Polymères de coordination à transition de spin : synthèse, élaboration de couches minces, nanostructuration et propriétés physiques

Depuis de nombreuses années, les demandes technologiques en termes de capacité de stockage de l'information numérique augmentent de façon considérable et sont en partie à l'origine du développement des nanosciences : l'objectif est de stocker toujours plus d'information dans un volume toujours plus petit et le plus rapidement possible. De même, la miniaturisation d'autres composants électroniques ou photonique tels que les capteurs, les dispositifs pour l'optique constitue un domaine en pleine essor. Les composés moléculaires organiques et inorganiques sont potentiellement prometteurs dans la perspective de telles applications. En particulier, les matériaux bistables présentant une hystérésis de leurs propriétés physiques sont des matériaux de choix pour la conception de dispositifs de stockage de l'information et capteur de gaz à très petite échelle. Cependant, le dépôt sous forme de couches minces et également la fabrication de nano-objets ainsi que leur intégration sans altérer la propriété de ces systèmes constituent des étapes incontournables dans la conception de tout dispositif. Dans ce contexte, ce manuscrit présente la conception, la synthèse et la caractérisation d'une nouvelle famille de composés tridimensionnels polymériques de coordination présentant des propriétés bistables. Il s'agit de polymères de coordination à transition de spin de la famille des clathrates de Hofmann. L'étude physico-chimique de ces composés a été centrée, notamment, sur les variations des caractéristiques de la transition de spin en fonction des compositions chimiques obtenues pour différentes méthodes de synthèse mises en œuvre. De plus, sont étudiés les changements des propriétés physiques et structurales qui se produisent lors de l'adsorption de molécules invitées dans les pores de ces polymères. D'autre part, ce travail de thèse montre une approche originale dite dépôt séquentiel ou plus largement dépôt "couche-par-couche" pour l'élaboration de couches minces et présente également la nano-structuration de ces matériaux par combinaison de techniques lithographiques et de l'assemblage séquentiel. La modification des conditions expérimentales de l'assemblage séquentiel nous a permis de maîtriser la croissance et la qualité de surface des dépôts (rugosité inférieure à quelques Angström). Ces couches minces et ces nano-objets ont été caractérisés par différentes techniques optiques (microscopie Raman, ellipsométrie, spectroscopie des plasmons de surface …) ou encore des techniques d'imagerie (AFM, microscopie optique en champ sombre). La combinaison des résultats obtenus sur les propriétés d'adsorption de ces composés bistables et leur mise en forme en tant que systèmes micro- et nano-structurés ouvre la voie vers des perspectives sérieuses pour la réalisation de dispositifs capteurs de gaz.In the last few years, the technological demand concerning information storage capacity has grown considerably and is, mostly, due to the development of nano-science and nano-technology: the aim is storing a large quantity of information in an increasingly reduced space. Moreover, miniaturization of other electronic or photonic devices such as gas sensors represents an expanding field. Organic and inorganic molecular compounds are highly promising for these applications. Particularly, bistable materials, which present a hysteresis loop in some of their physical properties are good candidates in the design of information storage and gas sensor devices on a very small scale. Nevertheless, deposition of thin films or the fabrication of nano-objects, as well as their implementation without modifying their properties represents an important step in the design of any device. In this context, this thesis presents the design, synthesis and characterization of a new family of coordination polymers presenting bistable properties. These materials are spin crossover Hofmann-like tridimensional networks. The physical and chemical studies carried out on these compounds are primarily focused on the changes to the spin crossover properties as a function of the chemical composition, which can be altered according to the synthetic methods. Furthermore, the influence of adsorbed guest molecules on the structural and physical properties of these compounds has been investigated. This manuscript contains an original method to the elaboration of thin films of such materials, the so-called “layer by layer” or “multilayer self-assembling” technique. Additionally, micro and nano-structures of these films have been prepared combining lithographic techniques and sequential assembling. The modification of experimental conditions for the layer by layer deposition has allowed control over growth and the surface quality thin films (roughness of a few Angström) to be realized. Thin films and nano-objects have been characterized by different optical (Raman microscopy, optical ellipsometry, surface plasmon spectroscopy…) and surface analysis (AFM, dark field microscopy) techniques

A sequential method to prepare polymorphsand solvatomorphs of [Fe(1,3-bpp)2](ClO4)2·nH2O( n=0, 1, 2) with varying spin-crossover behaviour

Two polymorphs of the spin crossover (SCO) compound [Fe(1,3bpp)2](ClO4)2 (1 and 2; 1,3bpp, 2-(pyrazol-1-yl)-6-(pyrazol-3-yl)-pyridine) are prepared using a novel, stepwise procedure. Crystals of 1 deposit from dry solvents while 2 is obtained from a solid-state procedure, by sequentially removing lattice H2O molecules from the solvatomorph [Fe(1,3bpp)2](ClO4)2·2H2O (2·2H2O), using single-crystal-to-single-crystal (SCSC) transformations. Hydrate 2·2H2O is obtained through the same reaction as 1, now with 2.5% of added water. Compounds 2 and 2·2H2O are unstable in the atmosphere and absorb or lose one equivalent of water, respectively, to both yield the stable solvatomorph [Fe(1,3bpp)2](ClO4)2·H2O (2·H2O), also following SCSC processes. The four derivatives have been characterized by single crystal X-ray diffraction (SCXRD). Furthermore, the homogeneity of the various compounds as well as their SCSC interconversions have been confirmed by powder X-ray diffraction (PXRD). Polymorphs 1/2 exhibit abrupt SCO near room temperature with T1/2↑ = 279/316 K and T1/2↓ = 276/314 K (near 40 K of shift) and different cooperativity

Allosteric Spin Crossover Induced by Ligand-Based Molecular Alloying

he spin crossover (SCO) phenomenon represents a source of multistability at the molecular level, and dilution into a nonactive host was originally key to understand its cooperative nature and the parameters governing it in the solid state. Here, we devise a molecular alloying approach in which all components are SCO-active, but with significantly different characteristic temperatures. Thus, the molecular material [Fe(Mebpp)2](ClO4)2 (2) has been doped with increasing amounts of the ligand Me2bpp (Mebpp and Me2bpp = methyl- and bis-methyl-substituted bis-pyrazolylpyridine ligands), yielding molecular alloys with the formula [Fe(Mebpp)2-2x(Me2bpp)2x](ClO4)2 (4x; 0.05 < x < 0.5). The effect of the composition on the SCO process is studied through single-crystal X-ray diffraction (SCXRD), magnetometry, and differential scanning calorimetry (DSC). While the attenuation of intermolecular interactions is shown to have a strong effect on the SCO cooperativity, the spin conversion was found to occur at intermediate temperatures and in one sole step for all components of the alloys, thus unveiling an unprecedented allosteric SCO process. This effect provides in turn a means of tuning the SCO temperature within a range of 42 K

A spin-crossover molecular material describing four distinct thermal pathways

Spin-crossover (SCO) molecular solids are valued switchable materials for their common abrupt and reversible thermal transitions, large thermal hysteresis, or guest-dependent effects. These properties usually involve crystallographic transitions coupled to the SCO events. These phenomena are of great value for the understanding of solidstate transformations and also for exploiting them. We present here a lattice of the complex [FeL(bbp)](ClO4)2 (1; L and bbp are tris-imine ligands) featuring an unprecedented rich succession of SCO and crystallographic phase transformations. Magnetometry measurements unveil a thermally irreversible sequence of spin conversions that delineate four different thermal pathways. All of these are single-crystal-tosingle- crystal processes and can thus be monitored by single crystal X-ray diffraction using one unique specimen. Fresh crystals of 1 contain one molecule of acetone per Fe center (1·ac) that abandons the lattice upon warming at the same time that a SCO from an ordered mixed spin state (1:1 high spin/low spin; HS/LS) to a fully HS state, 1α, occurs. This crystallographic phase, accessed through a template effect by the solvent, converts into another one, 1β, upon cooling, as triggered by a HS to LS SCO. Warming of 1β induces a new SCO (LS to ordered HS/LS) coupled to another crystallographic phase transition, 1β → 1γ. The fully HS state of 1γ can not be reached before decomposition of the compound. Instead, this phase cycles between the HS/LS and the LS states through superimposable pathways, different from that of the prerequired 1β → 1γ phase change. Analysis of the thermal variation of the free energy, G, through density functional theory methods provides trends in agreement with the observation of these transformations and clarifies the possible metastable nature of the various phases identified. This unique behavior allows the access to four different magnetic responses depending on the thermal history of the sample, within a given range of temperatures near the ambient conditions

A probe of steric ligand substituent effects on the spin crossover of Fe(II) complexes

Identifying and quantifying the individual factors affecting the temperature and properties of the spin crossover in transition metal complexes is a challenging task, because many variables are involved. While the most decisive factor is the crystal field imparted by ligands around the active metal center, some less common actors are intramolecular steric repulsions or non-covalent interactions. A series of three Fe(ii) complexes of 1,3bpp derivatives of (2-(pyrazol-1-yl)-6-(1H-pyrazol-3-yl)pyridine) have been prepared and characterized crystallographically to probe these effects: [Fe(1,3bpp)2](ClO4)2 (1), [Fe(met1,3bpp)2](ClO4)2 (2) and [Fe(dimet1,3bpp)2](ClO4)2 (3). The ligands exhibit none, one or two methyl substituents on the pyrazol-1-yl heterocycle. These groups exert a dramatic effect on the SCO temperature in the solid state, and, most significantly, in solution (with TSCO (3) > TSCO (1) > TSCO (2)). Extensive DFT calculations have unveiled the origin of these effects which lie in the intramolecular non-covalent or steric interactions rather than resulting from crystal field effects

Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic Chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.inorgchem.7b02272[EN] Understanding the origin of cooperativity and the equilibrium temperature of transition (T1/2) displayed by the spin-crossover (SCO) compounds as well as controlling these parameters are of paramount importance for future applications. For this task, the occurrence of polymorphism, presented by a number of SCO complexes, may provide deep insight into the influence of the supramolecular organization on the SCO behavior. In this context, herein we present a novel family of mononuclear octahedral FeII complexes with formula cis- [Fe(bqen)(NCX)2], where bqen is the chelating tetradentate ligand N,N¿-bis(8-quinolyl)ethane-1,2-diamine and X = S, Se. Depending on the preparation method, these compounds crystallize in either the orthorhombic or the trigonal symmetry systems. While the orthorhombic phase is composed of a racemic mixture of mononuclear complexes (polymorph I), the trigonal phase contains only one of the two possible enantiomers (¿ or ¿), thereby generating a chiral crystal (polymorph II). The four derivatives undergo SCO behavior with well-differentiated T1/2 values occurring in the interval 90¿233 K. On one hand, T1/2 is about 110 K (polymorph I) and 87 K (polymorph II) higher for the selenocyanate derivatives in comparison to those for their thiocyanate counterparts. These differences in T1/2 are ascribed not only to the higher ligand field induced by the selenocyanate anion but also to a remarkable difference in the structural reorganization of the [FeN6] coordination core upon SCO. Likewise, the higher cooperativity observed for the thiocyanate derivatives seems to be related to their stronger intermolecular interactions within the crystal. On the other hand, T1/2 is about 53 K (thiocyanate) and 29 K (selenocyanate) higher for the trigonal polymorph II in comparison to those for the orthorhombic polymorph I. These differences, and the small changes observed in cooperativity, stem from the slightly different hetero- and homochiral crystal packing generated by the cis-[Fe(bqen)(NCX)2] molecules, which determines subtle adaptations in the intermolecular contacts and the FeII coordination core.We thank the Spanish Ministerio de Economia y Competitividad (MINECO) and FEDER funds (CTQ2013-46275-P and CTQ2016-78341-P and Unidad de Excelencia Maria de Maeztu MDM-2015-0538), Generalitat Valenciana (PROM-ETEO/2016/147), and EU Framework Program for Research and Innovation (RISE project number 734322). FJ.V.-M. thanks MINECO for a predoctoral FPI grantBartual-Murgui, C.; Piñeiro-López, L.; Valverde-Muñoz, FJ.; Muñoz Roca, MDC.; Seredyuk, M.; Real, JA. (2017). Chiral and Racemic Spin Crossover Polymorphs in a Family of Mononuclear Iron(II) Compounds. Inorganic Chemistry. 56(21):13535-13546. https://doi.org/10.1021/acs.inorgchem.7b022721353513546562

Homoleptic versus heteroleptic formation of mononuclear Fe(II) complexes with tris-imine ligands

We show a marked tendency of Fe(II) to form heteroleptic [Fe(L)(L′)](ClO4)2 complexes from pairs of chelating tris-imine 3bpp, tpy, or 2bbp ligands. New synthetic avenues for spin crossover research become thus available, here illustrated with three new heteroleptic compounds with differing magnetic behaviors: [Fe(H4L1)(Cl-tpy)](ClO4)2·C3H6O (1), [Fe(H2L3)- (Me3bpp)](ClO4)2·C3H6O (2), [Fe(H4L1)(2bbp)](ClO4)2· 3C3H6O (3). Structural studies demonstrate that 1 is in the lowspin (LS) state up to 350 K, while complexes 2 and 3 are, by contrast, in the high-spin (HS) state down to 2 K, as corroborated through magnetic susceptibility measurements. Upon exposure to the atmosphere, the latter exhibits the release of three molecules of acetone per complex, turning into the solvent-free analogue [Fe(H4L1)(2bbp)](ClO4)2 (3a), through a singlecrystal- to-single-crystal transformation. This guest extrusion process is accompanied by a spin switch, from HS to LS

Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

[EN] Aiming at investigating the suitability of Hofmann-type two-dimensional ( 2D) coordination polymers {Fe-II(L-ax)(2)[M-II(CN)(4)]} to be processed as single monolayers and probed as spin crossover (SCO) junctions in spintronic devices, the synthesis and characterization of the MII derivatives (M-II = Pd and Pt) with sulfur-rich axial ligands (L-ax = 4-methyl- and 4-ethyl-disulfanylpyridine) have been conducted. The thermal dependence of the magnetic and calorimetric properties confirmed the occurrence of strong cooperative SCO behavior in the temperature interval of 100-225 K, featuring hysteresis loops 44 and 32.5 K/21 K wide for PtII-methyl and Pt-II/Pd-II- ethyl derivatives, while the Pd-II-methyl derivative undergoes a much less cooperative multistep SCO. Excluding Pt-II-methyl, the remaining compounds display light-induced excited spin-state trapping at 10 K with T-LIESST temperatures in the range of 50-70 K. Single-crystal studies performed in the temperature interval 100-250 K confirmed the layered structure and the occurrence of complete transformation between the high- and low-spin states of the Fe-II center for the four compounds. Strong positional disorder seems to be the source of elastic frustration driving the multistep SCO observed for the Pd-II-methyl derivative. It is expected that the peripheral disulfanyl groups will favor anchoring and growing of the monolayer on gold substrates and optimal electron transport in the device.This work was supported by the Spanish Ministerio de Ciencia e Innovacion (MICINN) and FEDER founds (No. PID2019-106147GB-I00), Unidad de Excelencia Maria de Maeztu (Nos. MDM2015-0538-18-2, CEX2019-000919-M) and EU Framework FET-OPEN project COSMICS (Grant Agreement No. 766726). R.T.C. and M.M.S. thank the MICINN for a predoctoral grant.Turo-Cortés, R.; Valverde-Muñoz, FJ.; Meneses-Sánchez, M.; Muñoz Roca, MDC.; Bartual-Murgui, C.; Real, JA. (2021). Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces. Inorganic Chemistry. 60(12):9040-9049. https://doi.org/10.1021/acs.inorgchem.1c0101090409049601

Ultrathin films of 2D Hofmann-type coordination polymers: influence of pillaring linkers on structural flexibility and vertical charge transport

Searching for novel materials and controlling their nanostructuration into electronic devices is a challenging task ahead of chemists and chemical engineers. Even more so when this new application requires an exquisite control over the morphology, crystallinity, roughness and orientation of the films produced. In this context, it is of critical importance to analyze the influence of the chemical composition of perspective materials on their properties at the nanoscale. We report the fabrication of ultrathin films (thickness < 30 nm) of a family of FeII Hofmann-like coordination polymers by using an optimized liquid phase epitaxy (LPE) set-up. The series [Fe(L)2{Pt(CN)4}] (L = pyridine, pyrimidine and isoquinoline) conform an ideal platform for correlating the effect of the axial nitrogenated ligand with changes to their structural response to guests or electrical resistance. All film properties relevant to device integration have been thoroughly analyzed with complementary surface techniques for a meaningful comparison. Our results reveal that changes to this ligand can hinder the structural transformation triggered by the absorption of guest molecules previously reported for the pyridine phase. Also important, it can substantially hinder vertical charge transport across the layers, even at the ultrathin film limit

Epitaxial thin-film vs single crystal growth of 2D Hofmann-type iron(II) materials: a comparative assessment of their bi-stable spin crossover properties

Integration of the ON−OFF cooperative spin crossover (SCO) properties of FeII coordination polymers as components of electronic and/or spintronic devices is currently an area of great interest for potential applications. This requires the selection and growth of thin films of the appropriate material onto selected substrates. In this context, two new series of cooperative SCO two-dimensional FeII coordination polymers of the Hofmann-type formulated {FeII(Pym)2[MII(CN)4]·xH2O}n and {FeII(Isoq)2[MII(CN)4]}n (Pym = pyrimidine, Isoq = isoquinoline; MII = Ni, Pd, Pt) have been synthesized, characterized, and the corresponding Pt derivatives selected for fabrication of thin films by liquid-phase epitaxy (LPE). At ambient pressure, variable-temperature single-crystal X-ray diffraction, magnetic, and calorimetric studies of the Pt and Pd microcrystalline materials of both series display strong cooperative thermal induced SCO properties. In contrast, this property is only observed for higher pressures in the Ni derivatives. The SCO behavior of the {FeII(L)2[PtII(CN)4]}n thin films (L = Pym, Isoq) were monitored by magnetization measurements in a SQUID magnetometer and compared with the homologous samples of the previously reported isostructural {FeII(Py)2[PtII(CN)4]}n (Py = pyridine). Application of the theory of regular solutions to the SCO of the three derivatives allowed us to evaluate the effect on the characteristic SCO temperatures and the hysteresis, as well as the associated thermodynamic parameters when moving from microcrystalline bulk solids to nanometric thin films