Two-Photon 3D Printing in Metal-Organic Framework Single Crystals

Two-photon polymerization (TPP) is a micro/nano-fabrication technology for additive manufacturing, enabling 3D printing of polymeric materials using ultrafast laser pulses. In this work, two-photon polymerization is realized inside a metal-organic framework (MOF) crystal. Intricate structures are built in the porous crystal to create a microstructure-in-crystal hybrid. Furthermore, the MOF can be removed by acid treatment to release the printed structure. The two-photon polymerization inside the crystal has the potential for MOF sensing device fabrication and data storage applications. In the future development, printing different materials in the same MOF crystal for creating functional 3D devices is hoped.

Linker Expansion and Its Impact on Switchability in Pillared-Layer MOFs

Linker elongation is an important method to systematically adjust porosity and pore size in isoreticular MOFs. In flexible structures, this approach opens the possibility for the systematic analysis of the building blocks and their contribution to the overall flexible behavior enabling tuning of the framework responsivity toward molecular stimuli. In this work, we report two new compounds isoreticular to the highly flexible pillared layer structure DUT-8(Ni) ([Ni₂(2,6-ndc)₂(dabco)]_n, 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicylo[2.2.2]octane). Aromatic linker 2,6-ndc was substituted by longer carboxylic linkers, namely, 4,4'-biphenyldicarboxylate (4,4'-bpdc) and 4,4'-stilbenedicarboxylate (4,4'-sdc), while the dabco pillar was retained. The structural response of the new compounds toward the desolvation and adsorption of various fluids was studied using advanced in situ PXRD techniques, demonstrating distinct differences in the flexible behavior of three compounds and disclosing the impact of linker structure on the framework response. Theoretical calculations provide mechanistic insights and an energetic rationale for the pronounced differences in switchability observed. The energetics of linker bending and linker-linker dispersion interactions govern the phase transitions in investigated MOFs.

Anthracene-Tagged UiO-67-MOF as Highly Selective Aqueous Sensor for Nanoscale Detection of Arginine Amino Acid

In the present paper, new functionalized UiO-67 metal-organic frameworks (MOF) which contain aromatic tagged groups such as phenyl, naphthyl, and anthracene have been synthesized, characterized, and used for sensing water-soluble amino acids. The results show that anthracene-tagged UiO-67-MOF is shown to act as a highly efficient and selective aqueous sensor for arginine over other water-soluble amino acids in nanoscale. Upon adding an increasing amount of arginine, PL bands of the anth-UiO-67 MOF quenched completely, while there is no perturbation in the PL bands for other amino acid observed. This MOF allows a selective ratiometric detection of arginine without any interference from other amino acids.

Large Differences in Carbon Dioxide and Water Sorption Capabilities in a System of Closely Related Isoreticular Cd(II)-based Mixed-Ligand Metal-Organic Frameworks

We report the synthesis of two isoreticular, mixed-ligand metal-organic frameworks (MOFs), [Cd(μ₂-mia)(μ₂-bpe)]_n·n(DMF)_0.5·n(H₂O)_0.5 (1) and [Cd(μ₂-nia)(μ₂-bpee)]_n·nDMF (2), where mia = 5-methoxyisophthalate, nia = 5-nitroisophthalate, bpe = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene, and DMF = N,N'-dimethylformamide. Variable-temperature powder X-ray diffraction studies confirmed that both MOFs remain crystalline with activation at high temperatures. Variable-temperature single-crystal X-ray diffraction studies were performed on 1, 2, and a previously published, isoreticular structure, [Cd(μ₂-nia)(μ₂-bpe)_1.5]_n·nDMF_0.84 (3). These studies show that upon desolvation that monocrystallinity is retained to significantly higher temperatures for 2 and 3 when compared to 1 for which only a partially desolvated crystal structure could be obtained. Carbon dioxide sorption is negligible for 1 at 195 and 298 K, while it is higher for 2 than 3 at 298 K and reversed at 195 K. Water vapor sorption increases in the order 1, 2, and 3. On the contrary, water liquid sorption was significantly higher for 1 when compared to 2. The variable-temperature structures of the (partially) desolvated forms of 1, 2, and 3 give some insight into the reasons for the remarkably different gas, vapor, and liquid sorption properties.

Gas Adsorption, Proton Conductivity, and Sensing Potential of a Nanoporous Gadolinium Coordination Framework

The new nanoporous framework [Gd₄(di-nitro-BPDC)₄(NO₂)₃(OH)(H₂O)₅]·(solvent) (I; di-nitro-BPDC^2- = 2,2'-dinitrobiphenyl-4,4'-dicarboxylate) has been designed and synthesized through a simple one-pot reaction. In addition to its exceptional thermal and water stabilities, I exhibited multifunctional properties. A sudden CO₂ uptake to a maximum of 4.51 mmol g^-1 (195 K and 1 bar) with notable selectivity over O₂ and N₂ (CO₂/O₂ = 39 at 195 K and 0.10 bar, CO₂/N₂ = 46 at 195 K and 0.10 bar) and an isosteric adsorption enthalpy of 20.7(4) kJ mol^-1 have been revealed. Depending on the temperature and humidity, I also showed distinguished superprotonic conductivities with a maximum value and activation energy of 6.17 × 10^-2 S cm^-1 (55 °C, 99 RH%, and 1 V AC voltage) and 0.43 eV, respectively. With respect to the linear dependence of conductivities on both temperature (25-55 °C at 99 RH%) and humidity (55-99 RH% at 25 °C), the potential of I in temperature and humidity sensing was evaluated, disclosing an excellent sensing resolution and exceptional accuracy, precision, and repeatability for the measurements.

Hydrogen-Bonding Linkers Yield a Large-Pore, Non-Catenated, Metal-Organic Framework with pcu Topology

Pillared paddle-wheel-based metal-organic framework (MOF) materials are an attractive target as they offer a reliable method for constructing well-defined, multifunctional materials. A drawback of these materials, which has limited their application, is their tendency to form catenated frameworks with little accessible volume. To eliminate this disadvantage, it is necessary to investigate strategies for constructing non-catenated pillared paddle-wheel MOFs. Hydrogen-bonding substituents on linkers have been postulated to prevent catenation in certain frameworks and, in this work, we present a new MOF to further bolster this theory. Using 2,2'-diamino-[1,1'-biphenyl]-4,4'-dicarboxylic acid, BPDC-(NH₂)₂, linkers and dipyridyl glycol, DPG, pillars, we assembled a MOF with pcu topology. The new material is non-catenated, exhibiting large accessible pores and low density. To the best of our knowledge, this material constitutes the pcu framework with the largest pore volume and lowest density. We attribute the lack of catenation to the presence of H-bonding substituents on both linkers.

Proton-conduction photomodulation in spiropyran-functionalized MOFs with large on–off ratio

A spiropyran functionalized metal–organic framework is presented, allowing to photomodulate the aqueous proton conductivity by two orders of magnitude.

The Elusive Nitro-Functionalised Member of the IRMOF-9 Family

The solvothermal reaction of 2-nitro-[1,1′‐biphenyl]‐4,4′‐dicarboxylic acid (H2bpdcNO2) with Zn(NO3)2·6H2O in DMF solvent does not give a functionalised variant of IRMOF-9. Single-crystal X-ray diffraction analysis shows the major initial product of this reaction, WUF-21 (WUF=Wollongong University Framework), is a porous interpenetrated diamondoid metal–organic framework (MOF) with a secondary building unit that ‘doubly straps’ eight bridging bpdcNO2 ligands in a distorted tetrahedral shape around an unusual pentazinc core. A second porous MOF phase (WUF-23) containing a large and novel dodecazinc secondary building unit forms in the same reaction and eventually predominates in solutions containing formate anion, which arises from the hydrolysis of DMF. Doping the starting ligand with [1,1′‐biphenyl]‐4,4′‐dicarboxylic acid (H2bpdc) provides a facile way to grow nitro-functionalised IRMOF-9, hereafter denoted as WUF-22, where the dopant is carried through into the product. Activated WUF-22 is a microporous solid with an apparent Brunauer–Emmett–Teller (BET) surface area of 2497m2g−1, which matches well with geometric surface area calculations. The CO2 adsorption properties of WUF-22 are reported.

Two Chemically Stable Cd(II) Polymers as Fluorescent Sensor and Photocatalyst for Aromatic Dyes

Two new 2D Cd(II)-based coordination polymers (CPs), viz. [Cd₂(H₂L)₂(2,2'-bipy)₂] (1) and [Cd(L)0.5(phen)·0.5H₂O] (2), have been constructed using ethylene glycol ether bridging tetracarboxylate ligand 5,5'(4,4'-phenylenebis(methyleneoxy)) diisophthalic acid (H₄L). Both CPs behaved as profound fluorescent sensor for Fe3+ ion and nitro-aromatics (NACs), specifically 2,4,6-trinitrophenol (TNP). The stability at elevated temperature and photocatalytic behaviors of both 1 and 2 for photo-decomposition of aromatic dyes have also been explored. An attempt has been made to explore the plausible mechanism related with the decrease in fluorescence intensity of 1 and 2 in presence of NACs using theoretical calculations. Additionally, the probable mechanism of photo catalysis by 1 and 2 to photo-degrade aromatic dyes has been explained using density of states (DOS) calculations.

Metal-Organic Frameworks with Internal Urea-Functionalized Dicarboxylate Linkers for SO2 and NH3 Adsorption

Introduction of a urea R-NH-CO-NH-R group as a seven-membered diazepine ring at the center of 4,4'-biphenyl-dicarboxylic acid leads to a urea-functionalized dicarboxylate linker (L1^2-) from which four zinc metal-organic frameworks (MOFs) could be obtained, having a {Zn₄(μ₄-O)(O₂C-)₆} SBU and IRMOF-9 topology (compound [Zn₄(μ₄-O)(L1)₃], 1, from dimethylformamide, DMF) or a {Zn₂(O₂C-)₄} paddle-wheel SBU in a 2D-network (compound [Zn₂(L1)₂(DEF)₂·2.5DEF], 2, from diethylformamide, DEF). Pillaring of the 2D-network of 2 with 4,4'-bipyridine (bipy) or 1,2-bis(4-pyridyl)ethane (bpe) gives 3D frameworks with rhombohedrally distorted pcu-a topologies ([Zn₂(L1)₂(bipy)], 3 and [Zn₂(L1)₂(bpe)], 4, respectively). The 3D-frameworks 1, 3, and 4 are 2-fold interpenetrated with ∼50% solvent-accessible volume, albeit of apparently dynamic porous character, such that N₂ adsorption at 77 K does not occur, while H₂ at 77 K (up to ∼1 wt %) and CO₂ at 293 K (∼5 wt %) are adsorbed with large hystereses in these flexible MOFs. The urea-functionalized MOF 3 exhibits an uptake of 10.9 mmol g^-1 (41 wt %) of SO₂ at 293 K, 1 bar, which appears to be the highest value observed so far. Compounds 3 and 4 adsorb 14.3 mmol g^-1 (20 wt %) and 17.8 mmol g^-1 (23 wt %) NH₃, respectively, which is at the top of the reported values. These high uptake values are traced to the urea functionality and its hydrogen-bonding interactions to the adsorbents. The gas uptake capacities follow the specific porosity of the frameworks, in combination with pore aperture size and affinity constants from fits of the adsorption isotherms.

Organic-inorganic hybrid materials from divalent metal cations and expanded N,N′-donor linkers

The rod-shaped linker (E,E)-N,N′-(3,3′-dimethyl-4,4′-biphenyldiyl)bis[1-(3-pyridinyl)methanimine] (L) is exploited for the first time in the synthesis of extended structures. Four new coordination polymers of composition {[ZnL(OAc)2]·EtOH}n (1), {[CdL(OAc)2]·MeOH}n (2), {[Cu2L(OAc)4]·CH2Cl2}n (3) and [MnL(N3)2]n (4) have been structurally characterized. The metal cations and the anionic ancillary ligands play pivotal roles for the topology of these compounds. In the crystalline reaction products of Zn(II), Cd(II) and Cu(II) acetate with the organic linker, the acetate anions connects two neighboring cations to dinuclear [M2(OAc)4] subunits. These secondary building units are further crosslinked by the N,N′-donor ligand, either perpendicular to the acetato bridges, leading to a ladder-like ribbon for 1 and 2, or in the direction of the metal···metal separation, resulting in a simple chain in the case of 3. Instead of dinuclear secondary building units, a different topology results from reaction of the N,N′ linker with Mn(ClO4)2 in the presence of azide anions: 1,3 bridging by the N3 − groups leads to infinite chains. These are crosslinked by L in perpendicular direction, and the layer structure 4 is obtained. Natural bond orbital (NBO) analyses revealed information on the basis of orbital interactions about the coordination environments of the metal ions. Thermogravimetric measurements indicate the highest thermal stability for 2. Strong antiferromagnetic coupling within the dinuclear subunits of 3 is observed as a consequence of superexchange via the acetato bridges.

Introduction of functionality, selection of topology, and enhancement of gas adsorption in multivariate metal-organic framework-177

Metal-organic framework-177 (MOF-177) is one of the most porous materials whose structure is composed of octahedral Zn4O(-COO)6 and triangular 1,3,5-benzenetribenzoate (BTB) units to make a three-dimensional extended network based on the qom topology. This topology violates a long-standing thesis where highly symmetric building units are expected to yield highly symmetric networks. In the case of octahedron and triangle combinations, MOFs based on pyrite (pyr) and rutile (rtl) nets were expected instead of qom. In this study, we have made 24 MOF-177 structures with different functional groups on the triangular BTB linker, having one or more functionalities. We find that the position of the functional groups on the BTB unit allows the selection for a specific net (qom, pyr, and rtl), and that mixing of functionalities (-H, -NH2, and -C4H4) is an important strategy for the incorporation of a specific functionality (-NO2) into MOF-177 where otherwise incorporation of such functionality would be difficult. Such mixing of functionalities to make multivariate MOF-177 structures leads to enhancement of hydrogen uptake by 25%.

A non-regular layer arrangement of a pillared-layer coordination polymer: avoiding interpenetration via symmetry breaking at nodes

A coordination terpolymerization strategy is exploited to generate a non-interpenetrated pillared-layer coordination polymer with a non-regular layer arrangement. The role of breaking nodal symmetry is discussed.

Solvent modulated assembly of two Zn metal–organic frameworks: syntheses, luminescence, and gas adsorption properties

Two new MOFs based on H₂btca and zinc – [Zn(btca)(2,2′-bipy)]_n (1) and [Zn₂(btca)₂(bpy)(H₂O)]_n·n(DMA) (2) – have been designed and synthesized by modulating the solvent.

A novel porous metal–organic framework from a new bis(acylhydrazone) ligand capable of reversibly adsorbing/desorbing water and small alcohol molecules

Weak coordination and π–π interactions link the metallomacrocycles into two interpenetrating 3D networks to form the material with hydrophilic pores.

A 3D MOF showing unprecedented solvent-induced single-crystal-to-single-crystal transformation and excellent CO2 adsorption selectivity at room temperature

A multifunctional MOF material showing rare solvent-induced structural transformation with substitution of bridging-water and guest-exchange at 298 K is reported.