Fluorocarbon-Functionalized Superhydrophobic Metal-Organic Framework: Enhanced CO2 Uptake via Photoinduced Postsynthetic Modification

The design and synthesis of porous materials for selective capture of CO₂ in the presence of water vapor is of paramount importance in the context of practical separation of CO₂ from the flue gas stream. Here, we report the synthesis and structural characterization of a photoresponsive fluorinated MOF {[Cd(bpee)(hfbba)]·EtOH}_n (1) constructed by using 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (hfbba), Cd(NO₃)₂, and 1,2-bis(4-pyridyl)ethylene (bpee) as building units. Due to the presence of the fluoroalkyl -CF₃ functionality, compound 1 exhibits superhydrophobicity, which is validated by both water vapor adsorption and contact angle measurements (152°). The parallel arrangement of the bpee linkers makes compound 1 a photoresponsive material that transforms to {[Cd₂(rctt-tpcb)(hfbba)₂]·2EtOH}_n (rctt-tpcb = regio cis,trans,trans-tetrakis(4-pyridyl)cyclobutane; 1IR) after a [2 + 2] cycloaddition reaction. The photomodified framework 1IR exhibits increased uptake of CO₂ in comparison to 1 under ambient conditions due to alteration of the pore surface that leads to additional weak electron donor-acceptor interactions with the -CF₃ groups, as examined through periodic density functional theory calculations. The enhanced uptake is also aided by an expansion of the pore window, which contributes to increasing the rotational entropy of CO₂, as demonstrated through force field based free energy calculations.

High-throughput screening of metal-organic frameworks for kinetic separation of propane and propene

We apply molecular simulations to screen a database of reported metal-organic framework structures from the computation-ready, experimental (CoRE) MOF database to identify materials potentially capable of separating propane and propene by diffusion. We report a screening workflow that uses descriptor analysis, conventional molecular dynamics (MD), and Nudged Elastic Band (NEB) energy barrier calculations at both classical force field and Density Functional Theory (DFT) levels. For the first time, the effects of framework flexibility on guest transport properties were fully considered in a screening process and led to the identification of candidate MOFs. The hits identified by this proof-of-concept workflow include ZIF-8 and ZIF-67 previously shown to have large differences in propane and propene diffusivities as well as two other materials that have not been tested experimentally yet. This work emphasises the importance of taking into account framework flexibility when studying guest transport in porous materials, demonstrates the potential of the data-driven identification of high-performance materials and highlights the ways of improving the predictive power of the screening workflow.

Unraveling the Effect on Luminescent Properties by Postsynthetic Covalent and Noncovalent Grafting of gfp Chromophore Analogues in Nanoscale MOF-808

Here, we demonstrate mimicking of photophysical properties of native green fluorescent protein (gfp) by immobilizing the gfp chromophore analogues in nanoscale MOF-808 and further exploring the bioimaging applications. The two virtually nonfluorescent gfp chromophore analogues carrying different functionalities, BDI-AE (COOH/COOMe) and BDI-EE (COOMe/COOMe) were immobilized in nanosized MOF-808 via postsynthetic modification. An ¹H NMR and IR study confirms that BDI-AE was coordinated in NMOF-808, whereas BDI-EE was just noncovalently encapsulated. Interestingly, the extremely weakly fluorescent monomers BDI-AE and BDI-EE (QY = 0.01-0.03%, lifetime = 0.01-0.03 ns) showed a 10²-fold increase in quantum efficiency with a significantly longer excited-state lifetime (QY = 1.8-5.6%, lifetime 0.89-1.49 ns) after immobilization in the NMOF-808 scaffold. Moreover, BDI-AE@MOF-808 has 4 times higher quantum efficiency as well as longer excited-state lifetime in comparison to BDI-EE@NMOF-808 due to the rigidity imposed in the chromophore upon coordination with Zr⁴⁺ in the former case. Further, a cell viability test performed for BDI-AE@NMOF-808 in HeLa cells confirmed the nontoxic nature of the material and, more importantly, bioimaging applications have also been explored successfully.

Double-click synthesis of polysiloxane third-order nonlinear optical polymers with donor–acceptor chromophores

A series of third-order nonlinear polysiloxane polymer materials were prepared by thiol–ene click polymerization and [2 + 2] click chemistry. All the polymers exhibit good electron transfer capabilities and nonlinear optical properties.

Reversible photo/thermal solid-state transformation of a coordination polymer

A 2D coordination polymer, 1, was synthesized, characterized, and investigated with single-crystal-to-single-crystal photoreaction and thermal pyrolysis.

CO2-induced single-crystal to single-crystal transformations of an interpenetrated flexible MOF explained by in situ crystallographic analysis and molecular modeling

A molecular-level investigation is reported on breathing behaviour of a metal-organic framework (1) in response to CO2 gas pressure. High-pressure gas adsorption shows a pronounced step corresponding to a gate-opening phase transformation from a closed (1cp ) to a large-pore (1lp ) form. A plateau is observed upon desorption corresponding to narrow-pore intermediate form 1np which does not occur during adsorption. These events are corroborated by pressure-gradient differential scanning calorimetry and in situ single-crystal X-ray diffraction analysis under controlled CO2 gas pressure. Complete crystallographic characterisation facilitated a rationalisation of each phase transformation in the series 1cp → 1lp → 1np → 1cp during adsorption and subsequent desorption. Metropolis grand-canonical Monte Carlo simulations and DFT-PBE-D3 interaction energy calculations strongly underpin this first detailed structural investigation of an intermediate phase encountered upon desorption.

A reversible photochemical solid-state transformation in an interpenetrated 3D metal-organic framework with mechanical softness

We synthesized a two-fold interpenetrated 3D MOF with two crystallographically distinct C[double bond, length as m-dash]C bonds, which undergoes [2+2] photo-cycloaddition and thermal reversible reaction, in a single-crystal-to-single-crystal (SCSC) manner. The softer nature and comparable mechanical properties of the crystals of the parent and cyclized MOFs revealed by nanoindentation allowed rationalizing their structural softness and SCSC transformation behaviour.

Spin-crossover modulation via single-crystal to single-crystal photochemical [2 + 2] reaction in Hofmann-type frameworks

This study reports the first modulation of spin-crossover (SCO) behavior via a photochemical [2 + 2] cycloaddition reaction. Here we construct two no-solvent Fe(ii)-Ag(i) bimetallic Hofmann-type frameworks, [Fe(4-spy)2{Ag(CN)2}2] (1) and [Fe(2,4-bpe)2{Ag(CN)2}2] (2) (4-spy = 4-styrylpyridine, 2,4-bpe = trans-1-(2-pyridyl)-2-(4-pyridyl)ethylene). For 1, the dimerization of 4-spy results in a single-crystal to single-crystal (SCSC) transformation from 2D interdigitated layers to a 3D interpenetrated structure. Additionally, a 3D → 3D structural transformation accompanied with Ag(i)-N bond breaking is achieved via the photochemical cycloaddition reaction of 2,4-bpe in 2. More importantly, both the spin transition temperatures and the SCO character are effectively modulated; thus, this approach provides a new strategy for constructing photo-responsive SCO magnetic materials.

Highly selective C2H2 and CO2 capture and magnetic properties of a robust Co-chain based metal–organic framework

A robust Co-MOF based on uncommon 1D alternate Co₄ chain units was synthesized, which efficiently takes up C₂H₂ and CO₂ with significant selectivity for C₂H₂ and CO₂ over CH₄, as well as shows a slow freezing process of magnetic behavior.

Functionalization of Metal-Organic Frameworks for Photoactive Materials

Metal-organic frameworks (MOFs) are intriguing platforms with multiple functionalities. Additional functionalization of MOFs generates novel materials for various applications. Here, three main topics are examined regarding the functionalization of MOFs for use as photoactive materials. The first is chemical approaches for postsynthetic modification of the metal clusters and organic linkers in MOFs; that is, sites on pore surfaces and chemical trapping of photoactive moieties within the pores, which create materials with chemical functionalities for water splitting and CO₂ reduction by light. The second topic focuses on the functionalization of MOFs for photochemical response and the versatile applications of such materials. State-of-the-art research on functionalizing MOFs through photochemical reactions on the pore surface and within the pores as guests is also summarized. The third topic introduces the functionalization of MOFs for photofunctional materials, including photoluminescent tuning and integration, photoluminescent LED devices and barcodes, and photophysical applications for chemical sensing. Finally, conclusions and perspectives on the fields are given.

Tetracarboxylate Linker-Based Flexible CuII Frameworks: Efficient Separation of CO2 from CO2/N2 and C2H2 from C2H2/C2H4 Mixtures

We report the synthesis, structure, and adsorption properties of two new metal-organic frameworks (MOFs) {[Cu₂(bpp)₃(L1)]·(bpp)·(4H₂O)} (1) and {[Cu₂(bipy)₂(L2)(H₂O)₂]·(bipy)·(5H₂O)} (2) obtained from two different flexible tetracarboxylate linkers (L1 and L2) of variable lengths and flexibility. While 1 comprising Cu^II, L1, and 1,3-bis(4-pyridyl) propane (bpp) is a 2D MOF with a cage-type structure, 2 consisting of Cu^II, L2, and 4,4'-bipyridine (bipy) has a 3D twofold interpenetrated structure. Both frameworks manifest permanent porosity, as realized from CO₂ adsorption at 195 K. 2 shows excellent CO₂/N₂ and C₂H₂/C₂H₄ adsorption selectivity at 298 K. This has been established by using 2 as a separating medium in a breakthrough column for separating mixtures of CO₂/N₂ (15:85, v/v) and C₂H₂/C₂H₄ (1:99, v/v). The selectivity of 2 toward CO₂ over N₂ and C₂H₂ over C₂H₄ is governed by favorable thermodynamic interactions owing to its structural flexibility, unsaturated metal sites, and polar carboxylate groups. Thus, 2 proves to be an extremely efficient material for specific gas separation.