Tatsuo Kojima

Rate-determining step in the self-assembly process of supramolecular coordination capsules

An in-depth understanding of the self-assembly process at the molecular level is crucial in both biological and materials science fields. However, such research is scarce due to the difficulty in monitoring a great deal of fragmentary species that are transiently produced in the process. We present a novel method for investigating the self-assembly process of supramolecular coordination assemblies by following the time variation of the average composition of the fragmentary species, which was indirectly determined by spectroscopy. With this method, we found that the final stage is the rate-determining step of the self-assembly of an octahedron-shaped coordination capsule, and that the relative energy barrier of each step is controllable by modifying the chemical structure of the building blocks.

Synthesis of π-Conjugated Dendrimers Based on Azaborines

pi-Conjugated dendrons and dendrimers based on dibenzoazaborine were synthesized. The azaborine dendrons exhibited strong light absorption and photoluminescence, reflecting the optical properties of the azaborine. The fluorescence from the azaborine dendrimers bearing a benzothiadiazole core was strongly red-shifted or quenched, indicating photoinduced electron transfer from the azaborine dendrons to the core unit.

Structural Dynamics of Overcrowded Alkene-Based Molecular Motors during Thermal Isomerization

Synthetic light-driven rotary molecular motors show complicated structural dynamics during the rotation process. A combination of DFT calculations and various spectroscopic techniques is employed to study the effect of the bridging group in the lower half of the molecule on the conformational dynamics. It was found that the extent to which the bridging group can accommodate the increased folding in the transition state is the main factor in rationalizing the differences in barrier height and, as a consequence, the rotary speed. These findings will be essential in designing future rotary molecular motors.

Self-Assembly Process of Dodecanuclear Pt(II)-Linked Cyclic Hexagon.

The self-assembly process of a Pt(II)-linked hexagonal macrocycle consisting of six linear dinuclear Pt(II) units and six organic ditopic bent ligands was investigated. The process was monitored by (1)H NMR, and the intermediates in the self-assembly were analyzed by the n-k analysis. It was found that a 1:2 complex of a dinuclear Pt(II) unit and an organic ditopic ligand was exclusively observed as an intermediate with a certain lifetime and that the reaction of the 1:2 complex is the rate-determining step in the supramolecular macrocycle formation. The key 1:2 complex was unambiguously characterized by (1)H and DOSY NMR and ESI-TOF mass measurement.

Selective alternate derivatization of the hexaphenylbenzene framework through a thermodynamically controlled halogen dance.

We report a practical synthetic protocol and mechanistic details for the selective alternate derivatization of the hexaphenylbenzene (HPB) framework through a thermodynamically controlled halogen dance. The stability of the alternately trilithiated species of HPB is interpreted by the through-space interaction at the ipso-carbons of the phenyl groups of the HPB framework. By using this approach, C3-symmetric and lower-symmetric HPB derivatives possessing two or three kinds of substituents on the periphery have become easily and practically available.

Quantitative Analysis of Self‐Assembly Process of a Pd2L4 Cage Consisting of Rigid Ditopic Ligands

The self-assembly process of a Pd2 L4 cage complex consisting of rigid ditopic ligands, in which two 3-pyridyl groups are connected to a benzene ring through acetylene bonds and PdII ions was revealed by a recently developed quantitative analysis of self-assembly process (QASAP), with which the self-assembly process of coordination assemblies can be investigated by monitoring the evolution with time of the average composition of all the intermediates. QASAP revealed that the rate-determining steps of the cage formation are the intramolecular ligand exchanges in the final stage of the self-assembly: [Pd2 L4 Py*2 ]4+ →[Pd2 L4 Py*1 ]4+ +Py* and [Pd2 L4 Py*1 ]4+ →[Pd2 L4 ]4+ +Py* (Py*: 3-chloropyridine, which was used as a leaving ligand on the metal source). The energy barriers for the two reactions were determined to be 22.3 and 21.9 kcal mol-1 , respectively. DFT calculations of the transition-state (TS) structures for the two steps indicated that the distortion of the trigonal-bipyramidal PdII center at the TS geometries increases the activation free energy of the two steps.

The Effect of Solvent and Coordination Environment of Metal Source on the Self-Assembly Pathway of a Pd(II)-Mediated Coordination Capsule

The effect of reaction environment on the self-assembly process of an octahedron-shaped Pd6L8 capsule was investigated. Quantitative analysis of self-assembly process with 1H NMR spectroscopy revealed that the self-assembly pathway of the capsule was altered by solvent and a leaving ligand coordinating to the metal source, which are not the components of the final self-assembly. Solvents definitively determine the pathway of the self-assembly at a very early stage of the self-assembly. Contrary to the expectation that the weaker the coordination ability of the leaving ligand is, the faster the formation of the final assembly becomes, a leaving ligand with weak coordination ability tends to generate a kinetically trapped species to prevent the capsule formation under mild conditions.

Quantitative Analysis of the Self‐Assembly Process of Hexagonal PtII Macrocyclic Complexes: Effect of the Solvent and the Components

The self-assembly process of three PtII -linked hexagonal macrocycles consisting of dinuclear PtII complexes and organic ditopic ligands was investigated in polar and less polar solvents by a recently developed approach: quantitative analysis of the self-assembly process (QASAP). In polar CD3 NO2 , for all the three macrocycles, an ML2 complex was the dominant intermediate during self-assembly, as a result of high positive allosteric cooperativity for the ligand exchange on the PtII centers of the dinuclear PtII complexes. On the other hand, in less polar CD2 Cl2 , the self-assembly process was affected by the components. For two of the three macrocycles, the chainlike oligomers that contain fewer metals and ligands than the corresponding macrocycles grew with time and the type of the chainlike intermediates formed correlated with the allostericity of the two binding sites in the organic ditopic ligands. In every case, no long oligomers containing more components than the macrocycles themselves were produced during the self-assembly even though free rotation around single bonds in the chainlike oligomers allows them to adopt various conformations that do not facilitate the cyclization. This result suggests that electrostatic and/or steric factors besides rigidity of the components make the cyclization advantageous not only thermodynamically but also kinetically.

Hyperthermostable cube-shaped assembly in water

Proteins in hyperthermophiles exhibit extremely high thermal stability unlike general proteins. These thermostable proteins are stabilized by weak molecular interactions such as hydrogen bonding, charge interactions and van der Waals (vdW) interactions, along with the hydrophobic effect. An in-depth understanding of the stabilization mechanisms will enable us to rationally design artificial molecules with very high thermal stability. Here we show thermally stable supramolecular assemblies composed of six identical amphiphilic molecules having an indented hydrophobic surface, held together by weak intermolecular interactions (vdW and cation-π interactions) and the hydrophobic effect in water. The disassembly temperature of one of the assemblies is over 150 °C, which is higher than that of the most hyperthermophilic protein reported to date (PhCutA1). Study of the relationship between the structure of the components and the stability of the assemblies indicates that the hyperthermostability is achieved only if all the weak interactions and the hydrophobic effect work cooperatively.The self-assembly of thermally stable structures in water is a challenge in supramolecular chemistry. Here, cooperativity between weak intramolecular forces allows amphiphiles to associate into cube-shaped assemblies that are thermally stable in water up to 150 °C.

Quantitative Analysis of Self-assembly Process of a Pd12L24 Coordination Sphere

The self-assembly process of a Pd12 L24 sphere was revealed by a quantitative approach (quantitative analysis of self-assembly process: QASAP) quantifying all the substrates, the products, and the observable intermediates, indicating that the Pd12 L24 sphere is produced through several pathways. Firstly, Pdn L2n (n=6, 8, and 9), which are perfectly closed structures smaller than the Pd12 L24 sphere, and a mixture of intermediates not observed by NMR (Int) were produced. Next, the sphere was assembled from intra-/intermolecular reaction of a certain class of Int (path A) and from the coordination of free pyridyl groups in Int to the PdII center of Pdn L2n (n=6, 8, and 9) (path B). While capping the free pyridyl groups in Int with PdII ions perfectly inhibited the sphere formation, the addition of free L to Int accelerated the formation of the sphere.