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.

In the iron(II) low-spin complex [Fe(bpy)₃]²⁺, the zero-point energy difference between the ⁵T_2g(t⁴_2ge²_g) high-spin and the ¹A_1g(t⁶_2g) low-spin states, ΔE⁰_HL, is estimated to lie in the range of 2500-5000 cm^-1. This estimate is based on the low-temperature dynamics of the high-spin→low-spin relaxation following the light-induced population of the high-spin state and on the assumption that the bond-length difference between the two states Δr_HL is equal to the average value of ≈0.2 Å, as found experimentally for the spin-crossover system. Calculations based on density functional theory (DFT) validate the structural assumption insofar as the low-spin-state optimised geometries are found to be in very good agreement with the experimental X-ray structure of the complex and the predicted high-spin geometries are all very close to one another for a whole series of common GGA (PB86, PW91, PBE, RPBE) and hybrid (B3LYP, B3LYP*, PBE1PBE) functionals. This confirmation of the structural assumption underlying the estimation of ΔE⁰_HL from experimental relaxation rate constants permits us to use this value to assess the ability of the density functionals for the calculation of the energy difference between the HS and LS states. Since the different functionals give values from -1000 to 12000 cm^-1, the comparison of the calculated values with the experimental estimate thus provides a stringent criterion for the performance of a given functional. Based on this comparison the RPBE and B3LYP* functionals give the best agreement with experiment.

The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon.

In the three-dimensional oxalate network structures of composition [Co_xM^II_1-x(bpy)₃][M^ICr(ox)₃], the spin state of the [Co_x(bpy)₃]²⁺ complex can be tuned by means of chemical pressure. With M^I=Na it is a classic high-spin complex. Substitution of Na by Li stabilises the complex and it becomes a spin-crossover complex. Dilution with M^II=Fe reinforces this effect, and M^II=Zn reverses it.

Incorporation of [Co(bpy)₃]²⁺ into the cavities of the three-dimensional oxalate network structure in [Co(bpy)₃][LiCr(ox)₃] produces chemical pressure that destabilises the normal high-spin ground state ⁴T₁ to such an extent that the [Co(bpy)₃]²⁺ complex becomes a spin-crossover complex. It shows a temperature-dependent equilibrium between the ²E low-spin and the ⁴T₁ high-spin states.

Short range correlations of the distribution of high spin (HS) and low spin (LS) states show up in thermal spin transition curves, decay curves of the light induced metastable HS state (LIESST state), and in structural features during the spin transitions. Correlations are due to short range interactions between the spin crossover molecules. Short range interactions may compete with omnipresent long range interactions and give rise to interesting spin transition phenomena. In this paper, the effect of correlations on the thermal spin transition in the mixed crystal system [Fe_xZn_1−x(pic)₃]Cl₂·EtOH (pic=picolylamine) is discussed. In particular the step in the thermal transition curve is a direct consequence of such correlations. In addition, the decay of the metastable HS state of the pure iron compound at ca. 20 K can be significantly changed by preparing metastable HS states with a random distribution over the lattice sites. Both experiments could be well reproduced by Monte Carlo simulations. In the orthorhombic modification of the compound Fe[5NO₂-sal-N(1,4,7,10)]([2,2′-(2,5,8,11-tetraazadodeca-1,11-diene-1,12-diyl)4-nitrophenolato] (2-)-N2, N2′,N2′′,N2′′′,O1, O1′]Fe(II)) a commensurable superstructure has been found. This compound represents the first example of a stable infinite range correlation of the spin states over the lattice sites.

In the [Fe(etz)₆](BF₄)₂ spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high-spin (HS) → low-spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light-induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS → LS conversion on site B reveal a light-induced HS→LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B-site complexes with a critical temperature T^↑_c K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first-order kinetics (self-acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self-retardation.

The [Fe(etz),](BF,), spin-cross-over system (etz = 1-ethyl-1 _H-tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a10.419(3), b=15.709(1), c = 18.890(2) Å, α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) ų and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites n_A:n_B is 2:l. Iron(II) on site A undergoes a thermal low-spin (LS) → high-spin (HS) transition with T_1/2I, = 105 K. whereas that on site B remains in the high-spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low-spin state on site B. On site A the high-spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light-induced population of the low-spin state can be achieved with λ = 820 nm.