Targeted internal radiotherapy is an emerging modality in nuclear medicine that relies on the systemic administration of radiopharmaceutical agents. The use of α-particle–emitting radionuclides for this application is particularly attractive due to the high cytotoxic potency and short tissue penetration range of α particles. However, the clinical translation of therapeutic α-emitters has been limited in part by challenges in their coordination chemistry, especially for radiometal ions with large ionic radii. In this Account, we describe our efforts to develop chelation strategies for therapeutically relevant α-emitting radiometals, including ²²⁵Ac, ²¹³Bi, ²¹²Pb, and ²²³Ra. A central obstacle in stabilizing these ions arises from their large size and low charge density, which render many established macrocyclic chelators suboptimal. We demonstrate how the 18-membered macrocyclic ligand macropa provides a platform for effective complexation of large radiometals under mild conditions, and how systematic structural modifications of this scaffold elucidate key structure–function relationships governing metal binding. In addition, we discuss the development of a complementary class of dual-size selective chelators capable of accommodating both large therapeutic α-emitters and smaller diagnostic radiometals, thereby enabling matched theranostic applications within a unified ligand framework. Collectively, this work illustrates how fundamental principles of coordination chemistry can guide the rational design of radiometal chelators required for the advancement of α-particle–based targeted internal radiotherapy.

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In this study, the kinetic aspects of molecular self-assembly were revealed. The self-assembly mechanisms of metal-organic cages were investigated by experimental and numerical analyses (QASAP–NASAP), found that the emergence of quasi-reversibility of certain elementary reactions in a reaction network is the key to pathway selection in the self-assembly of M₆L₄ truncated tetrahedron (TT) and higher acceleration of the later stage of the self-assembly of M₆L₄ square-based pyramid (SP) by ReO₄ ^– led to a very high yield. Novel strategies were developed for the formation of large metal–organic cages based on the remote geometric communication of multitopic ligands and kinetic control of the assembly of heteroleptic Pd(II)–coordination cages. It was also discovered that unusual reactivities of structurally very similar Pd₄L₈ interpenetrated cages that have F^– and/or Cl^– in their three pockets and interpreted the different reactivities based on a new index that can evaluate the suitability of anions to the pockets.

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Development of catalytic transformations of ubiquitous small molecules (e.g., CO₂, N₂, and H₂O) using renewable energy (especially solar energy) is one of the most important research topics for achieving a carbon-neutral society. In this context, we have developed various visible-light-driven catalytic transformations of small molecules using a combination of transition-metal complexes. In this paper, we review our recent achievements, particularly in photocatalytic CO₂ reduction and nitrogen fixation. First, we have successfully synthesized multinuclear complexes, which function as effective photocatalysts (or electrocatalysts) for the reduction of low-concentration CO₂. The reasons for their high catalytic activity and the reaction mechanism of photocatalytic CO₂ reduction were investigated in detail using Ru(II)-Re(I) binuclear complexes, constructed with a Ru(II) photosensitizer unit and a Re(I) catalyst unit, by means of time-resolved spectroscopy. Second, molecular photocathodes, prepared by electrochemical polymerization of metal complexes on semiconductor electrodes, enabled durable photocatalytic CO₂ reduction using water as an electron source. Third, we have successfully developed the following photocatalytic nitrogen fixation as the first examples: (1) uphilltype photocatalytic NH₃ formation using dihydroacridine as both electron and proton sources, and (2) photocatalytic NH₃ formation from N₂ and water utilizing photochemical activation of water with a tertiary phosphine. These studies will provide important insights into the design of efficient photocatalytic systems for small-molecule transformations driven by renewable energy.

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This account summarizes our systematic development of rigid π-conjugated dendrimers as precisely synthetic platforms that enable functions inaccessible to conventional linear polymers. By focusing on the intrinsic geometric density gradient of dendritic architectures, we designed phenylazomethine dendrimers (DPAs) that generate well-defined intramolecular electronic and potential gradients. These gradients drive a unique stepwise metal ion coordination behavior, allowing metal ions to be accumulated in a highly ordered manner with atomic-level precision. Building on this concept, we established the “atom-hybridization” which enables the synthesis of subnanometer metal and alloy clusters with strictly defined atomic counts and compositions, including multi-element subnano alloy particles and superatoms. Such atomically precise subnano particles exhibit unprecedented atom-number-dependent catalytic activities, strong alloy effects, and low-temperature reactivity beyond the scope of conventional nanoparticles. Furthermore, the rigid dendritic framework provides stable internal nanospaces that support enzyme-like molecular shape recognition, induced-fit catalysis, and long-lived charge-separated states driven by designed potential gradients. Together, these studies establish dendrimers as a versatile molecular platform that bridges polymer chemistry, coordination chemistry, catalysis, and quantum materials science, offering a general strategy for designing functional materials based on atomic-scale control.

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Discrete stacked complexes constructed from planar and macrocyclic molecules have emerged as versatile platforms for precise molecular organization and exploration of their functions. This review summarizes recent advances in the design and construction of such complexes based on coordination-driven self-assembly. Three representative strategies are highlighted: (i) Metal-mediated sandwich stacking, (ii) pillared architectures using bridging organic ligands, and (iii) self-complementary stacking driven by intermolecular coordination. These approaches enable precise control over stacking number, interplanar distance, and the spatial arrangement of metal centers and coordination sites. As a result, a variety of discrete assemblies with well-defined molecular stacking and/or internal spaces have been realized. These confined environments provide unique opportunities for molecular recognition, selective catalysis, and photofunctional processes, often through cooperative interactions such as multipoint coordination and aromatic interactions. In addition, recent studies demonstrate dynamic control of stacking structures triggered by external stimuli or guest molecules, expanding the scope of functional design. The concepts and strategies discussed herein offer useful guidelines for constructing sophisticated supramolecular architectures and are expected to inspire future developments in coordination chemistry and functional molecular systems.

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