Flexible Covalent Organic Frameworks:Design,Synthesis,and Applications

Structural flexibility is an intriguing characteristic observed in materials that endows them with dynamic properties,allowing for intelligent adaptation and deformation in response to varying conditions.Covalent organic frameworks(COFs),a promising class of crystalline porous polymers,have garnered significant attention due to their unique features,including well-ordered,predesignable,and tunable structures,as well as their light-weighted nature and high thermal/chemical stability.Integrating flexibility into COFs offers a captivating pathway for developing advanced materials with dynamic properties and versatile functionalities.The flexible COF field is currently experiencing rapid expansion,and there is a growing need for systematic review articles that offer a comprehensive overview of the developments in this domain.In this mini-review,we delve into the factors that contribute to structural flexibility in COFs from the point of structural design.We summarize and categorize various modes of achieving flexibility across different dimensions,and discuss the potential applications of flexible COFs in vapor/gas adsorption,sensing,and smart membrane separations;further,we highlight their prospects and future research advancement.

Dynamic properties of a flexible metal-organic framework exhibiting a unique"picture frame"-like crystal morphology

The precise control of the crystal morphology of metal-organic frameworks(MOFs)enables optimization of its adsorptive properties,as well as enables better integration within functional devices.However,the influence of such modifications on the dynamic properties of flexible MOFs is poorly understood Here,we report the synthesis of a series of Cu_(2)(bdc)_(2)(bpy)(bdc^(2-)=1,4-benzenedicarboxylate;bpy=4,4,-bipyridine)crystals having an unusual picture frame-like morphology that results from a restriction in the quantity of bpy pillars added to the reaction mixture during the intercalation of the Cu_(2)(bdc)_(2)(MeOH)_(2)layers.The width of the frames is found to correlate with the quantity of bpy,and importantly,causes the dynamic properties of the resulting Cu_(2)(bdc)_(2)(bpy)material to vary between rigid,elastic,and shape memory modes.In all,the results demonstrate the potential for the properties of MOFs to be optimized via subtle manipulations in the crystal morphology rather than changes in the overall material composition.

Cucurbit[7]uril-threaded flexible organic frameworks: Quantitative polycatenation through dynamic covalent chemistry

A three-dimensional flexible organic framework FOF-1 has been synthesized from the condensation of a tetratopic acylhydrazine and a rigid 4,4-diphenyl-4,4-bipyridinium dialdehyde in water through the quantitative formation of hydrazone bond. FOF-1 is further applied to construct a polycatenane framework FOF-pc-1 through the quantitative cucurbit[7]uril encapsulation for the diphenylbipyridinium subunits of the framework by making use of the dynamic nature of the hydrazone bond in water. The bipyridinium subunits in both frameworks can be reduced their radical cation counterparts to produce conjugated radical cation-linked dynamic organic frameworks rc-FOF-1 or rc-FOF-pc-1. Polycatenation is revealed to enhance the stability of the dynamic frameworks in water, whereas depolycatenation can be reached for both FOF-pc-1 and rc-FOF-pc-1 by using a ferrocene guest to form a more stable complex with CB[7].

Flexible Organic Framework-Modified Membranes for Osmotic Energy Harvesting

Bioinspired membranes are advantageous in capturing the osmotic energy.However,the conventional hybrid membranes possess low harvesting power density due to their low interfacial ionic transport efficiency and high internal resistance.Herein,a new kind of hybrid membranes consisting of porous polymer and flexible organic frameworks was developed.The 3D porous framework enables ions to flux with high efficiency at the polymer-framework interface,resulting in high osmotic energy harvesting efficiency.By systematically screening the pore size of the frameworks,the output power density as high as 5.7 W/m^(2) was achieved under 50-fold KCl salinity gradient.

Thermal expansion behaviors of Mn(Ⅱ)-pyridylbenzoate frameworks based on metal-carboxylate chains

Solvothermal reactions of MnCl2with pyridylbenzoic acids gave three-dimensional metal-carboxylate frameworks(MCFs),named MCF-34,MCF-43,and MCF-44,based on one-dimensional Mn-carboxylate chains.The crystal structure,stability,porosity,and framework flexibility of the new compound MCF-44 were studied in detail and compared with its analogs.Depending on their shapes and the bridging angles of the ligands,these compounds possess different network connectivities and porosities.Considering the pyridylbenzote ligands and Mn(II)ions as,respectively,3-and 6-connected nodes,they resemble either the anatase(ant)or rutile(rtl)polymorph of TiO2.Variable-temperature single-crystal X-ray diffraction studies revealed large thermal expansion coefficients for these compounds,which are probably related to the relatively flexible edge-sharing polyhedral structure of their Mn-carboxylate chains.Interestingly,the new compound MCF-44,with its highly porous rtl structure exhibits the largest thermal expansion coefficienct among the coordination polymers reported so far.