The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

Spin crossover complexes are a route toward designing molecular devices with a facile readout due to the change in conductance that accompanies the change in spin state. Because substrate effects are important for any molecular device, there are increased efforts to characterize the influence of the substrate on the spin state transition. Several classes of spin crossover molecules deposited on different types of surface, including metallic and non-metallic substrates, are comprehensively reviewed here. While some non-metallic substrates like graphite seem to be promising from experimental measurements, theoretical and experimental studies indicate that 2D semiconductor surfaces will have minimum interaction with spin crossover molecules. Most metallic substrates, such as Au and Cu, tend to suppress changes in spin state and affect the spin state switching process due to the interaction at the molecule-substrate interface that lock spin crossover molecules in a particular spin state or mixed spin state. Of course, the influence of the substrate on a spin crossover thin film depends on the molecular film thickness and perhaps the method used to deposit the molecular film.

Thermal hysteresis of stress and strain in spin-crossover@polymer composites: towards a rational design of actuator devices

Polymer composites of molecular spin crossover complexes have emerged as promising mechanical actuator materials, but their effective thermomechanical properties remain elusive. In this work, we investigated a series of iron(ii)-triazole@P(VDF-TrFE) particulate composites using a tensile testing stage with temperature control. From these measurements, we assessed the temperature dependence of the Young's modulus as well as the free deformation and blocking stress, associated with the thermally-induced spin transition. The results denote that the expansion of the particles at the spin transition is effectively transferred to the macroscopic composite material, providing ca. 1-3% axial strain for 25% particle load. This strain is in excess of the 'neat' particle strain, which we attribute to particle-matrix mechanical coupling. On the other hand, the blocking stress (∼1 MPa) appears reduced by the softening of the composite around the spin transition temperature.

Morphological Studies of Composite Spin Crossover@SiO2 Nanoparticles

Spin crossover (SCO) iron (II) 1,2,4-triazole-based coordination compounds in the form of composite SCO@SiO2 nanoparticles were prepared using a reverse microemulsion technique. The thickness of the silica shell and the morphology of the as obtained core@shell nanoparticles were studied by modifying the polar phase/surfactant ratio (ω), as well as the quantity and the insertion phase (organic, aqueous and micellar phases) of the tetraethylorthosilicate (TEOS) precursor, the quantity of ammonia and the reaction temperature. The morphology of the nanoparticles was monitored by transmission electron microscopy (TEM/HRTEM) while their composition probed by combined elemental analyses, thermogravimetry and EDX analyses. We report that not only the particle size can be controlled but also the size of the silica shell, allowing for interesting perspectives in post-synthetic modification of the shell. The evolution of the spin crossover properties associated with the change in morphology was investigated by variable temperature optical and magnetic measurements.

Magneto-elastic properties of a spin crossover membrane deposited on a deformable substrate

Spin-crossover (SCO) solids have been studied for several years due to their fascinating physical properties and their potential applications as optical switches and reversible high-density memories for information storage. Through this article, we will examine in details the effects of substrate's lattice parameters, on a deformable spin crossover membrane, simulated using an electro-elastic model taking into account the volume change at the transition. The molecules of the membrane can be either in the low spin state (LS) or the high spin state (HS), while those of the substrate are electronically neutral. Magnetic properties of the SCO membrane and the pressure distribution as a function of the lattice parameter of the substrate have been investigated. We demonstrated that the thermally induced first-order spin transition is significantly affected by the structural properties of the substrate, where a rise in the lattice parameter of the latter lowers the transition temperature and reduces the width of the thermal hysteresis loop. The investigations on the spatiotemporal aspects of the spin transition in the membrane demonstrates that the nucleation and growth processes are sensitive to the structural properties of the elastic misfit between the substrate and the SCO membrane.

Temperature dependence of the cooperative out-of-equilibrium elastic switching in a spin-crossover material

We explore the key parameters that influence the efficiency of the cooperative low-spin to high-spin conversion through long range elastic intermolecular interactions during the so-called elastic step, triggered by instantaneous photo-induced conversion.

An electro-elastic theory for the mechanically-assisted photo-induced spin transition in core-shell spin-crossover nanoparticles

The development of heterostructure materials may lead to new features that cannot be obtained with natural materials. Here we simulate a model structurally hybrid core-shell nanoparticle with different lattice parameters between an electronically inert shell and an active spin crossover core. The nanoparticle consists of a 2D core with 20 × 20 size with square symmetry, surrounded by a shell made of 10 atomic layers. The low temperature photoexcitation of the core shows a significant environment-dependent behavior. In particular, we demonstrate that a shell with a large lattice parameter accelerates the low-spin to high-spin photoexcitation process of the core through the single domain nucleation mechanism while a moderate shell lattice parameter leads to spatially-homogeneous growth of the high-spin fraction. We found that the mechanical retro-action of the shell may cause elastic instability of the core leading to efficient control and manipulation of its photo-conversion.

Strain engineering of photo-induced phase transformations in Prussian blue analogue heterostructures

Heterostructures based on Prussian blue analogues (PBA) combining photo- and magneto-striction have shown a large potential for the development of light-induced magnetization switching. However, studies of the microscopic parameters that control the transfer of the mechanical stresses across the interface and their propagation in the magnetic material are still too scarce to efficiently improve the elastic coupling. Here, this coupling strength is tentatively controlled by strain engineering in heteroepitaxial PBA core-shell heterostructures involving the same Rb0.5Co[Fe(CN)6]0.8·zH2O photostrictive core and isostructural shells of similar thickness and variable mismatch with the core lattice. The shell deformation and the optical electron transfer at the origin of photostriction are monitored by combined in situ and real time synchrotron X-ray powder diffraction and X-ray absorption spectroscopy under visible light irradiation. These experiments show that rather large strains, up to +0.9%, are developed within the shell in response to the tensile stresses associated with the expansion of the core lattice upon illumination. The shell behavior is, however, complex, with contributions in dilatation, in compression or unchanged. We show that a tailored photo-response in terms of strain amplitude and kinetics with potential applications for a magnetic manipulation using light requires a trade-off between the quality of the interface (which needs a small lattice mismatch i.e. a small a-cubic parameter for the shell) and the shell rigidity (decreased for a large a-parameter). A shell with a high compressibility that is further increased by the presence of misfit dislocations will show a decrease in its mechanical retroaction on the photo-switching properties of the core particles.

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state-of-the-art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.

The influence of the sample dispersion on a solid surface in the thermal spin transition of [Fe(pz)Pt(CN)4] nanoparticles

The dispersion on a Sapphire surface of [Fe(pz)Pt(CN)₄], pz = pyrazine nanoparticles influences the thermal spin transition, as shown using magnetic, spectroscopic and diffraction data. This is explained within the framework of the mechanoelastic model.

Pressure-induced switching properties of the iron(iii) spin-transition complex [FeIII(3-OMeSalEen)2]PF6

The volume-dependent properties of a spin-crossover Fe(iii) prototypical compound revealed by combined magnetic, vibrational and structural investigations of the pressure effect.

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-changing materials.

Abrupt versus Gradual Spin-Crossover in Fe(II)(phen)2(NCS)2 and Fe(III)(dedtc)3 Compared by X-ray Absorption and Emission Spectroscopy and Quantum-Chemical Calculations

Molecular spin-crossover (SCO) compounds are attractive for information storage and photovoltaic technologies. We compared two prototypic SCO compounds with Fe(II)N6 (1, [Fe(phen)2(NCS)2], with phen = 1,10-phenanthroline) or Fe(III)S6 (2, [Fe(dedtc)3], with dedtc = N,N'-diethyldithiocarbamate) centers, which show abrupt (1) or gradual (2) thermally induced SCO, using K-edge X-ray absorption and Kβ emission spectroscopy (XAS/XES) in a 8-315 K temperature range, single-crystal X-ray diffraction (XRD), and density functional theory (DFT). Core-to-valence and valence-to-core electronic transitions in the XAS/XES spectra and bond lengths change from XRD provided benchmark data, verifying the adequacy of the TPSSh/TZVP DFT approach for the description of low-spin (LS) and high-spin (HS) species. Determination of the spin densities, charge distributions, bonding descriptors, and valence-level configurations, as well as similar experimental and calculated enthalpy changes (ΔH), suggested that the varying metal-ligand bonding properties and deviating electronic structures converge to similar enthalpic contributions to the free-energy change (ΔG) and thus presumably are not decisive for the differing SCO behavior of 1 and 2. Rather, SCO seems to be governed by vibrational contributions to the entropy changes (ΔS) in both complexes. Intra- and intermolecular interactions in crystals of 1 and 2 were identified by atoms-in-molecules analysis. Thermal excitation of individual dedtc ligand vibrations accompanies the gradual SCO in 2. In contrast, extensive inter- and intramolecular phen/NCS vibrational mode coupling may be an important factor in the cooperative SCO behavior of 1.

Predicting the spin state of paramagnetic iron complexes by DFT calculation of proton NMR spectra

Many transition-metal complexes easily change their spin state S in response to external perturbations (spin crossover). Determining such states and their dynamics can play a central role in the understanding of useful properties such as molecular magnetism or catalytic behavior, but is often far from straightforward. In this work we demonstrate that, at a moderate computational cost, density functional calculations can predict the correct ground spin state of Fe(ii) and Fe(iii) complexes and can then be used to determine the (1)H NMR spectra of all spin states. Since the spectral features are remarkably different according to the spin state, calculated (1)H NMR resonances can be used to infer the correct spin state, along with supporting the structure elucidation of numerous paramagnetic complexes.

Nanocrystals of Fe(phen)2(NCS)2 and the size-dependent spin-crossover characteristics

We describe the preparation of nano- and microcrystals of the Fe(phen)₂(NCS)₂ spin-crossover prototypical compound based on the solvent-assisted technique applied to an ionic and soluble precursor and analyze the size-dependent characteristics of the thermal spin-crossover.

Fe(Me2-bpy)2(NCSe)2spin-crossover micro- and nanoparticles showing spin-state switching above 250 K

Nano- and microparticles or polycrystalline powders of the Fe(Me₂-bpy)₂(NCSe)₂spin-crossover complex were easily elaborated from the diamagnetic precursor [Fe(Me₂-bpy)₃](NCSe)₂·S by precipitation in an anti-solvent or by solid-state thermolysis.