Benoit Fleury

Magnetic coordination complexes interact strongly with maghemite nanoparticles, increasing their magnetic anisotropy and remaining coupled up to room temperature.

Inducing chirality in semiconductor nanoparticles is a recent trend motivated by the possible applications in circularly polarized light emission, spintronics, or stereoselective synthesis. However, the previous reports on CdSe nanoplatelets (NPLs) exclusively rely on cysteine or its derivatives as chiral ligands to induce optical activity. Here, we show a strong induction of chirality with derivatives of tartaric acid obtained by a single-step synthesis. The ligand exchange procedure in organic solvent was optimized for five-monolayer (5 ML) NPLs but can also be performed on 4, 3, and 2 ML. We show that the features of the CD spectra change with structural modification of the ligands and that these chiral ligands interact mainly with the first light-hole (lh1) band rather than the first heavy-hole (hh1) band, contrary to cysteine. This result suggests that chiroptical properties could be used to probe CdSe nanoplatelets' surface ligands.

CdSe nanoplatelets are a recently discovered class of colloidal semiconducting nanocrystals. Atomic control over their thickness allows achieving control over quantum size effects, and in particular, these platelets exhibit monochromatic light emission because of the confinement of photo-generated excitons only in their thickness. These nanoplatelets can self-organize into supra-particular polymers, depending on their environment, which means that their shape anisotropy can be expressed at the microscale. Here, the self-assembly of semiconducting nanoplatelets is controlled remotely by light, in a dynamic nanoparticulate system that integrates light-responsive molecular switches covalently. Azobenzene ligands were thus designed to (i) be grafted on the nanoplatelets (ii) ensure their colloidal stability in chloroform when confined on their surface in the E-configuration. Upon irradiation, the ligands isomerize into their Z-configuration, leading to a modification of the dipolar moment of the particles and to the formation of one-dimensional stacks. The self-assembly is reversible, as thermal relaxation of the ligands yields the initial dispersion back. This reversible hybrid system can be used in the design of responsive optical systems, as illustrated by photo-patterning experiments leading to controlled spatial resolution of the luminescence intensity in thin films.

The successful utilization of the “direct synthesis” approach yielded the unprecedented hexanuclear complex of formula [Co₂Mn^II ₂Mn^IV ₂(L¹)₄Cl₂(μ₃-O)₂(dmf)₄]·2dmf (1) (H₃L is the Schiff base derived from the condensation of salicylaldehyde and 3-aminopropane-1,2-diol). Single crystal X-ray analysis revealed that 1 crystallizes in the monoclinic system P2₁/c and it contains a rare mixed-valence {Co^III ₂Mn^II ₂Mn^IV ₂(μ₂-O)₈(μ₃-O)₂} core where all metal ions are linked through the phenolato and alkoxo groups of the L³⁻ ligand. Besides the charge balance resulting from the X-ray structure, the oxidation state of the metal ions has been confirmed by XPS spectroscopy. Cryomagnetic studies indicate the coexistence of ferro- (Mn^IV–Mn^II, J₂ = +1.10(3) cm⁻¹, J₃ = +2.19(3) cm⁻¹; Mn^II–Mn^II, j = +0.283(3) cm⁻¹) and antiferromagnetic interactions (Mn^IV–Mn^IV, J₁ = −17.31(4) cm⁻¹), with the six-coordinate Co^III ions being diamagnetic. DFT type calculations were carried out to substantiate these values. The energy diagram for the different spin states using the best-fit parameters shows the occurrence of six low-lying spin states (S = 0–5) which are close in energy but clearly separated from the remaining ones, with the ground spin state being S = 5. Complex 1 is found to be the first example where weak ferromagnetic exchange between Mn^II ions through the long –O–Mn^IV–O– pathway takes place.

Octahedral molecular sieves (OMS) are built of transition metal-oxygen octahedra that delimit sub-nanoscale cavities. Compared to other microporous solids, OMS exhibit larger versatility in properties, provided by various redox states and magnetic behaviors of transition metals. Hence, OMS offer opportunities in electrochemical energy harnessing devices, including batteries, electrochemical capacitors and electrochromic systems, provided two conditions are met: fast exchange of ions in the micropores and stability upon exchange. Here we unveil a novel OMS hexagonal polymorph of tungsten oxide called h’-WO₃, built of (WO₆)₆ tunnel cavities. h’-WO₃ is prepared by a one-step soft chemistry aqueous route leading to the hydrogen bronze h’-H_0.07WO₃. Gentle heating results in h’-WO₃ with framework retention. The material exhibits an unusual combination of 1-dimensional crystal structure and 2-dimensional nanostructure that enhances and fastens proton (de)insertion for stable electrochromic devices. This discovery paves the way to a new family of mixed valence functional materials with tunable behaviors.

Superparamagnetic nanoparticles are promising objects for data storage or medical applications. In the smallest—and more attractive—systems, the properties are governed by the magnetic anisotropy. Here we report a molecule-based synthetic strategy to enhance this anisotropy in sub-10-nm nanoparticles. It consists of the fabrication of composite materials where anisotropic molecular complexes are coordinated to the surface of the nanoparticles. Reacting 5 nm γ-Fe₂O₃ nanoparticles with the [Co^II(TPMA)Cl₂] complex (TPMA: tris(2-pyridylmethyl)amine) leads to the desired composite materials and the characterization of the functionalized nanoparticles evidences the successful coordination—without nanoparticle aggregation and without complex dissociation—of the molecular complexes to the nanoparticles surface. Magnetic measurements indicate the significant enhancement of the anisotropy in the final objects. Indeed, the functionalized nanoparticles show a threefold increase of the blocking temperature and a coercive field increased by one order of magnitude.

The aim of this paper is to demonstrate the possibility of expressing molecular magnetic phenomena on hybrid nano-particles. The advantage of such composite materials is their wonderful versatility, in terms of choice of molecular compounds to be grafted on, and in terms of possibilities for integration in devices. As a first example, we designed a composite made of silica nanoparticles as cores on which Fe(II) coordination complexes displaying spin-crossover (SCO) phenomenon have been grafted in three steps involving the silanization of the SiO₂ core, the anchoring of bis(1-methylimidazolyl)-type ligands and at last the coordination of octahedral Fe(II) complexes at the surface of the nanoparticles. This step-by-step procedure enables an adequate coverage of the Fe(II) complexes at the surface of the nanoparticles that allows for the appearance of the spin-transition (ST) behavior at a level detectable by SQUID magnetometry. These tailor-made particles can thus be considered as nanometric switchable units.

The strategic recombination of preformed coordination clusters in the presence of polymodal bridging ligands has successfully led to the characterisation of five new compounds of structural and magnetic interest. Indeed using the dinuclear complex [M₂(H₂O)(piv)₄(Hpiv)₄] (M = Co, Ni; Hpiv = pivalic acid) as starting material and reacting it with phenylhydroxamic acid (H₂pha) has yielded the four tetrametallic coordination clusters [Co₄(Hpha)₂(piv)₆(Hpiv)₄] (1), [Ni₄(Hpha)₂(piv)₆(Hpiv)₂(DMF)₂] (2), [Co₄(Hpha)₂(piv)₆(EtOH)₂(H₂O)₂] (3), [Ni₄(Hpha)₂(piv)₆(EtOH)₂(H₂O)₂] (4) and the hexanuclear complex [Co₆(Hpha)₄(piv)₈(EtOH)₂]·EtOH (5). All the compounds have been structurally characterised revealing a particular binding mode for the hydroxamate ligand. The study of their magnetic properties has been performed and the modelling of these properties has been done using the appropriate hamiltonians for each compound. The experimental data and their modelling show non-zero spin ground states for compounds 4 and 5.

Organizing magnetic nanoparticles into long‐range and dynamic assemblies would not only provide new insights into physical phenomena but also open opportunities for a wide spectrum of applications. In particular, a major challenge consists of the development of nanoparticle‐based materials for which the remnant magnetization and coercive field can be controlled at room temperature. Our approach consists of promoting the self‐organization of magnetic nanoparticles in liquid crystals (LCs). Using liquid crystals as organizing templates allows us to envision the design of tunable self‐assemblies of magnetic nanoparticles, because liquid crystals are known to reorganize under a variety of external stimuli. Herein, we show that twisted liquid crystals can be used as efficient anisotropic templates for superparamagnetic nanoparticles and demonstrate the formation of hybrid soft magnets at room temperature.