Keita Kuroiwa

Lipid-packaged linear iron(II) triazole complexes in solution: controlled spin conversion via solvophobic self-assembly.

Linear Fe(II) 1,2,4-triazole complexes with lipid counteranions are newly developed. These complexes show sharp and reversible spin conversion in toluene, with temperatures significantly higher (by 20-100 K) than the spin crossover temperatures observed in the crystalline states. This is accounted for in terms of increased metal-ligand interactions in organic media, which is caused by solvophobic compaction of charged coordination chains. In atomic force microscopy, developed nanowires are observed for low spin (LS) complexes. On the other hand, fragmented nanostructures are seen for high spin (HS) complexes, indicating that the spin conversion in solution is governed by a self-assembly process. The lipid packaging of charged coordination chains thus provides powerful means to improve and regulate their functions via solvophobic self-assembly.

Spin crossover characteristics of nanofibrous FeII-1,2,4-triazole complexes in liquid crystals

A lipophilic linear Fe(II) complex is dispersed as nanofibers in nematic liquid crystals and displays spin-crossover at temperatures higher than those observed for the bulk crystalline state; thermal bistability is also induced in the liquid crystal environment, reflecting increased ligand field splitting energy and enhanced cooperativity by liquid crystals.

Controlled Polymerization and Self-Assembly of Halogen-Bridged Diruthenium Complexes in Organic Media and Their Dielectrophoretic Alignment

Lipophilic paddlewheel biruthenium complexes [Ru(2)(μ-O(2)CR)(3)X](n) (O(2)CR = 3,4,5-tridodecyloxybenzoate, X = Cl, I) self-assemble in organic media to form halogen-bridged coordination polymers. The polymerization is accompanied by spectral changes in π(RuO,Ru(2)) → π*(Ru(2)) and π(axial ligand) → π*(Ru(2)) absorption bands. These polymeric complexes form lyotropic liquid crystals in n-decane at concentrations above ~100 unit mM. The bridging halogen axial ligands (X = Cl or I) exert significant influences on their electronic structures and self-assembling characteristics: the chloride-bridged polymers give hexagonally aligned ordered columnar structure (columnar hexagonal phase, Col(h)), whereas the iodide-bridged polymers form less ordered columnar nematic (Col(n)) phase, as revealed by small-angle X-ray diffraction measurements. Chloro-bridged coordination polymers dispersed in n-decane are thermally intact even at the elevated temperature of 70 °C. In contrast, iodo-bridged polymers show reversible dissociation and reassembly phenomena depending on temperature. These halogen-bridged coordination polymers show unidirectional alignment upon applying alternating current (ac) electric field as investigated by crossed polarizing optical microscopy and scanning electron microscopy. The unidirectionally oriented columns of chloro-bridged polymers are accumulated upon repetitive application of the ac voltage, whereas iodo-bridged coordination polymers show faster and reversible alignment changes in response to turning on-and-off the electric field. The controlled self-assembly of electronically conjugated linear complexes provide a potential platform to design electric field-responsive nanomaterials.

Self-Assembly of Tubular Microstructures from Mixed-Valence Metal Complexes and Their Reversible Transformation by External Stimuli†

Controlled self-assembly of metal complexes is of high scientific and technological importance for the development of multi-functional materials and devices. Among various types of metal complexes, mixed-valence complexes have attracted much attention because of their wide range of interesting physical and chemical properties from chargetransfer interactions between metal ions linked via bridging ligands. In particular, low-dimensional assembly of such mixed-valence complexes gives rise to specific electronic, magnetic, and optical properties. Moreover, the assembly of discrete binuclear mixed-valence complexes has been suggested as a basis for forming molecular communication system such as quantum cellular automata. Ideally, the characteristics of such systems would be tunable by controlling the spatial arrangement of the mixed-valence complexes, resulting in electric interaction among metal complexes without covalent or coordinative linkage. In this context, supramolecular strategies have been developed to construct nanoassemblies of coordination compounds, such as one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) metal complexes. However, studies to date have focused on the conversion of crystalline coordination polymers to nanowires, nanosheets, and nanoparticles, which can be regarded as isolation of lowdimensional structures from 3D solids (Scheme 1a). In contrast, there exist no reports on the reversible and hierarchical self-assembly of discrete mixed-valence metal complexes (which could not interact with each other) into 1D nanowires, 2D nanosheets, or 3D nanoarchitectures, a concept which would lie at the very heart of bottom-up nanotechnology (Scheme 1b). However, we have developed selfassembled nanowires by amphiphilically modifying such

Supramolecular control of reverse spin transitions in cobalt(II) terpyridine complexes with diblock copolypeptide amphiphiles

Three composites composed of the cobalt terpyridine complex [CoII(MeO-terpy)2] and the diblock copolypeptide amphiphiles 1 and 2 or the polypeptide 3 (including glutamic acid and leucine) were prepared. Supramolecular structures such as rectangular morphologies were obtained from composites of 1 and 2. A perfectly reversible reverse spin transition was successfully generated in the case of composites made with 1.

Graphene oxide and reduced graphene oxide hybrids with spin crossover iron(III) complexes

Graphene (rGO) based hybrid materials exhibiting electrical conductivity and spin crossover (SCO) behavior are reported. The non-conductive [Fe(qnal)2]nGO (1·GO) and [Fe(qsal)2]nGO (2·GO) hybrids have been prepared by employing the electrostatic interaction between the negatively charged graphene oxide (GO) nanosheet and the respective iron(III) complex cations in [Fe(qnal)2]+Cl− and [Fe(qsal)2]+Cl−. The conductive [Fe(qnal)2]nrGO (1·rGO) and [Fe(qsal)2]nrGO (2·rGO) hybrids were obtained by thermal reduction of 1·GO and 2·GO. 1·GO and 1·rGO exhibit SCO behavior, and 1·rGO also shows a light-induced excited spin state trapping (LIESST) effect. Thus, in 1·rGO the electrical conductivity of rGO and the SCO behavior of [Fe(qnal)2]+ coexist in a single structure. We propose that the observed cooperativity for the rGO nanosheet-bound iron(III) [Fe(qnal)2]+ SCO material occurs through formation of large domains via π–π stacking between the graphene skeleton and the [Fe(qnal)2]+ cations.

Self-assembly of discrete metal complexes in aqueous solution via block copolypeptide amphiphiles

The integration of discrete metal complexes has been attracting significant interest due to the potential of these materials for soft metal-metal interactions and supramolecular assembly. Additionally, block copolypeptide amphiphiles have been investigated concerning their capacity for self-assembly into structures such as nanoparticles, nanosheets and nanofibers. In this study, we combined these two concepts by investigating the self-assembly of discrete metal complexes in aqueous solution using block copolypeptides. Normally, discrete metal complexes such as [Au(CN)2]−, when molecularly dispersed in water, cannot interact with one another. Our results demonstrated, however, that the addition of block copolypeptide amphiphiles such as K183L19 to [Au(CN)2]− solutions induced one-dimensional integration of the discrete metal complex, resulting in photoluminescence originating from multinuclear complexes with metal-metal interactions. Transmission electron microscopy (TEM) showed a fibrous nanostructure with lengths and widths of approximately 100 and 20 nm, respectively, which grew to form advanced nanoarchitectures, including those resembling the weave patterns of Waraji (traditional Japanese straw sandals). This concept of combining block copolypeptide amphiphiles with discrete coordination compounds allows the design of flexible and functional supramolecular coordination systems in water.

Supramolecular solvatochromism. Effect of solvents on the self-assembly and charge transfer absorption characteristics of lipid-packaged, linear mixed-valence platinum complexes

Abstract Lipid-packaged, mixed-valence linear platinum complexes are dispersed in chloroform, chlorocyclohexane and in methylcyclohexane. They show indigo-colors, which are identical to solid samples. These lipid-complexes display supramolecular thermochromism, resulting from heat-induced dissociation and recovery of coordination chains. Unexpectedly, reassembled samples show altered CT absorption spectra. In chloroform, the reassembled complex gives indigo-color, while the chlorocyclohexane and methylcyclohexane dispersions show blue and red-colors, respectively. These color changes indicate enhanced solvation of lipid-packaged platinum complexes after the heat treatment. The thermal re-assembly also affects the aggregate morphology, and leads to the formation of nanowires, nanofibers and nanoparticles depending on the solvents employed. The conversion of pseudo one-dimensional (1D) inorganic complexes to lipophilic supramolecular assemblies thus gives solvatochromic properties, which are elements not existing in the conventional solid-state inorganic chemistry.

Coal Oxide as a Thermally Robust Carbon-Based Proton Conductor

Inexpensive solid proton conducting materials with high proton conductivity and thermal stability are necessary for practical solid state electrochemical devices. Here we report that coal oxide (CO) is a promising carbon-based proton conductor with remarkable thermal robustness. The CO produced by simple liquid-phase oxidation of coal demonstrates excellent dispersibility in water owing to the surface carboxyl groups. The proton conductivity of CO, 3.9 × 10(-3) S cm(-1) at 90% relative humidity, is as high as that of graphene oxide (GO). Remarkably, CO exhibits much higher thermal stability than GO, with CO retaining the excellent proton conductivity as well as the capacitance performance even after thermal annealing at 200 °C. Our study demonstrates that the chemical modification of the abundant coal provides proton conductors that can be used in practical applications for a wide range of energy devices.

Preparation and electrochemical behaviour of hydrophobic vitamin B12 covalently immobilized onto platinum electrode

Hydrophobic vitamin B(12) was covalently immobilized onto a platinum electrode surface, and the immobilized complex exhibits Co(ii)/Co(i) redox couple and in situ the Co(i) species reacts with phenethyl bromide to form styrene under irradiation with visible light with a turnover number of over 6000 for 1 h.