Department of Chemistry and Biochemistry,Graduate School of Engineering, Kyushu University
KIMIZUKALABORATORY

Department of Chemistry and

Biochemistry, Graduate School of

Engineering, Kyushu University

 

744 Moto-oka, Nishi-ku

Fukuoka 819-0395

 

TEL +81-92-802-2832

FAX +81-92-802-2838

Home » Research

Current Research

  Towards the development of Molecular Systems Chemistry: A next frontier in Chemistry

Self-assembly of functional molecules into ordered molecular assemblies and fulfillment of potentials unique to their nano-to-mesoscopic structures have been one of the central challenges in chemistry.  Thus far, chemistry has been compiled information how to make specific molecular self-assemblies or supermolecules based on the design of molecular components. We believe that the next essential challenges would involve the development of chemistry relevant to the design and development of molecular systems that do useful work (Molecular Systems Chemistry).1 

My research group is broadly currently focusing on the development of Molecular Systems Chemistry especially for solving energy issues on the basis of molecular self-assembly. Our approach involves the design of unique molecular self-assemblies which are composed of amphiphilic molecules, functional organic molecules, metal complexes, metal clusters, biomolecules, and study their structures and properties by using a variety of physical techniques. Nano-scale interfaces formed by these self-assemblies are also important in many disciplines and we also investigate self-assembly phenomena and interfacial materials chemistry in ionic liquids & crystals (Soft-Ionics).  (Figure 1)

 

 


Figure   Four main fields and interdisciplinary area studied in our group.

 

Resarch 1: Conversion of photon energy based on molecular systems chemistry

Photon upconversion in self-assembled molecular systems.

Development of new photon upconversion phenomena

Molecular Solar Thermal Fuels based on molecular assemblies

Resarch 2: Soft Ionics based on molecular self-assembly

Self-assembly phenomena in Ionic liquids, Ionic crystals and their applications

Interfacial Materials Chemistry in Ionic liquids 

Research 3: Coordination Chemistry and nanochemistry

Self-assembling coordination nanowires & nanoparticles

Adaptive self-assembly by coordination networks

Self-assembly of polyoxometalates into ordered nanostructures and their functions

Supramolecular ferroelectrics based on coordination chemistry and soft matter

Research 4: Development of coordination nanomembranes and their functions*

*In collaboration with Dr. Shigenori Fujikawa, I2CNER, Kyushu University

Research 5:  Molecular Self-assembly and Dissipative nanostructures

 

 

Resarch 1: Conversion of photon energy based on molecular systems chemistry1,2

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) has attracted much attention as a promising wavelength up-shifting technology which can utilize non-coherent light sources. The characteristics of TTA-UC including the low-power excitation and high quantum yield are beneficial for practical applications, ranging from sunlight-powered renewable energy production including photovoltanics, photocatalysis to bioimaging and phototherapy.

 

Recent review articles:

1.      N. Kimizuka, N. Yanai, M-a. Morikawa, Langmuir, 2016, 32, 12304-12322 (Invited Feature Article).

2.      N. Yanai, N. Kimizuka, Chem. Commun. 2016, 52, 5354-5370 (Feature Article).

 

 

 

This TTA-UC process occurs in association with multistep photochemical events. First, a triplet excited state of donor (sensitizer) is formed by intersystem crossing from the photo-excited singlet state, and acceptor (emitter) excited triplets are populated by triplet-triplet energy transfer (TTET) from the donor triplets. When two acceptor molecules in the triplet state diffuse and come into collision during their lifetime, a higher singlet energy level is formed by TTA, which consequently produces delayed upconverted fluorescence. The excitation and emission wavelength of TTA-UC can be regulated by independent selection of sensitizers and emitters. Thanks to the long lifetime of the triplet species, the excitation power density can be reduced to as low as a few mW cm-2 that is in the order of sunlight intensity at a specific excitation wavelength.

 

 

Figure 2. Schematic energy-level diagram of TTA-based upconversion for a model donor/acceptor pair. Solid arrows indicate transitions. The process involves the population of the singlet excited state of a donor (S1) upon absorption of incident light, which is followed by intersystem crossing (ISC) to the triplet excited state (T1). When the triplet donor encounters the ground-state acceptor, triplet−triplet energy transfer (TTET) from the donor to an acceptor yields an acceptor triplet state.

When two acceptors in their triplet state undergo triplet−triplet annihilation (TTA), one of the acceptors is excited to its excited singlet state while the other acceptor is relaxed to its ground state. The photon emitted from the acceptor singlet state (UC emission, turquoise blue arrow) has a higher energy than that of the initially absorbed photons (green arrow).  

 

 

To date, most efficient TTA-UC systems have been achieved in molecularly dispersed solutions because they allow fast diffusion of excited molecules. However, they suffer from overwhelming deactivation of excited triplets by dissolved oxygen which severely limits their operation to strictly deaerated conditions. The realization of efficient TTA-UC in the ambient, oxygen-rich environment should lead not only to an improvement of efficiency but also to a myriad of applications including the sunlight-powered energy productions.

To solve these problems, we have initiated a project to introduce the essence of molecular self-assembly to photon upconversion. That is, we focus on triplet energy migration in designed molecular self-assemblies to facilitate photon upconversion in varied molecular systems. On the basis of the integration of molecular self-assembly and photon energy harvesting, triplet energy migration-based TTAUC has been achieved in varied molecular systems.

 

 

Figure 3. Schematic representation of the triplet−triplet annihilation-based photon upconversion (TTA-UC) process. D and A represent donor and acceptor molecules, respectively. (a) Conventional molecular diffusion-based TTA-UC.

(b) Supramolecular TTA-UC. Acceptor molecules self-assemble with a regular array of acceptor chromophores. Donor molecules are coassembled with (preferably) controlled spatial molecular orientation. Upon photoexcitation of the donor and succeeding ISC, efficient TTET occurs to the neighboring acceptor, and the excited triplet state of the acceptor migrates in the selfassembled chromophore arrays. Eventually, two excited states collide and TTA occurs to give an excited acceptor, which gives higher-energy UC emission (blue arrow).

 

Research Topic 1

A nonvolatile, in-air functioning liquid photon upconverting system is developed. A rationally designed triplet sensitizer (branched alkyl chain-modified Pt(II) porphyrin) is homogeneously doped in energyharvesting liquid acceptors with a 9,10-diphenylanthracene unit. A significantly high upconversion quantum yield of 28% is achieved in the solvent-free liquid state, even under aerated conditions.

 

 

 

 

Research Topic 2

Air-stable TTA-UC was realized by dense accumulation of dye molecules in supramolecular gel fibers. By just adding donor and acceptor molecules in the gelation process, clear TTA-UC emission was successfully observed in the air-saturated condition. Moreover, the air-stable TTA-UC in supramolecular gel nanofibers was observed for a wide combination of donor-acceptor pairs which enabled near IR-to-yellow, red-to-cyan, green-to-blue, and blue-to-UV wavelength conversions.

 

 

 

 

Research Topic 3

To resolve the biggest problemin visible-to-UV photon upconversion based on sensitized triplet–triplet annihilation—the quenching of upconverted fluorescence by sensitizers—we discovered a superior sensitizer with less UV absorption intensity that enables highly efficient, low-power (0.78 mW cm-2) visible-to-UV upconversion.

 

 

 

 

 

 

 

Molecular self-assemblies in ionic liquids and ionogels

  We have launched self-assembling systems in ionic liquids. Room temperature ionic liquids are attracting much interest in many fields of chemistry and industry, due to their potential as “green” recyclable alternative to the traditional organic solvents. They are nonvolatile and provide an ultimately polar environment for chemical synthesis, liquid-liquid extraction, and electrochemical applications. However to date, two major issues remain unexplored in the chemistry of ionic liquids: one is the development of ionic liquids that can dissolve proteins and carbohydrates, and the other is formation of supramolecular assemblies.

 

Chemical structures of “sugar-philic” ionic liquids

 

  We have recently developed

ether-containing ionic liquids, which are capable of dissolving carbohydrates such as β-D-glucose, α-cyclodextrin, amylose, agarose and a glycosylated protein - glucose oxidase. When glycolipids are dispersed in these sugar-philic ionic liquids, stable bilayer membranes are formed. They display reversible thermal transformation from fibrous assemblies to vesicles. Physical gelation of ionic liquids occurs by dissolving amide group-enriched glycolipids, providing a first example of self-assembling ionogels. They are the first example of ordered molecuar assemblies formed in ionic liquids. The ability to dissolve carbohydrates and the use of such unique media for supramolecular assemblies should lead to unique characteristics and functions which are not available from the conventional molecular assemblies in aqueous or in organic media.

 

  Reference: N. Kimizuka and T. Nakashima, “Spontaneous Self-assembly of
  Glycolipid Bilayer Membranes in Sugar-philic Ionic Liquids and Formation of
  Ionogels” Langmuir, 17, 6759-6761 (2001).

 

Interfacial Materials Chemistry of Ionic liquids - controlled inorganic synthesis

  An interfacial sol-gel synthesis of inorganic hollow microspheres in room temperature ionic liquids is newly developed. When metal alkoxides such as titanium tetrabutoxide, Ti(OBu)4, are dissolved in unhydrous toluene and they are injected into 1-buthyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6) under vigorous stirring, hollow titania microspheres are formed. The present technique is widely applicable to the reactive metal alkoxides such as Zr(OBu)4, Hf(OBu)4, Nb(OBu)4, and InSn3(OR)x, giving a general route to the metal oxide microspheres. When gold nanoparicles and carboxylate-containing dyes such as fluorescein isothiocyanate (FITC) are

 

 

dissolved in the toluene microdroplets, they are stably immobilized in the microsphere shells. Calcination of the titania gel microspheres gives anatase TiO2 microspheres. The present method provides the first example of inorganic hollow microspheres formed in ionic liquids, and the ability to modify microspheres with metal nanoparticles or functional organic molecules would be widely applied to the design of smart organic/inorganic hybrid materials.

 

Self-Assembling Systems, Nanochemistry and Chemical Cybernetics

  A second area of interest involves self-assembling systems. Inspired by neurons, we are working to create self-assembling molecular wires. One of the targets is inorganic molecular wires or nanowires which are assembled through non-covalent interactions into well-defined structures in solution.

 

 

Self-assembling molecular wires. Molecular wires are indispensable elements of future molecular-scale electronic devises, and their fabrication has been one of the central issues in nanochemistry. Conventional researches are focused on the synthesis of π-conjugated oligomers and polymers, and they suffer from limitations on the type of elements that can be incorporated into the wires.

 

  We have recently developed a new strategy to manipulate nano-metal complexes by “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 their unique physicochemical properties such as intense intervalence charge-transfer (CT) absorption, semiconductivity, and large third-order nonlinear optical susceptibilities. However, these one-dimensional complexes have not been considered as a candidate for molecular wires, since they exist only in three-dimensional solids.

 

  The supramolecular packaging of the one-dimensional complex provides solubility to the solvophobic Pt-chains. The structure of lipids exerts remarkable influence on the CT band, and thus the electronic structures of one-dimensional complexes are tunable (supramolecular band gap engineering). The packaging of low-dimensional inorganic solid enables creation of novel polymer molecules that have not been dealt as molecules. This strategy should open a new dimension in mesoscopic supramolecular assemblies 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 by self-assembly.

 

 

  References: N. Kimizuka, “Towards Self-Assembling Inorganic Molecular Wires”
  Adv. Mater., 12, 1461-1463 (2000), C.-S. Lee and N. Kimizuka, PNAS, and
references therein

 

  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 alkylchain moiety, a blue gel-like phase is formed in chloroform, even at very low concentration (ca. 0.01 wt%, at room temperature). The blue color is accompanied by a structured absorption around 580-730 nm, which is characteristic of cobalt (II) in the 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 a molecular length of the triazole ligand (ca. 2.2 nm) and they are consisted of aggregates of Td coordination polymers. Very interestingly, the blue gel-like phase turned into a solution by cooling below 25 °C. A pale pink solution is obtained at 0 °C, indicating the formation of octahedral (Oh) complexes. The observed thermochromic transition is totally reversible. The formation of gel-like networks by heating is contrary to the conventional organogels, which dissolve upon heating. These observations indicate that Oh complexes present as low-molecular weight species are self-assembled to polymeric Td complexes by heating and form gel-like networks. The observed unique thermochromic transition (pink solution → blue gel-like phase) is shown to be an enthalpy-driven process. The lipophilic modification of one-dimensional coordination systems provides unique solution properties and it would be widely applicable to the design of thermoresponsive, self-assembling molecular wires.

 

  References: T. Yonezawa, S. Onoue and N. Kimizuka, “Self-Organized
  Superstructures of Fluorocarbon-Stabilized Silver Nanoparticles”, Adv .Mater. 13,
  140-142 (2001). H. Matsune, N. Nakashima and N. Kimizuka, manuscript in
  preparation.