Antoine Tissot

Structure and properties of complex hydride perovskite materials

Perovskite materials host an incredible variety of functionalities. Although the lightest element, hydrogen, is rarely encountered in oxide perovskite lattices, it was recently observed as the hydride anion H(-), substituting for the oxide anion in BaTiO3. Here we present a series of 30 new complex hydride perovskite-type materials, based on the non-spherical tetrahydroborate anion BH4(-) and new synthesis protocols involving rare-earth elements. Photophysical, electronic and hydrogen storage properties are discussed, along with counterintuitive trends in structural behaviour. The electronic structure is investigated theoretically with density functional theory solid-state calculations. BH4-specific anion dynamics are introduced to perovskites, mediating mechanisms that freeze lattice instabilities and generate supercells of up to 16 × the unit cell volume in AB(BH4)3. In this view, homopolar hydridic di-hydrogen contacts arise as a potential tool with which to tailor crystal symmetries, thus merging concepts of molecular chemistry with ceramic-like host lattices. Furthermore, anion mixing BH4(-)←X(-) (X(-)=Cl(-), Br(-), I(-)) provides a link to the known ABX3 halides.

Single-nanoparticle phase transitions visualized by four-dimensional electron microscopy

The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal-organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations.

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

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.

Ligand-Driven Light-Induced Spin Change Activity and Bidirectional Photomagnetism of Styrylpyridine Iron(II) Complexes in Polymeric Media

Two pseudo-octahedral iron(II) complexes, Fe(stpy)(4)(NCSe)(2), containing photoresponsive ligands (cis <--> trans isomerization of -CHCH-) were prepared with trans- or cis-styrylpyridine (stpy) isomers. The magnetic behavior of the polycrystalline solids was previously shown to depend on the configuration of the stpy ligand. The crystal X-ray structures were determined at 293 and 104 K for both isomers. The all-trans and all-cis compounds crystallize in the orthorhombic (Pna2(1)) and the monoclinic space groups (C2/c), respectively. No symmetry change occurs upon cooling to 104 K. The Fe(II) centers lie in axially compressed octahedra with NCSe anions in the apical position and the four pyridinic nitrogens in the meridional plane. The variation of metal-ligand bond lengths as a function of temperature reflects the thermal S = 0 <--> S = 2 crossover of all-trans complexes and the S = 2 ground state of all-cis complexes. The unit-cell volumes per metal ion also change accordingly, and the relative variation due to the spin-crossover compares those associated with the formal change of configuration of the four stpy isomers. The photomagnetic responses were investigated at 130 K with doped polymer thin films containing all-cis (high-spin) or all-trans species (partly low-spin). The 130 K illumination of these doped poly(methyl methacrylate) (PMMA) films leads to the UV-vis absorption features typical for the cis <--> trans photoisomerization of the stilbenoid moiety. The direct magnetic measurements have unambiguously established the photomagnetic effect named ligand-driven light-induced spin change (LD-LISC). The 355 nm excitation of doped thin films produces very long lifetime states that are manifested by high-spin to low-spin (all-cis complex) and low-spin to high-spin (all-trans complex) changes of the Fe(II) magnetic behavior; the process is bidirectional. A structural analysis based on the single-crystal X-ray diffraction data has been proposed to rationalize the LD-LISC activity detected here for doped PMMA thin films.

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-changingxa0materials.

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.

Photoinduced Relaxation Dynamics in Iron(II) Spin-Crossover Nanoparticles: The Significance of Crystallinity

A switch in time: A fast precipitation technique was used to prepare 75 nm FeII spin-crossover nanocrystals. Their photoswitching dynamics, based on the light-induced excited spin-state trapping effect, has been investigated by means of optical spectroscopy. A significant variation of the switching proprieties is observed compared to similar but amorphous nanoparticles.

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]⋅2.6u2009H2O (pz = pyrazine) shows a cooperative thermal spin transition at around 270u2005K. 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 10u2005K 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.

Photoswitchable spin crossover nanoparticles

During the last few decades, the photoswitching of spin crossover compounds, based on the Light-Induced Excited Spin State Trapping (LIESST) effect, has been widely studied. More recently, the synthesis and characterization of spin crossover nanoparticles have been developed. This focused review describes the recent progress in the characterization of the photo-excitation and relaxation processes in spin crossover nanoparticles, for both large assemblies and single nano-objects. A discussion of the size-reduction effect on such processes is presented.

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.