption induced switchability characteristics of the system. After desolvation, the dense cp phase of DUT-8(Zn) shows no adsorption-induced reopening and therefore is non-porous for N2 at 77 K and CO2 at 195 K. However, polar molecules with a higher adsorption enthalpy, such as the polar molecules such as chloromethane at 249 K and dichloromethane (DCM) at 298 K can reopen the macro-sized crystals upon adsorption. For macro-sized particles, the outer surface energy is negligible and only the type of metal (Zn, Co, Ni) controls the DCM-induced gate opening pressure. The framework stiffness increases from Zn to Ni as confirmed by DFT calculations, X-ray crystal structural analyses, and low frequency Raman spectroscopy. The partial disintegration of the Zn based node hinges produces an overall increased stabilization of cp vs. op phase shifts the critical particle size at which switchability starts to become suppressed to even lower values (dcrit dcrit ≈ 500 nm). Hence, the three factors affecting switchability (energetics of the empty host, (Eop-Ecp) (I), particle size (II), and desolvation stress (III)) appear to be of the same order of magnitude and should be considered collectively, not individually

Spatiotemporal Design of the Metal-Organic Framework DUT-8(M)

Switchable metal-organic frameworks change their structure in time and selectively open their pores adsorbing guest molecules, leading to highly selective separation, pressure amplification, sensing and actuation applications. The three-dimensional engineering of metal-organic frameworks has reached a high level of maturity, but spatiotemporal evolution opens a new perspective towards engineering materials in the 4th dimension (time) by t-axis design, in essence exploiting the deliberate tuning of activation barriers. This work demonstrates the first example in which an explicit temporal engineering of a switchable metal-organic framework (DUT-8, M1M2(ndc)2dabco, ndc = 2,6,-naphthalenedicarboxylate, dabco = 1,4 diazabicyclo[2.2.2]octane, M1 = Ni, M2 = Co) is presented. The temporal response is deliberately tuned by variation of cobalt content. We present a spectrum of advanced analytical methods for analyzing the switching kinetics stimulated by vapor adsorption using in situ time resolved techniques ranging from ensemble adsorption and advanced synchrotron X-ray diffraction experiments to individual crystal analysis. A novel analysis technique based on microscopic observation of individual crystals in a microfluidic channel reveals the lowest limit for adsorption switching reported so far. The time constants for the bulk ensembles range from 2 - 300 s. Differences in spatiotemporal response of crystal ensembles originate from a delay (induction) time that varies statistically and widens characteristically with increasing cobalt content reflecting increasing activation barriers