Soichiro Ogi

Living supramolecular polymerization realized through a biomimetic approach

Various conventional reactions in polymer chemistry have been translated to the supramolecular domain, yet it has remained challenging to devise living supramolecular polymerization. To achieve this, self-organization occurring far from thermodynamic equilibrium—ubiquitously observed in nature—must take place. Prion infection is one example that can be observed in biological systems. Here, we present an ‘artificial infection’ process in which porphyrin-based monomers assemble into nanoparticles, and are then converted into nanofibres in the presence of an aliquot of the nanofibre, which acts as a ‘pathogen’. We have investigated the assembly phenomenon using isodesmic and cooperative models and found that it occurs through a delicate interplay of these two aggregation pathways. Using this understanding of the mechanism taking place, we have designed a living supramolecular polymerization of the porphyrin-based monomers. Despite the fact that the polymerization is non-covalent, the reaction kinetics are analogous to that of conventional chain growth polymerization, and the supramolecular polymers were synthesized with controlled length and narrow polydispersity. Self-organization that occurs far from thermodynamic equilibrium is ubiquitous in nature but has remained challenging to control in synthetic supramolecular systems. A complex system has now been devised that displays such behaviour. Porphyrin derivative monomers undergo living supramolecular polymerization, a reaction underpinned by the interplay of two supramolecular polymerization pathways.

Mechanism of Self-Assembly Process and Seeded Supramolecular Polymerization of Perylene Bisimide Organogelator

The mechanism of supramolecular polymerization has been elucidated for an archetype organogelator molecule composed of a perylene bisimide aromatic scaffold and two amide substituents. This molecule self-assembles into elongated one-dimensional nanofibers through a cooperative nucleation-growth process. Thermodynamic and kinetic analyses have been applied to discover conditions (temperature, solvent, concentration) where the spontaneous nucleation can be retarded by trapping of the monomers in an inactive conformation, leading to lag times up to more than 1 h. The unique kinetics in the nucleation process was confirmed as a thermal hysteresis in a cycle of assembly and disassembly processes. Under appropriate conditions within the hysteresis loop, addition of preassembled nanofiber seeds leads to seeded polymerization from the termini of the seeds in a living supramolecular polymerization process. These results demonstrate that seeded polymerizations are not limited to special situations where off-pathway aggregates sequester the monomeric reactant species but may be applicable to a large number of known and to be developed molecules from the large family of molecules that self-assemble into one-dimensional nanofibrous structures. Generalizing from the mechanistic insight into our seeded polymerization, we assert that a cooperative nucleation-growth supramolecular polymerization accompanied by thermal hysteresis can be controlled in a living manner.

Impact of Alkyl Spacer Length on Aggregation Pathways in Kinetically Controlled Supramolecular Polymerization.

We have investigated the kinetic and thermodynamic supramolecular polymerizations of a series of amide-functionalized perylene bisimide (PBI) organogelator molecules bearing alkyl spacers of varied lengths (ethylene to pentylene chains, PBI-1-C2 to PBI-1-C5) between the amide and PBI imide groups. These amide-functionalized PBIs form one-dimensional fibrous nanostructures as the thermodynamically favored states in solvents of low polarity. Our in-depth studies revealed, however, that the kinetic behavior of their supramolecular polymerization is dependent on the spacer length. Propylene- and pentylene-tethered PBIs follow a similar polymerization process as previously observed for the ethylene-tethered PBI. Thus, the monomers of these PBIs are kinetically trapped in conformationally restricted states through intramolecular hydrogen bonding between the amide and imide groups. In contrast, the intramolecularly hydrogen-bonded monomers of butylene-tethered PBI spontaneously self-assemble into nanoparticles, which constitute an off-pathway aggregate state with regard to the thermodynamically stable fibrous supramolecular polymers obtained. Thus, for this class of π-conjugated system, an unprecedented off-pathway aggregate with high kinetic stability could be realized for the first time by introducing an alkyl linker of optimum length (C4 chain) between the amide and imide groups. Our current system with an energy landscape of two competing nucleated aggregation pathways is applicable to the kinetic control over the supramolecular polymerization by the seeding approach.

Kinetic Control over Pathway Complexity in Supramolecular Polymerization through Modulating the Energy Landscape by Rational Molecular Design

Far-from-equilibrium thermodynamic systems that are established as a consequence of coupled equilibria are the origin of the complex behavior of biological systems. Therefore, research in supramolecular chemistry has recently been shifting emphasis from a thermodynamic standpoint to a kinetic one; however, control over the complex kinetic processes is still in its infancy. Herein, we report our attempt to control the time evolution of supramolecular assembly in a process in which the supramolecular assembly transforms from a J-aggregate to an H-aggregate over time. The transformation proceeds through a delicate interplay of these two aggregation pathways. We have succeeded in modulating the energy landscape of the respective aggregates by a rational molecular design. On the basis of this understanding of the energy landscape, programming of the time evolution was achieved through adjusting the balance between the coupled equilibria.

Oligofluorene-based nanoparticles in aqueous medium: hydrogen bond assisted modulation of functional properties and color tunable FRET emission

Fluorene-based linear π-conjugated oligomers with different end functional groups having zero- (OF1), one- (OF2), two- (OF3) and three-point (OF4) hydrogen bonding sites were synthesized and characterized. By using a reprecipitation method, self-assembled nanoparticles were prepared in aqueous medium. The spherical shape and amorphous nature of nanoparticles were established by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Zeta potential measurements showed that nanoparticles of OF2–4 have good colloidal stability, whereas those of OF1 have only moderate stability indicating that the hydrogen bonding groups in OF2–4 interact with the polar water molecules providing stability to the assembly. However, the interior of the nanoparticles remained non-polar, thus providing a conducive medium for hydrogen bonding between the oligofluorene molecules. This leads to varying interchromophore interactions in OF1–4 in the nanoparticle state depending on the H-bonding strength of the end groups. Dynamic light scattering (DLS) studies revealed that under identical conditions, the size of the nanoparticles decreased with increasing number of hydrogen bonding sites in the molecule. The interchromophore interactions were evident from the UV-Vis absorption and fluorescence studies. Bright blue fluorescence of the molecules in solution undergoes quenching in the nanoparticle state. The fluorescence quenching significantly increases from OF1 to OF4 indicating enhanced interaction between chromophores with increasing number of hydrogen bonding sites in the molecules. The nanoparticles were used as a donor scaffold for fluorescence resonance energy transfer (FRET) by encapsulating varying amounts of an orange red emitting neutral dye (D1) thereby achieving colour tunable emission including white. FRET studies were also conducted with a cationic dye (D2) adsorbed on the negatively charged nanoparticle surface. The FRET efficiency with both dyes showed direct correlation with the number of hydrogen bonding sites in the molecules.

Stimuli‐Responsive Folding and Unfolding of a Polymer Bearing Multiple Cerium(IV) Bis(porphyrinate) Joints: Mechano‐imitation of the Action of a Folding Ruler

A pivotal guest role: a new porphyrin polymer, poly(PorZn⋅DD) (pink/purple), composed of a porphyrinatozinc and a porphyrin double-decker complex as a repeating unit was synthesized. In poly(PorZn⋅DD), porphyrinatozinc complexes recognize a divalent amine (tan/red) to induce an intramolecular pivoting motion through the rotation of porphyrin double-decker complexes and the polymer undergoes shortening and compaction.

Synthesis of Self-Threading Bithiophenes and their Structure–Property Relationships Regarding Cyclic Side-Chains with Atomic Precision

We have recently reported a self-threading polythiophene as a new family of insulated molecular wires. Herein, we focused on the structure-property relationships of the unique three-dimensional architecture of the monomer. We have synthesized nine self-threading bithiophene monomers that have cyclic side-chains of different size and flexibility: i.e., 21-, 22-, 23-, 24-, 26-, and 30-membered rings composed of paraffinic, olefinic, or alkynic chains. To investigate their structure-property relationships, (1) H NMR spectroscopy, UV absorption, and fluorescence spectroscopy measurements were conducted. We found that cyclic side-chains define the movable range of the dihedral angle of the bithiophene backbone, thereby affecting its photophysical properties. Therefore, the ability to design a structure with atomic precision as described herein would lead to the fine-tuning of the electronic properties of insulated molecular wires.

Synthesis of polyaniline with low polydispersity by using a supramolecular ionic assembly as the reaction medium.

A supramolecular ionic assembly comprised of an anionic oligo(phenylene ethynylene) and anilinium cations provides a unique reaction medium in which anilinum cations are concentrated and aligned. The oxidative polymerization (see figure) of aniline using the supramolecular ionic assembly (gray) yielded polyaniline (green/blue) with a number-average molar mass of 20,500 and polydispersity of 1.3.