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Development of Self-Assembling Coordination Nanowires and their Functions

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Developments of self-wiring
coordination nanowires


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Self-wiring coordination nanowires
and their thermochromic transition

Our research group is broadly interested in self-assembling materials, and is involved identifying problems of fundamental significance to nanochemistry. Our approach involves the synthesis of materials that contain molecular, biomolecular and inorganic components, and the study of their structure and properties using a variety of physical techniques. In general, we use the tools of synthetic and physical organic chemistry to address problems at the interface of chemistry, biochemistry and materials science.

Inspired by neurons, we are working to create self-assembling molecular wires. One of our focuses is on inorganic molecular wires, also called nanowires, which are assembled through non-covalent interactions into well-defined structures in a solution. These self-assembling molecular wires will be indispensable elements of future molecular-scale electronic devices, and their fabrication has been one of the central issues of nanochemistry. Conventional research is focused on the synthesis of π-conjugated oligomers and polymers, but they suffer from limitations on the types of elements that can be incorporated into the wires. We have recently developed a new strategy to manipulate nano-metal complexes via the "supramolecular packaging" of one-dimensional inorganic complexes [M(en)2][M'Cl2 (en) 2] (M, M' = Pt, Pd and Ni, en: 1,2,-diaminoethane). A family of halogen-bridged one-dimensional MII/MIV mixed valence complexes [M(en) 2][M'X2 (en) 2]Y4 (X = Cl, Br, I, Y : counterions such as ClO4) has been attracting considerable interest due to its unique physicochemical properties, including intense intervalence charge-transfer (CT) absorption, semiconductivity, and large third-order nonlinear optical susceptibilities. However, these one-dimensional complexes have not yet been considered as candidates for molecular wires, since they exist only in three-dimensional solids.

The supramolecular packaging of this one-dimensional complex provides solubility to solvophobic Pt-chains. The structure of lipids exerts a remarkable influence on the CT band, and the electronic structures of one-dimensional complexes become tunable (supramolecular band gap engineering). The packaging of low-dimensional inorganic solids enables the creation of novel polymer molecules that have not yet been dealt with as molecules. This strategy should open a new dimension in mesoscopic supramolecular assembly as well as in molecular wire research. The goal of this aspect of the work is to learn how to make micro- and nano-electronic memories and other complex circuits using self-assembly.

Another class of thermally responsive supramolecular assemblies is formed from the lipophilic cobalt (II) complexes of 4-alkylated 1,2,4-triazoles. When an ether-linkage is introduced in the alkyl chain moiety, a blue gel-like phase is formed in chloroform, even at very low concentrations (ca. 0.01 wt%, at room temperature). The blue color is accompanied by a structured absorption at around 580-730 nm, which is characteristic of cobalt (II) in a tetrahedral (Td) coordination. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) of the gel-like phase confirms the formation of networks of fibrous nanoassemblies with widths of 5 - 30 nm. The observed widths are larger than the molecular length of the triazole ligand (ca. 2.2 nm) and they consist of aggregates of Td coordination polymers. Very interestingly, the blue gel-like phase turns into a solution when cooled below 25°C. A pale pink solution is obtained at 0°C, indicating the formation of octahedral (Oh) complexes. This observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the behavior of conventional organogels, which dissolve upon heating. These observations indicate that Oh complexes present as low molecular weight species self-assemble to become polymeric Td complexes upon heating, and form gel-like networks. The observed unique thermochromic transition (pink solution → blue gel-like phase) has been shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides solutions with unique properties, and it could be widely applicable to the design of thermoresponsive, self-assembling molecular wires.

Biofunctional Chemistry, Department of Applied Chemistry
Professor Nobuo Kimizuka
Associate Professor Teppei Yamada
Associate Professor Nobuhiro Yanai
Assistant Professor Masa-aki Morikawa
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