Spin Transition Curves Research Articles

Bipyrimidine‐Bridged Dinuclear Iron(II) Spin Crossover Compounds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Total free energy of a spin-crossover molecular system

The free energy of spin-crossover molecular systems studied so far deal with the inner degrees of freedom of the spin-crossover molecules and a variety of interaction schemes between the molecules in the high spin (HS) and low spin (LS) states. Different types of transition curves, gradual, abrupt, hysteresis, and also two step transitions have been simulated or even satisfactorily fitted to experimental data. However, in the last decade spin transition curves were measured, especially under pressure, which could not be explained within these theoretical models. In this contribution the total free energy of an anharmonic lattice incorporating spin-crossover molecules which have a certain misfit to the lattice and interact elastically by their change in volume and shape has been constructed for a finite spherical crystal treated as a homogeneous isotropic elastic medium. The simulations demonstrate that already the knowledge of average properties of the crystal, as elastic constants and the anharmonicity of the potential of the lattice, and relative effective sizes of the molecules and their misfit to lattice is sufficient to interpret spin transition behavior. Almost all known anomalous spin transitions behaviors have been reproduced within reasonable limits of such parameters.

Infrared detection of the hysteresis in the thermally induced spin-crossover in bis(4,4′-bis-1,2,4-triazole)bis(thiocyanato- N)iron(II) monohydrate

By the use of VT infrared spectroscopy, the thermally induced spin transition in the complex [Fe(btr) 2(NCS) 2]·H 2O has been analyzed. The abbreviation btr stands for 4,4′-bis-1,2,4-triazole. This complex was reported earlier to exhibit an abrupt spin transition with a large hysteresis effect. The spin-transition curve calculated from the change in the absorption of the CN-stretching vibrations of the thiocyanate ligand is similar to the curve found by other techniques. The transition temperatures were found to be T c↓=115±5 K and T c↑=141±5 K, leading to a hysteresis of 26 K. These values for the same compound, deduced from magnetic and calorimetric measurements and Mössbauer spectroscopy are T c↓=121 K and T c↑=145 K. The spin-transition curve was analyzed thermodynamically, leading to the following parameters: Δ H=9.7 kJ mol −1, Δ S=79 J mol −1 K −1 and Γ=3.15 kJ mol −1 in agreement with earlier determined values.

Pressure effect on a novel spin transition polymeric chain compound

[Fe(hyptrz)3]A2·H2O (hyptrz = 4-(3′-hydroxypropyl)-1,2,4-triazole and A = 4-chlorobenzenesulfonate) represents a novel iron(II) polymeric chain compound. A discontinuous spin transition has been observed by temperature dependent magnetic susceptibility measurements. The spin transition curves are shifted from 181 K to 324 K as the pressure varies from 1 bar to 5.9 kbar. The shapes of these curves are not strongly modified, and this is interpreted as being due to effective cooperative interactions along the chain. This behaviour under pressure can be extended to the family of iron(II)-1,2,4-triazole polymeric chain spin crossover materials.

57Fe Mössbauer spectroscopic and magnetic study of a spin-crossover polymer complex, Fe(3-chloropyridine)2Ni(CN)4

The coordination polymer Fe(3-chloropyridine)2Ni(CN)4 (2) has been prepared by a method similar to that for Fe(pyridine)2Ni(CN)4 (1). The complex (2) has been characterized by57Fe Mossbauer spectroscopy and a SQUID technique.57Fe Mossbauer and magnetic susceptibility data show that complex (2) exhibits spin-crossover behavior. The spin transition of (2) occurs between 120 and 80 K with very small hysteresis or without hysteresis. The temperature range of the spin transition in (2) is lower than that in (1). A residual high spin iron(II) fraction is observed at low temperatures in (2), being different from (1). SQUID data also show that samples treated differently yield different spin transition curves.

Modelling of two step high spin⇌low spin transitions using the cluster variation method

A thermodynamic description of high spin (HS)⇌low spin (LS) transition curves beyond the Bragg–Williams approximation is given using the Kikuchi cluster variation method (CVM). Transition curves of unusual behaviour (i.e. two step transition) are reproduced by short range interaction energies which are present in addition to long range elastic interaction between the spin changing molecules. The correlations in the distribution of the spin changing centers can be expressed analytically. They give rise to a reduced mixing entropy which was found experimentally in compounds with two step transitions.

Magnetic Properties and Mössbauer Spectroscopy for Complex Fe(tpa)(NCS)2

The studies of Mössbauer spectroscopy and magnetic properties for complex Fe(tpa)(NCS)2 exhibit that low spin → high spin (HS) transition may be thermally induced. The impurities such as HS-Fe2+ residue and HS-Fe3+, that were caused by compound preparation, play a significant role in the investigation of spin transition curve. The Mössbauer spectroscopy is used to observe the impurities as separated forms from the spin transition phase directly, and the magnetic susceptibility measurement is used to determine the total paramagnetism from all HS phases. With these two useful tools, an exact spin transition curve was defined by removing the magnetic contribution of the impurities.

Thermal and Light‐Induced Spin Transition in [Fe(bpen)X2] (bpen = 1,6‐Bis(2‐pyridyl)‐2,5‐diazahexane, X = NCS‐, NCSe‐)

AbstractThe thermal and light‐induced spin transitions in [Fe(bpen)(NCS)2] and [Fe(bpen)(NCSe)2] have been investigated by means of magnetic susceptibility and Mössbauer measurements. The HS → LS transition in [Fe(bpen)(NCS)2] is detected at temperatures below 120 K (T1/2 [when the fraction of high‐spin species γHS = 0.5] = 68 K) with a residual HS fraction of 8% at 20 K. The transition is fairly abrupt and no hysteresis is observed, in contrast to earlier studies (ref. [8]). The generation of metastable HS states in [Fe(bpen)(NCS)2] is achieved by rapid cooling of the sample and by irradiating the compound at low temperatures with light of appropriate wavelengths (LIESST). The HS → LS relaxation of the metastable spin states after rapid cooling is observed at temperatures between 35 and 42.5 K and shows strong deviations from single exponential behavior. The spin‐transition behavior changes drastically on substitution of the thiocyanate ions by NCSe‐. In [Fe(bpen)(NCSe)2] a gradual and complete spin transition between 140 and 250 K is observed (T1/2 = 181 K). Irradiation of the compound with green light at low temperatures does not result in formation of long‐lived metastable HS states. The spintransition curves derived from magnetic susceptibility and Mössbauer studies show good agreement for each compound, and no hints of significantly different Lamb–Mössbauer factors for the HS and LS states are observed.

Thermal and Light-Induced Spin Transition in the High- and Low-Temperature Structure of [Fe(0.35)Ni(0.65)(mtz)(6)](ClO(4))(2).

The thermal and light induced spin transition in [Fe(0.35)Ni(0.65)(mtz)(6)](ClO(4))(2) (mtz = 1-methyl-1H-tetrazole) was studied by (57)Fe Mössbauer spectroscopy and magnetic susceptibility measurements. In addition to the spin transition of the iron(II) complexes the compound undergoes a structural phase transition. The high-temperature structure could be determined by X-ray crystallography of the isomorphous [Fe(0.25)Ni(0.75)(mtz)(6)](ClO(4))(2) complex at room temperature. The X-ray structural analysis shows this complex to be rhombohedric, space group R&thremacr;, with a = 10.865(2) Å and c = 23.65(1) Å with three molecules in the unit cell. The transition to the low-temperature structure occurs at approximately 60 K without changing the spin state of the molecules. By subsequent heating of the complex the high-temperature structure is reached again between ca. 170 and 200 K. The spin transition behavior is strongly influenced by the structural changes, and the observed spin transition curves are completely different for the high- and low-temperature phases. In the high-temperature structure a complete and gradual spin transition between 220 and 120 K (T(1/2)(gamma(HS) = 0.5) = 185 K) is detected; the high-spin (HS) state is represented by one HS doublet in the Mössbauer spectra. In the low-temperature structure a two-step transition curve is detected in the heating mode. About 36% of the molecules show a LS (low-spin) --> HS transition between ca 50 and 75 K. Then the HS fraction stays constant up to 150 K. A further increase in the high-spin fraction is observed at temperatures above 150 K. In this structural phase the HS state is represented by two different HS doublets in the Mössbauer spectra. The formation of metastable HS states by making use of the LIESST effect is only possible in the low-temperature structure. By excitation of the LS molecules with green light, two different HS states are populated which show very different relaxation behavior. One HS state shows a relaxation to the LS state even at 10 K; the other HS state shows a very slow HS --> LS relaxation at 60 K (within days), leading to the HS fraction corresponding to the thermal equilibrium value.

Thermal and optical switching of bistable iron compounds and possible applications

Iron(II) coordination compounds with ligand field strengths close to the mean spin pairing energy may undergo thermal spin transition in the solid as well as in the liquid state. The thermodynamically stable state is high spin (HS) 5T 2 (under O h symmetry) with (t 2) 4 e 2 electron configuration above the critical transition temperature T c , and low spin (LS) 1A 1 with (t 2) 6 electron configuration below T c . The spin transition curve γ HS( T) (molar fraction of HS molecules as function of temperature) can be abrupt or gradual, with or without hysteresis, or even with a plateau near T c . T c itself can vary over the whole temperature scale between the liquid helium region up to above room temperature, depending on the nature of the ligand molecules which may be chosen such as to “fine-tune” the ligand field strength via steric and electronic effects. Such bistable systems exhibiting thermal spin-crossover have the potential to be used as temperature sensors and thermal switches. The phase detection (HS or LS) can be carried out by Mössbauer or optical spectroscopy, magnetic measurements, and other techniques. Bistable iron(II) compounds of the above mentioned nature can also be switched by irradiation with light. Green light switches the LS( 1A 1) state to the HS( 5T 2) state, which surprisingly may have practically infinitely long lifetimes at sufficiently low temperatures. Red light switches from HS to LS. The switching is faster than nanoseconds, and the quantum yields are very high. This phenomenon of “Light-Induced Excited Spin State Trapping”, which we have abbreviated as LIESST, occurs in single crystals, polymer foils, and KBr pellets. Such bistable materials exhibiting the LIESST effect show promising features for possible applications in optical information technology. First holography experiments have been successfully carried out in our laboratory.