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 overview of crystalline architectures utilizing Hofmann-type MOFs at the nanoscale.•Thin film fabrication and nanoscale patterning by means of step-by-step technique.•Crystalline oriented thin films confirmed by X-ray diffraction study.•Crystal-downsizing effects observed in nanoparticles and thin films.•Potential uses for future optical, electronic and sensing device applications.Metal–organic frameworks (MOFs) have been of particular interest to researchers because of their structural designability, robustness, and rich science arising from uniform porosity. Among them, Hofmann-type MOFs, which are one of the earliest examples of a MOF, have been intensively studied. For the bulk state of Hofmann-type MOFs, their spin transition behavior induced by guest molecules and external stimuli has long been discussed as a key subject. On the other hand, the use of Hofmann-type MOFs as a nanoscale architecture has recently attracted significant attention toward future practical applications, such as stimuli-responsive sensors and switching devices. In this review, we present an overview of the recent development of fabrication of crystalline architectures at the nanoscale including thin films and nanoparticles utilizing Hofmann-type MOFs.Download high-res image (287KB)Download full-size image

We report a step-by-step route to fabricate the first example of a crystalline oriented metal–organic framework thin film having an anionic inorganic pillar ligand, {Cu(4,4′-bipyridyl)2(SiF6)}. X-ray study and sorption analysis revealed its high crystallinity and porous character.

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 (Film-D) as confirmed by synchrotron X-ray diffraction (XRD). Surprisingly, under water and pyridine vapors, Film-D showed chemisorption-induced dynamic structural transformations to Fe(L)2[Pt(CN)4] thin films [L = H2O (Film-H), pyridine (Film-P)], 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.

We describe the synthesis and sorption properties of a new metal–organic framework (MOF), Fe(H2O)2(bpy)[Pt(CN)4]·H2O (bpy = 4,4′-bipyridine), with a three-dimensional accordion-like structure. Although crystalline oriented MOF thin films reported to date have been mainly limited to a layer-type structure, we succeeded in the fabrication of its crystalline oriented thin film.

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 (1R,2R)-(−)-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.

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.

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.

Ladder materials situated in a dimensional crossover region have attracted significant attention because of their unique physical properties, which depend strongly on the number of their constituent legs. Among them, metal–organic halogen-bridged ladder systems are currently of particular interest. These new series of ladder materials are composed of two or four halogen-bridged transition-metal complexes, the so-called MX-chains, as legs that are connected to each other by organic rung units. To date, a variety of two- and four-legged halogen-bridged ladder compounds, called MX-ladders, have been synthesized. We present an overview of design strategies, crystal structures, and unique electronic properties of MX-ladder materials with even numbers of legs. Utilization of coordination chemistry provides a high number of structural and electronic degrees of freedom, a striking feature of the MX-ladder system compared with transition-metal-oxide ladder systems. Not only their structural features including tunability via rung substitution but also their unique electronic properties depending on the number of legs and electronic states have recently been revealed.

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 in situ XRD measurements. The results presented here would promise useful insights for fabrication of MOF-based nanodevices in the future.

We describe the synthesis and sorption properties of a new metal–organic framework (MOF), Fe(H2O)2(bpy)[Pt(CN)4]·H2O (bpy = 4,4′-bipyridine), with a three-dimensional accordion-like structure. Although crystalline oriented MOF thin films reported to date have been mainly limited to a layer-type structure, we succeeded in the fabrication of its crystalline oriented thin film.