Spontaneous Synthesis of [FeII (Atrz)3 ]SO4 and its Analogues Through Accelerated Ageing: New Insights from Small-Scale Reactions.

Accelerated ageing reactions that take place between two solid materials on contact in the absence of added solvent have been used to synthesize two spin-crossover-active 1D coordination polymers and one of their Cu(II) analogues. The hygroscopy of the ligands and the relative humidity of the reaction chamber have been shown to be particularly important factors in the rate of reaction. Small-scale reactions between a few individual crystals have allowed observation of deliquescence of the 4-aminotriazole ligand at high humidity. The metal salt does not dissolve, and the ligand diffuses into the crystal of the metal salt during the reaction. In the case of the Cu analogue, the formation of the product causes the crystal of the metal salt to deform with the formation of pseudocrystals, which have a fibrous structure.

Investigations of the Magnetocaloric and Thermal Expansion Properties of the Ln3(adipate)4.5(DMF)2 (Ln = Gd-Er) Framework Series.

The development of sustainable and efficient cryogenic cooling materials is currently the subject of extensive research, with the aim of relieving the dependence of current low-temperature cooling methods on expensive and nonrenewable liquid helium. One potential method to achieve this is the use of materials demonstrating the magnetocaloric effect, where the cycling of an applied magnetic field leads to a net cooling effect due to changes in magnetic entropy upon application and removal of an external magnetic field. This study details the synthesis and characterization of a Ln3(adipate)4.5(DMF)2 series (where Ln = Gd-Er) of metal-organic framework (MOF) materials incorporating a flexible adipate ligand and their associated magnetocaloric and thermal expansion properties. The magnetocaloric performance of the Gd3(adipate)4.5(DMF)2 material was found to exhibit the highest magnetic entropy changes of the series, with a peak entropy change of 36.4 J kg-1 K-1 for a 5-0 T field change at a temperature of 2 K, which is suited for ultra-low-temperature cooling applications. Thermal expansion properties were also investigated within these materials, demonstrating modest negative and large positive thermal expansion identified along the different crystallographic axes within the MOF structures over a 100-300 K temperature range that demonstrated the novel mechanical properties of these adipate framework structures.

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.

Through-Space Intervalence Charge Transfer as a Mechanism for Charge Delocalization in Metal-Organic Frameworks.

Understanding the nature of charge transfer mechanisms in 3-dimensional metal-organic frameworks (MOFs) is an important goal owing to the possibility of harnessing this knowledge to design electroactive and conductive frameworks. These materials have been proposed as the basis for the next generation of technological devices for applications in energy storage and conversion, including electrochromic devices, electrocatalysts, and battery materials. After nearly two decades of intense research into MOFs, the mechanisms of charge transfer remain relatively poorly understood, and new strategies to achieve charge mobility remain elusive and challenging to experimentally explore, validate, and model. We now demonstrate that aromatic stacking interactions in Zn(II) frameworks containing cofacial thiazolo[5,4- d]thiazole (TzTz) units lead to a mixed-valence state upon electrochemical or chemical reduction. This through-space intervalence charge transfer (IVCT) phenomenon represents a new mechanism for charge transfer in MOFs. Computational modeling of the optical data combined with application of Marcus-Hush theory to the IVCT bands for the mixed-valence framework has enabled quantification of the degree of charge transfer using both in situ and ex situ electro- and spectro-electrochemical methods. A distance dependence for the through-space electron transfer has also been identified on the basis of experimental studies and computational calculations. This work provides a new window into electron transfer phenomena in 3-dimensional coordination space, of relevance to electroactive MOFs where new mechanisms for charge transfer are highly sought after, and to understanding biological light-harvesting systems where through-space mixed-valence interactions are operative.

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.

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.

Quantification of the mixed-valence and intervalence charge transfer properties of a cofacial metal-organic framework via single crystal electronic absorption spectroscopy.

Gaining a fundamental understanding of charge transfer mechanisms in three-dimensional Metal-Organic Frameworks (MOFs) is crucial to the development of electroactive and conductive porous materials. These materials have potential in applications in porous conductors, electrocatalysts and energy storage devices; however the structure-property relationships pertaining to charge transfer and its quantification are relatively poorly understood. Here, the cofacial Cd(ii)-based MOF [Cd(BPPTzTz)(tdc)]·2DMF (where BPPTzTz = 2,5-bis(4-(pyridin-4-yl)phenyl)thiazolo[5,4-d]thiazole, tdc2- = 2,5-thiophene dicarboxylate) exhibits Intervalence Charge Transfer (IVCT) within its three-dimensional structure by virtue of the close, cofacial stacking of its redox-active BPPTzTz ligands. The mixed-valence and IVCT properties are characterised using a combined electrochemical, spectroelectrochemical and computational approach. Single crystal electronic absorption spectroscopy was employed to obtain the solid-state extinction coefficient, enabling the application of Marcus-Hush theory. The electronic coupling constant, H ab, of 145 cm-1 was consistent with the localised mixed-valence properties of both this framework and analogous systems that use alternative methods to obtain the H ab parameter. This work demonstrates the first report of the successful characterisation of IVCT in a MOF material using single crystal electronic absorption spectroscopy and serves as an attractive alternative to more complex methods due to its simplicity and applicability.

Increasing spin crossover cooperativity in 2D Hofmann-type materials with guest molecule removal.

Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (ΔT = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host-host interactions in their interdigitated lattices, which optimises long-range communication pathways. With the presence of water molecules in the interlayer pore space in the hydrated phases, competing host-host and host-guest interactions occur, whilst water removal dramatically increases the framework cooperativity, thus affording systematic insight into the structural features that favour optimal spin crossover properties.