New Architecture Based on Metal-Organic Frameworks and Spin Crossover Complexes to Detect Volatile Organic Compounds.

We present here the encapsulation of a spin crossover complex C1 [FeII(L)] (L: 4-amino-, 2-(2-pyridinylmethylene)hydrazide) inside MOF-808(Zr), a chemically robust Metal-Organic Framework. The compound C1⊂MOF-808 retains its crystallinity as well as a partial porosity compared to pristine MOF and shows solvatochromism under Volatile Organic compounds (VOCs) sorption associated to a spin state change of the guest complex. More specifically, this compound shows an interesting reversible color change under formaldehyde and formic acid vapor sorption and can therefore be considered as a new kind of optical VOCs chemosensor, opening new doors for developing a broad range of VOCs optical sensors.

Ln-MOF Based Ratiometric Luminescent Sensor for the Detection of Potential COVID-19 Drugs.

Countless people have been affected by the COVID-19 pandemic on a global scale. Favipiravir, has shown potential as an effective drug for SARS-CoV-2, attracting scientists' attention. However, overuse of Favipiravir easily leads to serious side effects, requiring real-time monitoring in body fluids. Given this, a new lanthanide metal-organic framework (MOF) was prepared under solvothermal conditions from either Eu (Eu-MOF or (1)) or Tb (Tb-MOF or (2)) using the highly delocalized imidazoledicarboxylic acid linker H2 L (H2 L=5-(4-(imidazol-1-yl) phenyl) isophthalic acid) and could be successfully applied to selective optical detection of Favipiravir. In this MOF framework, the organic linker H2 L provides a high excitation energy transfer efficiency that can sensitize luminescence in lanthanides. In addition, through deliberate tuning of Eu/Tb molar ratio and reaction concentration in the lanthanide framework, ratiometric recyclable luminescent Eux Tb1-x -MOF nanoparticles with open metal sites have been constructed, which present a high detection sensitivity (Ksv =1×107 [M-1 ], detection limit is 4.63 nM) for Favipiravir. The detection mechanism is discussed with the help of Density Functional Theory (DFT) calculations that sheds light over the selective sensing of Favipiravir over other related COVID-19 drug candidates. Finally, to explore the practical application of Favipiravir sensing, MOF based thin films have been used for visual detection of Favipiravir and recycled 5 times.

Ultrasmall Copper Nanoclusters in Zirconium Metal-Organic Frameworks for the Photoreduction of CO2.

Encapsulating ultrasmall Cu nanoparticles inside Zr-MOFs to form core-shell architecture is very challenging but of interest for CO2 reduction. We report for the first time the incorporation of ultrasmall Cu NCs into a series of benchmark Zr-MOFs, without Cu NCs aggregation, via a scalable room temperature fabrication approach. The Cu NCs@MOFs core-shell composites show much enhanced reactivity in comparison to the Cu NCs confined in the pore of MOFs, regardless of their very similar intrinsic properties at the atomic level. Moreover, introducing polar groups on the MOF structure can further improve both the catalytic reactivity and selectivity. Mechanistic investigation reveals that the CuI sites located at the interface between Cu NCs and support serve as the active sites and efficiently catalyze CO2 photoreduction. This synergetic effect may pave the way for the design of low-cost and efficient catalysts for CO2 photoreduction into high-value chemical feedstock.

One-Step Room-Temperature Synthesis of Metal(IV) Carboxylate Metal-Organic Frameworks.

Room-temperature syntheses of metal-organic frameworks (MOFs) are of interest to meet the demand of the sustainable chemistry and are a pre-requisite for the incorporation of functional compounds in water-stable MOFs. However, only few routes under ambient conditions have been reported to produce metal(IV)-based MOFs. Reported here is a new versatile one-step synthesis of a series of highly porous M6 -oxocluster-based MOFs (M=Zr, Hf, Ce) at room temperature, including 8- or 12-connected micro/mesoporous solids with different functionalized organic ligands. The compounds show varying degrees of defects, particularly for 12-connected phases, while maintaining the chemical stability of the parent MOFs. Proposed here are first insights into in situ kinetics observations for efficient MOF preparation. Remarkably, the synthesis has a high space-time yield and also provides the possibility to tune the particle size, therefore paving the way for their practical use.

Engineering Structural Dynamics of Zirconium Metal-Organic Frameworks Based on Natural C4 Linkers.

Engineering the structural flexibility of metal-organic framework (MOF) materials for separation-related applications remains a great challenge. We present here a strategy of mixing rigid and soft linkers in a MOF structure to achieve tunable structural flexibility, as exemplified in a series of stable isostructural Zr-MOFs built with natural C4 linkers (fumaric acid, succinic acid, and malic acid). As shown by the differences in linker bond stretching and bending freedom, these MOFs display distinct responsive dynamics to external stimuli, namely, changes in temperature or guest molecule type. Comprehensive in situ characterizations reveal a clear correlation between linker character and MOF dynamic behavior, which leads to the discovery of a multivariate flexible MOF. It shows an optimal combination of both good working capacity and significantly enhanced selectivity for CO2/N2 separation. In principle, it provides a new avenue for potentially improving the ability of microporous MOFs to separate other gaseous and liquid mixtures.

Temperature dependence of the cooperative out-of-equilibrium elastic switching in a spin-crossover material.

We present a study of a molecular material, [Feiii(3-MeO-SalEen)2]PF6, undergoing cooperative reversible photo-induced transition between low-spin state and high-spin state. By using temporally multiscale pump-probe laser spectroscopy, we explore the key parameters that influence the low-spin to high-spin conversion efficiency through long range elastic intermolecular interactions during the so-called elastic step, where crystalline volume expansion takes place. We rationalize our findings using Monte Carlo simulations, and a mechano-elastic model. The experimental results and the simulations support the existence of a fast mechanism by which molecules cooperatively switch through coupling to the lattice strain. The efficiency of the coupling process is shown to depend on several parameters including the initial thermal population and the instantaneous photo-induced population among others. Far below the crossover temperature, the elastic self-amplification occurs above a threshold photo-excitation. On approaching the thermal crossover, the threshold disappears and the photo-elastic conversion increases.

Very Long-Lived Photogenerated High-Spin Phase of a Multistable Spin-Crossover Molecular Material.

The spin-crossover compound [Fe( n-Bu-im)3(tren)](PF6)2 shows an unusual long relaxation time of 20 h after light-induced excited spin state trapping when irradiating at 80 K. This is more than 40 times longer than when irradiating at 10 K. Optical absorption spectroscopy, magnetometry, and X-ray diffraction using synchrotron radiation were used to characterize and explain the different relaxation behaviors of this compound after irradiation below and above 70 K. Rearrangement of the butyl chains of the ligands occurring during the relaxation after irradiation above 70 K is thought to be responsible for the unusually long relaxation time at this temperature.

The influence of the sample dispersion on a solid surface in the thermal spin transition of [Fe(pz)Pt(CN)4] nanoparticles.

The thermal spin transition of [Fe(pz)Pt(CN)4], pz = pyrazine, nanoparticles is compared with the one of the microcrystalline powder by magnetic susceptibility measurements, absorption spectroscopy and X-ray powder diffraction (XRPD) using synchrotron radiation. The thermal transition shows less cooperativity when decreasing the size due to the reduction of cluster formation. Surprisingly, the dispersion of the nanoparticles on a surface entails important effects on the spin crossover properties of the system. These effects are simulated and explained within the framework of the mechanoelastic model.

Elastically driven cooperative response of a molecular material impacted by a laser pulse.

Photoinduced phase transformations occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities. The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny, coherently propagating cell deformations over acoustic timescales have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.

Structural investigation of the high spin→low spin relaxation dynamics of the porous coordination network [Fe(pz)Pt(CN)4]⋠2.6 H2O.

The Hoffman-type coordination compound [Fe(pz)Pt(CN)4]⋅2.6 H2O (pz = pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X-Ray diffraction studies reveal that a quantitative photoinduced conversion from the low-spin (LS) state into the high-spin (HS) state, based on the light-induced excited spin-state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time-resolved measurements evidence that the HS→LS relaxation proceeds by a two-step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached.

Highly defective ultra-small tetravalent MOF nanocrystals.

The size and defects in crystalline inorganic materials are of importance in many applications, particularly catalysis, as it often results in enhanced/emerging properties. So far, applying the strategy of modulation chemistry has been unable to afford high-quality functional Metal-Organic Frameworks (MOFs) nanocrystals with minimized size while exhibiting maximized defects. We report here a general sustainable strategy for the design of highly defective and ultra-small tetravalent MOFs (Zr, Hf) crystals (ca. 35% missing linker, 4-6 nm). Advanced characterizations have been performed to shed light on the main factors governing the crystallization mechanism and to identify the nature of the defects. The ultra-small nanoMOFs showed exceptional performance in peptide hydrolysis reaction, including high reactivity, selectivity, diffusion, stability, and show emerging tailorable reactivity and selectivity towards peptide bond formation simply by changing the reaction solvent. Therefore, these highly defective ultra-small M(IV)-MOFs particles open new perspectives for the development of heterogeneous MOF catalysts with dual functions.

Lanthanide MOF-based luminescent sensor arrays for the detection of castration-resistant prostate cancer curing drugs and biomarkers.

In recent years, castration-resistant prostate cancer (CRPC) has profoundly impacted the lives of many men, and early diagnosis of medication and illness is crucial. Therefore, a highly efficient detection method for CRPC biomarkers and curing drugs is required. However, the complex and diverse structures of CRPC drugs pose significant challenges for their detection and differentiation. Lanthanide metal-organic frameworks (Ln-MOFs) show great potential for sensing applications due to their intense and characteristic luminescence. In this work, a series of new bimetallic Ln-MOFs (EuxTb1-x-MOF) based luminescent sensor arrays have been developed to identify CRPC drugs, including in mixtures, via principal component analysis (PCA) and hierarchical cluster analysis (HCA) methods. These Ln-MOFs are built with a highly conjugated H2L linker (H2L = 5-(4-(triazole-1-yl)phenyl)isophthalic acid) and exhibit robust strong luminescence emissions (mainly located at 543 and 614 nm) and high energy transfer efficiencies. More specifically, Eu0.096Tb0.904-MOF (MOF 3) has demonstrated good sensing performances for CRPC curing drugs in real human serum samples. Furthermore, the curing drug hydroxyflutamide has been combined with MOF 3, to construct a robust composite sensing platform MOF 3@hydroxyflutamide for highly efficient detection of CRPC biomarkers such as the androgen receptor (AR) and prostate-specific antigen (PSA). Finally, luminescence lifetime measurements, zeta potential measurements, and density functional theory (DFT) calculations were performed to gain insights into the sensing mechanism.

Ultrasmall Functionalized UiO-66 Nanoparticle/Polymer Pebax 1657 Thin-Film Nanocomposite Membranes for Optimal CO2 Separation.

Ultrasmall 4 to 6 nm nanoparticles of the metal-organic framework (MOF) UiO-66 (University of Oslo-66) were successfully prepared and embedded into the polymer Pebax 1657 to fabricate thin-film nanocomposite (TFN) membranes for CO2/N2 and CO2/CH4 separations. Furthermore, it has been demonstrated that ligand functionalization with amino (-NH2) and nitro (-NO2) groups significantly enhances the gas separation performance of the membranes. For CO2/N2 separation, 7.5 wt % UiO-66-NH2 nanoparticles provided a 53% improvement in CO2 permeance over the pristine membrane (from 181 to 277 GPU). Regarding the CO2/N2 selectivity, the membranes prepared with 5 wt % UiO-66-NO2 nanoparticles provided an increment of 17% over the membrane without the MOF (from 43.5 to 51.0). However, the CO2 permeance of this membrane dropped to 155 GPU. The addition of 10 wt % ZIF-94 particles with an average particle size of ∼45 nm into the 5 wt % UiO-66-NO2 membrane allowed to increase the CO2 permeance to 192 GPU while maintaining the CO2/N2 selectivity at ca. 51 due to the synergistic interaction between the MOFs and the polymer matrix provided by the hydrophilic nature of ZIF-94. In the case of CO2/CH4 separation, the 7.5 wt % UiO-66-NH2 membrane exhibited the best performance with an increase of the CO2 permeance from 201 to 245 GPU.

Stimuli responsive hybrid magnets: tuning the photoinduced spin-crossover in Fe(III) complexes inserted into layered magnets.

The insertion of a [Fe(sal2-trien)](+) complex cation into a 2D oxalate network in the presence of different solvents results in a family of hybrid magnets with coexistence of magnetic ordering and photoinduced spin-crossover (LIESST effect) in compounds [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHCl3 (1·CHCl3), [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CHBr3 (1·CHBr3), and [Fe(III)(sal2-trien)][Mn(II)Cr(III)(ox)3]·CH2Br2 (1·CH2Br2). The three compounds crystallize in a 2D honeycomb anionic layer formed by Mn(II) and Cr(III) ions linked through oxalate ligands and a layer of [Fe(sal2-trien)](+) complexes and solvent molecules (CHCl3, CHBr3, or CH2Br2) intercalated between the 2D oxalate network. The magnetic properties and Mössbauer spectroscopy indicate that they undergo long-range ferromagnetic ordering at 5.6 K and a spin crossover of the intercalated [Fe(sal2-trien)](+) complexes at different temperatures T1/2. The three compounds present a LIESST effect with a relaxation temperature TLIESST inversely proportional to T1/2. The isostructural paramagnetic compound, [Fe(III)(sal2-trien)][Zn(II)Cr(III)(ox)3]·CH2Cl2 (2·CH2Cl2) was also prepared. This compound presents a partial spin crossover of the inserted Fe(III) complex as well as a LIESST effect. Finally, spectroscopic characterization of the Fe(III) doped compound [Ga0.99Fe0.01(sal2trien)][Mn(II)Cr(III)(ox)3]·CH2Cl2 (3·CH2Cl2) shows a gradual and complete thermal spin crossover and a LIESST effect on the isolated Fe(III) complexes. This result confirms that cooperativity is not a necessary condition to observe the LIESST effect in an Fe(III) compound.

Unusual molecular material formed through irreversible transformation and revealed by 4D electron microscopy.

Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate the in situ irreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN)4. The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal-cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. This in situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems.

Room temperature design of Ce(iv)-MOFs: from photocatalytic HER and OER to overall water splitting under simulated sunlight irradiation.

The development of MOF-based efficient and reusable catalysts for hydrogen production under simulated sunlight irradiation, especially through overall water splitting, remains challenging. This is mainly due to either the inappropriate optical features or poor chemical stability of the given MOFs. Room temperature synthesis (RTS) of tetravalent MOFs is a promising strategy to design robust MOFs and their related (nano)composites. By employing these mild conditions, herein, we report for the first time that RTS leads to the efficient formation of highly redox active Ce(iv)-MOFs that are inaccessible at elevated temperatures. Consequently, not only highly crystalline Ce-UiO-66-NH2 is synthesized, but also many other derivatives and topologies (8 and 6-connected phases) without compromise in space-time yield. Their photocatalytic HER and OER activities under simulated sunlight irradiation are in good agreement with their energy level band diagrams: Ce-UiO-66-NH2 and Ce-UiO-66-NO2 are the most active photocatalysts for the HER and OER, respectively, with a higher activity than other metal-based UiO-type MOFs. Combining Ce-UiO-66-NH2 with supported Pt NPs results finally in one of the most active and reusable photocatalysts for overall water splitting into H2 and O2 under simulated sunlight irradiation, due to its efficient photoinduced charge separation evidenced by laser flash photolysis and photoluminescence spectroscopies.

A MOF/DNA luminescent sensing platform for detection of potential COVID-19 biomarkers and drugs.

COVID-19 has afflicted people's lives worldwide. Interleukin-6 (IL-6) is an important COVID-19 biomarker in human body fluids that can be used as a reference to monitor COVID-19 in real-time and therefore to reduce the risk of virus transmission. On the other hand, oseltamivir is a potential COVID-19 curing drug, but its overuse easily leads to hazardous side effects, calling for its real time monitoring in body fluids. For these purposes, a new yttrium metal-organic framework (Y-MOF) has been synthesized, in which the 5-(4-(imidazole-1-yl)phenyl)isophthalic linker contains a large aromatic backbone capable of strongly interacting with DNA sequences through π-π stacking interactions, which makes it appealing to build a unique sensor based on DNA functionalized MOFs. The MOF/DNA sequence hybrid luminescent sensing platform presents excellent optical properties associated with a high Förster resonance energy transfer (FRET) efficiency. Furthermore, to construct a dual emission sensing platform, a 5'-carboxylfluorescein (FAM) labeled DNA sequence (S2) with a stem-loop structure that can specifically interact with IL-6 has been associated with the Y-MOF. The resulting Y-MOF@S2 exhibits an efficient ratiometric detection of IL-6 in human body fluids with an extremely high Ksv value 4.3 × 108 M-1 and a low detection limit (LOD) of 70 pM. Finally, the Y-MOF@S2@IL-6 hybrid platform allows the detection of oseltamivir with high sensitivity (Ksv value is as high as 5.6 × 105 M-1 and LOD is 54 nM), due to the fact that oseltamivir can disconnect the loop stem structure constructed by S2, leading to a strong quenching effect towards Y-MOF@S2@IL-6. The nature of the interactions between oseltamivir and Y-MOF has been elucidated using density functional theory calculations while the sensing mechanism for the dual detection of IL-6 and oseltamivir has been deciphered based on luminescence lifetime tests and confocal laser scanning microscopy.

100†picosecond diffraction catches structural transients of laser-pulse triggered switching in a spin-crossover crystal.

We study by 100 picosecond X-ray diffraction the photo-switching dynamics of single crystal of the orthorhombic polymorph of the spin-crossover complex [(TPA)Fe(TCC)]PF(6), in which TPA = tris(2-pyridyl methyl)amine, TCC(2-) = 3,4,5,6-Cl(4)-Catecholate(2-). In the frame of the emerging field of dynamical structural science, this is made possible by using optical pump/X-ray probe techniques, which allow following in real time structural reorganization at intra- and intermolecular levels associated with the change of spin state in the crystal. We use here the time structure of the synchrotron radiation generating 100 picosecond X-ray pulses, coupled to 100 fs laser excitation. This study has revealed a rich variety of structural reorganizations, associated with the different steps of the dynamical process. Three consecutive regimes are evidenced in the time domain: 1) local molecular photo-switching with structural reorganization at constant volume, 2) volume relaxation with inhomogeneous distribution of local temperatures, 3) homogenization of the crystal in the transient state 100 µs after laser excitation. These findings are fundamentally different from those of conventional diffraction studies of long-lived photoinduced high spin states. The time-resolution used here with picosecond X-ray diffraction probes different physical quantities on their intrinsic time-scale, shedding new light on the successive processes driving macroscopic switching in a functionalized material. These results pave the way for structural studies away from equilibrium and represent a first step toward femtosecond crystallography.

Ultrafast spin-state photoswitching in a crystal and slower consecutive processes investigated by femtosecond optical spectroscopy and picosecond X-ray diffraction.

We report the spin state photo-switching dynamics in two polymorphs of a spin-crossover molecular complex triggered by a femtosecond laser flash, as determined by combining femtosecond optical pump-probe spectroscopy and picosecond X-ray diffraction techniques. The light-driven transformations in the two polymorphs are compared. Combining both techniques and tracking how the X-ray data correlate with optical signals allow understanding of how electronic and structural degrees of freedom couple and play their role when the switchable molecules interact in the active crystalline medium. The study sheds light on crossing the border between femtochemistry at the molecular scale and femtoswitching at the material scale.

Recent progress on MOF-based optical sensors for VOC sensing.

The raising apprehension of volatile organic compound (VOC) exposures urges the exploration of advanced monitoring platforms. Metal-organic frameworks (MOFs) provide many attractive features including tailorable porosity, high surface areas, good chemical/thermal stability, and various host-guest interactions, making them appealing candidates for VOC capture and sensing. To comprehensively exploit the potential of MOFs as sensing materials, great efforts have been dedicated to the shaping and patterning of MOFs for next-level device integration. Among different types of sensors (chemiresistive sensors, gravimetric sensors, optical sensors, etc.), MOFs coupled with optical sensors feature distinctive strength. This review summarized the latest advancements in MOF-based optical sensors with a particular focus on VOC sensing. The subject is discussed by different mechanisms: colorimetry, luminescence, and sensors based on optical index modulations. Critical analysis for each system highlighting practical aspects was also deliberated.