Photoswitchable Nonlinear-Optical Crystal Based on a Dysprosium-Iron Nitrosyl Metal Assembly.

Nitrosyl metal complexes (M-NO), in which nitrosyl ligands are coordinated to transition-metal ions, have been studied from the viewpoints of physiological activity, catalytic activity, and photosensitivity. The structural flexibility and electric polarization of the nitrosyl ligand are attractive characteristics. Herein we show a photoswitchable nonlinear-optical (NLO) crystal based on a dysprosium-iron nitrosyl assembly. This crystal is composed of a one-dimensional chain structure in the polar Pna21 space group. Because of spontaneous electric polarization, it exhibits a NLO effect of second harmonic generation (SHG). The SHG signal reversibly changes by alternate irradiation with 473 and 804 nm laser lights. The observed photoreversible switching effect on SHG is caused by photoinduced linkage isomerization of the metal nitrosyl sites, i.e., M-N+═O ↔ M-O═N+. Such an optically switchable NLO crystal should be useful for optical devices such as optical filters and optical shutters as well as probes in SHG microscopy.

Magnetic Properties and Second Harmonic Generation of Noncentrosymmetric Cyanido-Bridged Ln(III)-W(V) Assemblies.

One-dimensional zigzag cyanido-bridged coordination polymers have been prepared as a result of self-assembly of lanthanide(III) ions with octacyanidotungstate(V) anions in the presence of N,N-dimethylacetamide (dma). All compounds crystallized in noncentrosymmetric space group P21 with a molecular formula of [LnIII(dma)5][WV(CN)8] [Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5), Tm (6), Yb (7), Lu (8), or Y (9)]. Magnetic studies revealed weak antiferromagnetic interactions through LnIII-NC-WV bridges and the formation of ferrimagnetically coupled chains at very low temperatures. Moreover, temperature dependencies of magnetic susceptibilities were fitted using the crystal field parameters for Ln(III) ions, determined by the ab initio calculations, yielding magnetic coupling constants in the range of -1 to -5 cm-1. The wide optical transparency of 1-9 has been determined using solid state absorption spectroscopy. Samples exhibited second harmonic (SH) generation properties with SH susceptibilities ranging from 4.7 × 10-12 to 9.4 × 10-11 esu due to the presence of nonlinear optical susceptibility tensor elements (χijk) χzxx, χzyy, χzzz, χzxy, χyyz, χyzx, χxyz, and χxzx, corresponding to space group P21. The determined values were also compared with the results of theoretical calculations and previous reports, indicating a potential relationship between the type of lanthanide ion and the SH intensity.

Exploring Ultrafast Photoswitching Pathways in RbMnFe Prussian Blue Analogue.

We study by femtosecond optical pump-probe spectroscopy the photoinduced charge transfer (CT) in the RbMnFe Prussian blue analogue. Previous studies evidenced the local nature of the photoinduced MnIII FeII → MnII FeIII process, occurring within less than 1 ps. Here we show experimentally that two photoswitching pathways exist, depending on the excitation pump wavelength, which is confirmed by band structure calculations. Photoexcitation of α spins corresponds to the Mn(d-d) band, which drives reverse Jahn-Teller distortion through the population of antibonding Mn-N orbitals, and induces CT within ≈190 fs. The process launches coherent lattice torsion during the self-trapping of the CT small-polaron. Photoexcitation of ß spins drives intervalence Fe→Mn CT towards non-bonding states and results in a slower dynamic.

Rapid Faraday Rotation on ε-Iron Oxide Magnetic Nanoparticles by Visible and Terahertz Pulsed Light.

Light- or electromagnetic wave-responsive magnetism is an attractive issue in spin chemistry and optical materials science. Herein we show the magnetization reversal induced by visible-light pulsed laser and the ultrafast dynamic magnetooptical effect caused by terahertz (THz) pulsed laser irradiation onto chemically synthesized magnetic films based on gallium-titanium-cobalt-substituted ε-Fe2O3 (GTC-ε-Fe2O3) and ε-Fe2O3 nanoparticles. Visible-light pulsed laser irradiation switches the sign of the Faraday effect in GTC-ε-Fe2O3 films. On the other hand, irradiating the ε-Fe2O3 film with pulsed THz light induces an ultrafast Faraday rotation in an extremely short time of 400 fs. The time evolution dynamics of these ultrafast magnetooptical effects are theoretically demonstrated by stochastic Landau-Lifshitz-Gilbert calculations of a nanoparticle model that considers all motions of the individual spins. These ε-iron oxide magnetic nanomaterials are expected to contribute to high-density magnetic memory media or high-speed operation circuit magnetic devices.

Humidity-Induced Switching between Two Magnetic and Structural Phases in a CoII -[WV (CN)8 ] Molecular Magnet.

The self-assembly of cobalt(II) with purine and octacyanidotungstate(V) results in the formation of the three-dimensional Co3 [W(CN)8 ]2 (purine)2 ⋅8.5H2 O (1) coordination polymer. This compound exhibits humidity-induced variation of the number of water molecules of crystallisation leading to a reversible structural phase transition and the alternation of the long-range ferromagnetic ordering temperature from TC =29 K for the pristine assembly (1) to TC =49 K for the sample stored in a low-humidity atmosphere (1-deh). This phenomenon can be attributed to a reversible change in the hydrogen-bonding network resulting in the modification of the local geometries of cobalt(II) as well as the cyanido bridges.

In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4- (M = Mo, Nb) Cyanido-Bridged Frameworks.

We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different FeII complexes into a single coordination framework. The 3-acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled FeII-[MIV(CN)8]4- (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {FeII2(3-OAcpy)5(3-OHpy)3[MIV(CN)8]}· nH2O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe3M2} n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)4(µ-NC)2], [Fe2(3-OHpy)4(µ-NC)2], and [Fe3(3-OAcpy)3(3-OHpy)(µ-NC)2], which lead to the thermal two-step FeII SCO, as proven by X-ray diffraction, magnetic susceptibility, UV-vis-NIR optical absorption, and 57Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150-170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50-140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep FeII SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

Large Coercive Field of 45 kOe in a Magnetic Film Based on Metal-Substituted ε-Iron Oxide.

Magnetic ferrites are stable, sustainable, and economical. Consequently, they have been used in various fields. The development of large coercive field (large Hc) magnetic ferrites is a very important but challenging issue to accelerate the spread of use and to expand practical applications. In this study, we prepared a rhodium-substituted ε-iron oxide film and observed a remarkably large Hc value of 35 kOe at room temperature. This is the largest value among magnetic ferrites to date. Such a large-Hc ferrite is expected to greatly expand the application of magnetic ferrites. Furthermore, when the temperature dependence of the magnetic properties was measured, an even larger Hc value of 45 kOe was recorded at 200 K. Such large Hc values are much larger than those of conventional hard magnetic ferrites.

Multimetal-Substituted Epsilon-Iron Oxide ϵ-Ga0.31 Ti0.05 Co0.05 Fe1.59 O3 for Next-Generation Magnetic Recording Tape in the Big-Data Era.

From the viewpoints of large capacity, long-term guarantee, and low cost, interest in magnetic recording tapes has undergone a revival as an archive storage media for big data. Herein, we prepared a new series of metal-substituted ϵ-Fe2 O3 , ϵ-Ga(III) 0.31 Ti(IV) 0.05 Co(II) 0.05 Fe(III) 1.59 O3 , nanoparticles with an average size of 18 nm. Ga, Ti, and Co cations tune the magnetic properties of ϵ-Fe2 O3 to the specifications demanded for a magnetic recording tape. The coercive field was tuned to 2.7 kOe by introduction of single-ion anisotropy on Co(II) (S=3/2) along the c-axis. The saturation magnetization was increased by 44 % with Ga(III) (S=0) and Ti(IV) (S=0) substitution through the enhancement of positive sublattice magnetizations. The magnetic tape media was fabricated using an actual production line and showed a very sharp signal response and a remarkably high signal-to-noise ratio compared to the currently used magnetic tape.

Selective synthesis of Co8S15 cluster in bowl-shaped template of the pentaaryl[60]fullerene ligand.

A cobalt-sulfur cluster Co8S15 with molecular formula of (C60Ar5)-Co8S15(n)Bu2-(C60Ar5) (Ar = 4-(t)BuC6H4) was selectively synthesized from a pentaaryl[60]fullerene cobalt trisulfide complex, (η(5)-C60Ar5)CoS3. The bowl-shaped steric templating fullerene ligand, as well as the metastable cobalt trisulfide moiety, played important roles in the exclusive cluster formation. X-ray crystallography, as well as electrochemical, time-resolved photophysical, and magnetic, measurements revealed a mixed-valence cluster with six cobalt(III) and two high-spin cobalt(II) centers showing reversible redox behavior and photoinduced charge separation.

Mechanical Exfoliation of Multilayer Pseudohalogen-Bridged Nanosheets.

The development of nanolayered materials is one of the greatest challenges in nanoscience. Until now, pseudohalogen-bridged nanosheets using the mechanical exfoliation method have not been reported. A state-of-the-art material, {[FeII(3-acetylpyridine)2][HgII(µ-SCN)4]}n (1), has been developed to achieve the goal. The compound forms a two-dimensional (2D) coordination polymer with weak out-of-plane van der Waals interactions and has an intrinsic tendency to form shear planes perpendicular to the crystallographic c-direction. These structural features predispose 1 to mechanical exfoliation realized by employing the "Scotch-tape method". As a result, nanosheets were fabricated and characterized by digital optical microscopy, scanning electron microscopy, and atomic force microscopy. The nanosheets were found to have a minimum thickness of ∼15 nm and a lateral size of several micrometers. As the first example of thiocyanato-bridged coordination nanosheets, these materials extend the scope of 2D materials and potentially pave the way toward developing nanolayered materials.

Giant adiabatic temperature change and its direct measurement of a barocaloric effect in a charge-transfer solid.

Solid refrigerants exhibiting a caloric effect upon applying external stimuli are receiving attention as one of the next-generation refrigeration technologies. Herein, we report a new inorganic refrigerant, rubidium cyano-bridged manganese-iron-cobalt ternary metal assembly (cyano-RbMnFeCo). Cyano-RbMnFeCo shows a reversible barocaloric effect with large reversible adiabatic temperature changes of 74 K (from 57 °C to -17 °C) at 340 MPa, and 85 K (from 88 °C to 3 °C) at 560 MPa. Such large reversible adiabatic temperature changes have yet to be reported among caloric effects in solid-solid phase transition refrigerants. The reversible refrigerant capacity is 26000 J kg-1 and the temperature window is 142 K. Additionally, cyano-RbMnFeCo shows barocaloric effects even at low pressures, e.g., reversible adiabatic temperature change is 21 K at 90 MPa. Furthermore, direct measurement of the temperature change using a thermocouple shows +44 K by applying pressure. The temperature increase and decrease upon pressure application and release are repeated over 100 cycles without any degradation of the performance. This material series also possesses a high thermal conductivity value of 20.4 W m-1 K-1. The present barocaloric material may realize a high-efficiency solid refrigerant.

Conjunction of chirality and slow magnetic relaxation in the supramolecular network constructed of crossed cyano-bridged Co(II)-W(V) molecular chains.

The addition of chiral 2,2'-(2,6-pyridinediyl)bis(4-isopropyl-2-oxazoline) (iPr-Pybox) to a self-assembled Co(II)­[W(V)(CN)(8)] magnetic system gives two enantiomorphic cyano-bridged chains, {[Co(II)((S,S)-iPr-Pybox)(MeOH)](3)[W(V)(CN)(8)](2)·5.5MeOH·0.5H(2)O}(n) (1-SS) and {[Co(II)((R,R)-iPr-Pybox) (MeOH)](3)[W(V)(CN)(8)](2)·5.5MeOH·0.5H(2)O}(n) (1-RR). Both compounds crystallize with a structure containing a unique crossed arrangement of one-dimensional chains that form a microporous supramolecular network with large channels (14.9 Å × 15.1 Å × 15.3 Å) filled with methanol. The investigated materials exhibited optical chirality, as confirmed by natural circular dichroism and UV-vis absorption spectra. 1-(SS) and 1-(RR) are paramagnets with cyano-mediated Co(II)-W(V) magnetic couplings that lead to a specific spin arrangement with half of the W(V) ions coupled ferromagnetically with their Co(II) neighbors and the other half coupled antiferromagnetically. Significant magnetic anisotropy with the easy axis along the [101] direction was confirmed by single-crystal magnetic studies and can be explained by the single-ion anisotropy of elongated octahedral Co(II) sites. Below 3 K, the frequency-dependent χ(M)"(T) signal indicated slow magnetic relaxation characteristic of single-chain magnets.

Second harmonic generation on chiral cyanido-bridged FeII-NbIV spin-crossover complexes.

Incorporating chiral organic ligands into cyanido-bridged FeII-NbIV assemblies synthesized chiral spin-crossover complexes, FeII2[NbIV(CN)8](L)8·6H2O (L = R-, S-, or rac-1-(3-pyridyl)ethanol: R-FeNb, S-FeNb, or rac-FeNb). Rietveld analyses based on a racemic complex of rac-FeNb indicate that the chiral complexes have a cubic crystal structure in the I213 space group with a three-dimensional cyanido-bridged FeII-NbIV coordination network. All the complexes exhibit spin crossover between the high-spin (HS) and the low-spin (LS) FeII states without thermal hysteresis. Chiral complexes of R-FeNb and S-FeNb show second harmonic generation (SHG) due to their non-centrosymmetric structure. The I213 space group provides second-order susceptibility tensor elements of χxyz, χyzx, and χzxy, which contribute to SHG. The temperature-dependent second harmonic light intensity change is due to spin crossover between FeIIHS and FeIILS.

High proton conductivity in prussian blue analogues and the interference effect by magnetic ordering.

We observed high proton conductivities of 1.2 x 10(-3) and 1.6 x 10(-3) S cm(-1) on Co[Cr(CN)(6)](2/3).zH(2)O and V[Cr(CN)(6)](2/3).zH(2)O, respectively, and an interference effect between magnetic ordering and ionic conduction below the magnetic phase transition temperature.

Poly[aqua-hexa-benzimidazole-octa-µ-cyanido-octa-cyanidotricopper(II)ditungstate(V)].

In the polymeric title compound, [Cu(3)W(2)(CN)(16)(C(7)H(6)N(2))(6)(H(2)O)](n), the coordination geometry of the W(V) atom is eight-coordinate dodeca-hedral, where four CN groups of [W(CN)(8)] are bridged to Cu(II) ions, and the other four CN groups are not bridged. The coordination geometries of the Cu(II) ions are five-coordinate pseudo-square-based pyramidal. There are two distinct Cu sites, which build and link the cyanido-bridged Cu-W ladder chains. Successive connections lead to the formation of a two-dimensional network. The H atoms of a coordinated water molecule and the imino groups form hydrogen bonds to the N atoms of non-bridged CN groups.

Extremely low-frequency phonon material and its temperature- and photo-induced switching effects.

Atomic vibrations due to stretching or bending modes cause optical phonon modes in the solid phase. These optical phonon modes typically lie in the frequency range of 102 to 104 cm-1. How much can the frequency of optical phonon modes be lowered? Herein we show an extremely low-frequency optical phonon mode of 19 cm-1 (0.58 THz) in a Rb-intercalated two-dimensional cyanide-bridged Co-W bimetal assembly. This ultralow frequency is attributed to a millefeuille-like structure where Rb ions are very softly sandwiched between the two-dimensional metal-organic framework, and the Rb ions slowly vibrate between the layers. Furthermore, we demonstrate temperature-induced and photo-induced switching of this low-frequency phonon mode. Such an external-stimulation-controllable sub-terahertz (sub-THz) phonon crystal, which has not been reported before, should be useful in devices and absorbers for high-speed wireless communications such as beyond 5G or THz communication systems.

A photoswitchable polar crystal that exhibits superionic conduction.

Ionic conductors serve as solid electrolytes for fuel cells and batteries, whereas polar crystals such as ferroelectrics and pyroelectrics-which are typically insulating materials-are used in electronic devices. Here we show a material that combines superionic conductivity with a polar crystal structure at room temperature. This three-dimensional anionic network is based on -Fe-N≡C-Mo- units, with Cs cations hosted in every other pore. In the resulting Cs1.1Fe0.95[Mo(CN)5(NO)]·4H2O material, the negative and positive charges of the framework and Cs+ ions, respectively, are non-symmetrically shifted in the c-axis direction of the unit cell, and spontaneous electric polarization is generated, in turn leading to second harmonic generation (SHG). Additionally, this material is a superionic conductor (with an ionic conductivity value of 4 × 10-3 S cm-1 at 318 K). Furthermore, the ionic conductivity significantly decreases under 532 nm light irradiation (from 1 × 10-3 S cm-1 to 6 × 10-5 S cm-1 at room temperature) and, when irradiation stops, returns to its original value within ~1 h.

Sigmoidally hydrochromic molecular porous crystal with rotatable dendrons.

Vapochromic behaviour of porous crystals is beneficial for facile and rapid detection of gaseous molecules without electricity. Toward this end, tailored molecular designs have been established for metal-organic, covalent-bonded and hydrogen-bonded frameworks. Here, we explore the hydrochromic chemistry of a van der Waals (VDW) porous crystal. The VDW porous crystal VPC-1 is formed from a novel aromatic dendrimer having a dibenzophenazine core and multibranched carbazole dendrons. Although the constituent molecules are connected via VDW forces, VPC-1 maintains its structural integrity even after desolvation. VPC-1 exhibits reversible colour changes upon uptake/release of water molecules due to the charge transfer character of the constituent dendrimer. Detailed structural analyses reveal that the outermost carbazole units alone are mobile in the crystal and twist simultaneously in response to water vapour. Thermodynamic analysis suggests that the sigmoidal water sorption is induced by the affinity alternation of the pore surface from hydrophobic to hydrophilic.

Magnetic-Pole Flip by Millimeter Wave.

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, "focused-millimeter-wave-assisted magnetic recording (F-MIMR)," is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau-Lifshitz-Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.

Vanadium octacyanoniobate-based magnet with a Curie temperature of 138 K.

In this work, we prepared a three-dimensional vanadium octacyanoniobate-based magnet, K(0.10)V(II)(0.54)V(III)(1.24)[Nb(IV)(CN)(8)].(SO(4))(0.45).6.8H(2)O. This compound exhibits ferrimagnetism with a Curie temperature of 138 K, in which the sublattice magnetizations of V(II) (S = (3)/(2)) and V(III) (S = 1) are antiparallelly ordered to that of Nb(IV) (S = (1)/(2)). The estimated superexchange interaction constants of V(II)-Nb(IV) and V(III)-Nb(IV) are -51 and -25 cm(-1), respectively.