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International audience; Light-Induced Excited Spin-State Trapping (LIESST) is a well-known phenomenon in spin-crossover (SCO) materials, studied since 1982 in solution and 1984 in the solid state, as it offers some interesting prospects in data storage. In this paper we discuss the influence of metal dilution on the lifetime of the photo-induced state in two 1D chain solid solution series: [FexZn1−x(btzp)3](BF4)2 and [FexZn1−x(endi)3](BF4)2 {btzp = 1,2-bis(tetrazol-1-yl)propane and endi = 1,2-bis(tetrazol-1-yl)ethane}. For each composition, we have determined the thermal spin transition temperature, T1/2, and the relaxation temperature of the photoinduced high-spin state, T(LIESST). A detailed analysis of the relaxation kinetics allowed a discussion of the evolution of this lifetime as a function of metal dilution

Combining Co(II) with the 2,6-bis-(pyrazol1-yl)pyridine ligand functionalized with a carboxylic acid group (bppCOOH) results in the compound [CoII(bppCOOH)2](ClO4)2·2Me2CO, which shows a field-induced slow relaxation of magnetization as a result of the magnetic anisotropy of the distorted octahedral coordination of high-spin Co(II). Co(II)/Fe(II) solid solutions of bppCOOH have been prepared to study the influence of Co(II) on the spin crossover properties of Fe(II). Magnetic characterization of [Fe0.92Co0.08(bppCOOH)2](ClO4)2 indicates that the Cobalt dopant leads to a reduction of T1/2 and a loss of cooperativity of the spin transition.

Four coordination complexes, namely, [Cu(HBCB)]n (1), {[Cu(HBCB)(o-bimb)]·H2O} (2), {[Cu1.5(BCB)(m-bimb)1.5]·DMF}n (3), and [Co1.5(BCB)(bpy)1.5(H2O)]n (4), have been assembled from the semirigid tripodal 3,5-bis(2-carboxylphenoxy)benzoic acid (H3BCB) in the presence or absence of the N-donors (o-bimb = 1,2-bis(imidazol-1-ylmethyl)benzene, m-bimb = 1,3-bis(imidazol-1-ylmethyl)benzene, bpy = 4,4′-bipyridine). Structural analyses revealed that complex 1 features a {Cu2(COO)4} SBUs based 3D (3,6)-connected {4·62}2{42·610·83}-rtl net. Complex 2 displays an interesting coordination molecule cage structure. Complex 3 exhibits a 2D (3,4,4)-connected {4·62}2{4·64·8}2{64·82} sheet. And complex 4 shows a 3D (3,4,6)-connected {5·62}2{56·64·7·82·92}{64·8·9} net based on the binuclear {Co2(COO)2} SBUs and Co2 ions. The temperature-dependent magnetic susceptibilities indicated there are gradual spin transition behavior in 1, and antiferromagnetic interactions in 4.

The preparation and magnetic properties of four Fe(II) Schiff-base complexes, [Fe(qsal X ) 2 ] (Hqsal X = N -(8′-quinolyl)-2-hydroxy-5-halogeno-1-salicylaldimine; X = F( 1 ), Cl( 2 ), Br( 3 ), I( 4 )) are reported. X-ray single crystal structure analyses of 1 and 2 reveal that Fe(II) ions in both complexes are coordinated by two qsal X ligands in meridional fashion. Molecular packing of 1 shows that a non-planar qsal F ligand interacts with neighboring two qsal F ligand’s through π–π interactions, while that of 2 shows that two planar qsal Cl ligands interact each other through π–π interaction. Although the magnetic property of 1 shows a high-spin state at all the temperature range measured, the χ T – T plots of 2 , 3 , and 4 show gradual spin crossover behaviors with T 1/2 of 308 K, 341 K, and 340 K, respectively.

Five ligands as monosubstituted derivatives of the basic skeleton of 2,6-bis(2-benzimidazyl)pyridine were synthesized, and characterized by NMR and IR spectra along with the X-ray structure analysis. Their complexation gave a set of hexacoordinate Fe(II) complexes showing predominantly the diamagnetism until ambient temperature. Some temperature-independent paramagnetism along with an onset of the spin transition is also detected.

Investigations of spin-crossover in an iron(II) tris(diimine) system, based on the ligands L1 (4,5-dimethy-2-(pyridine-2-yl)imidazole) and L2 (1-acetic ethyl-2-(pyridine-2-yl)-imidazole), have been presented in this paper. The methyl groups at adjacent sites to the donor atoms in the ligand L1 generated the crossover situation for complexes Fe(L1)3(ClO4−)2 (1), Fe(L1)3(BF4−)2 (2), Fe(L1)3(PF6−)2 (3) and Fe(L1)3(SbF6−)2 (4), while the acetic ethyl group at the non-coordinating NH site of the ligand L2 resulted in an increase in the ligand field and generated a low-spin (LS) state for the complex Fe(L2)3(ClO4−)2 (5). Complexes 1 and 5 were structurally characterized by single-crystal X-ray diffraction at different spin-states, while the phase purity of complexes 2–4 was confirmed by PXRD. Raman, IR and UV–Vis absorption spectra and magnetic properties of 1–4 were obtained, which revealed the influence of different counter anions on the spin transition temperatures. The blue-shift of IR and Raman spectra, red-shift and decrease in molar absorptivity of UV–Vis absorption spectra all follow the sequence 1

International audience; In this paper we report on the integration of micro-rods of the [Fe(Htrz)2(trz)](BF4) spin crossover compound into an electronic switch-type device. The device has been fabricated by organizing the micrometric particles between interdigitated gold electrodes using dielectrophoresis and the influence of dielectrophoresis parameters on their integration is also examined. A particular attention was devoted to the investigation of the stability of the spin transition and the associated device properties. The stability of the particles before dielectrophoresis was investigated using variable temperature diffuse reflectivity experiments, which revealed a remarkable stability of the spin crossover even after performing 3000 switching cycles. We show also that the spin transition is not influenced by the solvents used for device fabrication, neither by the atmosphere (ambient air or vacuum) in which the device is used. The spin transition remains reproducible over several tens of cycles even at the device level, but the current in the device decreases continuously.

New nitronyl nitroxides LMe and LMe-CP containing a 4-methylpyridin-3-yl substituent were synthesized. It was found that the interaction of Сu(hfac)2 with LMe and LMe-CP gave binuclear [Cu(hfac)2LMe]2 and [Cu(hfac)2LMe-CP]2·Solv (Solv = n-C6H14, n-C10H22, n-C16H34) and chain polymer {[[Cu(hfac)2]2LMe2][Cu(hfac)2]}∞ heterospin complexes. An important structural peculiarity of LMe and LMe-CP is a large dihedral angle between the planes of the O –N−C N → O paramagnetic fragment and the pyridine ring: 55.2 and 56.1°, respectively. The presence of a methyl group in the pyridine ring of the nitroxide molecule in {[[Cu(hfac)2]2LMe2][Cu(hfac)2]}∞ proved favorable for spin transition at nearly room temperature.

A new family of wire-like molecules with butadiyndiyl (1a/b), hexadiyndiyl (2) and phenylenediethynyl (3) bridges linking three diruthenium units have been prepared. The single crystal X-ray diffraction study of 1a revealed structural details of the trimeric compounds, including a distance of 18 A between two Ru2 termini. Both the voltammetric and spectroscopic data point to an extensive electronic delocalization in trimers 1 and 2, and the lack of delocalization in 3, demonstrating the advantage of linear polyyndiyl in mediating intermetallic electronic couplings. Magnetic studies of compound 1a reveal that a spin transition may occur as the temperature increases from 1.8 to 300 K.

The Prussian blue analogue K 0.4 Co 1.3 [Fe(CN) 6 ]· n H 2 O has previously been shown to exhibit three distinct phases at low temperature. In addition to the well-studied high-temperature ‘high spin’ and low-temperature ‘low spin’ phases, with unit cells of ∼10.3 and ∼9.95 A, respectively, an intermediate phase with unit cell a = 10.09 A was recently reported, although it existed as a minority constituent in a mixed-phase material. Herein we report the synthesis of 130 nm K 0.4 Co 1.3 [Fe(CN) 6 ]· 4.4 H 2 O particles that form solely the intermediate phase at temperatures below the normal charge transfer induced spin transition. Compared with those formed from a bulk synthesis, the nanosized particles are shown to have the same chemical composition at high temperature, but they undergo different structural transitions with thermal cycling.

Raman spectra of cis-Fe(1,10-phenanthroline)2(NCS)2 and trans-Fe(pyridine)4(NCS)2 are recorded at variable temperature and pressure and used to characterize these complexes via the vibrational modes of the [NCS]− ligands. Vibrational frequencies of the [NCS]− anion in KNCS are 2052, 748 and 484 cm−1 for the ν(C–N) stretching, ν(C–S) stretching and δ(NCS) bending modes, respectively. The two stretching frequencies show average increases of 20 and 60 cm−1 upon coordination to high-spin iron(II) ions in both complexes. For low-spin iron(II), observed for cis-Fe(1,10-phenanthroline)2(NCS)2 at 83 K or 15 kbar, the ν(C–N) stretching frequency is 2114 cm−1, an increase by approximately 40 cm−1 compared to the high-spin complex. This frequency can therefore be used as a sensitive probe, even if C–N bond lengths as determined by X-ray diffraction are identical within experimental precision for all conditions. The frequencies of [NCS]− ligands show pressure-induced changes of +0.16 and +0.20 cm−1/kbar for the ν(C–N) modes of high-spin and low-spin cis-Fe(1,10-phenanthroline)2(NCS)2, and +0.31 cm−1/kbar for high-spin trans-Fe(pyridine)4(NCS)2.

Two new iron(II) complexes, [FeII(HLMe)2](ClO4)2 (1) and its ethanol adduct [FeII(HLMe)2](ClO4)2·EtOH (2) were synthesized by selective crystallization in methanol and ethanol, respectively, where HLMe is a tridentate N3 ligand, 2-methylimidazol-4-yl-methylideneamino-2-ethylpyridine, and their structures and magnetic properties were investigated. Each compound contains the same octahedrally N6 coordinated complex-cation [FeII(HLMe)2]2+ and two ClO4− ions. The non-solvated compound 1 is the HS (S = 2) complex without spin transition over the 5–300 K temperature range, and has the homochiral 1D helical-chain structure constructed of the intermolecular NH···O− hydrogen bonds between ClO4− ions and neighboring imidazole NH groups of the [FeII(HLMe)2]2+ cations. The ethanol adduct 2 shows a gradual one-step spin transition between the HS (S = 2) and LS (S = 0) states, and has two crystallographically independent iron(II) sites with two NH(imidazole)···OH(EtOH)···O(ClO4−) hydrogen bonds of Fe(1) site and two NH(imidazole)···O(ClO4−) hydrogen bonds of Fe(2) site. There are no further intermolecular interactions to produce an extended structure in a crystal lattice of 2. The presence or absence of thermal spin transition is related to the structural distortion of each HS molecule of these perchlorate salts associated with their surroundings of intermolecular hydrogen bonds.

The complex [ FeL 2 1 ](ClO4)2·H2O (1a) (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine) was synthesized by the reaction of Fe(ClO4)2·6H2O with new NNN-tridentate pyrazolylpyrimidine ligand L1 in MeOH/EtOH mixture. It is the first example of iron(II) spin crossover complexes with 2,4(6)-di(1H-pyrazol-1-yl)pyrimidines and manifests gradual spin transition centered around 150 K on the χMT versus T curve without hysteresis. A broad heat capacity anomaly with the maximum at 150 K was observed on the Cp(T) dependence, ΔtrsH = 5.50 kJ/mol and ΔtrsS = 42.0 J/(mol K). Surprisingly, the syntheses of iron(II) perchlorate compound with L1 performed in EtOH and iPrOH afford another modification of the complex, [ FeL 2 1 ](ClO4)2·H2O (1b), being high spin in the 78–300 K temperature range. The complexes with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L2), [ FeL 2 2 ](ClO4)2·EtOH (2), [ FeL 2 2 ](CF3SO3)2 (3), [FeL2Cl2] 0.5H2O (4) and [FeL2(NCS)2] (5) were synthesized in EtOH solutions. They are in the high spin state within 78–300 K temperature range.

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy) 4 (X) 2 ] (stpy = styrylpyridine, X = NCS, NCBH 3 ) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe( trans -stpy) 4 (NCS) 2 ], while [Fe( cis -stpy) 4 (NCS) 2 ] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH 3 , due to the stronger ligand field of NCBH 3 , the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe( trans -stpy) 4 (NCBH 3 ) 2 ]. In the case of [Fe( cis -stpy) 4 (NCBH 3 ) 2 ], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65–1.09 GPa where [Fe(stpy) 4 (NCBH 3 ) 2 ] undergoes LD-LISC at room temperature.

A novel two-dimensional network bimetallic Fe Au spin crossover coordination polymer based on 3-phenylpyridine-coordinated iron centers and linear gold cyanide bridges {Fe(3-phenylpyridine)2[Au(CN)2]2}n (1), has been synthesized. The compound is characterized by elemental analysis, IR, single-crystal X-ray analysis at 300 and 90 K and magnetic measurements. The FeII ions in 1 have octahedral FeIIN6 coordination geometries, which are linked by [Au(CN)2]− units at the equatorial plane to form a polymeric 2D sheet architecture. The two pyridine rings coordinate in axial position. Variable-temperature (2–300 K) magnetic susceptibility measurements of 1 were performed to determine the spin transition behavior. SQUID data show that high and low spin states exist in a 1:1 ratio at 90 K. However, only one kind of FeII atom is apparent crystallographically at 90 K, indicating that the high and low spin sites are disordered in the polymeric 2D framework.

The bzimpy iron(II) complexes, 1–3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T1/2↑ = 236 K to T1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.

The precursor [Fe III (L)Cl] (LH 2 = N , N ′-bis(2′-hydroxy-benzyliden)-1,6-diamino-3-N-hexane is a high-spin ( S = 5/2) complex. This precursor is combined with the bridging units [Sn IV (X) 4 ] (X = CN − , NCS − ) to yield star-shaped pentanuclear clusters, [(LFe III –X) 4 Sn]Cl 4 . For X = CN − the 57 Fe-Mossbauer data show a multiple spin transition between iron(III) in the high-spin and low-spin state, while the 119m Sn-Mossbauer data indicate a valence tautomerism between Sn(IV) and Sn(II). Changing the bridging unit from X = CN − to X = NCS − turns the switchability off.

New interesting two-dimensional (2D) bi-metallic coordination polymers {FeII(3-fluoropyridine)2[AuI(CN)2]2}n (1), {MnII(3-fluoropyridine)2[AuI(CN)2]2}n (2) were synthesized and characterized by using single-crystal X-ray analysis and IR and elemental analyses. Both complexes are isostructural to each other and construct 2D {MII[AuI(CN)2]2} (M = Fe, Mn) sheet network structures, including the rhombus { M 4 II [ Au I ( CN ) 2 ] 4 }, which are called Hofmann-like structure. Both structures exhibit bilayer connections via Au–Au interactions. Complex 1 which shows two-step spin transition has been already found. However, only the structure of 1 in high-spin state (293 K) was completely solved. Therefore, in this report, the high quality of crystal data of 1 in 100% low-spin state (80 K) and 50% low-spin state (123 K) are completely solved. Also magnetic property of 2 and structure of 2 at 293 K have been studied. Furthermore, we have found that both compounds show strong fluorescent emissions in the solid state at room temperature.

The metal complexes with long alkyl chains [Co(C16-terpy) 3 ](BF 4 ) 2 ( 1 ) and [Fe(C16-terpy) 2 ](BF 4 ) 2 ( 2 ) were synthesized and the physical properties of the complex were characterized by magnetic susceptibility, Mossbauer spectroscopy, polarizing optical microscopy, differential scanning calorimetry, and X-ray scattering, where C16-terpy is 4′-hexadecyloxy-2,2′:6′,2′′-terpyridine. Variable-temperature magnetic susceptibility measurements and/or Mossbauer studies revealed that the complex 1 exhibited unique spin transition ( T 1/2 ↓ = 217 K and T 1/2 ↑ = 260 K) induced by structural phase transition, and the complex 2 was in the low-spin state in the temperature region of 5–400 K before the first mesophase transition. The cobalt(II) and iron(II) complexes exhibited liquid-crystal properties in the temperature range of 371–528 K and 466–556 K, respectively. After mesophase transition, the complex 1 exhibited only slight spin transition ( T 1/2 ↓ = 266 K and T 1/2 ↑ = 279 K), and the complex 2 was in the low-spin state. The compounds with multifunction, i.e., magnetic property and liquid-crystal properties, are important in the development of molecular materials.

A novel Fe(III) spin-crossover complex, [Fe(qsal)2][Ni(dmise)2] · 2CH3CN 1 [qsalH = N-(8-quinolyl)-salicylaldimine, dmise = 4,5-dithiolato-1,3-dithiole-2-selone] was prepared. The magnetic susceptibility measurements revealed 1 exhibited a cooperative spin transition with a thermal hysteresis loop of 15 K. The high and the low temperature structures of 1 indicated three-dimensional intermolecular π−π interactions play a key role in the cooperative spin transition, accompanying a reversible molecular slipping of π-dimer of Ni(dmise)2 along the molecular long axis. The transfer integral calculation for 1 suggested the π-dimer of Ni(dmise)2 is in the spin singlet state.

A nonmagnetic compound, [NO2BzPy][Cu(mnt)2] (mnt2− = maleonitriledithiolate; NO2BzPy+ = 1-(4′-nitrobenzyl)pyridinium), is isostructural with [NO2BzPy][Ni(mnt)2], which is a quasi-1D spin system and exhibits a spin-Peierls-like transition with J = 192 K in the gapless state and spin energy gap = 738 K in the dimerization state, respectively. Further, five nonmagnetic impurity doped compounds [NO2BzPy][CuxNi1−x(mnt)2] (x = 0.04–0.74) were prepared, and their crystal structures as well as magnetic properties were investigated. The nonmagnetic doping causes the suppression of the spin transition with an average rate of 139(13) K/percentage of dopant concentration, and the transition collapse is estimated at around x > 0.5.

Variable temperature magnetic susceptibility, Mossbauer spectroscopic and X-ray crystallographic studies are described on two structurally similar families of dinuclear iron(II) spin crossover (SCO) complexes of formula [Fe(NCX)(py)]2(μ-L)2, where L is either a 3,5-bis(2-pyridyl)-pyrazolate bridging ligand, bpypz−, examples of which have been earlier reported by Kaizaki and coworkers, or a corresponding 3,5-bis(2-pyridyl)-1,2,4-triazolate, bpytz−. Compounds synthesised were [Fe(NCS)(py)]2(μ-bpypz)2 (1), [Fe(NCSe)(py)]2(μ-bpypz)2 (2), [Fe(NCS)(py)]2(μ-bpytz)2 (3), [Fe(NCSe)(py)]2(μ-bpytz)2 (4), [Fe(NCBH3)(py)]2(μ-bpytz)2 (5). The crystal and molecular structures of 1 and 3 are very similar in their HS–HS forms (HS = high spin d6). In contrast to reported SCO behaviour for precipitated samples of 1, also repeated here, crystals of 1 show only HS–HS behaviour with no spin crossover transition. Complex 3 likewise displays HS–HS magnetism, with very weak antiferromagnetic coupling. Compound 5 displays a well resolved two-step, full spin transition from HS–HS to LS–LS states while compound 2 shows a one step transition. The Mossbauer data for 2 and 5 show unusual features at low temperatures.

The iron(II) compounds [Fe(1C16-O-bzimpy) 2 ](BF 4 ) 2 ( 1 ) and [Fe(2C16-bzimpy) 2 ](BF 4 ) 2 ( 2 ) were synthesized and the physical properties of the compounds were characterized by polarizing optical microscopy, differential scanning calorimetry, and powder X-ray analysis, where 1C16-O-bzimpy and 2C16-bzimpy are 2,6-bis(benzimidazol-2′-yl)-4-hexadecyloxypyridine and 2,6-bis( N -hexadecylbenzimidazol-2′-yl)pyridine, respectively. The iron(II) compounds 1 and 2 have exhibited crystal-liquid crystal transition at 547 K and 322 K, respectively. Variable-temperature magnetic susceptibility measurements and/or Mossbauer studies reveal that the compound 1 exhibits spin-crossover behavior between high-spin and low-spin states and photo-induced spin transition from a low-spin state to a metastable high-spin state, and the compound 2 is in the low-spin state. Therefore, the iron(II) compound 1 can undergo spin-crossover, photo-induced spin transition and liquid-crystal properties in a compound. The compounds with multifunction, i.e. spin-crossover and photo-induced spin transition material with liquid-crystal properties, are important in the development of molecular switches and optical materials.

New one-dimensional spin transition coordination polymers of formula [Fe(NH(2)trZ)(3)](AF(6)) center dot nH(2)O (A = Ti, Zr, Sn; NH(2)trz = 4-amino-1,2,4-triazole) have been synthesized in MeOH and EtOH media. These materials display an abrupt and hysteretic spin transition around 200 K as well as a reversible thermochromic effect on, cooling. A preliminary evaluation of the lattice dynamics in the high-spin and low-spin states is presented. (c) 2006 Elsevier Ltd. All rights reserved.

We present the synthesis and structural characterization of the salt [Fe(bpp)2][MnCr(ox)3]2 · bpp · CH3OH. It crystallizes in the monoclinic space group. This material contains an anionic [MnCr(ox)3]− 3D 10-gon ferromagnetic network, that orders below 3.0 K. The channels created by this architecture are filled by the spin crossover cations [Fe(bpp)2]2+ (bpp = 2,6(bispyrazol-3-yl)pyridine), free ligand and solvent molecules. No spin transition has been observed at ambient pressure.

Using the FORC method, we present in this communication a study on the diluted spin transition materials with the formula [FexM1 − x(btr)2 (NCS)2] · H2O with M = Zn, Co and Ni. We compare the evolution of distributions obtained for different dilutions with the three impurities and we notice that these distributions depend not only on the dilution (1 − x) but also on the impurity used. The statistical analysis shows that the maximum values of the distributions are close to a linear dependence for Zn and Co. The impurity that does not obey this dependence is Ni, which volume is close to the average volumes of high spin and low spin iron atoms. In addition, opposite to others, the complex with Ni impurities presents a rotation of the principal axes.

A series of mixed-metal solid solutions MxCu1 − x(hfac)2L (M = Mn, Ni, and Co) liable to undergo thermally induced spin transitions have been synthesized. The structure and magnetic properties of the compounds have been investigated. By varying the metal to be incorporated in the complex (Mn, Ni, or Co) and the value of x, one can control the character of the magnetic anomaly.

The iron(II) compounds [Fe(1C16-L)2(NCS)2] (1), [Fe(2C16-L)2(NCS)2] (2) and [Fe(3C16-L)2 (NCS)2] (3) were synthesized and the physical properties of the compounds were characterized by polarizing optical microscopy, differential scanning calorimetry, and Powder X-ray analysis, where 1C16-L, 2C16-L and 3C16-L are 4-hexadecyloxy, 4,5-bis(hexadecyloxy) and 3,4,5-tris(hexadecyloxy)-N-(2-pyridinylmethylene)-benzenamine, respectively. The iron(II) compounds have exhibited liquid-crystal properties in the temperature range from 345 to 400 K. Variable-temperature magnetic susceptibility measurements and/or Mossbauer studies reveal that the compounds exhibit spin-crossover behavior between high-spin and low-spin states and photo-induced spin transition from a low-spin state to a metastable high-spin state. Therefore the iron(II) compounds can undergo spin-crossover, photo-induced spin transition and liquid-crystal properties in a compound. The compounds with multifunction, i.e., spin-crossover and photo-induced spin transition material with liquid-crystal properties, are important in the development of molecular switches and optical materials.

Transparent spin crossover complex films [Fe(R-trz)3]-Nafion (R = H, NH2), have been prepared by using an ion-exchange resin (Nafion) as counter anion. We observed the spin crossover phenomenon around room temperature and revealed the LIESST (light-induced excited spin state trapping) effect at 4.2 K, for these films. In ICMM2004, we have demonstrated the spin crossover behavior as thermochromism for the transparent [Fe(H-trz)3]-Nafion film, where the spin transition takes place between the high spin (HS) state (colorless at 300 K) and the low spin (LS) state (purple color at 77 K). Here, we review the synthesis and the spin crossover phenomenon for the spin crossover complex film [Fe(H-trz)3]-Nafion.

Cytochrome c-551 from the mesophile Pseudomonas aeruginosa is an electronic transfer protein that contains 82 amino-acid residues and a c-type heme as the prosthetic group with low spin Fe(II) in the reduced form and low spin Fe(III) in the oxidized form of cytochrome c-551. We have studied the electronic properties of ferricytochrome c-551 from P. aeruginosa at high pH (9–11.4) by means of paramagnetic 1H NMR spectra and the T1 and T2 values of isotropically shifted proton resonances. We have also analyzed the temperature dependence of the hyperfine-shifts. Resonance assignment of some signals was based on 2D saturation transfer experiments, EXSY. These results indicate the existence of high-spin iron(III) cytochrome c-551 (S = 5/2, 6A1) with penta or hexacoordinated symmetry at high pH. A spin transition from low-spin iron(III) to high-spin iron(III) has been produced. This transition can be associated with the deprotonation of the axially coordinated amino-acid to iron, His-16. We also discuss the spin crossover between different ferricytochromes c.

Iron (II), cobalt (II) and nickel (II) complexes of 2,6-bis(pyrazol-3-yl)pyridine (bpp) with [Cr(C2O4)3]3− have been prepared. They were characterised by single-crystal X-ray diffraction, magnetic susceptibility measurements and thermal gravimetric analyses. All three compounds are isostructural and they are formed by isolated [MII(bpp)2]2+ and [Cr(C2O4)3]3− complexes and free ClO4 −. As expected, only the salt [Fe(bpp)2]2[Cr(C2O4)3]ClO4·5H2O shows a thermal spin transition with transition temperature (T1/2) around 375 K that is correlated to the loss of water molecules.

The Monte Carlo entropic sampling method previously presented in [ I. Shteto, J. Linares, F. Varret, Phys. Rev. E 56 (1997) 5128 ] is adapted here to an Ising-like system with short- and long-range interactions. Such model is suited to spin crossover solids [ [2] , [3] ] where the long interaction is due to elastic coupling mediated by the lattice, while the short-range interaction originates from the bonding between the spin crossover units [J. Linares, H. Spiering, F. Varret, Eur. J. Phys. B 10 (1999) 271]. Taking into account the different degeneracies gHS for high-spin (HS) and gLS for low-spin (LS) states, the Ising Hamiltonian associated with fictitious spins is written: H=−h∑σ i −J∑σ i σ j with h=− Δ 2 + k B T ln g HS g LS 2 +G〈σ〉 where J and G are the short- and long-range interactions, respectively, and Δ the energy gap of ligand field such that the LS state is the ground state. The numerical method has been tested successfully by comparison to the exact solution for a 1D system: [Fe(Htrz)2(trz)](BF4)2 [ J. Linares, H. Spiering, F. Varret, Eur. J. Phys. B 10 (1999) 271; J. Krober, J.P. Audiere, R. Claude, O. Kahn, J. Hassnoot, F. Groliere, C. Jay, A. Bousseksou, J. Linares, F. Varret, A. Gonthier-Vassal, Chem. Mater. 6 (1994) 1404 ]. We describe here the results obtained for 2D systems, and show that the squareness of the thermal hystersis loop, associated with the spin-transition, can be correlated to the strength of short-range interactions.

In the current study we report on the unusual results obtained in the framework of the atom-phonon coupling model: two metastables states at low temperatures. These results can explain the experimental results reported by Ould Moussa et al. [9] and Matsumoto et al. [10] . This type of SCO behavior is due to the competition between the Light-Induced Excited Spin State Trapping (LIESST) effect, the elastic interaction between the molecules and the thermal activation. In order to determine the stability of the states founded at low temperature, the free energy F is presented for the case of the two step spin transition of a SCO compound.

The quasi-static nature of a light induced thermal hysteresis was studied on the spin-transition compound [Fe(ptz)6](BF4)2 ,b y means of optical spectroscopy and magnetic measurements in the temperature interval between 10 and 80 K. Various experimental procedures are discussed in relation to the competition between the two processes considered, namely the photoexitation and the high-spinlow-spin relaxation. A detailed discussion of the experimental parameters, which should be considered in order to avoid erroneous interpretations of LITH, is given. © 2001 Elsevier Science Ltd. All rights reserved.

The synthesis and characterization of the spin transition compound hexakis(1-methyltetrazole-N4)iron(II) trifluoromethanesulfonate obtained from the reaction of 1-methyltetrazole and iron(II) triflate are described. The spin transition of [Fe(mtz)6](CF3SO3)2 (1) presents a hysteresis loop in magnetism with T↓=157.0 and T↑=179.5 K. Only one-third of the Fe(II) ions undergo this transition, the other two-thirds of the ions remain in the high-spin state. Mossbauer spectroscopy and relaxation studies have shown that the actual width of the hysteresis is only 12 K, with T↓=167.5 and T↑=179.5 K. Thermal spin-state trapping appears to be possible. The trapped high-spin state is stable up to 103 K. The relaxation is not first-order. After irradiation with red light, reverse light-induced excited spin state trapping (LIESST) is observed. The new low-spin state is stable up to 42 K. Also, its relaxation to HS is non-first-order. A reverse light-induced thermal hysteresis loop of approximately 7 K width is present too. No LIESST effect could be observed. Finally, a ferromagnetic type of coupling has been recognized, which is temperature and field dependent.

In [Fe(mtz) 6 ](BF 4 ) 2 (mtz=1-methyltetrazole), a spin transition of 50% of the Fe(II) ions corresponding to one crystallographic site occurs at 74 K. Thermal trapping in the HS state of the spin transiting site is achieved quantitatively. Relaxation back to the LS state occurs in the range 45–70 K. Non-first-order kinetics are reproduced using a phenomenological model including a self-acceleration parameter α =1.8. The Arrhenius plot shows that a thermally activated process is dominant in the range of temperatures studied, with an activation energy E a =6.02 kJ mol −1 . The speed of cooling and the applied field are shown to be of small influence on the kinetics. A new member of the propyltetrazole family, [Fe(ptz) 6 ](CF 3 SO 3 ) 2 ·2ptz (ptz=1-propyltetrazole), has been synthesized, which has two non-coordinated ptz molecules in the lattice. Its steep transition at 178 K is characterized by magnetic measurements, Mossbauer spectroscopy, reflectivity and calorimetry. The LIESST effect is shown to be present at 10 K, but only on the surface and with an extremely short lifetime.

The precursor [FeIII(L)Cl] (LH2 = N,N′-bis(2′-hydroxy-benzyliden)-1,6-diamino-3-azahexane) has been prepared and Mossbauer spectroscopy assigned a high-spin (S = 5/2) state at room temperature. The precursor is combined with the bridging units [SbV(X)6]− (X = CN−, NCS−) to yield star-shaped heptanuclear clusters [(LFeIII–X)6SbV]Cl5. The star-shaped compounds are in general high-spin systems at room temperature. On cooling to 20 K some of the iron(III) centers switch to the low-spin state as indicated by Mossbauer spectroscopy, i.e. multiple electronic transitions. While the cyano-bridged complex performs a multiple spin transition the thiocyanate-compound shows no significant population at both temperatures.

New complexes of iron(II) iodide and thiocyanate with 4-amino-1,2,4-triazole were synthesized. The complexes were characterized by magnetic measurements, reflection, Mossbauer and IR spectra. The coordination of iron(II) is discussed on the basis of these data. The compounds posses reversible thermochromism (colour change from white to pink) due to the spin transition 5 T 2 ⇌ 1 A 1 .

The FeII ion in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O) has distorted tetragonal symmetry with two trans-oriented NCS− ligands. It shows a very abrupt high-spin α low-spin transition at 123.5 K on cooling and 144.5 K on warming. The compound is rather stable in vacuo at room temperature; however, samples which have passed the spin transition once lose their water of hydration above 240 K in vacuo. The dehydrated substance does not show a spin transition; it is high spin in the whole temperature range. Mossbauer ligand-field spectra and magnetic behaviour of both the hydrated and non-hydrated compounds are discussed. The spin transition has been followed by EPR measurements with the aid of traces of Cu2+ ions which could be substituted for Fe2+ in the tetragonal structure. In the high-spin phase the EPR signal is very broad and featureless; in the diamagnetic low-spin phase it is very sharp and resolved in hyperfine and superhyperfine structures. This unusual method to follow the spin transition was shown to be quite generally applicable. In the crystal structure of Fe(NCS)2(C4H4N6)2(H2O) the distances FENCS are 2.125(3) A, and FeN(ligand) 2.180(3) and 2.188(2) A. The water molecule is connected to the non-coordinating ligand N-atom by hydrogen bonding.