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Proceedings of the Japan Academy. Series B, Physical and Biological Sciences logoLink to Proceedings of the Japan Academy. Series B, Physical and Biological Sciences
. 2012 Jun 11;88(6):213–225. doi: 10.2183/pjab.88.213

Switchable molecular magnets

Osamu SATO *1,†
Editor: Hiroo INOKUCHI
PMCID: PMC3410140  PMID: 22728438

Abstract

Various molecular magnetic compounds whose magnetic properties can be controlled by external stimuli have been developed, including electrochemically, photochemically, and chemically tunable bulk magnets as well as a phototunable antiferromagnetic phase of single chain magnet. In addition, we present tunable paramagnetic mononuclear complexes ranging from spin crossover complexes and valence tautomeric complexes to Co complexes in which orbital angular momentum can be switched. Furthermore, we recently developed several switchable clusters and one-dimensional coordination polymers. The switching of magnetic properties can be achieved by modulating metals, ligands, and molecules/ions in the second sphere of the complexes.

Keywords: molecule-based magnets, photomagnets, quantum magnets, switching, clusters, one-dimensional compounds

Introduction

The study of magnetic properties of molecular compounds has recently attracted considerable attention.1–6) The development of pure organic magnets has been carried out for a long time, and the first pure organic magnet was reported ca. 20 years ago. This study aimed at developing new molecular magnets having magnetic properties that can be controlled by light, electrochemical reaction, and chemical stimuli.7,8) The switchable molecular magnets can be used for future memory and switching devices. In addition, our approach should lead to the development of an interdisciplinary field between molecular magnetism and photochemistry as well as molecular magnetism and electrochemistry.1–3,7)

The magnetic properties of molecular magnets can be controlled by various methods including the direct modulation of metal centers carrying spins. Changes in the redox state and spin multiplicity can be induced by charge transfer, spin crossover, and electrochemical reaction, resulting in the significant modulation of magnetic properties. Moreover, it is possible to control magnetic behavior through the modulation of ligands located in the first coordination sphere. We used polymerization reactions involving ligand exchange. Furthermore, because the modification of the second sphere is useful to control magnetic properties, we used guest exchange reactions in the magnetic coordination polymers. Although the metal center carrying spins is not directly modulated, it is possible to significantly control magnetic properties when magnetic materials have a bistable nature and possess nearly degenerated electronic states. We applied these techniques to molecular magnets, quantum magnets, paramagnetic mononuclear complexes, clusters, and one-dimensional (1-D) complexes.

This paper is organized as follows: switchable bulk magnets are first described. Next, phototunable quantum magnets are introduced. Finally, switchable paramagnetic mononuclear complexes, clusters, and 1-D complexes are described.

Bulk magnets

Electrochemical reaction.

Prussian blue analogs are redox active complexes (Fig. 1) with a redox labile property that makes these complexes suitable for use as electrochromic materials. The color in electrochromic materials can be switched between two or more colors by electrochemical reduction and oxidation. The color in Prussian blue can be reversibly switched between blue and colorless in the reaction of FeIII4[FeII(CN)6]3 (blue) + 4 K+ + 4e− ↔s K4FeII4[FeII(CN)6]3 (colorless). However, Prussian blue has ferromagnetic properties. Therefore, we proposed the development of electrochemically tunable magnets and demonstrated that the magnetic properties can be controlled by the electrochemical reaction, FeIII4[FeII(CN)6]3 (ferromagnet) + 4 K+ + 4e− ↔s K4FeII4[FeII(CN)6]3 (paramagnet).9) In particular, when the redox state is modulated in a high Tc (critical temperature) magnet, the change in magnetic properties can be observed at around room temperature. Indeed, Cr1.43[Cr(CN)6] exhibited a change in Tc between 240 and 100 K because of the redox reaction, CrIII–CN–CrII ↔s CrII–CN–CrII (Fig. 2).10) The magnetic properties could be directly controlled by the modulation of the oxidation state through electrochemistry by using the redox ability of the metal center and the zeolitic property of the Prussian blue framework.

Figure 1.

Figure 1.

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Structure of the unit cell of a Prussian blue analog (M and M′ denote metal ions). Random distribution of [M(CN)6]n− vacancies is omitted for clarity.

Figure 2.

Figure 2.

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Field-cooled magnetization versus temperature curves before electrochemical reduction (solid symbols, right axis) and after electrochemical reduction (open symbols, left axis). The material property can be switched from ferrimagnetism and paramagnetism between 100 and 240 K. We should note that although SI unit is recommended to be used in scientific journals, cgs unit “G” is used in this Fig. 2 and Fig. 3. This is because cgs unit is usually used in the field of molecular magnetism.

Photoinduced charge transfer.

We have reported a photochemically tunable magnet on the basis of an FeCo Prussian blue analog.11–14) At room temperature, the oxidation state in the complex having the formula A0.4Co1.3[Fe(CN)6]·nH2O (A = alkali cation) can be expressed as AI0.4CoII–HS1.3[FeIII(CN)6]·nH2O (HS = high spin).12) On cooling, this complex exhibits charge transfer that is coupled to spin crossover. The phase change can be expressed by FeIII–CN–CoII–HS (high temperature phase) ↔s FeII–CN–CoIII–LS (low temperature phase) (LS = low spin). This is a novel system that exhibits charge transfer coupled to spin transition. The driving force of this phase transition is entropy. It should be noted that the charge transfer coupled to spin transition is frequently called charge transfer induced spin transition (CTIST).

The phase transition temperature depends on the number of vacancies in [FeIII(CN)6]. In the case of NaCoII–HS[FeIII(CN)6] (Co/Fe = 1.0), Co is fully coordinated by the N atoms of the CN ligands. On the other hand, CoII–HS[FeIII(CN)6]2/3 (Co/Fe = 1.5) has a vacancy in the [FeIII(CN)6] sites. Hence, a Co ion in CoII–HS[FeIII(CN)6]2/3 is coordinated by 4 N atoms of CN ligands and 2O atoms of H2O ligands on an average. Therefore, the ligand field strength and energy level of Co ions depend on the number of vacancies. Indeed, the phase transition temperature decreases with the increase of the number of vacancies in [FeIII(CN)6] sites. In particular, CoII–HS[FeIII(CN)6]2/3 does not exhibit thermally induced charge transfer. The number of vacancies can be controlled by changing the Na+ concentration during the synthesis. Hence, we could control the phase transition temperature via a chemical method. In Fig. 3, the magnetic properties for Na0.07Co1.50Fe(CN)6·6.3H2O (Co/Fe = 1.50), Na0.37Co1.37Fe(CN)6·4.8H2O (Co/Fe = 1.37), and Na0.94Co1.15Fe(CN)6·3.0H2O (Co/Fe = 1.15) are depicted.

Figure 3.

Figure 3.

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(a) χMT versus T for Na0.07Co1.50Fe(CN)6·6.3H2O (Co/Fe = 1.50), Na0.37Co1.37Fe(CN)6·4.8H2O (Co/Fe = 1.37), and Na0.94Co1.15Fe(CN)6·3.0H2O (Co/Fe = 1.15) during cooling and warming at H = 5000 G. (b) Field-cooled magnetization curves for Na0.37Co1.37Fe(CN)6·4.8H2O (Co/Fe = 1.37) before (●) and after (○) light irradiation at H = 5 G.

FeCo Prussian blue has a charge transfer band from FeII to CoIII–LS at around 400–600 nm. When the FeCo Prussian blue analog is irradiated with 532-nm light, magnetic properties are switched between ferrimagnetism and paramagnetism. This is because charge transfer is induced by light, and the resultant metastable state could be trapped at low temperature. The photoinduced reaction for Na0.37Co1.37Fe(CN)6·4.8H2O is expressed as

FeII–CN–CoIII–LS→FeIII–CN–CoII–HS,

where Tc after irradiation is 26 K (Fig. 3). Before irradiation, Na0.37Co1.37Fe(CN)6·4.8H2O contains a large number of diamagnetic CoIII–LS(S = 0) and FeII(S = 0) moieties, which break the magnetic interaction between paramagnetic centers CoII–HS(S = 3/2). On the other hand, after irradiation, it consists of paramagnetic centers CoII–HS(S = 3/2, t2g5eg2) and FeIII(S = 1/2, t2g5eg0). The unpaired spins interact through the CN bridge. t2g in FeIII and t2g in CoII–HS typically give rise to antiferromagnetic interactions, while t2g in FeIII and eg in CoII–HS typically give rise to ferromagnetic interactions due to orthogonality. In the present case, the antiferromagnetic interaction is dominant, leading to the formation of a ferrimagnet after irradiation. When the metastable compound is heated, it recovers the original ground state at approximately 145 K.

Furthermore, photoinduced magnetization was also observed at around the charge transfer phase transition temperature.15) When the complex was irradiated within the hysteresis loop at just below the phase transition temperature from FeII–CN–CoIII–LS to FeIII–CN–CoII–HS and just above the phase transition temperature from FeIII–CN–CoII–HS to FeII–CN–CoIII–LS, transformation from FeII–CN–CoIII–LS to FeIII–CN–CoII–HS and FeIII–CN–CoII–HS to FeII–CN–CoIII–LS, respectively, could be induced with a pulsed laser (λ = 532 nm). In contrast to typical photomagnetic properties such as light-induced excited spin state trapping (LIESST) effect, this behavior could be observed even at around room temperature.

In addition, quenching effects called temperature induced excited spin state trapping (TIESST) were observed in the FeCo Prussian blue analog. When the FeCo sample was quickly introduced into the SQUID cavity at 5 K, the high temperature phase with the FeIII–CN–CoII–HS structure was trapped without exhibiting phase transition from FeIII–CN–CoII–HS to FeII–CN–CoIII–LS. This is because the sample is cooled rapidly down to 5 K before the phase change occurs. Furthermore, electric-field-induced conductance switching and reversible electrochemical redox reactions were observed.12,16) The electrochemical reaction in NaCl aqueous solution is expressed by FeII–CN–CoII ↔s FeIII–CN–CoII ↔s FeIII–CN–CoIII.

Polymerization reaction.

As described above, we could achieve the switching of magnetic properties by using electrochemical reactions and photo-induced metal-to-metal charge transfer. However, another technique called the solid-state polymerization reaction can be used to control the magnetic interaction. We recently revealed that the hexanuclear cluster demonstrates the solid-state reversible polymerization reaction. Because of the formation of the M–CN–M′ bridge (M and M′ denote metal ions) after the polymerization, long-range magnetic ordering was observed in the polymer structure.

Figure 4 shows the tunable hexanuclear Fe complex, [{FeIII(Tp)(CN)3}4{FeII(MeCN)(H2O)2}2]·10H2O·2MeCN (Tp = hydrotris(pyrazolyl)borate).17) The cluster has a free CN ligand that serves as a coordination donor site. When the sample was heated to 150 ℃, water molecules and coordinated CH3CN desorbed from the crystal, and the coordination reaction proceeded between CN in [FeIII(Tp)(CN)3]− and FeII in neighboring clusters. Single-crystal analysis shows the 1-D structure formed because of the polymerization reaction. Following this structural change, paramagnetic properties changed to metamagnetic ones because of the formation of magnetically coupled –NC–FeIII–CN–FeII– moieties. Furthermore, the original cluster was achiral, while after heating, it changed into a chiral one, indicating that chiral and achiral structures could be switched in this system.

Figure 4.

Figure 4.

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(a) Top: Side view of [{FeIII(Tp)(CN)3}4{FeII(MeCN)(H2O)2}2]·10H2O·2MeCN. Bottom: Side view of [FeIII(Tp)(CN)3]4FeII(H2O)2FeII with 1-D double zigzag chain structure after heating. (b) M versus H plots at 2 K for [{FeIII(Tp)(CN)3}4{FeII(MeCN)(H2O)2}2]·10H2O·2MeCN (■) and [FeIII(Tp)(CN)3]4FeII(H2O)2FeII (●).

Using this polymerization technique, various magnetic properties can be controlled. Indeed, we could prepare tunable porous magnets for FeCo hexanuclear systems, [{FeIII(Tp)(CN)3}4{CoII(MeCN)(H2O)2}2]·10H2O·2MeCN.18) As in the case of the above complex, the present FeCo complex showed reversible interconversion between clusters and 1-D polymers by heating and exposing the compound to solvent vapors, respectively. The structure after polymerization was microporous, having pores with the channel size of 1.9 Å × 3.6 Å. Gas adsorption measurements show that CO2 and H2 were absorbed in the pores while N2 was not. This result is consistent with a general correlation between the pore size and kinetic diameters of adsorbates. H2 and CO2 could be adsorbed because their kinetic diameters, 2.827–2.89 Å for H2 and 3.3 Å for CO2, are smaller than the effective channel size. Conversely, N2 could not be adsorbed because it has a kinetic diameter (3.64–3.80 Å) larger than the effective channel size. As in the case of the previous compounds, magnetic properties switched between paramagnetism and metamagnetism.

Guest exchange.

Prussian blue analogs are typical examples of porous coordination polymers, containing alkali cations and water molecules in interstitial sites. An important property of Prussian blue analogs is their cation exchange ability. We found that cation exchange in FeCo Prussian blue induces charge transfer between Fe and Co through CN in the framework of the porous coordination polymer.19) At room temperature, Na0.4CoII–HS1.3[FeIII(CN)6] consists of FeIII–CN–CoII–HS moieties. When this is dipped into the solution containing only K+ ions, cation exchange from Na+ to K+ ions is induced in the complex and is expressed as Na0.4CoII–HS1.3[FeIII(CN)6] + 0.4 K+ ↔s K0.4CoII–HS0.3CoIII–LS[FeII(CN)6] + 0.4Na+. The interaction between the Prussian blue framework and alkali cations induced the modulation of the electronic structure of the metal center, leading to cation-exchange-induced charge transfer. This suggests that magnetic properties can be controlled by guest ions.

Magnetic properties of the 1-D compound [FeTp(CN)3]2Co(bpe)·5H2O can also be controlled by guest molecules (Fig. 5).20) This complex exhibited reversible temperature-induced charge transfer between FeIII–CN–CoII–HS and FeII–CN–CoIII–LS at around 190 K. Furthermore, when the FeCo complex was irradiated, it exhibited charge transfer from FeII–CN–CoIII–LS to FeIII–CN–CoII–HS states. When the FeCo complex is heated, the dehydrated sample [FeTp(CN)3]2Co(bpe) was obtained. The dehydrated sample acquires the FeIII–CN–CoII–HS state throughout the entire temperature range of 5–300 K. Furthermore, the sample did not exhibit photoinduced magnetization. This indicates that the FeCo complex shows appreciable crystal-water-dependent magnetic and photomagnetic properties. Transformation between the two states involves changes in the switching of hydrogen bonds between uncoordinated water molecules and terminal cyanide nitrogen atoms in [FeIIITp(CN)3]−. When hydrogen bonds were removed, the redox potential of FeII shifted toward the negative potential, destabilizing FeII–CN–CoIII–LS pairs, while stabilizing FeIII–CN–CoII–HS pairs. Therefore, the hydrated compound exhibiting phase transition transformed into the dehydrated complex that does not exhibit metal-to-metal charge transfer.

Figure 5.

Figure 5.

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Temperature-dependent susceptibilities of [FeTp(CN)3]2Co(bpe) (line), and [FeTp(CN)3]2Co(bpe)·5H2O (□). (Inset) Side view of the double zigzag chain in [FeTp(CN)3]2Co(bpe)·5H2O along the c axis.

Quantum magnets

Recently, the study of quantum magnets has attracted considerable attention.21) We synthesized several clusters showing nanomagnetic properties. One of the complexes exhibited photofunctional properties.

The quantum magnets we synthesized were an FeII-based cyano-bridged linear trinuclear single molecule magnet (SMM), {[{FeII(L-N3O2)(H2O)}2CrIII(CN)6][ClO4]}·3H2O (L-N3O2 = azaoxa-macrocycle ligand),22) and a single chain magnet (SCM), [MnIII3O(Meppz)3(EtOH)4(OAc)] (Meppz = 3-(5-methyl-2-phenolate)pyrazolate).23) In addition, we have synthesized a quantum magnet with a wheel structure (Fig. 6).24) When [FeIII(bpmb)(CN)2] (H2bpmb = 1,2-bis(pyridine-2-carboxamido)-4-methylbenzene) and Mn molecules [MnIII(salen)]+ (salen2− = N,N′-ethylenebis(salicylideneaminato)dianion) are mixed, a CN-bridged dodecanuclear macrocyclic complex, [{MnIII(salen)}6{FeIII(bpmb)(CN)2}6]·7H2O (S = 15), was obtained. The ac magnetic susceptibility measurements exhibit frequency-dependent behavior below 4 K. Furthermore, the complex showed a magnetic hysteresis loop below 0.5 K.

Figure 6.

Figure 6.

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(a) Molecular structure of [{Mn(salen)}6{Fe(bpmb)(CN)2}6]·7H2O. (b) Hysteresis loop at different temperatures measured at a scan magnetic field speed of 0.14 T s−1.

Unfortunately, the aforementioned complexes did not exhibit photoswitching properties. However, more recently, we have succeeded in synthesizing phototunable SCM (antiferromagnetic phase) by using photoinduced metal-to-metal charge transfer (Fig. 7).25) In order to prepare a phototunable SCM, designing of 1-D magnets with negative anisotropy, strong intrachain magnetic interaction, and well-isolated structure between 1-D chains was required. Moreover, 1-D compounds were expected to exhibit charge transfer or spin crossover, which can be achieved by designing the appropriate energy level and ligand field of metal ions. On the basis of this strategy, we synthesized an FeCo complex, [Fe(bpy)(CN)4]2Co(4,4′-bipyridine)·4H2O, with neutral bimetallic layers. The FeCo complex exhibited thermally induced intramolecular charge transfer between FeIII–CN–CoII–HS and FeII–CN–CoIII–LS. Furthermore, on light irradiation at 5 K, charge transfer from FeII to CoIII was induced and a metastable FeIIICoII state was trapped. Variable-temperature ac susceptibility measurements revealed a significant frequency dependence of both in-phase (χ′) and out-of-phase components (χ″), as observed in other SCMs. The pre-exponential factor and the relaxation energy barrier were τ0 = 1.4 × 10−9 s and Δτ/kB = 29 K, respectively. A semicircular Cole–Cole diagram gave an α value of 0.34. In-phase ac susceptibility showed a frequency-independent peak at 3.8 K, which is due to weak interchain antiferromagnetic interactions. Hence, the magnetic properties after irradiation should be an antiferromagnetic phase of SCM.

Figure 7.

Figure 7.

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(a) Side view of the 1-D double zigzag chain of the FeCo complex. (b) Temperature dependence of the imaginary part of ac susceptibility after irradiation in zero dc field at varying ac frequency and a 3 Oe ac field.

Paramagnetic mononuclear complexes

In addition to magnets and nanomagnets, we have reported several tunable paramagnetic mononuclear complexes showing spin crossover, valence tautomerism, and changes in orbital angular momentum.

Spin crossover.

Metal complexes with a d4–d7 configuration can have two different spin states, high spin (HS) and low spin (LS).26) When the ligand field is situated in the critical region, the complex exhibits thermally induced spin transition; moreover, LIESST effects have been reported.27) Since the first observation of the LIESST effect, many LIESST complexes have been reported. However, the relaxation temperature is typically below ca. 80 K. In contrast, we have recently succeeded in observing a higher relaxation temperature of 130 K for the FeIIL(CN)2 complex (L–N3O2 = azaoxa-macrocycle ligand).28)

Furthermore, with respect to solid-state samples, only the FeII complex showed the LIESST effect to date, and it was believed that LIESST effects did not occur in FeIII complexes.26) However, we have succeeded in observing the LIESST effect in solid-state FeIII complexes, [Fe(pap)2]ClO4·H2O and [Fe(pap)2]PF6·MeOH (Hpap = 2-(2-pyridylmethyleneamino)phenol), as shown in Fig. 8.29,30) Magnetization measurements show that spin crossover occurs at approximately 165 K on cooling and at approximately 180 K on warming in [Fe(pap)2]ClO4·H2O.29) On illumination of ligand-to-metal charge transfer (LMCT) band in the FeIII complex (400 to 600 nm), Mössbauer spectra clearly show that FeIII transformed from a LS to HS state. This suggests that LIESST can be induced in an FeIII complex. The LIESST effects were also observed in several other FeIII complexes.31)

Figure 8.

Figure 8.

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Photomagnetic behavior: χMT of [Fe(pap)2]PF6·MeOH as a function of temperature on cooling (▽), after irradiated at 5 K and warming up (▲).30)

More recently, a spin crossover conductor, [FeIII(qsal)2][Ni(dmit)2]3·CH3CN·H2O,32) in which [Ni(dmit)2] (dmit2− = 1,3-dithiole-2-thione-4,5-dithiolato) is a potential conducting molecule and [Fe(qsal)2]+ (qsalH = N-(8-quinolyl)salicylaldimine) is a spin crossover complex,33) was reported by Takahashi. This indicated an anomaly in conductivity exists at the spin transition temperature of [FeIII(qsal)2]+. This anomaly is caused by the chemical pressure to the [Ni(dmit)2] layer, which is induced by the change in the molecular volume involving the spin transition of [Fe(qsal)2]+ in the crystal. Although several attempts have been made to develop spin crossover conductors, this is a first example in which synergy is exhibited between spin crossover and conductivity. [FeIII(qnal)2][Pd(dmit)2]5·acetone [qnalH = N-(8-quinolyl)-2-hydroxy-1-naphthaldimine] also showed synergistic effects between spin crossover and conductivity.34)

Valence tautomerism.

The observation of a long-lived metastable state for Fe LIESST complexes and FeCo Prussian blue analogs prompted us to explore new complexes showing a photoinduced long-lived metastable state using a different mechanism. We focused on valence tautomeric complexes,35,36) and found that several Co compounds show photoinduced magnetization effects because of the long-lived LMCT.37,38) Figure 9 shows the magnetic properties of a switchable complex, [CoIII–LS(3,5-DBSQ)(3,5-DBCat)(tmeda)] (3,5-DBSQ = 3,5-di-tert-butyl-1,2-semiquinonate; 3,5-DBCat = 3,5-di-tert-butyl-1,2-catecholate; tmeda = N,N,N′,N′-tetramethylethylenediamine).39) At high temperature, molecules adopt the HS CoII state and incorporate two semiquinone ligands [CoII–HS(3,5-DBSQ)2(tmeda)]. On lowering the temperature, an electron is transferred from Co to semiquinone, resulting in a CoIII–LS species with catechole and semiquinone ligands [CoIII–LS(3,5-DBSQ)(3,5-DBCat)(tmeda)]. Furthermore, on irradiation to excite the LMCT band, magnetization at 5 K increases significantly. This suggests that the following reaction has occurred: [CoIII–LS(3,5-DBSQ)(3,5-DBCat)(tmeda)] ↔s [CoII–HS(3,5-DBSQ)2(tmeda)]. The change in magnetization could be maintained below ca. 50 K. Similar photoeffects were also observed in many other Co-valence tautomeric compounds.38) The relaxation behavior is the Arrhenius type at high temperature, while it is temperature independent at low temperature, indicating that relaxation is a tunneling process.

Figure 9.

Figure 9.

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μeff versus T plots before and after illumination. Inset: changes in the magnetization at 5 K. Variables hν and Δ represent illumination at 5 K and thermal treatment at 60 K, respectively.

Orbital angular momentum.

Control of magnetic properties can be achieved by changing spin multiplicity. The change in spin multiplicity is realized by inducing charge transfer and spin crossover. However, magnetic susceptibility is also affected by orbital angular momentum. Hence, the bistability of magnetization can be achieved by changing the contribution of the orbital angular momentum term. However, the control of magnetic properties through the modulation of orbital angular momentum has been less investigated thus far. It is well known that the quenching of orbital angular momentum depends on the ligand field and hence on the structure around metal ions. Hence, in order to realize thermally induced change in orbital angular momentum, we synthesized a Co complex, [CoII(NO3)2(L′)] (L′ = 2,6-di(pyrazol-1-yl)pyrazine), incorporating NO3 ligands.40) The octahedral HS CoII complex has typically unquenched orbital angular momentum. Furthermore, NO3 is a flexible ligand; therefore, we anticipated that the temperature-dependent structural change would be induced in the complex. Indeed, this complex exhibited structural phase transition at approximately 235 K (Fig. 10). As a result, the change in magnetization accompanying the hysteresis loop was clearly observed. Because spin multiplicity does not change, the switching in magnetization can be achieved from the change in orbital contribution, which is induced by the modification of the structure around CoII–HS ions. This conclusion is supported by density functional theory calculation.

Figure 10.

Figure 10.

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(a) Splitting of d orbitals of high- and low-temperature phases. (b) Temperature dependence of χMT in cooling (○) and heating mode (●).

Clusters

More recently, we have focused on the development of nanomaterials because, in order to apply these materials to a high-density recording medium, the bulk materials must be divided into nanometer-scale pieces. Therefore, it is preferable that clusters are synthesized on a nanometer scale from the beginning. Indeed, we synthesized several nanosized clusters that exhibit photofunctional properties.

Tetranuclear spin crossover.

We have synthesized a spin-crossover grid, [Fe4(HL1)4]·(BF4)4·(H2O)2·CH3OH, showing synergy between spin crossover and the magnetic interaction (Fig. 11).41) The magnetic property exhibits a two-step decrease in the magnetization value. On cooling from room temperature, spin crossover occurs from the FeII–HS4 to FeII–HS2FeII–LS2 state at around 170 K. These two FeII–HS have a cis-type arrangement in the cluster. The formation of two HS and LS states results from the presence of intracluster anticooperative interactions. On further cooling, the χMT value approaches 0 cm3 mol−1 K with decreasing temperature because of the presence of antiferromagnetic interactions between two FeII–HS sites. When this complex is irradiated at 5 K by 532-nm light to excite the metal to ligand charge transfer (MLCT) band, only a small increase was observed in magnetization. This is due to the operation of the antiferromagnetic interaction in the cluster. The ground state with an FeII–HS2FeII–LS2 structure before irradiation has S = 0 because of the antiferromagnetic interaction between two FeII–HS ions. Furthermore, the ground state after irradiation also has S = 0 because of the antiferromagnetic interaction between neighboring FeII ions in the FeII–HS4 state. Hence, only a small increase in magnetization was observed at low temperatures although two HS species were generated after irradiation. This demonstrated that the grid is a spin-crossover cluster in which the magnetic interaction operates between FeII–HS sites, which is distinguishable from typical spin crossover complexes.

Figure 11.

Figure 11.

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Temperature dependence of χMT–T plots and photoinduced changes after irradiation at 532 nm (inset).

Dinuclear valence tautomerism.

We have succeeded in preparing a dinuclear Co complex, [{CoIII–LS(tpa)}(dhbq){CoIII–LS(tpa)}]·(PF6)3 (tpa = tris(2-pyridylmethyl)amine, dhbq = deprotonated 2,5-dihydroxy-1,4-benzoquinone), which exhibits room temperature hysteresis and photomagnetization effects (Fig. 12).42) The phase transition temperatures are Tc↓ = 297 K and Tc↑ = 310 K with a hysteresis width of 13 K. When the LMCT band around the visible region was irradiated at 5 K, charge transfer from (dhbq)3− to CoIII was induced. The photoprocess that occurred by photoirradiation can be expressed by [{CoIII–LS(tpa)}(dhbq)3−{CoIII–LS(tpa)}]·(PF6)3 ↔s [{CoIII–LS(tpa)}(dhbq)2−{CoII–HS(tpa)}]·(PF6)3. The change could be reversibly induced.

Figure 12.

Figure 12.

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Temperature dependence of χMT–T plots and photoinduced changes after irradiation at 532 nm (inset).

Structural isomerism.

Molecule-based magnets are usually composed of metal ions with spin and bridging ligands mediating the magnetic interaction between metal ions. Hence, when the electronic properties of the bridging ligand change, the magnetic interaction between metal ions can be switched. On the basis of this, we constructed a photoswitchable spin device, NiII[FeII(CN)5NO].43,44) In the present compound, nickel nitroprusside, [Fe(CN)5NO]2−, can be regarded as photofunctional bridging complex ligands. It has been reported that when the MLCT band in the visible region in Na2[FeII(CN)5NO] is excited, the coordination structure of the nitrosyl group can be modified. Hence, we can expect that the magnetic interaction between NiII ions through [Fe(CN)5NO]2− can be modulated by light. In fact, magnetic coupling in NiII[FeII(CN)5NO] could be switched by the excitation of the MLCT band. Photoirradiation finally results in the formation of magnetic clusters with S = 5 in the compound.

One-dimensional compounds

Spin crossover.

Only a few examples of magnetically coupled phototunable 1-D compounds have been reported, including one magnetically coupled spin crossover compound. We reported a 1-D LIESST compound, [FeII(L)(CN)2][MnII(hfac)2] (L-N3O2 = azaoxa macrocycle ligand; hfac = (1,1,1,5,5,5)-hexafluoroacetylacetonate), having the –NC–FeII–CN–MnII– heterometallic structure.45) The temperature dependence of magnetization showed that on cooling, spin crossover occurs at 110 K, forming 50% FeII–HS and 50% FeII–LS at low temperature. When the sample was illuminated at 5 K, a LIESST effect was observed.

Charge transfer.

Another example is the homochiral MoIVCuII 1-D compound, [CuII(tren)][CuII(33)][MoIV(CN)8]·4H2O (tren = tris(2-aminoethyl)amine; 33 = bis(3-aminopropyl)amine).46) This compound exhibits spontaneous resolution from achiral components and crystallizes into the chiral space group P21. On light irradiation, the complexes exhibit reversible photoinduced magnetic properties because of intramolecular charge transfer between MoIV and CuII centers. After irradiation, a ferromagnetic interaction occurs between CuII (S = 1/2) and photogenerated MoV (S = 1/2) centers.

Conclusions

We reported electrochemically switchable molecular magnets. The direct switching of the oxidation state induced a shift in Tc of the magnets. Furthermore, several compounds showing photoinduced magnetization effects were successfully synthesized, including an FeCo Prussian blue analog, which is the first example of a molecular photomagnet. Furthermore, the 1-D FeCo complex with uniaxial anisotropy exhibited slow magnetization relaxation after irradiation; an antiferromagnetic phase of SCM. Magnetic properties can also be successfully controlled by using polymerization reactions and guest exchange reactions. A considerable change in magnetic properties could be observed even through the modification of the second sphere of the complex. Moreover, we developed FeIII LIESST complexes, phototunable Co-valence tautomeric complexes, and a Co complex exhibiting the change in orbital angular momentum. Furthermore, we extended our work to phototunable clusters and 1-D complexes.

An interesting aspect of our work is the synergy of various properties, in particular the coupling of light and spin. On the other hand, spin electronics have recently attracted considerable attention. Therefore, one challenging issue in molecular magnetism would be the development of a new interdisciplinary field of optospinics and spin electronics. Furthermore, it would be interesting to develop a new field in which light, spin, and polarization show synergistic effects. Indeed, such systems are already being developed with oxides; however, their development is still challenging in terms of molecular compounds. Through the works reported herein, it is expected that new compounds, concepts, fields, and applications will be developed.

Acknowledgements

The author acknowledges all co-workers who contributed to above works, in particular Profs. Akira Fujishima of Tokyo University of Science, Kazuhito Hashimoto of the University of Tokyo, Tomokazu Iyoda of Tokyo Institute of Technology, and Ryo Baba of Tokyo University of Marine Science and Technology.

Profile

Osamu Sato received his Ph.D. in 1994 from the University of Tokyo under the direction of Professor Akira Fujishima. He began his academic career in 1994 as a researcher with the Photochemical Conversion Materials Project at the Kanagawa Academy of Science and Technology (KAST). In 1998, he became the director of the Special Research Laboratory for Optical Science at KAST. Since 2005, he has been a Professor at the Institute for Materials Chemistry and Engineering, Kyushu University. His research interests include the development of phototunable molecular magnets, nanomagnets, spin crossover complexes, and valence tautomeric compounds. He was awarded the Chemical Society of Japan Award for Young Chemists in 1998, the Japanese Photochemistry Association Award in 2007, and the JSPS Prize from the Japan Society for the Promotion of Science in 2009. graphic file with name pjab-88-213-p001.gif

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