Hideyuki Otsuka

Water‐curing capability of cementitious materials, commonly known as hydraulicity, is key to their widespread use through simply mixing powder with water to form desired robust structures. However, hydraulicity has been difficult to achieve for organic materials due to the lack of a strategy for wet curing while ensuring water resistance after curing. Here, we report unprecedented hydraulicity in an organic polymer enabled by an unusual water‐induced polarity inversion, in which the reaction of a hydrophilic polymer with water induces hydrophobization via sol–gel reaction of silatrane. A model study using a low‐molecular‐weight silatrane revealed that water drives the sol–gel reaction, which is autocatalyzed by triethanolamine generated in situ. A silatrane‐containing polymer, prepared by conventional free‐radical polymerization, was obtained as a powder but rapidly plasticized upon addition of water. The formed sticky putty gradually transformed first into a soft hydrogel and finally into a rigid solid through hydrophobization associated with the formation of silsesquioxane structures. This unique behavior enabled not only wet molding into desired structures but also underwater adhesion, as demonstrated by repairing a broken vial and bonding substrates. These findings provide a versatile design strategy for organic materials exhibiting hydraulicity.

Water‐curing capability of cementitious materials, commonly known as hydraulicity, is key to their widespread use through simply mixing powder with water to form desired robust structures. However, hydraulicity has been difficult to achieve for organic materials due to the lack of a strategy for wet curing while ensuring water resistance after curing. Here, we report unprecedented hydraulicity in an organic polymer enabled by an unusual water‐induced polarity inversion, in which the reaction of a hydrophilic polymer with water induces hydrophobization via sol–gel reaction of silatrane. A model study using a low‐molecular‐weight silatrane revealed that water drives the sol–gel reaction, which is autocatalyzed by triethanolamine generated in situ. A silatrane‐containing polymer, prepared by conventional free‐radical polymerization, was obtained as a powder but rapidly plasticized upon addition of water. The formed sticky putty gradually transformed first into a soft hydrogel and finally into a rigid solid through hydrophobization associated with the formation of silsesquioxane structures. This unique behavior enabled not only wet molding into desired structures but also underwater adhesion, as demonstrated by repairing a broken vial and bonding substrates. These findings provide a versatile design strategy for organic materials exhibiting hydraulicity.

Ladder‐type polycyclic conjugated hydrocarbons (PCHs) incorporating four‐membered rings represent a unique class of organic materials with local antiaromaticity and dearomatization induced by the four‐membered rings. [ N ]Phenylenes represent the only systematically synthesized series of PCHs containing multiple four‐membered rings or biphenylene units. Linear [ N ]phenylenes become increasingly unstable as the conjugation length extends. Here, we report the regioselective synthesis of linear [4] and [5]naphthylenes, representing the longest PCHs containing multiple four‐membered rings synthesized in solution to date. These structures were accessed through sequential catalytic arene oxanorbornene annulation reactions followed by aromatization. Raman spectroscopy and computation revealed significant dearomatization of naphthalenoid units and enhanced bond localization with increasing the conjugation length, contrasting with the typical bond delocalization trend in conventional conjugated molecules. These [ N ]naphthylenes exhibited high stability, markedly narrow blue emission profiles with very small Stokes shifts, high fluorescence quantum yields, as well as small shifts in the energy levels of frontier molecular orbitals upon increasing conjugation length. Skeletal flexibility was also observed in [4] and [5]naphthylene, but absent in [3]naphthylene, suggesting dynamic structural behaviors of longer [ N ]naphthylenes.

Ladder‐type polycyclic conjugated hydrocarbons (PCHs) incorporating four‐membered rings represent a unique class of organic materials with local antiaromaticity and dearomatization induced by the four‐membered rings. [ N ]Phenylenes represent the only systematically synthesized series of PCHs containing multiple four‐membered rings or biphenylene units. Linear [ N ]phenylenes become increasingly unstable as the conjugation length extends. Here, we report the regioselective synthesis of linear [4] and [5]naphthylenes, representing the longest PCHs containing multiple four‐membered rings synthesized in solution to date. These structures were accessed through sequential catalytic arene oxanorbornene annulation reactions followed by aromatization. Raman spectroscopy and computation revealed significant dearomatization of naphthalenoid units and enhanced bond localization with increasing the conjugation length, contrasting with the typical bond delocalization trend in conventional conjugated molecules. These [ N ]naphthylenes exhibited high stability, markedly narrow blue emission profiles with very small Stokes shifts, high fluorescence quantum yields, as well as small shifts in the energy levels of frontier molecular orbitals upon increasing conjugation length. Skeletal flexibility was also observed in [4] and [5]naphthylene, but absent in [3]naphthylene, suggesting dynamic structural behaviors of longer [ N ]naphthylenes.

Sacrificial bonds serve as key energy‐dissipating motifs, governing the toughness of elastomers. Herein, we propose a robust and versatile strategy to toughen elastomers by precisely tuning the mechanochemical reactivity of sacrificial mechanical bonds through the systematic modification of stopper structures in rotaxane crosslinkers. By varying the size and number of stoppers, the quantitative relationship between the mechanochemical response of sacrificial bonds at the molecular level and the macroscopic mechanical performance of elastomers is elucidated. This is a breakthrough that expands the practical potential of elastomers and establishes a conceptual framework for the rational design of sacrificial bonds and mechanochemicallyinspired polymer systems.

Mechanoresponsive polymers, which exhibit changes in properties such as color in response to mechanical stimuli, have attracted increasing attention for smart materials. Among various design strategies, mechanophore‐incorporated systems are widely used approaches that enable precise molecular‐level design and control; however, this approach often suffers from complicated synthesis and limited applicability to existing materials such as block copolymers, which have found widespread industrial use as high‐performance materials. Here, we present a post‐synthetic strategy enabled by exploiting microphase‐separated structures to impart mechanochromism to existing polymer systems while retaining the advantages of mechanophore‐based approaches. By exploiting the microphase‐separated structure of styrene–butadiene–styrene (SBS) block copolymers, blending with polystyrene bearing a tetraarylsuccinonitrile (TASN) mechanophore enables domain‐selective localization of the mechanophore within the rigid domains without chemical modification of the host polymer. The resulting materials exhibit fluorescence upon tensile deformation originating from force‐induced TASN cleavage, demonstrating mechanoactivation even under minimal deformation of the hard domains. The mechanoresponse is governed by the molecular weight and content of the mechanophore‐containing polystyrene. Importantly, bulk mechanical properties are preserved and can even be enhanced. This strategy establishes a scalable platform for integrating mechanochemical functionality into existing polymer systems.

Helical nanographenes (HNGs) have attracted much attention owing to their red‐region chiroptical properties. These HNGs, typically based on hexa‐peri‐hexabenzocoronene (HBC), are commonly synthesized via the Diels–Alder reaction followed by the Scholl reaction; however, the former generally requires harsh conditions (> 200°C), thereby limiting modular tuning and structural diversity. Here, we report the tunable synthesis of NH‐containing HNGs based on carbazole frameworks that enable π‐extension without protecting the NH group. Multifold Scholl reactions of 1,3,6,8‐tetrakis(oligophenyl)‐appended carbazoles, derived from a common 1,3,6,8‐tetrabromocarbazole platform, enabling uniform installation of arene units for π‐extension, afford HNGs through the formation of up to 12 C─C bonds at defined positions. The use of identical arenes suppresses regioisomer formation, while the symmetric framework enforces a single reaction pathway, avoiding regioisomeric scrambling. The resulting HNGs were successfully resolved into their enantiopure forms by chiral HPLC and exhibit distinct chiroptical properties. Notably, the unprotected NH groups enable a pronounced redshift and reversible on/off switching of circularly polarized luminescence (CPL) upon treatment with base and acid, respectively, indicating pH‐driven chiroptical switching. Furthermore, excited‐state proton transfer in dimethyl sulfoxide generates anionic species showing redshifted fluorescence and CPL even under neutral conditions.

Helical nanographenes (HNGs) have attracted much attention owing to their red‐region chiroptical properties. These HNGs, typically based on hexa‐peri‐hexabenzocoronene (HBC), are commonly synthesized via the Diels–Alder reaction followed by the Scholl reaction; however, the former generally requires harsh conditions (> 200°C), thereby limiting modular tuning and structural diversity. Here, we report the tunable synthesis of NH‐containing HNGs based on carbazole frameworks that enable π‐extension without protecting the NH group. Multifold Scholl reactions of 1,3,6,8‐tetrakis(oligophenyl)‐appended carbazoles, derived from a common 1,3,6,8‐tetrabromocarbazole platform, enabling uniform installation of arene units for π‐extension, afford HNGs through the formation of up to 12 C─C bonds at defined positions. The use of identical arenes suppresses regioisomer formation, while the symmetric framework enforces a single reaction pathway, avoiding regioisomeric scrambling. The resulting HNGs were successfully resolved into their enantiopure forms by chiral HPLC and exhibit distinct chiroptical properties. Notably, the unprotected NH groups enable a pronounced redshift and reversible on/off switching of circularly polarized luminescence (CPL) upon treatment with base and acid, respectively, indicating pH‐driven chiroptical switching. Furthermore, excited‐state proton transfer in dimethyl sulfoxide generates anionic species showing redshifted fluorescence and CPL even under neutral conditions.

Mechanochemical polymer radicals enable solvent-free oxidative cleavage of 1,1-disubstituted alkenes using molecular oxygen in the solid state.

A triazine-based succinonitrile mechanophore was designed using radical stabilization energy calculations and simulations. It shows high thermal stability and mechanochromism via C–C bond cleavage, enabling its use in radical polymerization.