The development of synergistic functions of ordered structures formed by molecular self-assembly has been a significant methodology in materials chemistry. Prof. Nobuo Kimizuka pioneered several new concepts in molecular self-assembly and functional nanomaterials chemistry. While conventional molecular assembly has been studied in the vicinity of thermodynamic equilibrium, Prof. Kimizuka discovered for the first time the formation of dissipative nanostructures based on molecular self-assembly under non-equilibrium conditions. Prof. Kimizuka also developed supramolecular one-dimensional metal complexes whose electronic interactions or spin states are controlled via self-assembly, guest-adaptive self-assembly of aqueous coordination networks, solvent-free supramolecular solar thermal fuels with photoliquefaction and phase-transition characteristics, and triplet fusion photon upconversion based on triplet energy migration in designed chromophore self-assemblies. His photon energy conversion works provided the perspective that the capability of pursuing valuable, energetically uphill work is an essential feature for being called a molecular system. The following is an overview of his significant achievements.

1. Solution chemistry of one-dimensional (1D) metal complexes and functional control based on molecular self-assembly
Prof. Kimizuka developed supramolecular methodologies to convert quasi-one-dimensional metal complexes, which had previously been the subject of research in solid-state science, into nanowire complexes dispersible in solutions. He realized control in electronic states and spin properties in response to the self-assembly of coordination main chains, thereby pioneering the solution chemistry of functional nanometal complexes. Discovery of spin conversion phenomena based on significant stabilization of low-spin states in lipid-packaged 1D Fe(II) triazole complexes (nano-interface effect) and their spin transition concerted with dissociation of the complex main chains, macroscopic dielectric orientation of 1D halogen-bridged dinuclear diruthenium complexes, aqueous self-assembly of polyoxometalates into giant nanosheets and their optical manipulation. These achievements laid the foundation for a unique solution functional chemistry in self-assembled nanometal complexes.

2. Discovery of dissipative nanostructures and spatio-temporal formation of metallic nanostructures
Although dissipative structures formed in non-equilibrium liberated systems have macroscopic scales, the existence of nanoscale dissipative structures is not known. Prof. Kimizuka investigated the photoreduction of Au(III) complexes at the water-organic interface and found that nanolevel dissipative structures, "dissipative nanostructures", are formed only in molecular fluxes across the interface under non-equilibrium conditions. The generality of this finding was confirmed in solid-liquid interfacial systems. Furthermore, the formation of metal nanocrystals was viewed from the unique perspective of "self-assembly of metal ions coupled with redox reactions". Photoreduction of Au(III) ions and an oxidative dissolution reaction using dissolved oxygen were investigated in one pot. As a result, gold nanoclusters with unique and complex shapes, such as flower crowns and propeller shapes, were obtained, which indicated that photoreduction and oxidative etching reactions proceeded at different spatiotemporal kinetics. This phenomenon was explained by the higher concept of "self-organization at the spatio-temporal level with chemical reactions. The discovery of dissipative nanostructures by Prof. Kimizuka has opened the field of molecular organization and materials chemistry using non-equilibrium interfacial systems.

3. Adaptive self-assembly ̶ aqueous coordination networks from nucleotides and lanthanide Ions
Prof. Kimizuka found that the self-assembly of nucleotides and lanthanide ions in water gives amorphous coordination networks that adaptively encapsulate guests of various shapes and sizes (dyes, metal complexes, metal nanoparticles, quantum dots, etc.). Nanoparticles formed from nucleotides and gadolinium ions showed superior performance as contrast MRI agents compared to commercial MRI agents. The platinum porphyrin molecules adaptively encapsulated in these nanoparticles emit phosphorescence even in the presence of dissolved oxygen, indicating that the dense coordination networks exhibit an oxygen barrier function. Protecting the excited triplet from oxygen by self-assembly led to the conception and development of molecular self-assembly-based photon upconversion chemistry.

4. Solvent-free molecular solar thermal fuels (STFs) with phase-transition characteristics
Photoisomerization reactions can store photon energy as molecular strain energy, which is released as heat upon reversed isomerization. These reversible photoisomerization processes have been applied to molecular STFs, and azobenzenes are popularly used because of their chemical stability. Meanwhile, photoisomerization of azobenzenes usually occurs in solutions, but not in the dense solid state. The dissolution in solvents inevitably reduces the energy density, and an open problem in the study of STFs was that the isomerization heat of azobenzenes limits the performance of molecular STFs. To solve these issues, Prof. Kimizuka developed room-temperature liquid azobenzenes, arylazopyrazoles that show reversible photoisomerization without dilution, and azobenzene crystals that exhibit photoliquefaction and phase-transition. The latter, photoliquefied cis-azobenzenes, exhibits cis→trans isomerization energy plus latent heat associated with the liquid-crystal phase transition, giving approximately twice the heat storage capacity. The concept of phase-transition STFs he created caused a paradigm shift in STF research around the world.

5. Development of triplet fusion upconversion based on molecular self-assembly.
Prof. Kimizuka developed supramolecular photon upconversion based on triplet-triplet annihilation following triplet energy migration among self-assembled chromophores. Metal complexes that exhibit heavy atom effects were used as triplet sensitizers, and are co-assembled with acceptor chromophores organized in various molecular assemblies such as π-electron liquids, ionic liquids, supramolecular gels, molecular membranes, organic nanoparticles, 1D coordination polymers, and polymer films. The energy landscapes of the excited states are controlled on the basis of molecular self-assembly, which allows UC even in the presence of dissolved oxygen. For the first time, Prof. Kimizuka further developed singlet fission (SF) in chiral molecular self-assemblies, providing a new perspective of SF by designed self-assembly.

As described above, Prof. Kimizuka has created a new interdisciplinary field of molecular self-assembly and various areas of coordination chemistry, inorganic chemistry, non-equilibrium science, and photochemistry, laying the foundation of molecular systems chemistry. These ingenious achievements have impacted multiple fields of chemistry and related disciplines. Therefore, his achievements are recognized as worthy of the Chemical Society of Japan Award.