Photomagnetic Properties Research Articles

Temperature-Dependent Interactions and Disorder in the Spin-Transition Compound [FeII(L)2][ClO4]2·C7H8Through Structural, Calorimetric, Magnetic, Photomagnetic, and Diffuse Reflectance Investigations

The title compound [Fe (II)(L) 2][ClO 4] 2.C 7H 8 (L = 2-[3-(2'-pyridyl)pyrazol-1-ylmethyl]pyridine) has been isolated while attempting to grow single crystals of the spin-transition (continuous-type) compound [Fe (II)(L) 2][ClO 4] 2, published earlier ( Dalton Trans. 2003, 3392-3397). Magnetic susceptibility measurements, as well as Mossbauer and calorimetric investigations on polycrystalline samples of [Fe(L) 2][ClO 4] 2.C 7H 8 revealed the occurrence of an abrupt HS ( (5) T 2) <--> LS ( (1) A 1) transition with steep and narrow (2 K) hysteresis at approximately 232 K. The photomagnetic properties exhibit features typical for a broad distribution of activation energies, with relaxation curves in the shape of stretched exponentials. We performed a crystal structure determination of the compound at 120, 240, and 270 K. A noteworthy temperature-dependent behavior of the structural parameters was observed, in terms of disorder of both the anions and solvent molecules, leading to a strong thermal dependence of the strength and dimensionality of the interaction network. Additional data were obtained by diffuse reflectance measurements. We model and discuss the antagonistic effects of interactions and disorder by using a two-level cooperative mean-field approach which includes a distribution of barrier energies at the microscopic scale.

Prediction of the Equilibrium Structures and Photomagnetic Properties of the Prussian Blue Analogue RbMn[Fe(CN)6] by Density Functional Theory

A periodic density functional theory method using the B3LYP hybrid exchange-correlation potential is applied to the Prussian blue analogue RbMn[Fe(CN)6] to evaluate the suitability of the method for studying, and predicting, the photomagnetic behavior of Prussian blue analogues and related materials. The method allows correct description of the equilibrium structures of the different electronic configurations with regard to the cell parameters and bond distances. In agreement with the experimental data, the calculations have shown that the low-temperature phase (LT; Fe(2+)(t(6)2g, S = 0)-CN-Mn(3+)(t(3)2g e(1)g, S = 2)) is the stable phase at low temperature instead of the high-temperature phase (HT; Fe(3+)(t(5)2g, S = 1/2)-CN-Mn(2+)(t(3)2g e(2)g, S = 5/2)). Additionally, the method gives an estimation for the enthalpy difference (HT <--> LT) with a value of 143 J mol(-1) K(-1). The comparison of our calculations with experimental data from the literature and from our calorimetric and X-ray photoelectron spectroscopy measurements on the Rb0.97Mn[Fe(CN)6]0.98 x 1.03 H2O compound is analyzed, and in general, a satisfactory agreement is obtained. The method also predicts the metastable nature of the electronic configuration of the high-temperature phase, a necessary condition to photoinduce that phase at low temperatures. It gives a photoactivation energy of 2.36 eV, which is in agreement with photoinduced demagnetization produced by a green laser.

Effect of the metal dilution on the thermal and light-induced spin transition in [Fe xMn 1− x(bpp) 2](NCSe) 2: When T(LIESST) reaches T1/2

The thermal and light-induced spin transition in [Fe x Mn 1− x (bpp) 2](NCSe) 2 (bpp = 2,6-bis(pyrazol-3-yl)pyridine) has been investigated by magnetic susceptibility, photomagnetism and diffuse reflectivity measurements. These complexes display a thermal spin transition and exhibit the light-induced excited spin state trapping (LIESST) effect at low temperature. For each mixed-crystal system, the thermal spin transition temperature, T 1/2, and the relaxation temperature of the photo-induced high-spin state, T(LIESST), have been systematically determined. It appears that T 1/2 decreases with the metal dilution while T(LIESST) remains unchanged, suggesting that the two interconversion processes are controlled by different factors; i.e. the photomagnetic properties are governed at the molecular scale and the thermal spin crossover regime is affected by both the ligand field and crystal packing effects. For highly metal-diluted complex with x < 0.2, it is found that when T 1/2 reaches the T(LIESST), the complex remains HS on the whole range of temperature.

Magnetic and photo-magnetic properties of Co dinuclear complexes

The magnetic and photo-magnetic properties of Co dinuclear compounds were studied. The Co compounds, [{Co(tpa)} 2(dhbq)](BF 4) 3 · 2H 2O and [{Co(tpa)} 2(dhbq)](ClO 4) 3 · 2H 2O, have the structure [(tpa)Co III-LS–(μ-dhbq) 3−–Co III-LS(tpa)] 3− (where LS, H 2dhbq, tpa denote low-spin, 2,5-di-hydroxy-1,4-benzoquinone and tris(2-pyridylmethyl)amine, respectively) at room temperature. On heating, these compounds exhibit valence-tautomeric inter-conversion; [(tpa)Co III-LS–(μ-dhbq) 3−–Co III-LS(tpa)] 3− ⇆ [(tpa)Co II-HS–(μ-dhbq) 2−–Co III-LS(tpa)] 3− (where HS denotes high-spin). Furthermore, both of these compounds exhibit photo-induced valence tautomerism at low temperature (<60 K). These results show that the electronic structures of these complexes can be controlled by modifying the counter anions.

Reversible photomagnetic properties of the molecular compound [{CuII(bipy)2}2{MoIV(CN)8}]·9H2O·CH3OH

The optical and photomagnetic properties of [{CuII(bipy)2}2{MoIV(CN)8}]·9H2O·CH3OH (1) have been reinvestigated. A comparison between spectra in solution and in the solid state revealed the presence of an intervalence band (or Metal–Metal Charge Transfer, hereafter noted MMCT) at 570 nm. The photomagnetic properties have been performed in a Superconducting QUantum Interference Device at 10 K with irradiation in the range of the MMCT: 488 nm, 520 nm and 647 nm at 10 K. An important increase of the magnetic signal has been measured after 1 h of irradiation at 488 nm, whereas a weaker increase has been obtained for the irradiation at 520 nm in the same conditions. Moreover, after an excitation at 488 nm, an irradiation at 647 nm has induced a decrease of the magnetic moment, which corresponds to a partial deexcitation. The complete characterization of the photoproduct has been realised after an irradiation of 4 h at 488 nm. The photomagnetic properties have shown an increase of the paramagnetism of 1 at low temperature. After a thermal heating at 300 K, the material goes back to its initial state before irradiation. It is the first time that a fully reversible photomagnetic behaviour for the compound [{CuII(bipy)2}2{MoIV(CN)8}]·9H2O·CH3OH has been described. The observed properties have been discussed in terms of an electron transfer mechanism Mo → Cu.

Cooperative Iron(II) Spin Crossover Complexes with N4O2Coordination Sphere

Two new spin crossover complexes [FeL(py)(2)] (1) and [FeL(DMAP)(2)] (2) with L being a tetradentate N(2)O(2)(2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentanedionato)(2-)-N,N',O(2),O(2)'], py = pyridine and DMAP = p-dimethylaminopyridine have been investigated using temperature-dependent susceptibility and thermogravimetric and photomagnetic measurements as well as Mössbauer spectroscopy and X-ray structure analysis. Both complexes show a cooperative spin transition with an approximately 9 K wide thermal hysteresis loop in the case of 2 (T(1/2) upward arrow = 183 K and T(1/2) downward arrow = 174 K) and an approximately 2 K wide thermal hysteresis loop in the case of the pyridine diadduct 1 (T(1/2) upward arrow = 191 K and T(1/2) downward arrow = 189 K). The spin transition was additionally followed by different temperature-scanning calorimetry and Mössbauer spectroscopy for 2, and a good agreement for the transition temperatures obtained with the different methods was found. Results from X-ray structure analysis indicate that the cooperative interactions are due to elastic interactions in both compounds. They are more pronounced in the case of 2 with very short intermolecular iron-iron distances of 7.2 A and several intense C-C contacts. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bit of the equatorial ligand, from 108 degrees in the high-spin state to 90 degrees in the low-spin state. The reflectivity measurements of both compounds give at low temperature indication that at the sample surface the light-induced excited spin state trapping (LIESST) effect occurs. In bulk condition using a SQUID magnetometer the complex 2 displays some photomagnetic properties with an photoexcitation level of 60% and a T(LIESST) value of 53 K.

Photomagnetism of A-DNAs Intercalated with Photoresponsive Molecules

We have reported that the natural DNA and DNAs intercalated with organic radicals were able to show the noble magnetic properties in dry state. Extending our study on the Magnetic properties of DNAs, we intercalated the photoresponsive proflavine (PF) molecules into DNAs when the intercalated DNA complexes were illuminated by a laser beam of 488 nm. Electron magnetic resonance (EMR) new signals appeared. We succeeded in measurement of photomagnetic properties of modified DNAs through electron magnetic resonance (EMR) spectroscopy and SQUID measurement. A significant increase in magnetization was observed for the photo-illuminated DNA/proflavine samples.

Photomagnetic Properties of an Iron(II) Low-Spin Complex with an Unusually Long-Lived Metastable LIESST State

A comprehensive study of the photomagnetic behavior of the [Fe(L222N5)(CN)2].H2O complex has been carried out. This complex is characterized by a low-spin (LS) iron(II)-metal center up to 400 K and exhibits at 10 K the well-known Light-Induced Excited Spin State Trapping (LIESST) effect. The critical LIESST temperature (T(LIESST)) has been measured to be 105 K. The kinetics of the transition from the metastable high-spin (HS) state to the low-spin state have been determined and used for reproducing the experimental T(LIESST) curve. This study represents a second example of a fully low-spin iron(II)-metal complex up to 400 K, which can be photoexcited at low temperature with an atypical long-lived metastable HS state. This underlines the preponderant role of the inner coordination sphere for stabilizing the lifetime of the photoinduced HS state.

A typical example of a photomagnetic study carried out on a spin-crossover material

The light-induced excited spin state trapping (LIESST) is a well known phenomenon in an iron(II) spin-crossover (SCO) material which offers some interesting prospects in data storage. In this work, we present a typical investigation of the photomagnetic properties of a SCO material. At the surface of the sample, the light-induced phenomenon is investigated by using a home-built reflectivity set up and in bulk material by using a quantum design SQUID magnetometer coupled to a light source.

Density Functional Theory Prediction of Enhanced Photomagnetic Properties of Nitronyl Nitroxide and Imino Nitroxide Diradicals with Substituded Dihydropyrene Couplers

We predict the photoswitching magnetic properties of four substituted dihydropyrenes from density functional broken-symmetry calculations. The magnetic exchange coupling constants differ up to 9.44 cm(-1). The intramolecular exchange interactions are ferromagnetic in nature. The calculated coupling constants are much larger than those reported earlier for photomagnetic organic molecules.

Photomagnetic properties of iron(ii) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.

EPR spectra analysis of photo-magnetic properties of EuO nanocrystals

The temperature dependencies of magnetic susceptibility c of the europium oxide (EuO) nanocrystals were examined. The experimental Bohr magnetons ( p) for EuO nanocrystals agreed well with the theoretical p for the 4f 7 configuration (Eu(II)). A dramatic increase in magnetization of the EuO nanocrystals under UV irradiation at room temperature was observed by photo-magnetic measurements superconducting quantum interface device (SQUID with optical fiber). This increase in magnetization under UV irradiation can be explained by the occurrence of a d–f exchange interaction of conductive electrons in the 5d band (magnetic exciton). We suggest a mechanism in which the photo-magnetization increases by the presence of an exciton band in the UV region for the polyurea-modified EuO nanocrystals. In order to confirm the magnetic exciton of EuO nanocrystals, we carried out the electron paramagnetic resonance (EPR) measurements under UV irradiation, showing the formation of photo-active species in EuO nanocrystals.

Electrochemical Synthesis, Crystal Structure, and Photomagnetic Properties of a Three‐Dimensional Cyano‐Bridged Copper—Molybdenum Complex.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Electrochemical Synthesis, Crystal Structure, and Photomagnetic Properties of a Three-Dimensional Cyano-Bridged Copper−Molybdenum Complex

A single crystal of Cs(I)2Cu(II)7[Mo(IV)CN8]4.6H2O was electrochemically prepared on a Pt wire electrode with a constant potential of +500 mV vs Ag/AgCl electrode. X-ray single-crystal structural analysis showed that this compound consists of a three-dimensional cyano-bridged Cu-Mo bimetallic assembly with a tetragonal structure of I4/mmm. The coordination geometry of Mo(IV) is bicapped trigonal prism, and that of Cu(II) is five-coordinate of square pyramidal or four-coordinate of square planar. This compound was also prepared as a 0.2-3.0 microm thick film on a SnO2-coated glass in the same electrochemical manner. When the sample, which shows paramagnetism due to Cu(II) (S = 1/2), was irradiated with 450-500 nm light at 5 K, spontaneous magnetization with a Curie temperature of 23 K was observed. This photoinduced change was recovered by a thermal treatment. In the infrared (IR) and electron spin resonance (ESR) spectra after light irradiation, variations in the stretching IR peak of CN bridged to Mo(IV) and the paramagnetic ESR peak of Cu(II) were observed, respectively. The data indicate that this photomagnetism is caused by the electron transfer from Mo(IV) to Cu(II) and the ferromagnetic ordering between Cu(II) (S = 1/2) and Mo(V) (S = 1/2).

Photomagnetic properties of the [Fe(L222(N3O2))(CN)2]·H2O complex: a fascinating example of multi-metastability

Today, it is known that the [Fe(L222(N3O2))(CN)2]·H2O complex exhibits an atypical thermal spin-crossover behaviour probably connected to an unusual change of the iron(II) environment from a six to a seven coordination sphere. Here, we show that the photomagnetic properties of this derivative are also atypical. When the sample is initially prepared in the LS form, by a cooling process of 3 K/min, the light irradiation gives access to a long-lived metastable HS state with a T(LIESST) value of 132 K. This photoinduced HS state is moreover characterised by a strong cooperative lattice, as reflected by the kinetic relaxations which follow a sigmoidal model and the existence of a Light-Induced Thermal Hysteresis (LITH) loop. At the opposite, when the sample is carefully placed into the LS/HS mixture state, by using a slow cooling process of less than 1 K/min, the light irradiation generates a metastable state with a T(LIESST) value of only 73 K and the influence of the cooperativity is weak. The relaxation curves are indeed satisfactory fitted by using a stretched exponential model and the LITH phenomenon is not present. All these data will be discussed in term of multimetastability.

Does cooperativity influence the lifetime of the photo-induced HS state?

We have first recalled the T(LIESST) procedure which consists to determine the temperature above which the photo-magnetic effect is erased. In addition we have selected to series of iron(II) spin crossover complexes, the [Fe(PM-L)2(NCS)2] and [Fe(bpp)2]X2·nH2O families, to analyse the influence of the cooperativity on the stability of the photo-induced HS state. Some of these complexes exhibit gradual thermal spin crossover behaviours while some others undergo an abrupt thermal transition, with and without hysteresis. Interestingly, whatever the cooperativity effect on the thermal spin crossover transition, the lifetime of the metastable state of all these derivates remains governed by the T(LIESST) = T0 - 0.31 T1/2 relation. Finally, we have investigated the magnetic and the photomagnetic properties of a [Fe(bpp)2]-Nafion film. Once more the role of the cooperativity on the stability of the photoinduced HS state appears minor. Conversely, the influence of the nature and the geometry of the inner coordination sphere appears preponderant.

Photomagnetism in Cyano-Bridged Hexanuclear Clusters [MnII(bpy)2]4[MIV(CN)8]2·xH2O (M = Mo, x = 14, and M = W, x = 9)

The magnetic and photomagnetic properties of two cyano-bridged, [Mn(bpy)2]4[Mo(CN)8]2·14H2O (1) and [Mn(bpy)2]4[W(CN)8]2·9H2O (2) (bpy = 2,2‘-bipyridyl), hexanuclear clusters (hereafter named MnII4MIV2) have been investigated. Before irradiation both compounds behave as paramagnets in almost the whole temperature range due to the presence of magnetic MnII centers separated by the diamagnetic MIV spacers in the molecules. After several hours of irradiation (337−356, 406−415, and 480 nm) in a superconducting quantum interference device at 10 K, the magnetic signal at 10 K has increased by a factor of almost 30% compared to that of the initial magnetic state. The magnetic characterization of the irradiated samples indicates the formation of a photoproduct exhibiting antiferromagnetic interactions. The X-ray photoelectron spectroscopy (XPS) data of the irradiated samples show the presence of MnII, MnI, MIV, and MV centers. The formation of MnI and MV centers is interpreted in terms of the reductive quenching of the excited state through an intramolecular electron-transfer mechanism in the MnII4MIV2 clusters. After irradiation at room temperature in the presence of air, both compounds behave as a paramagnet, but with a lower magnetic response than that of the initial state. The XPS data show the presence of MIV, MVI, and only MnII centers. The photochemical pathway in the presence of O2 is discussed in relation to the ligand-field photochemistry of octacyanometalates(IV), leading ultimately to the formation of [MVI(CN)4(O2)] and low-spin [MnII(bpy)2(CN)2] species in the solid state.

Polymeric compositions with hematite nanoparticles and organic compounds with charge transfer

The magnetoresistance, electric and photoconductivity of polymeric films based on polyvinylbutyral doped with compounds with intramolecular charge transfer (CICT) and containing nanoparticles Fe2O3 have been investigated. The photoconductivity of such polymeric compositions is governed by photogeneration of non-equilibrium charge carriers from excited with light CICT molecules and by their transport inside and between the particles. Revealing of inner photoeffect in investigated nanocomposite are of interest for development of new mediums with photomagnetic properties. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atypical photomagnetic properties in a series of binuclear iron(ii) spin crossover complexes

In this work, we have re-examined the photomagnetic properties of a series of binuclear iron(II) complexes of formula [{Fe(L)(NCX)2}2bpym], where L is bpym = 2,2′-bipyrimidine and bt = 2,2′-bithiazoline and X = S, Se, by light irradiation at 647.1–676.4 nm and at 830 nm and we confirm that after optical excitation the magnetic behaviors of [{Fe(bpym)(NCSe)2}2bpym] (2), [{Fe(bt)(NCS)2}2bpym] (3) and [{Fe(bt)(NCSe)2}2bpym] (4) are close to the antiferromagnetic response of [{Fe(bpym)(NCS)2}2bpym] (1) characterized by two iron(II) metal ions in the HS electronic configuration. This hypothesis of quantitative HS–HS photoconversion is also supported by the reflectivity experiments and the infrared spectroscopies performed on 3. We have also investigated the relaxation process of 2–4 by using a SQUID magnetometer. Atypical magnetic behavior is found for compounds 3 and 4. More precisely, while the magnetic response of 2versus time continuously decreases whatever the temperature, the signal of 3 increases below 17 K, becomes stable at around 18–20 K and decreases at higher temperature. We thus propose a direct HS–HS→LS–HS relaxation process for 2 and an indirect HS–HS→LS–HS→LS–LS relaxation process for 3 and 4. The agreement found between the experimental T(LIESST) and the simulated T(LIESST) curves, calculated using the measured kinetic parameters, guarantees the validity of our approach.

Photochromism of the Inclusion Crystals of a New Hydroperoxybiindenylidene.

AbstractFor Abstract see ChemInform Abstract in Full Text.