Exploring the Optical and Energetic Properties of a Co(II)-Based Mixed Ligand MOF.

Due to their remarkable properties, including remarkable porosity and extensive surface area, metal-organic frameworks (MOFs) are being investigated for various applications. Herein, we report the first Co(II)-based mixed ligand MOF, formulated Co4(HTrz)2(d-cam)2.5(µ-OH)3. Its 3D structure framework is composed of helical chains {[Co4(µ3-HTrz)4]8+}n connected by d-camphorate ligand building blocks and featured as an extended structure in an AB-AB fashion. The investigated compound displays a wide absorption range across the visible spectrum, characterized by an optical gap energy of 3.7 eV, indicating its semiconducting nature and efficient sunlight absorption capabilities across various wavelengths. The electrochemical performance demonstrated an excellent reversibility, cyclability, structural stability, as well as a specific capacity of up to 100 cycles at a scan rate of 0.1 mV·s-1 and a current density of 50 mA·g-1. Thus, it showcases its ability to retain the capacity over numerous charge-discharge cycles. Additionally, the investigated sample displayed an impressive rate capability during the Li-ion charge/discharge process. Therefore, the material's remarkable electrochemical properties can be ascribed to the synergistic effects of its large specific surface area of 348.294 m2·g-1 and well-defined pore size distribution of 20.448 Å, making it a promising candidate for high-performance Li-ion batteries.

Cu(I)-Glutathione Assembly Supported on ZIF-8 as Robust and Efficient Catalyst for Mild CO2 Conversions.

The Cu-glutathione (GSH) redox system, essential in biology, is designed here as a supramacromolecular assembly in which the tetrahedral 18e Cu(I) center loses a thiol ligand upon adsorption onto ZIF-8, as shown by EXAFS and DFT calculation, to generate a very robust 16e planar trigonal single-atom Cu(I) catalyst. Synergy between Cu(I) and ZIF-8, revealed by catalytic experiments and DFT, affords CO2 conversion into high-value-added chemicals with a wide scope of substrates by reaction with terminal alkynes or propargyl amines in excellent yields under mild conditions and reuse at least 10 times without significant decrease in catalytic efficiency.

Laser-Driven Transient Phase Oscillations in Individual Spin Crossover Particles.

An unusual expansion dynamics of individual spin crossover nanoparticles is studied by ultrafast transmission electron microscopy. After exposure to nanosecond laser pulses, the particles exhibit considerable length oscillations during and after their expansion. The vibration period of 50-100 ns is of the same order of magnitude as the time that the particles need for a transition from the low-spin to the high-spin state. The observations are explained in Monte Carlo calculations using a model where elastic and thermal coupling between the molecules within a crystalline spin crossover particle govern the phase transition between the two spin states. The experimentally observed length oscillations are in agreement with the calculations, and it is shown that the system undergoes repeated transitions between the two spin states until relaxation in the high-spin state occurs due to energy dissipation. Spin crossover particles are therefore a unique system where a resonant transition between two phases occurs in a phase transformation of first order.

Rational Direct Synthesis of RbMnFe Nanoparticles (RbMnFe = RbxMn[Fe(CN)6](2+x)/3·nH2O Prussian Blue Analogue).

In this paper, we report the chemical strategy followed to obtain, in a direct way, nanoparticles of the RbxMn[Fe(CN)6](x+2)/3·nH2O (RbMnFe) Prussian blue analogue with the aim of keeping the switching ability of this compound at the nanoscale. The switching properties come from a reversible electron transfer between the iron and manganese ions and depends on the rubidium content in the structure that has to be higher than 0.6. Despite the multifunctionality of this family of compounds and its interest in various applications, no systematic studies were performed to obtain well-defined nanoparticles. This paper relates to such an investigation. To draw relationship between size reduction, composition, and switching properties, a special attention was brought to the determination of the composition through elemental analysis and structure refinement of powder X-ray diffraction patterns together with infrared spectroscopy and elemental analysis. Several chemical parameters were explored to control both the size reduction and the composition following a direct synthetic approach. The results show that the smaller the particles, the lower the rubidium content. This observation might prevent the observation of switching properties on very small particles. Despite this antagonist effect, we achieved switchable particles of around 200 nm without any use of surfactant. Moreover, the size reduction is associated with the observation of the electron transfer down to 52% of rubidium in the nanoparticles against 64% in microparticles. This work is of particular interest in processing such nanoparticles into devices.

One-Step Synthesis of Spin Crossover Nanoparticles Using Flow Chemistry and Supercritical CO2.

Switchable materials are increasingly considered for implementation in devices or multifunctional composites leading to a strong need in terms of reliable synthetic productions of well-defined objects. Here, an innovative and robust template-free continuous process was developed to synthesize nanoparticles of a switchable coordination polymer, including the use of supercritical CO2 , aiming at both quenching the particle growth and drying the powder. This all-in-one process offers a 12-fold size reduction in a few minutes while maintaining the switching properties of the selected spin crossover coordination polymer.

Unprecedented Reverse Volume Expansion in Spin-Transition Crystals.

The current craze for research around the spin crossover phenomenon can be justified to some extent by the mechanical properties due to the decrease of volume associated with the transition of the metal ion from the HS state to the LS state. As demonstrated here, the molecular complex [Fe(PM-pBrA)2 (NCS)2 ] exhibits, on the contrary, an increase of the unit-cell volume from HS to LS. This counter-intuitive and unprecedented behavior that concerns both the thermal and the photoexcited spin conversions is revealed by a combination of single-crystal and powder X-ray diffraction complemented by magnetic measurements. Interestingly, this abnormal volume change appears concomitant with the wide rotation of a phenyl ring which induces a drastic modification, though reversible, of the structural packing within the crystal. In addition, the light-induced HS state obtained through the Light-Induced Excited Spin-State Trapping shows a remarkably high relaxation temperature, namely T(LIESST), of 109 K, one of the highest so far reported. The above set of quite unusual characteristics opens up new fields of possibilities within the development of spin crossover materials.

Pressure-Induced Spin-Crossover Features at Variable Temperature Revealed by In Situ Synchrotron Powder X-ray Diffraction.

An accurate high-pressure X-ray diffraction investigation, at various temperatures, on a powder of a spin-crossover (SCO) complex has allowed the rare deconvolution of the structural features of the high-spin and low-spin phases. As a result, the pressure dependence of the structural parameters of the high-spin and low-spin phases can be discussed independently in the pressure domain where both phases co-exist within the powder. Consequently, crucial unprecedented information is given, such as the variation of bulk moduli with temperature, similar here in amplitude for both spin phases, the temperature-dependence of the pressure-induced SCO abruptness, the temperature dependence of the pressure at which SCO occurs, and arguments for a possible piezo-hysteresis. Performed on the molecular complex [Fe(PM-PeA)2 (NCSe)2 ] (PM-PeA=N-(2'-pyridylmethylene)-4-(phenylethynyl) aniline), this study reveals a pressure-induced SCO at 0.16 GPa and demonstrates that, when increasing temperature, the pressure of transition increases linearly, the abruptness of the pressure-induced SCO strongly increases, and the bulk moduli decrease.

A Nickel-Based Semiconductor Hybrid Material with Significant Dielectric Constant for Electronic Capacitors.

A novel semiconducting Ni(II)-based hybrid material with the formula (C7H12N2) NiCl4, which exhibits interesting optical and electrical properties, is reported. The crystal structure was investigated using SCXRD, whereas physical properties were studied by means of thermal analysis, Ft-Infrared, optical, and electrical measurements. Its crystal packing is formed through organic rings surrounded by inorganic [NiCl4]2- tetrahedral and stacked along the a-crystallographic axis. This arrangement is stabilized by a dense network of intermolecular hydrogen bonds. The investigated compound displayed a wide absorption range across the visible spectrum, characterized by an optical gap energy of 2.64 eV, indicating its semiconducting nature and efficient sunlight absorption capabilities across various wavelengths. Such features are of utmost importance in achieving a high energy conversion efficiency in solar cell applications. Further analyses of the thermal behavior using differential scanning calorimetry revealed a single-phase transition occurring at around 413 K, which was further confirmed through electrical measurements. A deep investigation of the electric and dielectric performances demonstrated a significant dielectric constant (ε' ∼ 104) at low frequencies and low dielectric loss at high frequencies. Thus, it highlights its exceptional dielectric potential, particularly in applications related to electronic capacitors.

Spin crossover complexes [Fe(NH2trz)3](X)2·nH2O investigated by means of polarized Raman scattering and DFT calculations.

Vibrational spectra of the spin crossover (SCO) polymers [Fe(NH2trz)3](X)2·nH2O where NH2trz = 4-NH2-1,2,4-triazole and X = Cl, Br, BF4, and NO3 have been analyzed. Our results show that the anions and water molecules have no significant influence on the vibrational properties of the Fe(NH2-trz)3 polymer chains. A detailed study of the nitrate derivative, based on the DFT analysis of the polarized spectra of single crystals, has been undertaken to propose the normal mode assignment of the Raman peaks in the low spin state of the compound. Changes in the Raman spectra in the high spin state could therefore be analyzed and interpreted by several Raman bands identified as molecular probes of the SCO phenomenon. Various factors (laser power, humidity, pressure) that influence the transition temperatures and the hysteresis loops have been identified and adjusted for obtaining reliable measurements. We demonstrate in particular that all the techniques used to probe the phase transition process give comparable results providing that the sample environment is well controlled.

Lattice Defects in Sub-Micrometer Spin-Crossover Crystals Studied by Electron Diffraction.

Spin-crossover particles of [Fe(Htrz)2trz](BF4) with sizes of some hundred nanometers are studied by in situ electron microscopy. Despite their high radiation sensitivity, it was possible to analyze the particles by imaging and diffraction so that a detailed analysis of crystallographic defects in individual particles became possible. The presence of one or several tilt boundaries, where the tilt axis is the direction of the polymer chains, is detected in each particle. An in situ exposure of the particles to temperature variations or short laser pulses to induce the spin crossover shows that the defect structure only changes after a high number of transformations between the low-spin and high-spin phases. The observations are explained by the anisotropy of the atomic architecture within the crystals, which facilitates defects between weakly linked crystallographic planes.

Room temperature optoelectronic devices operating with spin crossover nanoparticles.

Molecular systems can exhibit multi-stimuli switching of their properties, with spin crossover materials having unique magnetic transition triggered by temperature and light, among others. Light-induced room temperature operation is however elusive, as optical changes between metastable spin states require cryogenic temperatures. Furthermore, electrical detection is hampered by the intrinsic low conductivity properties of these materials. We show here how a graphene underlayer reveals the light-induced heating that triggers a spin transition, paving the way for using these molecules for room temperature optoelectronic applications.

Photo-Thermal Switching of Individual Plasmonically Activated Spin Crossover Nanoparticle Imaged by Ultrafast Transmission Electron Microscopy.

Spin crossover (SCO) is a promising switching phenomenon when implemented in electronic devices as molecules, thin films or nanoparticles. Among the properties modulated along this phenomenon, optically induced mechanical changes are of tremendous importance as they can work as fast light-induced mechanical switches or allow to investigate and control microstructural strains and fatigability. The development of characterization techniques probing nanoscopic behavior with high spatio-temporal resolution allows to trigger and visualize such mechanical changes of individual nanoscopic objects. Here, ultrafast transmission electron microscopy (UTEM) is used to precisely probe the length changes of individual switchable nanoparticles induced thermally by nanosecond laser pulses. This allows revealing of the mechanisms of spin switching, leading to the macroscopic expansion of SCO materials. This study is conducted on individual pure SCO nanoparticles and SCO nanoparticles encapsulating gold nanorods that serve for plasmonic heating under laser pulses. Length changes are compared with time-resolved optical measurements performed on an assembly of these particles.

Mechanism for optical switching of the spin crossover [Fe(NH(2)-trz)(3)](Br)(2).3H(2)O compound at room temperature.

This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH(2)-trz)(3)] (Br)(2).3H(2)O (with NH(2)trz = 4-amino-1,2,4-triazole) around room temperature and in the close vicinity of the thermal hysteresis loop. The measurements are carried out using a time-resolved pump-probe experiment and by recording the reflectivity change at various temperatures and laser intensities. The dynamics of the optically induced reflectivity changes are presented and discussed. We propose a simple model that describes well the recorded phenomena. It takes into account the physical and optical properties of the sample that directly impact the amplitude and the dynamics of the laser-induced heating of the compound.

Nanoparticles of [Fe(NH2-trz)3]Br2.3H2O (NH2-trz=2-amino-1,2,4-triazole) prepared by the reverse micelle technique: influence of particle and coherent domain sizes on spin-crossover properties.

This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2-trz)3]Br2.3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320 K. We find that when the size is reduced down to 50 nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50 nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30 nm individual particles consist of one coherent domain.

Nanoparticles of iron(II) spin-crossover.

We report the synthesis of spin crossover 69 nm spherical nanoparticles of [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2trz = 4-amino-1,2,4-triazole, surfactant = Lauropal), prepared by the reverse micelle technique, which exhibit at room temperature a thermal hysteresis characterized by magnetic, diffuse reflectivity and Raman studies.

Extreme downsizing in the surfactant-free synthesis of spin-crossover nanoparticles in a microfluidic flow-focusing junction.

A new surfactant-free, flow-focusing droplet microfluidic approach was developed as an important alternative to existing synthesis techniques for the preparation of spin crossover nanoparticles. It enables great control of the mixing of the reactants, and produces crystals of [Fe(pyrazine)(Pt(CN)4)] with an unexpected 20-fold downsizing compared to classical bulk synthesis.

Spray-Drying to Get Spin-Crossover Materials.

Spin-crossover (SCO) triazole-based coordination polymers can be synthesized by micelle techniques, which almost always lead to rod-shaped nanoparticles. In order to notably reach new morphologies, we explore here the potentiality of the spray-drying (SD) method to get SCO materials. Three SCO coordination polymers and a mononuclear complex are investigated. In all cases, the SD method obtains particles definitely showing SCO. The features of the latter are yet always different from those of the referenced materials, in the sense that SCO is more gradual and incomplete, in adequacy with the poor crystallinity of the powders obtained by SD. In the case of coordination polymers, the particles are preferentially spherical. Indications of possible polymorphism and/or new materials induced by the use of the SD method are evidenced. In the case of the mononuclear complex, the SD method has allowed reproducing, in a quick and easy way, the well-known bulk compound. This exploratory work demonstrates the relevance of the concept and opens the way to a systematic scrutiny of all the experimental parameters to tune the size, morphology, and properties of the SD-synthesized SCO particles.

Grafting of gold onto spin-crossover nanoparticles: SCO@Au.

Nanoparticles of gold were successfully grafted onto nanoparticles of a 1D polymeric spin-crossover material leading to singular SCO@Au hybrid particles. The result is equally obtained using a large range of gold-particle sizes, from 4 to 45 nm, which first allows definition of the best experimental conditions, notably in terms of gold-particle concentration, and then demonstrates the robustness and the efficiency of the method.

A New Photomagnetic Molecular System Based on Photoinduced Self-Assembly of Radicals.

An irreversible trans→cis isomerization of the imino group occurs during the irradiation of the new ferrocenyl Schiff-base polychlorotriphenylmethyl radical 1 by light. Low-temperature ESR investigations of frozen solutions revealed that the cis isomer exists as the (cis-1)2 dimer with strong antiferromagnetic interactions. The radical 1 constitutes an example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground-state species is promoted by a photoinduced self-assembly process driven by the formation of hydrogen bonds.

The 1-D polymeric structure of the [Fe(NH2trz)3](NO3)2·nH2O (with n = 2) spin crossover compound proven by single crystal investigations.

The first single crystal-based description of a spin transition compound from the patented [Fe(Rtrz)(3)]X(2)·nH(2)O family is reported, properly the compound with Rtrz = 4-amino-1,2,4-triazole, X = NO(3) and n = 2. This study definitively proves the 1-D polymeric nature of the crystal structure and allows a description of the [Fe(Rtrz)(3)] chains, as well as of the anions and water implementation within the crystal lattice opening the possibility of subsequent investigations of the structure-property correlation within this peculiar family of spin transition compounds.