Scientific activities, education, networks

To imagine, conceive, characterize, understand, optimize and develop new molecular magnetic materials exhibiting variable properties under external stimulation (temperature, light, pressure, current…). It's your aim - Team "Switchable Molecules and Materials"

 

The molecular engineering involved then consists in perfectly designing the ligand so as to promote the greatest structural rearrangement during the photo-induced transition. The greater this reorganization (distortion, elongation), the longer the lifetime of the photo-induced state (Chem. Squared 2018, 2, 2; C. R. Chimie 2018, 21, 1075).

We have shown over the years that this lifetime is also strongly influenced by intermolecular interactions, structural transitions, elastic competition … Indeed, elastic competition can induce symmetry breaking giving rise to multistability, opening up perspectives for multilevels information processing. In addition, this controlled competition allows access to hidden phases, that only light or pressure can reveal, giving rise to switches between states with a very long lifetimes (Inorg. Chem. 2016, 55, 11652; Eur. J. Inorg. Chem. 2018, 305).

Among the additional functions that we can provide thanks to molecular chemistry, the combination of chirality with coordination chemistry is particularly attractive. Indeed the manipulation of chiral objects allows a rigorous control of the obtaining of non-centrosymmetric solids. This opens the way to a variety of properties such as: optical activity in a wide energy range from THz to X-rays, through the visible and infrared domains; ferro / piezo / pyroelectricity; spin polarization observed on thin chiral molecular layers; non-linear optics in solution and / or solid; etc … (Appl. Phys. Lett. 2017, 110, 161908; Inorg. Chem. 2018, 57, 14501; J. Phys. Chem. Lett. 2019, 10, 5975).  The synthetic methods of chiral molecular complexes cover the case study of spontaneous resolution, the synthesis of ligands from the chiral pool, and resolution through the use of optically active anions.In addition, for several years, electron transfer between two metal centers linked by a cyanide bridge has proven to be a strategy of choice for creating reversible switchable molecular materials with temperature and light (Chem. Soc. Rev. 2016, 45,203;  Eur. J. Inorg. Chem. 2018, 248). The group, in collaboration with the M3 team of CRPP, designed using a top-down approach, the smallest electron transfer molecules by taking inspiration from photomagnetic networks of the Prussian blue family of general formula AxCoy[Fe(CN)6]z,nH2O. Using the molecular building blocks approach, blocking ligands on both the Co and Fe ions have been developed, making it possible to obtain cubes, squares and molecular pairs. This approach combined with a careful study of the redox activity of the precursors involved in the construction of the pairs allowed to define the conditions required to obtain the molecules with thermo- and photo-switchable electron transfer, in solution (Chem. Sci. 2013, 4, 2463) and in the solid state (J. Am. Chem. Soc. 2014, 136, 15461).

For more than 20 years, the group gathers together chemists, physical-chemists and physicists around photo-, piezo- and / or thermo-switchable molecular materials. This research activity fits into the general context of the development of new materials for information processing devices, pigments and sensors.

One of the great originality of these switchable molecular materials, developed and studied by the group, lies in their bistable nature, namely the presence of two different electronic states, depending on external stimulation such as pressure, temperature and / or light irradiation. Switching between these states, associated with the phenomenon of spin crossover (SCO) and / or electron transfer (ET), is accompanied by significant modifications of the mechanical, structural, dielectric, magnetic, optical and thermal properties. In addition, these modifications can be accompanied by memory effect, of interest for data processing in the solid state and at room temperature.

Our activity is based on proven expertise in molecular chemistry for the design and synthesis of new architectures with original, controlled and optimized properties. Our recognition in the field of photomagnetism, in close connection with an expertise in crystallography, is based on rigorous characterizations of the switching properties, leading us to a fine understanding of the structure-properties relationships at different scales of the material. Particular attention has been paid to the shaping of molecular materials (nanoparticles, molecular ceramics, thin films, etc.), particularly for issues of electrical transport and molecular electronic.

 


Molecular solid-state chemistry

Figure : Scheme summarizing the molecular engineering activities developped @Gr6 ICMCB.

Molecular chemistry, combining organic chemistry and coordination chemistry, is at the heart of the design of molecular solids with switchable properties. Its versatility makes it possible to target certain properties and also to promote multifunctionality.One of the main targets of our group concerns photoswitching, in particular by LIESST (Light-Induced Excited Spin-State Trapping) effect. This switching takes place at low temperature (typically 10 K), and one of the major challenges is to be able to observe it at higher temperature. The molecular engineering involved then consists in perfectly designing the ligand so as to promote the greatest structural rearrangement during the photo-induced transition. The greater this reorganization (distortion, elongation), the longer the lifetime of the photo-induced state (Chem. Squared 2018, 2, 2; C. R. Chimie 2018, 21, 1075).

We have shown over the years that this lifetime is also strongly influenced by intermolecular interactions, structural transitions, elastic competition … Indeed, elastic competition can induce symmetry breaking giving rise to multistability, opening up perspectives for multilevels information processing. In addition, this controlled competition allows access to hidden phases, that only light or pressure can reveal, giving rise to switches between states with a very long lifetimes (Inorg. Chem. 2016, 55, 11652; Eur. J. Inorg. Chem. 2018, 305).

Among the additional functions that we can provide thanks to molecular chemistry, the combination of chirality with coordination chemistry is particularly attractive. Indeed the manipulation of chiral objects allows a rigorous control of the obtaining of non-centrosymmetric solids. This opens the way to a variety of properties such as: optical activity in a wide energy range from THz to X-rays, through the visible and infrared domains; ferro / piezo / pyroelectricity; spin polarization observed on thin chiral molecular layers; non-linear optics in solution and / or solid; etc … (Appl. Phys. Lett. 2017, 110, 161908; Inorg. Chem. 2018, 57, 14501; J. Phys. Chem. Lett. 2019, 10, 5975).  The synthetic methods of chiral molecular complexes cover the case study of spontaneous resolution, the synthesis of ligands from the chiral pool, and resolution through the use of optically active anions.In addition, for several years, electron transfer between two metal centers linked by a cyanide bridge has proven to be a strategy of choice for creating reversible switchable molecular materials with temperature and light (Chem. Soc. Rev. 2016, 45,203;  Eur. J. Inorg. Chem. 2018, 248). The group, in collaboration with the M3 team of CRPP, designed using a top-down approach, the smallest electron transfer molecules by taking inspiration from photomagnetic networks of the Prussian blue family of general formula AxCoy[Fe(CN)6]z,nH2O. Using the molecular building blocks approach, blocking ligands on both the Co and Fe ions have been developed, making it possible to obtain cubes, squares and molecular pairs. This approach combined with a careful study of the redox activity of the precursors involved in the construction of the pairs allowed to define the conditions required to obtain the molecules with thermo- and photo-switchable electron transfer, in solution (Chem. Sci. 2013, 4, 2463) and in the solid state (J. Am. Chem. Soc. 2014, 136, 15461).

 


Multi-scale, multi-constraint crystallography

Figure: the different physical scales involved in the spin conversion phenomenon and the information obtained from the crystallographic studies, @Gr6 ICMCB.

Based on robust, long-standing and constantly evolving knowledge and know-how covering the different fields of Crystal Science, i.e. crystallography, one of the main objectives here is to determine and understand the structure-property relationships of materials at all physical scales, from the immediate environment of an atom to the crystal itself via the crystal arrangement and microstructures. The study of phase transitions and resulting diagrams, the determination of new atomic and molecular architectures, the detailed description of interatomic topologies of interactions within solids and the analysis of symmetries including the concept of chirality require that the possibilities for investigation be pushed to their limits. X-ray diffraction is one of the tools we master in its many aspects, such as in situ studies – high pressure, high and low temperature, light irradiation -, operando – transition cycling -, on samples of very diverse shapes – single crystals, polycrystals (powders), nanocrystals, thin films and composites – and using both laboratory experiments and large instruments. The emphasis is placed on the quality and reliability of the results by targeting difficult problems involving notably molecular crystals that can lead to real advances in the materials considered or in crystallography knowledge itself.

A particular focus has been carried out for many years on switchable materials and in particular spin-crossover molecular materials (figure). An appropriate investigation of the structural properties has allowed a complete and pioneering description of the switching phenomenon in the crystalline solid, the achievement of novel crystal structures but also the design of relationships between certain switching characteristics and structural properties. Beyond this switching, we also work on many topics such as organic-inorganic hybrid materials, conductive materials, molecular compounds of pharmaceutical interest or occasionally on any problem that requires expertise in crystallography, with the stated objective of determining new atomic architectures for new properties and, incidentally, to achieve a real paradigm shift in the knowledge of crystal matter.

Selection of five publications from 2018 representative of our work on structure-property relationships:

[1] E. Tailleur, M. Marchivie, J.-P. Itié, P. Rosa, N. Daro & P. Guionneau “Pressure-Induced Spin-Crossover Features at Variable Temperature Revealed by In Situ Synchrotron Powder X-ray Diffraction“, Chem. – A Eur. J. 2018, 24, 14495–14499.

[2] A. Djemel, O. Stefanczyk, M. Marchivie, E. Trzop, E. Collet, C. Desplanches, R. Delimi & G. Chastanet “Solvatomorphism-Induced 45 K Hysteresis Width in a Spin-Crossover Mononuclear Compound“, Chem. – A Eur. J. 2018, 24, 14760–14767.

[3] E. Collet and P. Guionneau “Structural analysis of spin-crossover materials: From molecules to materials“, C. R. Chimie, 2018, 21, 1133-1151.

[4] A. Naim, Y. Bouhadja, M. Cortijo, E. Duverger-Nédellec, H. D. Flack, E. Freysz, P. Guionneau, A. Iazzolino, A. Ould Hamouda, P. Rosa, O. Stefańczyk, A. Valentín-Pérez, and M. Zeggar “Design and Study of Structural Linear and Nonlinear Optical Properties of Chiral [Fe(phen)3]2+ ComplexesInorg. Chem. 57, 2018, 14501−14512.

[5] K. Ziach, C. Chollet, V. Parissi, P. Prabhakaran, M. Marchivie, V. Corvaglia, P. P. Bose, K. Laxmi-Reddy, F. Godde, J.-M. Schmitter, S. Chaignepain, P. Pourquier & I. Huc “Single helically folded aromatic oligoamides that mimic the charge surface of double-stranded B-DNA“, Nat. Chem. 2018, 10, 511–518.

 


Molecular materials science

Figure: Scheme summarizing the activities around molecular materials sciences developed @Gr6 ICMCB.

Nanoparticles. The group has been a pioneer in the development of spinswitchable nanoparticles since 2005. We are exploring original and versatile synthesis processes with the aim of developing nanomaterials and nanocomposites for their use into electronic devices or for multifunctional materials and molecular ceramics. While micellar synthesis remains the most developed (Magnetochemistry 2016, 2, 10), we have explored unusual processes such as spray drying (Materials 2017, 10, 60), microfluidics (Chem. Comm. 2018, 54, 8040) and flow chemistry coupled to supercritical fluids in collaboration with the G7 (France Brevet FR 2941458. 2010-07-10. International Patent WO 2010086550 (2010-08-05)) to obtain nanoparticles by eliminating the use of surfactants. We systematically study the effect of the process and the shaping on the switching properties.

Thin films. The synthesis of neutral spin crossover molecules of low molecular weight, allows them to be evaporated on different surfaces and under clean conditions  (J. Phys. Chem. C, 2012), down to isolated molecules (J. Mater. Chem., 2012) to study their photoswitching (J. Phys. Chem. Lett., 2013). This approach opens the way to the elaboration of hybrid architectures and transport measurements.

Composite and hybid materials. Several approaches are developed to obtain composite materials. Nanoparticles have the advantage of having very short photoswitching times (nanoseconds). In order to optimize this photoswitching due to photothermal effects, nanocomposites associating metallic nanoparticles (gold) and spin crossover have been synthesized (Chem. Commun. 2016, 52, 13213). The synergy between the plasmon resonance (SPR) of the metallic part and the spin crossover has considerably reduced the photoswitching energy while inducing a remarkable modulation of the SPR.Thin layers of spin crossover molecules deposited on a ferroelectric substrate have shown that the polarization of the substrate induces the switching of the spin state of the SCO layer (Chem. Commun. 2014, 50, 2255). In continuation of these results, switchable nanoparticles are dispersed in the ferroelectric organic polymer so as to study the synergy between spin crossover and ferroelectricity.Molecular ceramics. Spin crossover is sensitive to pressure effects and microstructure. We explore the effect of densification of powders into functional molecular ceramics on spin crossover properties and dielectric properties, in collaboration with G1. This unique technique allows densification under pressure (up to 6 kbar) and temperature (up to 250 ° C).

 


Electronic transport

The group has been interested for years in charge transport phenomena involving molecular compounds. Our activity is divided into two main parts: on the one hand the study of transport at small scales in nanoparticles and thin layers of spin crossover compounds and on the other hand the study of superconductivity in massive samples (single crystals).

Charge transport in switchable thin films and nanoparticles.  During the spin croosver, the electrons of the system are redistributed within the orbitals of the metal centers involved. This change in electronic configuration (stimulable by variation in temperature, light, chemical environment, etc.) should logically modify the charge transport properties of these compounds. However, the usual spin crossover compounds are generally insulating. If we wish to develop devices in which transport is modulated by spin crossover, it becomes necessary to work on small scales (for which it is still possible to measure significant currents). Since spin crossover is a subtle phenomenon, its manifestation on small scales is sometimes unpredictable. This requires molecular engineering and materials science work that we have been developing and studying for several years for the formulation of nanostructured spin crossover compounds (thin layers on different substrates or nanoparticles). These studies have so far focused on two types of devices (see Figure): lateral devices, in which nanoparticles of several tens or even hundreds of nanometers serve as a junction between two interdigitated electrodes,[1] and vertical metal-insulator-metal devices in which thin layers of only a few nanometers of our compounds serve as a stimulable component. In this latter configuration, we use ultra-flat electrodes on which we evaporate or graft ultrathin layers of spin crossover compound which we then contact using a liquid indium and gallium eutectic electrode (EGaIn) for measuring the current-voltage characteristics of nanometric molecular junctions in a reproducible manner, without damaging the molecular layer as a function of temperature.[2,3]

Unconventionnal superconductivity: charge transport in the normal state. Unconventional superconductors share a common phase diagram: a superconducting dome appears when an external parameter (chemical doping, pressure) is used to destabilize an ordered phase (generally magnetic) within the limit T = 0 K. At higher doping / pressure, a metallic phase of the Fermi liquid type is generally restored. The strong similarity between the phase diagram of intrinsically very different compounds (cuprates and molecular compounds) suggests that a universal mechanism is at the origin of superconductivity. The nature of this mechanism represents one of the greatest challenges in condensed matter physics and remains to be established.

The ability to shape and adjust the electronic properties of the system through subtle changes in the molecule at play makes molecular superconducting compounds of particular interest. This versatility of molecular solids is a major advantage compared to inorganic superconducting compounds. For example, molecular compounds offer the possibility of combining superconductivity and chirality. In fact, it is possible to introduce a chiral center into a molecule in a controlled manner (high enantiomeric purity) before crystallizing it in the form of a chiral monocrystal. The mixture of superconductivity and chirality has been proposed as a possible elementary block of quantum computing.

We are studying the characteristics of charge transport (resistivity, Hall effect) in the normal state when a magnetic field is used to destroy the superconducting state at low temperatures. For this we use the magnetic fields produced at the ICMCB (- 9 T to + 9 T) and at the LNCMI (90 T) in combination with a hydrostatic pressure applied to the sample (up to 3 GPa), over a range from 2 K to 300 K.

 

[1]   C. Etrillard, V. Faramarzi, J.-F. Dayen, J.-F. Letard, B. Doudin, Chem. Commun. 2011, 47, 9663.
[2]   L. Poggini, M. Gonidec, J. H. J. H. González-Estefan, G. Pecastaings, B. Gobaut, P. Rosa, Adv. Electron. Mater. 2018, 4, 1800204.
[3]   L. Poggini, M. Gonidec, R. K. Canjeevaram Balasubramanyam, L. Squillantini, G. Pecastaings, A. Caneschi, P. Rosa, J. Mater. Chem. C 2019, 7, 5343.