Manipulating electron redistribution to achieve electronic pyroelectricity in molecular [FeCo] crystals.

Pyroelectricity plays a crucial role in modern sensors and energy conversion devices. However, obtaining materials with large and nearly constant pyroelectric coefficients over a wide temperature range for practical uses remains a formidable challenge. Attempting to discover a solution to this obstacle, we combined molecular design of labile electronic structure with the crystal engineering of the molecular orientation in lattice. This combination results in electronic pyroelectricity of purely molecular origin. Here, we report a polar crystal of an [FeCo] dinuclear complex exhibiting a peculiar pyroelectric behavior (a substantial sharp pyroelectric current peak and an unusual continuous pyroelectric current at higher temperatures) which is caused by a combination of Fe spin crossover (SCO) and electron transfer between the high-spin Fe ion and redox-active ligand, namely valence tautomerism (VT). As a result, temperature dependence of the pyroelectric behavior reported here is opposite from conventional ferroelectrics and originates from a transition between three distinct electronic structures. The obtained pyroelectric coefficient is comparable to that of polyvinylidene difluoride at room temperature.

Transparent Ion-Exchange Membrane Exhibiting Intense Emission under a Specific pH Condition Based on Polypyridyl Ruthenium(II) Complex with Two Imidazophenanthroline Groups.

The development and the photophysical behavior of a transparent ion-exchange membrane based on a pH-sensitive polypyridyl ruthenium(II) complex, [(bpy)2RuII(H2bpib)RuII(bpy)2](ClO4)4 (bpy = 2,2'-bipyridine, H2bpib = 1,4-bis([1,10]phenanthroline[5,6-d]-imidazol-2-yl)benzene), are experimentally and theoretically reported. The emission spectra of [(bpy)2RuII(H2bpib)RuII(bpy)2]@Nafion film were observed between pH 2 and pH 11 and showed the highest relative emission intensity at pH 5 (λmaxem = 594.4 nm). The relative emission intensity of the film significantly decreased down to 75% at pH 2 and 11 compared to that of pH 5. The quantum yields (Φ) and lifetimes (τ) showed similar correlations with respect to pH, Φ = 0.13 and τ = 1237 ns at pH 5, and Φ = 0.087 and τ = 1014 ns and Φ = 0.069 and τ = 954 ns at pH 2 and pH 11, respectively. These photophysical data are overall considerably superior to those of the solution, with the radiative- (kr) and non-radiative rate constants (knr) at pH 5 estimated to be kr = 1.06 × 105 s-1 and knr = 7.03 × 105 s-1. Density functional theory calculations suggested the contribution of ligand-to-ligand- and intraligand charge transfer to the imidazolium moiety in Ru-H3bpib species, implying that the positive charge on the H3bpib ligand works as a quencher. The Ru-Hbpib species seems to enhance non-radiative deactivation by reducing the energy of the upper-lying metal-centered excited state. These would be responsible for the pH-dependent "off-on-off" emission behavior.

Three-Step Spin State Transition and Hysteretic Proton Transfer in the Crystal of an Iron(II) Hydrazone Complex.

A proton-electron coupling system, exhibiting unique bistability or multistability of the protonated state, is an attractive target for developing new switchable materials based on proton dynamics. Herein, we present an iron(II) hydrazone crystalline compound, which displays the stepwise transition and bistability of proton transfer at the crystal level. These phenomena are realized through the coupling with spin transition. Although the multi-step transition with hysteresis has been observed in various systems, the corresponding behavior of proton transfer has not been reported in crystalline systems; thus, the described iron(II) complex is the first example. Furthermore, because proton transfer occurs only in one of the two ligands and π electrons redistribute in it, the dipole moment of the iron(II) complexes changes with the proton transfer, wherein the total dipole moment in the crystal was canceled out owing to the antiferroelectric-like arrangement.

Temperature dependence of spherical electron transfer in a nanosized [Fe14] complex.

The study of transition metal clusters exhibiting fast electron hopping or delocalization remains challenging, because intermetallic communications mediated through bridging ligands are normally weak. Herein, we report the synthesis of a nanosized complex, [Fe(Tp)(CN)3]8[Fe(H2O)(DMSO)]6 (abbreviated as [Fe14], Tp-, hydrotris(pyrazolyl)borate; DMSO, dimethyl sulfoxide), which has a fluctuating valence due to two mobile d-electrons in its atomic layer shell. The rate of electron transfer of [Fe14] complex demonstrates the Arrhenius-type temperature dependence in the nanosized spheric surface, wherein high-spin centers are ferromagnetically coupled, producing an S = 14 ground state. The electron-hopping rate at room temperature is faster than the time scale of Mössbauer measurements (<~10-8 s). Partial reduction of N-terminal high spin FeIII sites and electron mediation ability of CN ligands lead to the observation of both an extensive electron transfer and magnetic coupling properties in a precisely atomic layered shell structure of a nanosized [Fe14] complex.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex.

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Structural Insight into Order-Disorder Transition and Charge-Transfer Phase Transition in an Iron Mixed-Valence Complex (n-C3H7)4N[FeIIFeIII(dto)3] with a Two-Dimensional Honeycomb Network.

The structural properties of the iron mixed-valence complex ( n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato, C2O2S2) have been investigated by single-crystal X-ray diffraction (SCXRD) at low temperatures. ( n-C3H7)4N[FeIIFeIII(dto)3] has two-dimensional (2D) honeycomb layers consisting of alternating FeII and FeIII arrays bonded by bis-bidentate dithiooxalato ligands. Upon cooling, a superlattice structure with q = (1/3, 1/3, 0) was observed below 260 K, which corresponds to an order-disorder transition of the ( n-C3H7)4N+ ions between the honeycomb layers. The charge-transfer phase transition (CTPT) occurs at TC↑1/2 ∼ 120 K and TC↓1/2 ∼ 90 K upon heating and cooling, respectively, with an electron transfer between the FeII and FeIII ions, accompanied by a spin-state change, FeII ( S = 2; HS)-O2C2S2-FeIII ( S = 1/2; LS) ↔ FeIII ( S = 5/2; HS)-O2C2S2-FeII ( S = 0; LS). During the CTPT, the intersheet [FeIIFeIII(dto)3] distance decreased monotonously upon cooling, and an abrupt structural contraction was observed in the hexagonal 2D network. The volume contraction during the CTPT was quite small (∼0.7%), and differences in the structural distortions at the FeS6 and FeO6 sites were not found in the vicinity of the CTPT. We also calculated the orbital energies and the occupied spin states for the [Fe(O2C2S2)3] and [Fe(S2C2O2)3] octahedra in the vicinity of the CTPT by density functional theory (DFT). Because the local symmetry around the two coordinating iron ions is already lowered to trigonal symmetry, the CTPT did not cause any further deformation. This symmetry invariance resulted in an absence of orbital contributions to the total entropy change (Δ S) in the CTPT, which is in agreement with the previous heat capacity measurements. [Nakamoto, T; Miyazaki, Y; Itoi, M; Ono, Y; Kojima, N; Sorai, M. Heat Capacity of the Mixed-Valence Complex {[( n-C3H7)4N][FeIIFeIII(dto)3]}∞, Phase Transition because of Electron Transfer, and a Change in Spin-State of the Whole System. Angew. Chem., Int. Ed. 2001, 40, 4716-4719.].

Unique Hydration/Dehydration-Induced Vapochromic Behavior of a Charge-Transfer Salt Comprising Viologen and Hexacyanidoferrate(II).

We successfully prepared and crystallographically characterized the first intermolecular charge-transfer (CT)-based vapochromic compound, (EV)(H3O)2[Fe(CN)6] (1-Wet, EV2+: 1,1'-diethyl-4,4'-bipyridine-1,1'-diium), an ethyl viologen-containing CT salt. 1-Wet, which is purple in color, is transformed into a brown powder (1-Dry) upon exposure to methanol vapor, drying over silica gel, or heating; 1-Dry returns to 1-Wet upon exposure to water vapor. These color changes are induced by hydration and dehydration, and gravimetric analyses suggest that 1-Dry is the dehydrated form of 1-Wet, namely, (EV)(H)2[Fe(CN)6]. Interestingly, desorption of water molecules from the oxonium ions in 1-Wet produces isolated protons (H+) that remain in 1-Dry as counter cations. Powder X-ray crystal structure analysis of 1-Dry reveals the presence of very short contacts between the nitrogen atoms of adjacent [Fe(CN)6]4- anions in the crystal. The isolated protons are trapped between the nitrogen atoms of cyanido ligands to form very short N···H···N hydrogen bonds. A detailed comparison of the crystal structures of 1-Wet and 1-Dry reveals that hydration and dehydration induce changes in crystal packing and intermolecular CT interactions, resulting in reversible color changes.

A tricky water molecule coordinated to a verdazyl radical-iron(II) complex: a multitechnique approach.

The first iron complexes of high-spin iron(II) species directly coordinated to verdazyl radicals, [Fe(II)(vdCOO)2(H2O)2]·2H2O (1; vdCOO(-) = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate) and [Fe(II)(vdCOO)2(D2O)2]·2D2O (2), were synthesized. The crystal structure of 1 was investigated by single-crystal X-ray diffraction at room temperature and at 90 K. The compound crystallizes in the P1 space group with no phase transition between 300 and 90 K. The crystals are composed of discrete [Fe(II)(vdCOO)2(H2O)2] complexes and crystallization water molecules. In the complex, two vdCOO(-) ligands coordinate to the iron(II) ion in a head-to-tail arrangement and two water molecules complete the coordination sphere. The Fe-X (X = O, N) distances vary in the 2.069-2.213 Å range at 300 K and in the 2.0679-2.2111 Å range at 90 K, indicating that the iron(II) ion is in its high-spin (HS) state at both temperatures. At 300 K, one of the coordinated water molecules is H-bonded to two crystallization water molecules whereas the second one appears as loosely H-bonded to the two oxygen atoms of the carboxylate group of two neighboring complexes. At 90 K, the former H-bonds remain essentially the same whereas the second coordinated water molecule reveals a complicated behavior appearing simultaneously as tightly H-bonded to two oxygen atoms and non-H-bonded. The (57)Fe Mössbauer spectra, recorded between 300 K and 10 K, give a clue to this situation. They show two sets of doublets typical of HS iron(II) species whose intensity ratio varies smoothly with temperature. It demonstrates the existence of an equilibrium between the high temperature and low temperature forms of the compounds. The solid-state magic angle spinning (2)H NMR spectra of 2 were recorded between 310 K and 193 K. The spectra suggest the existence of a strongly temperature-dependent motion of one of the coordinated water molecules in the whole temperature range. Variable-temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction (J(Fe-vd) = -27.1 cm(-1); H = -J(ij)S(i)S(j)) of the HS iron(II) ion and the radical spins with high g(Fe) and D(Fe) values (g(Fe) = 2.25, D(Fe) = +3.37 cm(-1)) for the HS iron(II) ion. Moreover, the radicals are strongly antiferromagnetically coupled through the iron(II) center (J(vd-vd) = -42.8 cm(-1)). These last results are analysed based on the framework of the magnetic orbitals formalism.

Abrupt spin transition with thermal hysteresis of iron(III) complex [Fe(III)(Him)2(hapen)]AsF6 (Him = imidazole, H2hapen = N,N'-bis(2-hydroxyacetophenylidene)ethylenediamine).

The solvent-free spin crossover iron(III) complex [Fe(III)(Him)2(hapen)]AsF6 (Him = imidazole, H2hapen = N,N'-bis(2-hydroxyacetophenylidene)ethylenediamine), exhibiting thermal hysteresis, was synthesized and characterized. The Fe(III) ion has an octahedral coordination geometry, with N2O2 donor atoms of the planar tetradentate ligand (hapen) and two nitrogen atoms of two imidazoles at the axial positions. One of two imidazoles is hydrogen-bonded to the phenoxo oxygen atom of hapen of the adjacent unit to give a hydrogen-bonded one-dimensional chain, while the other imidazole group is free from hydrogen bonding. The temperature dependencies of the magnetic susceptibilities and Mössbauer spectra revealed an abrupt spin transition between the high-spin (S = 5/2) and low-spin (S = 1/2) states, with thermal hysteresis.

Exchange coupling and its chemical trend studied by high-frequency EPR on heterometallic [Ln2Ni] complexes.

We applied high-frequency electron paramagnetic resonance to trinuclear 4f-3d heterometallic complexes, [{Ln(hfac)3}2{Ni(dpk)2(py)2}] (Ln = Y, Gd, Tb, and Ho; hfac = hexafluoroacetylacetonate, dpk = di-2-pyridyl ketoximate, and py = pyridine), and determined the exchange parameter J(Ln-Ni) as well as nickel(II) zero-field splitting parameters. In contrast to the antiferromagnetic Dy analogue, ferromagnetic couplings were precisely characterized as J(Gd-Ni)/kB = +0.301(4) K, J(Tb-Ni)/kB = +0.216(12) K, and J(Ho-Ni)/kB = +0.110(3) K (defined as -J(Ln-Ni)∑J(Ln)(z)S(Ni)).

Reversible solid state redox of an octacyanometallate-bridged coordination polymer by electrochemical ion insertion/extraction.

Coordination polymers have significant potential for new functionality paradigms due to the intrinsic tunability of both their electronic and structural properties. In particular, octacyanometallate-bridged coordination polymers have the extended structural and magnetic diversity to achieve novel functionalities. We demonstrate that [Mn(H2O)][Mn(HCOO)(2/3)(H2O)(2/3)](3/4)[Mo(CN)8]·H2O can exhibit electrochemical alkali-ion insertion/extraction with high durability. The high durability is explained by the small lattice change of less than 1% during the reaction, as evidenced by ex situ X-ray diffraction analysis. The ex situ X-ray absorption spectroscopy revealed reversible redox of the octacyanometallate. Furthermore, the solid state redox of the paramagnetic [Mo(V)(CN)8](3-)/diamagnetic[Mo(IV)(CN)8](4-) couple realizes magnetic switching.

Long-range and short-range orderings in K4Fe4P5O20 with a natrolite-like framework.

K4Fe4P5O20 shows an interesting natrolite-like structure with a spin-tetrahedron lattice built by mixed valence Fe ions. Single crystals of the title compound are successfully grown by the flux method using KF as flux. Magnetic results combined from magnetic, heat capacity, and (57)Fe Mössbauer spectra measurements show that K4Fe4P5O20 possesses a short-range magnetic ordering at ∼13 K and a long-range ordering at ∼7 K. Magnetic anisotropy of K4Fe4P5O20 is observed between H‖c and H⊥c, suggesting that the c-axis is the magnetic easy-axis. The spin arrangements in the system are suggested to be ferrimagnetic along the natrolite chains.

Precise electrochemical control of ferromagnetism in a cyanide-bridged bimetallic coordination polymer.

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K(0.2)Ni[Fe(CN)(6)](0.7)·4.7H(2)O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe(3+)/Fe(2+) reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature T(C) gradually decreases due to reduction of paramagnetic Fe(3+) to diamagnetic Fe(2+), and the ferromagnetic transition is completely suppressed for Li(0.6)(NiFe-PBA). On the other hand, T(C) increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe(2+) to paramagnetic Fe(3+), and the ferromagnetic transition is nearly recovered for Li(0)(NiFe-PBA). Furthermore, the plots of T(C) as a function of the amount of inserted/extracted Li ion x are well consistent with the theoretical values calculated by the molecular-field approximation.

Effect of nonmagnetic substitution on the magnetic properties and charge-transfer phase transition of an iron mixed-valence complex, (n-C3H7)4N[Fe(II)Fe(III)(dto)3] (dto = C2O2S2).

The iron mixed-valence complex (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)] exhibits a novel type of phase transition called charge-transfer phase transition (CTPT), where the thermally induced electron transfer between Fe(II) and Fe(III) occurs reversibly at ~120 K, in addition to the ferromagnetic phase transition at T(C) = 7 K. To investigate the mechanism of the CTPT, we have synthesized a series of magnetically diluted complexes (n-C(3)H(7))(4)N[Fe(II)(1-x)Zn(II)(x)Fe(III)(dto)(3)] (dto = C(2)O(2)S(2); x = 0-1), and carried out magnetic susceptibility and dielectric constant measurements and (57)Fe Mössbauer spectroscopy. With increasing Zn(II) concentration (x), the CTPT is gradually suppressed and disappears at x ≈ 0.13. On the other hand, the ferromagnetic transition temperature (T(C)) is initially enhanced from 7 K to 12 K between x = 0.00 and 0.05, despite the nonmagnetic nature of Zn(II) ions, and then it decreases monotonically from 12 K to 3 K with increasing Zn(II) concentration. This anomalous dependence of T(C) on Zn(II) concentration is related to a change in the spin configuration of the ferromagnetic state caused by the partial suppression of the CTPT.

Exchange coupling in TbCu and DyCu single-molecule magnets and related lanthanide and vanadium analogs.

Heterometallic coordination compounds [Cu(II)(L)(C(3)H(6)O)Ln(III)(NO(3))(3)] and [V(IV)O(L)(C(3)H(6)O)Ln(III)(NO(3))(3)] (abbreviated as LnCu and LnV, respectively; H(2)L = N,N'-bis(3-methoxysalicylidene)-1,3-diamino-2,2-dimethylpropane; Ln = Gd, Tb, Dy, Ho, and Er) were synthesized, and the X-ray crystallographic analysis shows that their structures are isomorphous for each series. The single-molecule magnet behavior was observed for TbCu and DyCu, and the activation energies of magnetization reversal were 42.3(4) and 11.5(10) K, respectively. The magnetic exchange couplings in LnCu and LnV were precisely evaluated by means of combined high-frequency EPR and pulsed-field magnetization studies, to give J(Tb-Cu)/k(B)≥ 3.3 K, J(Dy-Cu)/k(B) = 1.63(1) K, J(Ho-Cu)/k(B) = 1.09(2) K, and J(Er-Cu)/k(B) = 0.24(1) K. A monotonic decrease of ferromagnetic J(Ln-Cu) was found in the order of the atomic number, (64)Gd to (68)Er. The corresponding exchange parameters in LnV are smaller than those of the Cu derivatives, and J(Gd-V) was antiferromagnetic (-3.0 K determined from the magnetization jump). A possible mechanism for the exchange coupling and chemical trend is discussed.

K4Fe4P5O20: a new mixed valence microporous compound with elliptical eight-ring channels.

A new small-pore compound K(4)Fe(4)P(5)O(20) was obtained by conventional solid-state reaction in a closed crucible. The crystal structure is constructed by Fe(4)P(5)O(20) units forming chains along the c axis and elliptical eight-ring channels on the a-b plane in which K cations locate inside. Such structural characteristics seem to be quite similar to those seen in the natrolite family. However, Fe ions in K(4)Fe(4)P(5)O(20) have trigonal-bipyramidal instead of common tetrahedral coordination. Furthermore, our experimental results combined from magnetic susceptibility and (57)Fe Mössbauer spectrum measurements show mixed valence Fe(3+)/Fe(2+) in the titled material. To the best of our knowledge, this is the first example that contains mixed valence iron ions in a so-called natrolite framework.

Ferromagnetic exchange couplings showing a chemical trend in Cu-Ln-Cu complexes (Ln = Gd, Tb, Dy, Ho, Er).

Exchange couplings in isomorphous [LnCu(2)] were evaluated by high-frequency electron paramagnetic resonance and magnetization studies. The exchange parameter J(Ln-Cu) was decreased with an increase in the atomic number; J(Ln-Cu)/k(B) = 4.45(11), 2.27(6), 0.902(10), 0.334(3), and 0.136(8) K for Ln = Gd, Tb, Dy, Ho, and Er, respectively.

Unusually large magnetic anisotropy in a CuO-based semiconductor Cu5V2O10.

A CuO-based material Cu(5)V(2)O(10) was successfully grown in a closed crucible using Sr(OH)(2)·8H(2)O as flux. The structure of Cu(5)V(2)O(10) can be viewed as being composed of two types of zigzag Cu-O chains running along the b- and c-axes, which shows a two-dimensional crosslike framework with 12-column square tunnels along the a-axis. Magnetic measurements show that Cu(5)V(2)O(10) exhibits unexpected large magnetic anisotropy, which is the first time magnetic anisotropy energy of ∼10(7) erg/cm(3) in the CuO-based materials has been observed. The origins of large anisotropy are suggested to arise from strong anisotropic exchanges due to the particular bonding geometry and the Jahn-Teller distortion of Cu(2+) ions. Further, the band structure investigated by the GGA+U method suggests that Cu(5)V(2)O(10) is a semiconductor.

Synthesis of triaxial LiFePO4 nanowire with a VGCF core column and a carbon shell through the electrospinning method.

A triaxial LiFePO4 nanowire with a multi wall carbon nanotube (VGCF:Vapor-grown carbon fiber) core column and an outer shell of amorphous carbon was successfully synthesized through the electrospinning method. The carbon nanotube core oriented in the direction of the wire played an important role in the conduction of electrons during the charge-discharge process, whereas the outer amorphous carbon shell suppressed the oxidation of Fe2+. An electrode with uniformly dispersed carbon and active materials was easily fabricated via a single process by heating after the electrospinning method is applied. Mossbauer spectroscopy for the nanowire showed a broadening of the line width, indicating a disordered coordination environment of the Fe ion near the surface. The electrospinning method was proven to be suitable for the fabrication of a triaxial nanostructure.