A stochastic cooperative dynamics of the Ising-like Hamiltonian describing the equilibrium properties of the spin-crossover solids is derived from the Glauber approach, where an appropriate choice of the microscopic dynamics is made with respect to experimental features. The model is used here in the mean-field approach, and supports the previous phenomenological dynamic equations. The expected sigmoidal shape of the relaxation curves from high-spin to low-spin states after photoexcitation is obtained, and the kinetic character of the thermal hysteresis loops (inherent to the experimental data) is properly accounted for.

Multifunctional mononuclear iron(ii) complex coordinated with six phosphorescent ligands exhibiting correlated spin-crossover transition and enhanced fluorescence.

We have introduced a photoexcitation contribution in the dynamical model we previously presented (see previous paper), as an additional term in the rate transition, in order to reproduce the light induced thermal hysteresis effect, experimentally observed in the spin-crossover materials. The set of motion equations governing the evolution of the high-spin (HS) fraction of cooperative spin-crossover systems under light, is obtained. The mean-field treatment leads to a macroscopic master equation previously given on a phenomenological basis. With both the $\mathrm{HS}\to$ LS (low-spin) and $\mathrm{LS}\to$ HS transition rates terms, a model of general use is derived. This model describes the static and dynamic properties under light. We have investigated this model analytically, as far as possible. The properties under light involve two instabilities different in nature: the high-temperature instability, chiefly entropy driven, and the low-temperature one, light induced. We have determined the conditions for these instabilities to occur, and have examined their possible collapse. The described phenomena are discussed with respect to the available data on the photoinduced bistable regime: thermal hysteresis loops, optical hysteresis loops, and transient regimes. The kinetic dependence of the photoinduced hysteresis loops is calculated.

We investigated by optical microscopy the propagation of the thermal spin transition in [Fe(btr)2(NCS)2]\ifmmode\cdot\else\textperiodcentered\fi{}H2O\ifmmode\cdot\else\textperiodcentered\fi{}(btr~= bis-triazole) single crystals. Using high-quality fresh crystals embedded in oil we could follow the transformation front of the on-cooling transition. We observed in detail a linear front propagating through the entire crystal at extremely slow velocity, $\sim${}2.5~$\mu${}m/s on average. The analysis of the temporal dependences of the optical densities (OD) at given positions within a Kolmogorov-Johnson-Mehl-Avrami (KJMA) law led to an exponent $\sim${}0.6, typical for a process governed by planar diffusion. It is inferred that the spin transition is a multiscale process. The agreement with a previous structural investigation, analyzed in terms of an average KJMA law, is shown. We conclude that the stresses generated by the volume change associated with the spin transformation are the driving force for the propagation of the nucleation and growth mechanism. The role of defects is discussed and optical effects specific of the experiment are evidenced.