The high-spin → low-spin relaxation in spin-crossover compounds can be described as non-adiabatic multi-phonon process in the strong coupling limit, in which the low-temperature tunnelling rate increases exponentially with the zero-point energy difference between the two states. Based on the hypothesis that the experimental bond length difference between the high-spin and the low-spin state of not, vert, similar0.2 Å is also valid for low-spin iron(II) complexes, extrapolation of the single configurational coordinate model allows an estimate of the zero-point energy difference for low-spin complexes from kinetic data. DFT calculations on low-spin [Fe(bpy)3]2+ support the structural assumption. However, for low-spin [Fe(terpy)2]2+ the relaxation rate constant shows an anomalous behaviour in so far as it is more in line with spin-crossover systems. This is attributed to very anisotropic bond length changes associated with the spin state change, and the subsequent breakdown of the single mode model

Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2'-bipyridine, terpy = 2,2':6',2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy

International audiencePhotoinduced phase transformations [1,2] occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently directing the functionalities [3,4,5,6,7]. The transient nature of photoinduced states has thus far severely limited the application scope. It is of paramount importance to explore whether structural feedback during the solid deformation has capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons long under scrutiny [8,9,10] , coherently propagating cell deformations over acoustic timescale [11,12,13,14] have not been explored to similar degree, particularly in light of cooperative elastic interactions. Herein we demonstrate experimentally and theoretically a self-amplified responsiveness in a spin-crossover material [15] during its delayed volume expansion. The cooperative response at material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically-driven cooperativity triggered by a light pulse offers a new efficient route to the generation and stabilization of photoinduced phases in many volume-changing materials

The relaxation in a spin transition compound is modeled on the basis of molecules interacting by the way of connecting springs and situated in a bidimensional open boundary hexagonal lattice. The switch of individual molecules is randomly checked using a standard Monte Carlo procedure. The switching probability depends on the energy gap between the two states in the absence of interactions and on the elongations of the nearest springs. The main characteristics of the experimental relaxation curves are reproduced and clustering and nucleation phenomena are detected

Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because of their ability to switch between high-spin and low-spin states with external stimuli such as voltage and light. In actual devices, the molecules are deposited on a substrate, which can modify their properties. However, surprisingly little is known about such molecule−substrate effects. Here we show for the first time, by grazing incidence X-ray diffraction, that an Fe II spin-crossover molecular layer displays a well-defined epitaxial relationship with a metal substrate. Then we show, by both density functional calculations and a mechanoelastic model, that the resulting epitaxial strain and the related internal pressure can induce a partial spin conversion at low temperatures, which has indeed been observed experimentally. Our results emphasize the importance of substrate-induced spin state transitions and raise the possibility of exploiting them.