Air-Stable Cyclohexasulfur as Cocrystal

A new stable cyclohexasulfur (cyclo-S₆) was discovered in the crystal of 3,5-diphenyl-1,2,4-dithiazol-1-ium (dpdti); dpdti was synthesized via an oxidation reaction of thiobenzamide with iodine in benzene, under reflux. Two kinds of crystal habitsbrick-shaped (crystal-<b>A</b>) and needle-shaped (crystal-<b>B</b>)were obtained following recrystallization from acetonitrile solvent. Single-crystal structure analysis using synchrotron X-ray radiation showed that both crystal-<b>A</b> and -<b>B</b> consist of a dpdti cation and an iodine anion. Furthermore, neutral cyclo-S₆ molecules are present in crystal-<b>A</b> as cocrystals, enclosed by the dpdti cation and the iodide anion. This is akin to cyclosulfur of <i>S</i>_<i>n</i> in zeolites and sodalities, suggesting a contribution to the stabilization of cyclo-S₆ molecules. The results show that crystal engineering of cocrystals may be used as a method to control the stability and activity of sulfur, for improved utilization

Edge-Dependent Transport Properties in Graphene

Graphene has two kinds of edges which have different electronic properties. A singular electronic state emerges at zigzag edges, while it disappears at armchair edges. We study the edge-dependent transport properties in few-layer graphene by applying a side gate voltage to the edge with an ionic liquid. The devices indicating a conductance peak at the charge neutrality point have zigzag edges, confirmed by micro-Raman spectroscopy mapping. The hopping transport between zigzag edges increases the conductance

Step-by-Step Fabrication of a Highly Oriented Crystalline Three-Dimensional Pillared-Layer-Type Metal–Organic Framework Thin Film Confirmed by Synchrotron X-ray Diffraction

Fabrication of a crystalline ordered thin film based on the porous metal–organic frameworks (MOFs) is one of the practical applications of the future functional nanomaterials. Here, we report the creation of a highly oriented three-dimensional (3-D) porous pillared-layer-type MOF thin film on a metal substrate using a step-by-step approach based on liquid-phase epitaxy. Synchrotron X-ray diffraction (XRD) study clearly indicates that the thin film is crystalline and its orientation is highly controlled in both horizontal and vertical directions relative to the substrate. This report provides the first confirmation of details of not only the crystallinity but also the orientation of 3-D MOF thin film using synchrotron XRD. Moreover, we also demonstrate its guest adsorption/desorption behavior by using <i>in situ</i> XRD measurements. The results presented here would promise useful insights for fabrication of MOF-based nanodevices in the future

Remarkable Lattice Shrinkage in Highly Oriented Crystalline Three-Dimensional Metal–Organic Framework Thin Films

Highly oriented crystalline thin films of metal–organic frameworks (MOFs) have promising practical applications, such as in gas separation, catalysis, and sensing. We report on the successful fabrication of highly oriented crystalline thin films of three-dimensional porous MOFs, Fe­(pz)­[M­(CN)₄] (M = Ni, Pd; pz = pyrazine). Synchrotron X-ray diffraction studies reveal not only the highly oriented crystalline nature but also the remarkable shrunken structure of the thin films (∼3–7% volume shrinkage) compared with bulk samples. Furthermore, because of lattice shrinkage, these films exhibit large lattice expansions upon guest adsorption, in marked contrast to the almost unchanged lattice in the bulk samples

Guest-Induced Two-Way Structural Transformation in a Layered Metal–Organic Framework Thin Film

Fabrication of thin films made of metal–organic frameworks (MOFs) has been intensively pursued for practical applications that use the structural response of MOFs. However, to date, only physisorption-induced structural response has been studied in these films. Chemisorption can be expected to provide a remarkable structural response because of the formation of bonds between guest molecules and reactive metal sites in host MOFs. Here, we report that chemisorption-induced two-way structural transformation in a nanometer-sized MOF thin film. We prepared a two-dimensional layered-type MOF Fe­[Pt­(CN)₄] thin film using a step-by-step approach. Although the as-synthesized film showed poor crystallinity, the dehydrated form of this thin film had a highly oriented crystalline nature (<b>Film-D</b>) as confirmed by synchrotron X-ray diffraction (XRD). Surprisingly, under water and pyridine vapors, <b>Film-D</b> showed chemisorption-induced dynamic structural transformations to Fe­(L)₂[Pt­(CN)₄] thin films [L = H₂O (<b>Film-H</b>), pyridine (<b>Film-P</b>)], where water and pyridine coordinated to the open Fe²⁺ site. Dynamic structural transformations were also confirmed by in situ XRD, sorption measurement, and infrared reflection absorption spectroscopy. This is the first report of chemisorption-induced dynamic structural response in a MOF thin film, and it provides useful insights, which would lead to future practical applications of MOFs utilizing chemisorption-induced structural responses

Variable-Rung Design for a Mixed-Valence Two-Legged Ladder System Situated in a Dimensional Crossover Region

Ladder systems situated in a crossover from one dimensionality to two dimensionalities have been an attractive research target, because the physical properties, which are associated with dimensionality, are strongly dependent on the number of constituent legs. However, control of the intraladder configuration and electronic properties based on the substitution of structural components remain challenging tasks in materials science. On the other hand, structural design using coordination chemistry offers crucial advantages for architectural and electronic variations through substitutions of metal–organic building blocks. Here, we show the rational design and electronic properties of novel metal complex-based two-legged ladder compounds with several organic rung units: 4,4′-bipyridine, trans-1,4-diaminocyclohexane, and 4,4′-azopyridine. Single-crystal X-ray studies show that these two-legged ladder compounds are composed of halogen-bridged mixed-valence one-dimensional chains (MX chains) as their constituent legs. Depending on the molecular shape of the organic rung units, unique configurations of two-legged ladder lattices with periodic distortion of the legs are achieved. In addition, the electronic absorption spectra show that intervalence charge-transfer (IVCT) band gap of the two-legged ladder system increases with increasing degree of distortion of the leg. We have demonstrated for the first time that a two-legged ladder system shows a unique relationship between IVCT energy and the distortion parameter of the leg, as distinct from a single MX chain system. These systematic investigations, not only of configurations based on the rung variation but also of electronic states in metal–organic ladder system, provide the possibility for wide and rational tunings of physical and electronic properties of metal complex-based functional materials

Neutral-Type One-Dimensional Mixed-Valence Halogen-Bridged Platinum Chain Complexes with Large Charge-Transfer Band Gaps

One-dimensional (1D) electronic systems have attracted significant attention for a long time because of their various physical properties. Among 1D electronic systems, 1D halogen-bridged mixed-valence transition-metal complexes (the so-called MX chains) have been thoroughly studied owing to designable structures and electronic states. Here, we report the syntheses, structures, and electronic properties of three kinds of novel neutral MX-chain complexes. The crystal structures consist of 1D chains of Pt–X repeating units with (1<i>R</i>,2<i>R</i>)-(−)-diaminocychlohexane and CN^– in-plane ligands. Because of the absence of a counteranion, the neutral MX chains have short interchain distances, so that strong interchain electronic interaction is expected. Resonance Raman spectra and diffuse-reflectance UV–vis spectra indicate that their electronic states are mixed-valence states (charge-density-wave state: Pt²⁺···X–Pt⁴⁺–X···Pt²⁺···X–Pt⁴⁺–X···). In addition, the relationship between the intervalence charge-transfer (IVCT) band gap and the degree of distortion of the 1D chain shows that the neutral MX chains have a larger IVCT band gap than that of cationic MX-chain complexes. These results provide new insight into the physical and electronic properties of 1D chain compounds

Fabrication and Structural Characterization of an Ultrathin Film of a Two-Dimensional-Layered Metal–Organic Framework, {Fe(py)₂[Ni(CN)₄]} (py = pyridine)

We report the fabrication and characterization of the first example of a tetracyanonickelate-based two-dimensional-layered metal–organic framework, {Fe­(py)₂Ni­(CN)₄} (py = pyridine), thin film. To fabricate a nanometer-sized thin film, we utilized the layer-by-layer method, whereby a substrate was alternately soaked in solutions of the structural components. Surface X-ray studies revealed that the fabricated film was crystalline with well-controlled growth directions both parallel and perpendicular to the substrate. In addition, lattice parameter analysis indicated that the crystal system is found to be close to higher symmetry by being downsized to a thin film

An Electrically Conductive Single-Component Donor–Acceptor–Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D–A–D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex <b>1H</b>_<b>2</b>). The crystal structure of complex <b>1H</b>_<b>2</b> has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex <b>1H</b>_<b>2</b> is semiconducting (<i>E</i>_a = 0.29 eV, ρ_rt = 9.1 × 10⁴ Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (<i>E</i>_a = 0.13 eV, ρ_rt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π–π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D–A–D aggregate

An Electrically Conductive Single-Component Donor–Acceptor–Donor Aggregate with Hydrogen-Bonding Lattice

An electrically conductive D–A–D aggregate composed of a single component was first constructed by use of a protonated bimetal dithiolate (complex <b>1H</b>_<b>2</b>). The crystal structure of complex <b>1H</b>_<b>2</b> has one-dimensional (1-D) π-stacking columns where the D and A moieties are placed in a segregated-stacking manner. In addition, these segregated-stacking 1-D columns are stabilized by hydrogen bonds. The result of a theoretical band calculation suggests that a conduction pathway forms along these 1-D columns. The transport property of complex <b>1H</b>_<b>2</b> is semiconducting (<i>E</i>_a = 0.29 eV, ρ_rt = 9.1 × 10⁴ Ω cm) at ambient pressure; however, the resistivity becomes much lower upon applying high pressure up to 8.8 GPa (<i>E</i>_a = 0.13 eV, ρ_rt = 6.2 × 10 Ω cm at 8.8 GPa). The pressure dependence of structural and optical changes indicates that the enhancement of conductivity is attributed to not only an increase of π–π overlapping but also a unique pressure-induced intramolecular charge transfer from D to A moieties in this D–A–D aggregate