Fumitaka Ishiwari

Rational control of a polyacetylene helix by a pendant rotaxane switch

Polyacetylene bearing a pendant rotaxane moiety with an optically active wheel component was synthesized to realize reversible structural control of its helical structure by position control of the wheel component. Polyacetylene formed a one-handed helical structure only when the optically active wheel component moved close to the main chain.

C−C Bond-Forming Click Synthesis of Rotaxanes Exploiting Nitrile N-Oxide

A click end-capping reaction exploiting nitrile N-oxide to rotaxane was described with emphasis of productivity of the protocol via stable C-C bond formation. Establishment of a pH-driven molecular shuttling system was also demonstrated by practical neutralization of the sec-ammonium group of the rotaxane axle with potassium hydroxide.

Reversible helix–random coil transition of poly(m-phenylenediethynylene) by a rotaxane switch

Pendant rotaxane switch-tethering poly(m-phenylene diethynylene) was synthesized by the polyoxidative coupling of a rotaxane containing an axle-terminal m-diethynylbenzene group and an optically active crown ether. The reversible helix-random coil transition of the polymer was successfully performed by the positional switching of the rotaxane wheel.

A Rational Design for the Directed Helicity Change of Polyacetylene Using Dynamic Rotaxane Mobility by Means of Through‐Space Chirality Transfer

Directed helicity control of a polyacetylene dynamic helix was achieved by hybridization with a rotaxane skeleton placed on the side chain. Rotaxane-tethering phenylacetylene monomers were synthesized in good yields by the ester end-capping of pseudorotaxanes that consisted of optically active crown ethers and sec-ammonium salts with an ethynyl benzoic acid. The monomers were polymerized with [{RhCl(nbd)}(2)] (nbd=norbornadiene) to give the corresponding polyacetylenes in high yields. Polymers with optically active wheel components that are far from the main chain show no Cotton effect, thereby indicating the formation of racemic helices. Our proposal that N-acylative neutralization of the sec-ammonium moieties of the side-chain rotaxane moieties enables asymmetric induction of a one-handed helix as the wheel components approach the main chain is strongly supported by observation of the Cotton effect around the main-chain absorption region. A polyacetylene with a side-chain rotaxane that has a shorter axle component shows a Cotton effect despite the ammonium structure of the side-chain rotaxane moiety, thereby suggesting the importance of proximity between the wheel and the main chain for the formation of a one-handed helix. Through-space chirality induction in the present systems proved to be as powerful as through-bond chirality induction for formation of a one-handed helix, as demonstrated in an experiment using non-rotaxane-based polyacetylene that had an optically active binaphthyl group. The present protocol for controlling the helical structure of polyacetylene therefore provides the basis for the rational design of one-handed helical polyacetylenes.

Rational synthesis of organic thin films with exceptional long-range structural integrity

Standing at order Thin films of organic molecules on solid substrates tend to nucleate at many sites and grow multiple domains. However, one large uniform film would be much more desirable in device applications. Seiki et al. designed organic molecules that filled space in a hexagonal tiling; a propeller-like triptycene base adhered to crystalline surfaces and alkyl tails extended away from it. The authors could make well-ordered multilayer films up to centimeter length scales. Science, this issue p. 1122 Tripodal triptycene building blocks that fill space form large-area organic thin films free of domain boundaries. Highly oriented, domain-boundary–free organic thin films could find use in various high-performance organic materials and devices. However, even with state-of-the-art supramolecular chemistry, it is difficult to construct organic thin films with structural integrity in a size regime beyond the micrometer length scale. We show that a space-filling design, relying on the two-dimensional (2D) nested hexagonal packing of a particular type of triptycene, enables the formation of large-area molecular films with long-range 2D structural integrity up to the centimeter length scale by vacuum evaporation, spin-coating, and cooling from the isotropic liquid of the triptycene. X-ray diffraction analysis and microscopic observations reveal that triptycene molecules form a completely oriented 2D (hexagonal triptycene array) + 1D (layer stacking) structure, which is key for the long-range propagation of structural order.

Bioinspired design of a polymer gel sensor for the realization of extracellular Ca(2+) imaging.

Although the role of extracellular Ca2+ draws increasing attention as a messenger in intercellular communications, there is currently no tool available for imaging Ca2+ dynamics in extracellular regions. Here we report the first solid-state fluorescent Ca2+ sensor that fulfills the essential requirements for realizing extracellular Ca2+ imaging. Inspired by natural extracellular Ca2+-sensing receptors, we designed a particular type of chemically-crosslinked polyacrylic acid gel, which can undergo single-chain aggregation in the presence of Ca2+. By attaching aggregation-induced emission luminogen to the polyacrylic acid as a pendant, the conformational state of the main chain at a given Ca2+ concentration is successfully translated into fluorescence property. The Ca2+ sensor has a millimolar-order apparent dissociation constant compatible with extracellular Ca2+ concentrations, and exhibits sufficient dynamic range and excellent selectivity in the presence of physiological concentrations of biologically relevant ions, thus enabling monitoring of submillimolar fluctuations of Ca2+ in flowing analytes containing millimolar Ca2+ concentrations.

An anion-conductive microporous membrane composed of a rigid ladder polymer with a spirobiindane backbone

A free-standing membrane that consists of a newly synthesized spirobiindane-based microporous polymer carrying trimethylammonium hydroxide groups exhibited a good OH− conductivity of 65 mS cm−1 at 80 °C under 100% relative humidity, which demonstrates its potential for use in solid-state alkaline fuel cells.

Induction of Single-Handed Helicity of Polyacetylenes Using Mechanically Chiral Rotaxanes as Chiral Sources

Effective induction of preferred-handed helicity of polyacetylenes by pendant mechanically chiral rotaxanes is discussed. Polyacetylenes possessing optically active mechanically chiral rotaxanes in the side chains were synthesized by the polymerization of the corresponding enantiopure [2]rotaxane-type ethynyl monomers prepared by the chiral-phase HPLC separations. The CD Cotton effects revealed that the polyacetylenes took preferred-handed helical conformations depending on the rotaxane chirality. The preferred-handed helix was not disturbed by an additional chiral substituent on the rotaxane side chain. These results demonstrate the significance and utility of mechanically chiral rotaxanes for the effective construction of asymmetric fields.

Highly efficient transformation of linear poly(phenylene ethynylene)s into zigzag-shaped π-conjugated microporous polymers through boron-mediated alkyne benzannulation

Porous polymers offer great advantages compared to other microporous materials in terms of solubility and processability. However, the design of porous polymers has suffered from the limited availability of suitable building blocks. Here we propose a conceptually new strategy for the design of porous polymers, which involves the transformation of a rigid linear polymer into a rigid zigzag polymer with a large free volume around the polymer backbone. This strategy relies on a boron-mediated alkyne benzannulation reaction, which was recently developed by our group. When the benzannulation reaction was applied to poly(phenylene ethynylene) (PPE) derivatives, a linear-to-zigzag structural transformation successfully occurred to give the corresponding π-conjugated polymers with a diarylphenanthrene unit in the main chain. As revealed by N2 adsorption experiments, while the parent PPEs were non-porous, the zigzag polymers in the solid state possessed porosity with a specific surface area of up to 366 m2 g−1, where the surface area largely depended on the steric bulkiness of the substituents on the polymer. Considering the fact that a wide variety of PPE derivatives have so far been synthesized, the present strategy may open a new avenue for the development of functional porous polymers.

Synthesis and Catalytic Applications of a Triptycene-Based Monophosphine Ligand for Palladium-Mediated Organic Transformations

1-Methoxy-8-(diphenylphosphino)triptycene (1), featuring high structural rigidity and steric bulkiness around the phosphine functionality, was synthesized as a new chiral monophosphine ligand for transition metal-catalyzed reactions. In the presence of 5–10 mol ppm (i.e., 0.0005–0.001 mol %) Pd(OAc)2 and 1 (2 equiv for Pd), Suzuki–Miyaura cross-coupling reactions of aryl bromides and arylboronic acids proceeded effectively under mild atmospheric conditions to give the corresponding biaryl compounds in a high yield. The single-crystal X-ray analysis of a Pd(II) complex of 1 revealed its coordination structure, in which two homochiral molecules form a dimer, suggesting that triptycene could provide a chiral environment for asymmetric organic transformations. In fact, optically active 1 obtained by optical resolution showed good enantioselectivity in the palladium-catalyzed asymmetric hydrosilylation of styrene, which represents, for the first time, the asymmetric catalytic activity of triptycene-based monophosphine ligands.