Lionel Salmon

Towards the Ultimate Size Limit of the Memory Effect in Spin‐Crossover Solids

Although the originof the spin-crossover phenomenon is purely molecular, themacroscopic behavior of these systems in the solid state isstrongly influenced by short- and long-range interactions (ofmainly elastic origin) between the transition-metal ions,giving rise to remarkable cooperative phenomena, such asfirst-order phase transitions.

Single-Laser-Shot-Induced Complete Bidirectional Spin Transition at Room Temperature in Single Crystals of (FeII(pyrazine)(Pt(CN)4))

Single crystals of the {Fe (II)(pyrazine)[Pt(CN) 4]} spin crossover complex were synthesized by a slow diffusion method. The crystals exhibit a thermal spin transition around room temperature (298 K), which is accompanied by a 14 K wide hysteresis loop. X-ray single-crystal analysis confirms that this compound crystallizes in the tetragonal P4/ mmm space group in both spin states. Within the thermal hysteresis region a complete bidirectional photoconversion was induced between the two phases (high spin right arrow over left arrow low spin) when a short single laser pulse (4 ns, 532 nm) was shined on the sample.

A novel approach for fluorescent thermometry and thermal imaging purposes using spin crossover nanoparticles

Temperature plays a fundamental role in all fields of science; hence the development of methods for measuring this property remains in vogue. Within this vast field, fluorescent thermometry appears as a simple, noninvasive and cost-effective method for providing good spatial, temporal and thermal resolution in both solid and liquid phases, even in distant or inaccessible environments. Here we describe the properties of a two-component fluorescent thermometry system comprised of Fe(II)-triazole type spin-crossover nanoparticles (temperature sensor) and an appropriate fluorophore (signal transducer). The primary advantage of this system is that the nanoparticles are modified easily, which enables fine control of the thermometric properties, while the optical properties (i.e. the signal detection) remain virtually unchanged. This system could thus be adapted in a straightforward manner to various problems where the use of fluorescent thermometry would be beneficial.

Synergetic effect of host-guest chemistry and spin crossover in 3D Hofmann-like metal-organic frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni).

The synthesis and characterization of a series of three-dimensional (3D) Hofmann-like clathrate porous metal-organic framework (MOF) materials [Fe(bpac)M(CN)(4)] (M=Pt, Pd, and Ni; bpac=bis(4-pyridyl)acetylene) that exhibit spin-crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin-crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)(4)]}(n) planar layers bridged by the bis-monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7% of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin-crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π-π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host-guest interaction and the spin-crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice.

Molecular actuators driven by cooperative spin-state switching

Molecular switches have great potential to convert different forms of energy into mechanical motion; however, their use is often limited by the narrow range of operating conditions. Here we report on the development of bilayer actuator devices using molecular spin crossover materials. Motion of the bilayer cantilever architecture results from the huge spontaneous strain accompanying the spin-state switching. The advantages of using spin crossover complexes here are substantial. The operating conditions used to switch the device can be manipulated through chemical modification, and there are many existing compounds to choose from. Spin crossover materials may be switched by diverse stimuli including light, temperature, pressure, guest molecules and magnetic field, allowing complex input combinations or highly specific operation. We demonstrate the versatility of this approach by fabricating actuators from four different spin crossover materials and by using both thermal variation and light to induce motion in a controlled direction.

Electric-Field-Induced Charge-Transfer Phase Transition: A Promising Approach Toward Electrically Switchable Devices

Much research has been directed toward the development of electrically switchable optical materials for applications in memory and display devices. Here we present experimental evidence for an electric-field-induced charge-transfer phase transition in two cyanometalate complexes: Rb(0.8)Mn[Fe(CN)(6)](0.93).1.62H(2)O and Co(3)[W(CN)(8)](2)(pyrimidine)(4).6H(2)O, involving changes in their magnetic, optical, and electronic properties as well. Application of an electric field above a threshold value and within the thermal hysteresis region leads to a transition from the high- to the low-temperature phase in these compounds. A model is proposed to explain the main observations on the basis of a para-ferroelectric transition. Our observations suggest that this new concept of electrical switching, based on materials exhibiting charge-transfer phase transitions with large thermal hysteresis loops, may open up doors for novel electro-optical devices.

Nano-electromanipulation of Spin Crossover Nanorods: Towards Switchable Nanoelectronic Devices

The nanoscale manipulation and charge transport properties of the [Fe(Htrz)2(trz)](BF4) spin-crossover compound is demonstrated. Such 1D spin-crossover nanostructures are attractive building blocks for nanoelectronic switching and memory devices.

Emerging properties and applications of spin crossover nanomaterials

In the past few years we have witnessed the spectacular development of nano-materials of spin crossover complexes of 3d4–3d7 transition metal ions, including thin films, nanoparticles, nanopatterns and nanoscale assemblies. These developments were primarily motivated by fundamental questions concerning size reduction effects on the bistability properties. On the other hand, spin crossover nano-objects require new characterization techniques (scanning probe microscopy, micro-magnetometry, etc.), revealing unknown details of the SCO mechanism at the nanoscale and giving access also to properties, which have been largely unexplored until now. In this Highlight, we review these recent experimental developments and discuss perspectives for societal applications as well.

The Effect of an Active Guest on the Spin Crossover Phenomenon

A straightforward method for the reversible modification ofsolid-state properties is a goal being constantly pursued in thedevelopment of molecular switches, and molecular electronicand photonic devices. As one of the most attractive molecule-based switchable materials, spin crossover (SCO) complexespresent different magnetic, optical, electrical and structuralproperties in response to external stimuli (such as temper-ature, pressure, light, or magnetic fields), driven by conver-sionoftheelectronconfigurationbetweenhigh spin(HS)andlow spin (LS) states.

Electrical properties and non-volatile memory effect of the [Fe(HB(pz)3)2] spin crossover complex integrated in a microelectrode device

We report on the deposition of thin films of the [Fe(HB(pz)3)2] (pz = pyrazolyl) molecular spin crossover complex by thermal evaporation. By means of impedance measurements and Raman microspectroscopy, we show that the films maintain the structure and properties of the bulk material. The conductivity of the films decreases by ca. 2 orders of magnitude when the freshly deposited compound goes through a first (irreversible) thermal phase change above ca. 380 K. This property can be exploited as a non-volatile (read-only) memory effect.