Cobalt substitution in a flexible metal-organic framework: modulating a soft paddle-wheel unit for tunable gate-opening adsorption

Spatial distribution modulation of mixed building blocks in metal-organic frameworks

The placement of mixed building blocks at precise locations in metal–organic frameworks is critical to creating pore environments suitable for advanced applications. Here we show that the spatial distribution of mixed building blocks in metal–organic frameworks can be modulated by exploiting the different temperature sensitivities of the diffusion coefficients and exchange rate constants of the building blocks. By tuning the reaction temperature of the forward linker exchange from one metal–organic framework to another isoreticular metal–organic framework, core–shell microstructural and uniform microstructural metal–organic frameworks are obtained. The strategy can be extended to the fabrication of inverted core–shell microstructures and multi-shell microstructures and applied for the modulation of the spatial distribution of framework metal ions during the post-synthetic metal exchange process of a Zn-based metal–organic framework to an isostructural Ni-based metal–organic framework.

Gaining control over the structure of metal organic–frameworks can be challenging. Here the authors report the modulation of the spatial distribution of mixed building blocks in a metal–organic framework from a uniform to a core–shell distribution; temperature control plays a crucial role.

Gating effect for gas adsorption in microporous materials-mechanisms and applications

In the past two decades, various microporous materials have been developed as useful adsorbents for gas adsorption for a wide range of industries. Considerable efforts have been made to regulate the pore accessibility in microporous materials for the manipulation of guest molecules' admission and release. It has long been known that some microporous adsorbents suddenly become highly accessible to guest molecules at specific conditions, e.g., above a threshold pressure or temperature. This anomalous adsorption behavior results from a gating effect, where a structural variation of the adsorbent leads to an abrupt change in the gas admission. This review summarizes the mechanisms of the gating effect, which can be a result of the deformation of the framework (e.g., expansion, contraction, reorientation, and sliding of the unit cells), the vibration of the pore-keeping groups (e.g., rotation, swing, and collapse of organic linkers), and the oscillation of the pore-keeping ions (e.g. cesium, potassium, etc.). These structural variations are induced either by the host-guest interaction or by an external stimulus, such as temperature or light, and account for the gating effect at a threshold value of the stimulus. Emphasis is given to the temperature-regulated gating effect, where the critical admission temperature is dictated by the combined effect of the gate opening and thermodynamic factors and plays a key role in regulating guest admission. Molecular simulations can improve our understanding of the gate opening/closing transitions at the atomic scale and enable the construction of quantitative models to describe the gated adsorption behaviour at the macroscale level. The gating effect in porous materials has been widely applied in highly selective gas separation and offers great potential for gas storage and sensing.

Pillared cobalt metal-organic frameworks act as chromatic polarizers

The ease with which molecular building blocks can be ordered in metal-organic frameworks is an invaluable asset for many potential applications. In this work, we exploit this inherent order to produce chromatic polarizers based on visible-light linear dichroism via cobalt paddlewheel chromophores.

Precise regulating synergistic effect in metal–organic framework for stepwise-controlled adsorption

MAC-20 shows a unique two-step pore-shape change and executes a stepwise-controlled adsorption of dyes mixture in order of their sizes.

Tunable Flexibility and Porosity of the Metal-Organic Framework DUT-49 through Postsynthetic Metal Exchange

As a prominent and representative example of flexible metal-organic frameworks (MOFs), DUT-49(Cu) has gained attention due to the unique phenomenon of negative gas adsorption (NGA), originating from an unprecedented structural contraction during the gas adsorption. Herein, postsynthetic metal exchange is demonstrated to afford DUT-49 frameworks with a wide variety of metal cations, e.g., Mn²⁺, Fe²⁺, Ni²⁺, Zn²⁺, Cu²⁺, and Cd²⁺. The single-crystal-to-single-crystal conversion allowed characterization of the new MOFs by single crystal X-ray diffraction, indicating identical structure and topology compared with that of previously explored DUT-49(Cu) framework. This approach is proven successful in achieving Mn-Mn and Cd-Cd dimers, which are rare examples of M-M paddle-wheel SBUs. The relative stability and flexibility of the resulted frameworks are observed to be highly sensitive to the metal ion of the framework, following the trends predicted by the Irving-Williams series. DUT-49(Ni) was recognized as a second material from the DUT-49 series showing adsorption-induced transitions. A sequential increase in framework flexibility from rigid to flexible and from flexible to NGA has been achieved through selective incorporation of metal centers into the structure. Finally, heterometallic structures are formed by selective and controlled exchange of metal ions to finely tune the flexibility and NGA phenomenon of the framework.

rtl-M-MOFs (M = Cu, Zn) with a T-shaped bifunctional pyrazole-isophthalate ligand showing flexibility and S-shaped Type F-IV sorption isotherms with high saturation uptakes for M = Cu

The flexible, activated MOF rtl-[Cu(HIsa-az-dmpz)] undergoes a reversible phase change into a closed form with gate opening at cryogenic temperatures for N₂ and CO₂.