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.

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.].

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.

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.