Porous Organic Cage Thin Films and Molecular‐Sieving Membranes

Porous organic cage molecules are fabricated into thin films and molecular-sieving membranes. Cage molecules are solution cast on various substrates to form amorphous thin films, with the structures tuned by tailoring the cage chemistry and processing conditions. For the first time, uniform and pinhole-free microporous cage thin films are formed and demonstrated as molecular-sieving membranes for selective gas separation. Microporous materials, such as conventional zeolites and activated carbons, are important in a wide range of applications including catalysis, gas storage, energy storage, and molecular separations. In recent years, organic or hybrid microporous materials with well-defined structure have attracted significant attention, such as metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) 1 and covalent organic frameworks (COFs).2 These extended 3D networks exhibit well-defined pore structures, chemical diversity, and in some cases ultrahigh surface areas. There is also increasing interest in the design and synthesis of porous molecular materials: that is, discrete organic molecules and macromolecules where intermolecular forces are dominated by noncovalent interactions in the solid state.3 Porous molecular materials have some potential advantages with respect to porous extended networks: for example, they can be dissolved in common organic solvents. This enhances their solution processability, allowing them to be cast or combined with other porous materials as composites.4 Likewise, due to the lack of intermolecular covalent bonding in porous molecular assemblies, porous molecules can exhibit structural mobility, which allows cooperative interactions between the host and guests.5 As an extreme case, it is also possible to prepare “porous liquids” by dissolving porous cages at high concentrations in a solvent that is size-excluded from the cage cavity.6 Porous materials have been extensively studied in the bulk, solid state for adsorption and separation processes. Compared to adsorption-based separation processes, membrane separation technology can often be more energy-efficient for molecular-level separations of gases and organic chemicals. In a broad context, fabrication of soluble, molecular microporous materials into functional thin films would also allow a wider range of applications beyond membranes, such as sensing, energy storage, and optoelectronics.7 However, the fabrication of crystalline molecular solids into thin films with well-defined structure is a challenging task. Significant progress has been made with the sophisticated fabrication of MOFs into thin films or membranes using techniques such as secondary crystal growth,8 in situ synthesis within a support,9 solution-stacking of MOF nanosheets,10 and polymer-MOF composites by coordination-driven in situ self-assembly11 or incorporation into a polymer matrix.12 By contrast, industrial membranes are dominated by polymers that are fabricated by simple one-step solution-processing techniques. Porous organic polymers and molecules that combine microporosity and solution processability are hence highly desirable to fabricate the next generation porous membranes. Notable examples are polymers of intrinsic microporosity (PIMs),13 a new class of polymers with unique rigid and contorted macromolecular structure and interconnected free volume that behaves like micropores (dimensions 10 wt%) generate hierarchically porous films with interconnected nanoparticles with sizes at ≈100 nm (Figure 2g), owing to the phase separation induced by solvent evaporation. XRD patterns confirmed that both dense and porous films are amorphous (Figure S8, Supporting Information). Similar phase separation phenomenon occurred for RCC3 cages and consequently macropores were formed in the films when the cage molecules were spin-coated at low speed (Figure Supporting Information). For the more FT-RCC3, hierarchically porous thin films can also be As the solubility of FT-RCC3 in chloroform or DCM was very a small amount of methanol was to the solvent to the solubility of cage molecules to wt%) that films could be but hierarchically porous films were then (Figure and Figure Supporting Information). In this case, the in the XRD the of small These hierarchically porous cage thin films be in a range of such as for films, or for These cage thin films have potential for on or Cage molecules were spin-coated on thin glass substrates to form thin films with a thickness of 100 the cage thin films were to solids in a glass of the was in the cage thin films, as by a in this film (Figure We the as a of adsorption As shown in Figure in the Supporting the spectra showed a strong at nm upon to The of to the amount of molecules in the cage films. The reactions of with POC molecules, and hence of the molecules is and that it is possible to similar reaction to the cage thin films. We further fabricated POC thin-film membranes by spin coating cage molecules on porous The films in this work were spin-coated on the membrane without any coating or The of cage molecules are small that they can through the surface layer of of and into the of (Figure and defects to low to diffusion and a significant of a continuous film is defect-free films could be prepared by spin coating cage solutions on the surface of the as shown in Figure and Figure in the Supporting Information. cage molecules to be in the nm in the but they form a continuous and defect-free We XRD of these thin films and confirmed that they are amorphous (Figure Supporting Information). The of defect-free films was significantly enhanced by using more cage solutions to wt%) (Figure concentration the viscosity of the cage solution and consequently results in films while the of the cage solution into the of the was also the spinning In some it is to the thickness for the thin-film membranes because of the into the in the that occurred for some cage molecules, including ASPOC, CC13, and the of of cage molecules into the substrates to be related to the structural of the cage molecules. For the RCC3 the cannot form a continuous film at the surface (Figure Supporting Information). In contrast, the more rigid FT-RCC3 cages showed phase separation as also observed on nonporous substrates (Figure Supporting Information). In polymer thin films were also prepared from a and a microporous polymer are given in Figure Supporting Information), as for of gas transport transport in materials the where the gas molecules are in the free volume and through the In the case of microporous cage thin films, gas transport can also be by the To have a of the gas solubility and and their with the we gas properties of cage solids using various techniques. As shown in Figure of ASPOC and amorphous CC3 confirmed the of microporosity in these disordered We also gas at (Figure and the gas solubility at 1 (Figure are given in Figure (Supporting Information). The gas solubility the of to the of gas molecules. These of gas solubility in cage molecules are to in the high in cage solids the solubility in micropores or free volume and intermolecular interactions between gas molecules and We further demonstrated that cage thin-film membranes show molecular-sieving properties in terms of high and molecular selectivity. gas transport properties of cage membranes were at with important gas molecules with different including and using a we the high gas through the (Figure Supporting Information), for example, and are and Therefore, the gas of the can be in membranes. In we also the gas transport properties of polymer thin-film membranes spin-cast from a dense and a microporous For example, thin film gives an high permeability of while gives a permeability of These gas transport properties are very to with polymer films in the Therefore, these polymer thin films as for the gas transport properties of cage thin films. for three representative cage thin-film membranes, CC3, ASPOC, and CC13, are shown in Figure The here are the at the when the More are shown in Figure and S1 in the Supporting Information. For a CC3 membrane spin-coated with 1 wt% cage solution in a co-solvent of DCM and methanol concentration of 2 wt%) at the gas the order of The of is as high as with high of and a to The is up to with of and a of The gas of CC3 thin films are than that for MOF membranes or membranes S1 in the Supporting because our cage films are much to In the case of ASPOC films, gas are owing to of the cages into the a solution wt% in was CC13, which has groups shows much but still shows molecular sieving For ASPOC, the of is high which is in with adsorption of and high solubility of is similar to that observed in polymers, such as and We then the gas permeability from by thickness We the as observed in SEM and this to be the thickness of the We then the We further the gas transport properties in the of gas is shown in Figure For a representative CC3 the permeability is while CC13 cages show permeability of these cage thin films have low permeability to polymer thin films permeability to but they are still much more than conventional polymers permeability of at Similar results were for other gas as shown in Figure in the Supporting Information. on the gas solubility and we the diffusion for various gas molecules The gas can be with molecular as in Figure For ASPOC, was as high as while and were and The cage thin films show high for gas and Similar results were for amorphous CC3 thin films. These the molecular sieving properties of cage thin films, their diffusion are than of rigid polymer on of polymer thin films have shown that of free volume with time result in significant of we observed significant in gas permeability for cage thin films (Figure and Supporting Information). with different structures show As shown in Figure ASPOC cages high permeability of but with low of The permeability for while for gas permeability to while the to permeability dropped to with increasing up to the gas but the gas permeability to For CC3 the was but the permeability also In the case of CC3 film both permeability and showed These films amorphous for The of cage thin films are In this study, a very thin layer of cage molecules were coated on the to form thin-film membranes, of cage membranes. Therefore, polymer and surface areas of cage membranes and use were easily the permeability in cage membranes significantly in for ASPOC gas of cage solids at high for permeability is a of solubility and the of gas permeability be to in the materials these amorphous cage thin films are in that are the spin coating, which leads to an within the The are in state and tend to pack more densely to free volume we a for amorphous cage and gas and diffusion in amorphous organic cage We that the of the extrinsic and intrinsic porosity to the porosity has a significant on pore connectivity and gas diffusion packing of cage molecules in the amorphous thin-film state would result in a of extrinsic which the gas diffusion of connectivity of free volume or whether intrinsic or and consequently the gas permeability in the cage thin films. The membranes fabricated from porous organic cage molecules are for applications due to and chemical and for example, in separation of or separation of at high For example, of the in cage molecules upon to water, and conditions. Further work is in progress to more films, for example, cage thin films to more as or intermolecular forces into cages to form rigid and networks by bonding interactions between or covalent the other the of cage solids and thin films be in other for example, of guest molecules or In we have demonstrated that porous organic cages can be solution-processed into thin films with tunable structure and The thin films show potential in molecular and selective membranes for molecular-level separations. The fabrication via spin coating is a and easily process that uniform and defect-free thin films with thickness of fabrication is by the solution processability of POC molecules as building work also leads to of the that the and in these with which we could generate hierarchically porous cage thin films that as for functional We further demonstrate the of membranes fabricated from porous organic cage molecules. The solution processability of POCs allows coating and of various membranes to form membranes for separation of and important gas molecules. these molecular cage films tend to time, as by in gas permeability they are as shown by work a simple and approach that the fabrication of cage thin films and membranes, for example in applications such as functional sensing, catalysis, separation membranes, and energy of Porous the CC3, CC13, ASPOC, and FT-RCC3 were synthesized CC3 was synthesized from reaction between 1,3,5-triformylbenzene (TFB) and (1R,2R)-1,2-cyclohexanediamine (CHDA) in dichloromethane at The were by and with and CC13 was synthesized by reactions between 2-methyl-1,2-propanediamine and by precipitation in and further Amorphous scrambled porous organic cages (ASPOC) were synthesized by of with a mixture of both and leads to an equilibrium distribution of both and vertices in a cage RCC3 was synthesized by CC3 with in a mixture of chloroform and FT-RCC3 was prepared by the RCC3 cages with in methanol at The reaction was to and the FT-RCC3 was and with methanol and further of Cage The cage solids were dissolved in solvents to cage solution with a In some co-solvents of methanol and dichloromethane (DCM) or chloroform was used with The cage solution was then purified by filters μm). cage thin films were prepared by spin coating the cage solution on different substrates. of cage solution is on the substrate which is spinning the substrate is at high speed at with an speed of were using a The films were in and coated with a layer of using an X-ray diffraction (XRD) was with a at 40 and 40 using with a of Cage films for XRD were prepared with various cages were coated on low-background spectra were on a Cage films were coated on and in spectra were using a with an range of adsorption and were at using or adsorption samples were at for dynamic of and were also at with up to 1 and The cage solution wt%) were spin-coated on substrate at a speed of to spin coating, were with and in for The thin films coated on were to solids to allow the adsorption of in the films. The spectra were as a of of membranes on POCs were prepared by spin-coating cage solutions on porous membrane with a surface layer of nm were used as porous The gas through the are significantly than in cage thin films. The membrane as ordered has an The membrane was on a PTFE substrate for The membrane is was to and the from spin-coating to gas gas were at of and of 1 using a in The membrane was in the and with a to gas The gas was The for a gas is from the of their The of the at the of are within and within permeability was from the with the thickness of the layer of POC film is in The solubility of gas molecules in POCs were from gas up to 1 at The diffusion 1 is from the permeability This work was by the and and an from the of an of the and As a to our and this by the materials are and be for but are or from than be to the The is for the or of any by the than be directed to the for the