Passivation-Free, Liquid-Metal-Based Electrosynthesis of Aluminum Metal-Organic Frameworks Mediated by Light Metal Activation.

The production of aluminum (Al) metal-organic frameworks (MOFs) by electrosynthesis using solid-state Al electrodes always faces significant challenges due to the formation of a passivating aluminum oxide layer in the process. Here, we developed a liquid-metal-based method to electrosynthesize an aluminum Al-MOF (MIL-53). This method uses a liquid-state gallium (Ga) anode as a reservoir and activator for a light metal, Al, in the form of Al-Ga alloys that releases Al3+ for the electrosynthesis of Al-MOFs. Introducing Ga into the system inhibits the formation of aluminum oxide passivation layer and promotes the electrochemical reaction for Al-MOF synthesis. The electrosynthesis using liquid Al-Ga alloy is conducted at ambient temperatures for long durations without requiring pretreatment for aluminum oxide removal. We show that the Al-MOF products synthesized from 0.40 wt % Al in liquid Ga lead to the highest crystallinity and possess a specific surface area greater than 800 m2 g-1 and a low capacity for CO2 adsorption that can be used as a potential matrix for CO2/N2 separation. This work provides evidence that employing liquid-metal electrodes offers a viable pathway to circumvent surface passivation effects that inevitably occur when using conventional solid metals. It also introduces an efficient electrosynthesis method based on liquid metals for producing atomically porous materials.

3D Printing of Monoliths Using TIFSIX-Ni-Based Formulations for the Electric Swing Adsorption of CO2.

The hybrid ultraporous material TIFSIX-Ni ([Ni(pyrazine)2(TiF6)]n) was incorporated into a composite ink for the first time for the three-dimensional (3D) printing of monoliths. The large-scale synthesis of TIFSIX-Ni was completed using two different Ni(II) salts, with CO2 uptakes of 1.90 mmol g-1 achieved using mechanochemically assisted thermal synthesis. The monoliths were then tested for the capture and release of CO2 gas using electric swing adsorption (ESA) under dry and humid conditions. A working capacity of 1.7% was achieved (comparing dynamic data with isotherm data) when a current of 2.1 A was applied for 10 min. The monolith could be cycled repeatedly for 6 h without impacting the performance of the material or loss of capacity. Part of this work explored the improvement of mechanochemically assisted synthetic methods of TIFISX-Ni in reducing the costs associated with large-scale production, allowing for improvements in the overall scale-up and processability of the material for industrial applications.

Chiral metal-organic frameworks for photonics.

Recently, there has been significant interest in the use of chiral metal-organic frameworks (MOFs) and coordination polymers (CPs) for photonics applications. The promise of these materials lies in the ability to tune their properties through judicious selection of the metal and ligand components. Additionally, the interaction of guest species with the host framework can be exploited to realise new functionalities. In this review, we outline the methods for synthesising chiral MOFs and CPs, then analyse the recent innovations in their use for various optical and photonics applications. We focus on two emerging directions in the field of MOF chemistry - circularly polarised luminescence (CPL) and chiroptical switching - as well as the latest developments in the use of these materials for second-order nonlinear optics (NLO), particularly second-harmonic generation (SHG). The current challenges encountered so far, their possible solutions, and key directions for further research are also outlined. Overall, given the results demonstrated to date, chiral MOFs and CPs show great promise for use in future technologies such as optical communication and computing, optical displays, and all-optical devices.

Electronic Spin Qubit Candidates Arrayed within Layered Two-Dimensional Polymers.

Molecular electronic spin qubits are promising candidates for quantum information science applications because they can be reliably produced and engineered via chemical design. Embedding electronic spin qubits within two-dimensional polymers (2DPs) offers the possibility to systematically engineer inter-qubit interactions while maintaining long coherence times, both of which are prerequisites to their technological utility. Here, we introduce electronic spin qubits into a diamagnetic 2DP by n-doping naphthalene diimide subunits with varying amounts of CoCp2 and analyze their spin densities by quantitative electronic paramagnetic resonance spectroscopy. Low spin densities (e.g., 6.0 × 1012 spins mm-3) enable lengthy spin-lattice (T1) and spin-spin relaxation (T2) times across a range of temperatures, ranging from T1 values of 164 ms at 10 K to 30.2 µs at 296 K and T2 values of 2.36 µs at 10 K to 0.49 µs at 296 K for the lowest spin density sample examined. Higher spin densities and temperatures were both found to diminish T1 times, which we attribute to detrimental cross-relaxation from spin-spin dipolar interactions and spin-phonon coupling, respectively. Higher spin densities decreased T2 times and modulated the T2 temperature dependence. We attribute these differences to the competition between hyperfine and dipolar interactions for electron spin decoherence, with the dominant interaction transitioning from the former to the latter as spin density and temperature increase. Overall, this investigation demonstrates that dispersing electronic spin qubits within layered 2DPs enables chemical control of their inter-qubit interactions and spin decoherence times.

Two-step spin crossover by guest-disorder induced local symmetry breaking within a 3D Hofmann-like framework.

A 3D Hofmann-like metal-organic framework has been prepared which contains a 2,1,3-benzothiadiazole-based pillaring ligand. Encapsulation of a polycyclic aromatic hydrocarbon, chrysene, within the pore structure leads to a new pathway to multi-step spin crossover behaviour in which the observed two-step spin transition arises due to the presence of multiple site environments associated with local guest positional effects within the host lattice.

Multi-Redox Responsive Behavior in a Mixed-Valence Semiconducting Framework Based on Bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane.

The two-dimensional (2-D) framework, [Cu(BTDAT)(MeOH)] {BTDAT = bis-[1,2,5]-thiadiazolo-tetracyanoquinodimethane}, possesses remarkable multi-step redox properties, with electrochemical studies revealing six quasi-stable redox states in the solid state. In situ electron paramagnetic resonance and visible-near infrared spectroelectrochemistry elucidated the mechanism for these multi-step redox processes, as well as the optical and electrochromic behavior of the BTDAT ligand and framework. In studying the structural, spectroscopic, and electronic properties of [Cu(BTDAT)(MeOH)], the as-synthesized framework was found to exist in a mixed-valence state with thermally-activated semiconducting behavior. In addition to pressed pellet conductivity measurements, single-crystal conductivity measurements using a pre-patterned polydimethylsiloxane layer on a silicon substrate provide important insights into the anisotropic conduction pathways. As an avenue to further understand the electronic state of [Cu(BTDAT)(MeOH)], computational band structure calculations predicted delocalized electronic transport in the framework. On the balance of probabilities, we propose that [Cu(BTDAT)(MeOH)] is a Mott insulator (i.e., electron correlations cause a metal-insulator transition). This implies that the conductivity is incoherent. However, we are unable to distinguish between activated transport due to Coulombically bound electron-hole pairs and a hopping mechanism. The combined electrochemical, electronic, and optical properties of [Cu(BTDAT)(MeOH)] shine a new light on the experimental and theoretical challenges for electroactive framework materials, which are implicated as the basis of advanced optoelectronic and electrochromic devices.

How Reproducible are Surface Areas Calculated from the BET Equation?

Porosity and surface area analysis play a prominent role in modern materials science. At the heart of this sits the Brunauer-Emmett-Teller (BET) theory, which has been a remarkably successful contribution to the field of materials science. The BET method was developed in the 1930s for open surfaces but is now the most widely used metric for the estimation of surface areas of micro- and mesoporous materials. Despite its widespread use, the calculation of BET surface areas causes a spread in reported areas, resulting in reproducibility problems in both academia and industry. To prove this, for this analysis, 18 already-measured raw adsorption isotherms were provided to sixty-one labs, who were asked to calculate the corresponding BET areas. This round-robin exercise resulted in a wide range of values. Here, the reproducibility of BET area determination from identical isotherms is demonstrated to be a largely ignored issue, raising critical concerns over the reliability of reported BET areas. To solve this major issue, a new computational approach to accurately and systematically determine the BET area of nanoporous materials is developed. The software, called "BET surface identification" (BETSI), expands on the well-known Rouquerol criteria and makes an unambiguous BET area assignment possible.

Metal-Organic Frameworks: Special Collection 2020.

MOF2020: This Editorial introduces a special collection of review and original research articles, dedicated to the MOF2020 web conference.

Breathing-Assisted Selective Adsorption of C8 Alkyl Aromatics in Zn-Based Metal-Organic Frameworks.

Invited for the cover of this issue are Purna Chandra Rao, Minyoung Yoon and co-workers at Kyungpook National University, Gachon University, POSTECH, Korea Atomic Energy Research Institute and the University of Sydney. The image depicts how single C8 isomers are selectively isolated from a mixture. Read the full text of the article at 10.1002/chem.202102640.

The electrochemical reduction of a flexible Mn(II) salen-based metal-organic framework.

A new metal-organic framework (MOF) containing a Mn(II) salen complex (BET surface area = 967 ± 6 m2 g-1) undergoes a reversible crystalline-to-amorphous transformation. Experimental studies and computational calculations show that the MOF is stable to a one-electron reduction at more anodic potentials than the corresponding discrete complex.

Breathing-Assisted Selective Adsorption of C8 Alkyl Aromatics in Zn-Based Metal-Organic Frameworks.

The breathing phenomenon in metal-organic frameworks (MOFs) has revealed supramolecular host-guest interactions that could be beneficial for chemical separation in numerous industrial applications. The cost-effective purification of C8 alkyl aromatics such as o-xylene, m-xylene, p-xylene, and ethylbenzene remains challenging owing to their similar molecular structures, boiling points, kinetic diameters, polarities, etc. Herein, we report two Zn-based pillar-bilayered MOFs, denoted [Zn2 (aip)2 (pillar)] (aip=5-aminoisophthalic acid; pillar: bpy=4,4'-bipyridine or bpe=1,2-bis(4-pyridyl)ethane) that exhibit a breathing effect depending on the adsorbed guest molecules. Guest-dependent sorption studies in organic solvents such as N,N-dimethylformamide, methanol, benzene, and water vapor display reversible structural flexibility through the breathing effect in both framework compounds. The experiments conducted on C8 -alkyl aromatics resulting in both MOF compounds can access these isomers in the shrunken pores, and thereby expand the pore size by framework breathing. In C8 binary mixtures, these Zn-MOFs exhibit selective sorption properties based on the different interactions between guest C8 aromatics and the framework structure.

Multifunctional Coordination Polymer Exhibiting Reversible Mechanical Motion Allowing Selective Uptake of Guests and Leading to Enhanced Electrical Conductivity.

A remarkably flexible, multifunctional, 2D coordination polymer exhibiting an unprecedented mode of reversible mechanical motion, enabling pores to open and close, is reported. Such multifunctional materials are highly sought after, owing to the potential to exploit coexisting electronic and mechanical functionalities that underpin useful technological applications such as actuators and ultrasensitive detectors. The coordination polymer, of composition Mn(F4TCNQ)(py)2 (F4TCNQ = 2,3,5,6-tetrafluoro-7,7,8,8-tetracycanoquinodimethane; py = pyridine), consists of Mn(II) centers bridged by F4TCNQ dianions and coordinated by py molecules that extend above and below the 2D network. Exposure of Mn(F4TCNQ)(py)2, in its collapsed state, to carbon dioxide results in a pore-opening process at a threshold pressure for a given temperature. In addition to carbon dioxide, a variety of volatile guests may be incorporated into the pores, which are lined with electron-rich F4TCNQ dianions. The inclusion of electron-deficient guests such as 1,4-benzoquinone, nitrobenzene, maleic anhydride, and iodine into the pores is accompanied by a striking color change associated with a new host-guest charge-transfer interaction and an improvement in the semiconductor behavior, with the iodine adduct showing an increase in conductivity of almost 5 orders of magnitude. Experimental and density functional theory calculations on this remarkable multifunctional material demonstrate a reduction in the optical band gap with increasing electron affinity of the guest.

Substituent effects on through-space intervalence charge transfer in cofacial metal-organic frameworks.

Electroactive metal-organic frameworks (MOFs) are an attractive class of materials owing to their multifunctional 3-dimensional structures, the properties of which can be modulated by changing the redox states of the components. In order to realise both fundamental and applied goals for these materials, a deeper understanding of the structure-function relationships that govern the charge transfer mechanisms is required. Chemical or electrochemical reduction of the framework [Zn(BPPFTzTz)(tdc)]·2DMF, hereafter denoted ZnFTzTz (where BPPFTzTz = 2,5-bis(3-fluoro-4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole), generates mixed-valence states with optical signatures indicative of through-space intervalence charge transfer (IVCT) between the cofacially stacked ligands. Fluorination of the TzTz ligands influences the IVCT band parameters relative to the unsubstituted parent system, as revealed through Marcus-Hush theory analysis and single crystal UV-Vis spectroscopy. Using a combined experimental, theoretical and density functional theory (DFT) analysis, important insights into the effects of structural modifications, such as ligand substitution, on the degree of electronic coupling and rate of electron transfer have been obtained.

A cofacial metal-organic framework based photocathode for carbon dioxide reduction.

Innovative and robust photosensitisation materials play a cardinal role in advancing the combined effort towards efficient solar energy harvesting. Here, we demonstrate the photocathode functionality of a Metal-Organic Framework (MOF) featuring cofacial pairs of photo- and electro-active 1,4,5,8-naphthalenediimide (NDI) ligands, which was successfully applied to markedly reduce the overpotential required for CO2 reduction to CO by a well-known rhenium molecular electrocatalyst. Reduction of [Cd(DPNDI)(TDC)] n (DPNDI = N,N'-di(4-pyridyl)-1,4,5,8-naphthalenediimide, H2TDC = thiophene-2,5-dicarboxylic acid) to its mixed-valence state induces through-space Intervalence Charge Transfer (IVCT) within cofacial DPNDI units. Irradiation of the mixed-valence MOF in the visible region generates a DPNDI photoexcited radical monoanion state, which is stabilised as a persistent species by the inherent IVCT interactions and has been rationalised using Density Functional Theory (DFT). This photoexcited radical monoanion state was able to undergo charge transfer (CT) reduction of the rhenium molecular electrocatalyst to effect CO generation at a lower overpotential than that required by the discrete electrocatalyst itself. The exploitation of cofacial MOFs opens new directions for the design philosophy behind light harvesting materials.

Visible Light Stimulated Bistable Photo-Switching in Defect Engineered Metal-Organic Frameworks.

The incorporation of photoactive donor-acceptor Stenhouse adduct (DASA) moieties into Metal-Organic Frameworks (MOFs) provides a new route to the development of visible light switching materials. Herein, a DUT-5 mixed-linker defect series was exploited to produce a derivative group of DASA-modified materials via postsynthetic modification (PSM). The photoactive MOFs exhibited conversion stimulated by visible wavelengths and were stable following multiple cycles. Thermodynamic and metastable states persisted over an extended time period.

Persistent Radical Tetrathiafulvalene-Based 2D Metal-Organic Frameworks and Their Application in Efficient Photothermal Conversion.

A series of stable radical 2D metal-organic frameworks has been assembled. (m-TTFTB)3 (m-Tetrathiafulvalene-tetrabenzoate) trimer building blocks are beneficial for the stability of the radicals due to delocalization of the unpaired electron. Hexanuclear rare-earth-cluster-based 1D chains further enhance the stability of the frameworks. The radical state of the middle TTF in the trimer has been observed by the change of central C-C and C-S bond distances and the configuration of the TTF by single-crystal X-ray diffraction. The radical characteristics are also confirmed by electron paramagnetic resonance, UV/Vis-NIR absorption, and X-ray photoelectron spectroscopy experiments. Stability tests showed that the radicals are stable even in solutions and under acid/base environments (pH 1-12). Owing to efficient light absorption due to intramolecular charge transfer, low thermal conductivity, and outstanding stability, the radical 2D Dy-MOF shows excellent photothermal properties, an increase of 34.7 °C within 240 s under one-sun illumination.