Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)₃](ClO₄)₂}_∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)₃](BF₄)₂}_∞, Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time and intensity. Crystallographic studies reveal a photo-induced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.

The mechanoelastic model is applied to reproduce the experimental relaxation and thermal transition curves as determined for crystals of pure and diluted {[Fe_xZn_1–x(bbtr)₃](ClO₄)₂}_∞ [bbtr = 1,4-di(1,2,3-triazol-1-yl)butane] spin-crossover systems. In the mechanoelastic model, the spin-crossover complexes are situated in a hexagonal planar lattice, which is similar to the 2D coordination polymer with (3,6) network topology of [Fe(bbtr)₃](ClO₄)₂. These complexes are linked by springs, which simulate the elastic interactions between them. Owing to the change in volume of the complexes during the spin transition, an elastic force accompanies the switch of every complex. This force propagates through the entire lattice and causes a shift of all molecules in the system and thus results in a new nuclear configuration. First, the ability of the model to reproduce various shapes of thermal transition and relaxation curves in pure compounds is analyzed; these range from gradual to very steep and include hysteresis behavior for the former and from single exponential to sigmoidal or with several steps for the latter. A structural phase transition can also be accounted for by changing the shape of the sample at a fixed temperature from a regular to an elongated hexagon. Furthermore, the effect of adding Zn as a dopant in a mixed crystal series is discussed. The role of dopants on the cluster evolution is also analyzed directly and by using the correlation factor.

In this paper we study the cluster formation and evolution in spin crossover systems during the thermal transition in the frame of a mechano-elastic model applied to open boundary hexagonal lattices. The switching processes between the high-spin (HS) and low-spin (LS) state are studied by a method combining a Monte Carlo standard procedure on the spin state and the lattice relaxation. In the present study, we adopt the transition probabilities of the spin state taking into account the energy gap between the two states, the degeneracy ratio and the local pressure determined by the elongations of the closest springs. It is found that clusters of molecules in the same state tend to grow starting from corners, as in available experimental data. Some qualitative differences between the processes of cluster formation for the two hysteresis branches, i.e., HS to LS and LS to HS are pointed out. Moreover, we have studied the dependence of cluster formation on the strength of the elastic interactions, and also on the system size. The size dependence of the ratio between the system size and the maximum cluster length is very weak, which indicates the appearance of macroscopic domains.

The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF₆^-, BPh₄^-; R-Cat=catecholate dianion substituted by R=NO₂, Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below ≈40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, k_HL(T→0), vary in the range from 0.58 to 8.84 s^-1. These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.

The compound {Fe(pmd)[Ag(CN)₂][Ag₂(CN)₃]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two-step high-spin (HS) ↔ low-spin (LS) transition. The critical temperatures are T_c1=185 and T_c2=148 K. Each step involves ~50 % of the iron centers, with the low-temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are ΔH₁=3.6±0.4 kJ mol^-1 and ΔS₁=19.5±3 J K^-1 mol^-1; ΔH₂=4.8±0.4 kJ mol^-1 and ΔS₂=33.5±3 J K^-1 mol^-1. Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green-red light (550-650 nm). HS-to-LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two-step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry (P2₁/c, Z=16) at all temperatures. There are five [FeN₆] pseudo-octahedral sites linked by pmd, [Ag(CN)₂]^- and [Ag₂(CN)₃]^- bridging ligands to form an unprecedented three-dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two-step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the Ag^…Ag distances. Correlations between the variation of the unit-cell volume and the change of Ag^…Ag interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred.