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 iron(II) spin-crossover compounds, the transition from the ¹A₁ low-spin state at low temperatures to the ⁵T₂ high-spin state at elevated temperatures is accompanied by a large increase in metal-ligand bond lengths. The resulting elastic interactions may be pictured as an internal pressure which is proportional to the concentration of the low-spin species. Because pressure stabilises the low-spin state relative to the high-spin state this results in a positive feedback. Thermal transition curves in neat iron(II) spin-crossover compounds are thus invariable much steeper than in diluted mixed crystals, and the high-spin→low-spin relaxation following the light-induced population of the high-spin state at low temperatures is self-accelerating. Strong interactions give rise to a thermal hysteresis, and light-induced bistabilities may be observed for compounds with initially a high-spin ground state and the potential for a light-induced population of the low-spin state. For such compounds, the increasing internal pressure may stabilise the low-spin state sufficiently so that it becomes the molecular ground state above some critical light-induced low-spin fraction. Secondary effects of the elastic interactions include crystallographic phase transitions, inhomogeneous distributions of sites, and anomalies such as steps in the transition curve.

The spin-crossover compound [Fe(pic)₃]Cl₂EtOH (pic = 2-picolylamine) shows an unusual two-step spin transition. This is thought to be caused by specific nearest-neighbour interactions and short-range correlations and requires a theoretical treatment of the elastic interactions between the spin-changing molecules beyond the mean-field approximation. Such short-range correlations also influence the high-spin → low-spin relaxation following the light-induced population of the high-spin state at cryogenic temperatures, leading to characteristic deviations from the predictions of a mean-field treatment. These deviations are directly observable by comparison of the full and unperturbed relaxation curves with curves for which the short-range correlations were destroyed using an appropriate irradiation technique. Monte Carlo simulations including both nearest-neighbour and long-range interactions give a description of the observed relaxation curves which is consistent with the thermal spin equilibrium.

A helium gas pressure cell for pressures up to 1 kbar (0.1 GPa) has been developed in conjunction with a closed-cycle He refrigerator allowing variable temperatures between 15 and 300 K. Both cell and refrigerator are equipped with optical windows suitable for photophysical measurements, such as temperature- and pressure-dependent absorption spectroscopy or laser flash photolysis. Examples of measurements on iron(II) spin-crossover systems are given. In these compounds, comparatively small external pressures induce significant changes in the thermodynamic equilibrium as well as in the relaxation dynamics.

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) Ã…, α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) Ã…³Â 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.

Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.

In der Übergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei ändern sich die magnetischen und optischen Eigenschaften, über die der thermische Spinübergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phänomen tritt sowohl in flüssiger Phase als auch im Festkörper auf. Eine herausragende Stellung nehmen Eisen(II) - Spincrossover - Verbindungen ein, in denen der Spinübergang im Festkörper auf sehr unterschiedliche Weise - graduell, abrupt, mit Hysterese oder stufenweise - verlaufen kann und mit Mößbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitäts- und Wärmekapazitätsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekülen kann befriedigend durch elastische Eigenschaften und durch die Änderung von Gestalt und Volumen der Komplexmoleküle beim Spinübergang erklärt werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, daß sich der Low-Spin-Zustand mit grünem Licht in den High-Spin-Zustand umschalten läßt, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht läßt sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zurückschalten. Der Mechanismus des LIESST-Effekts ist aufgeklärt, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Übergänge verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.

[Fe(ptz)6l(BF4)2 (ptz= 1-propyltetrazole) and the mixed crystals [Znl,Fe,(ptz)6] (BF4)2 are Fe(I1) spin-crossover compoundsthat exhibit light-induced excited-spin-state trapping. It is shown that (a) for x I 0.1 a single-ion treatment of both the spinequilibrium ( M H L = 510 (12) cm-', ASHL = 5.1 (2) cm-'/K at T = 100 K) and the relaxation from the excited high-spin state (.Eao = 810 (30) cm-I, A - 105/s) is appropriate and (b) for 0.2 I x I 1 cooperative effects observed in the relaxation from the high-spin state are of long-range nature and therefore of elastic rather than of electronic origin.