Katsuhiro Maeda

Memory of macromolecular helicity assisted by interaction with achiral small molecules

The helicity of biological macromolecules such as DNA and proteins is largely governed by the homochirality of their components (D-sugars and L-amino acids). In polymer and supramolecular chemistry, control of helicity is an attractive goal because of possible applications in materials science, chemical sensing and enantioselective catalysis. We reported recently that macromolecular helicity can be induced in a polymer by an optically active amine. Here we show that this helicity can be ‘memorized’ when the amine is replaced by various achiral amines. Although the maintenance of helicity in the polymer is not perfect, it can ‘repair’ itself over time. Small structural changes in the achiral amines influence the efficiency of helicity retention markedly.

Single- and Double-Stranded Helical Polymers: Synthesis, Structures, and Functions

Biological macromolecules, such as DNA and proteins, possess a unique and specific ordered structure, such as a right-handed double helix or a single alpha-helix. Those structures direct the sophisticated functions of these molecules in living systems. Inspired by biological helices, chemists have worked to synthesize polymers with controlled helicity, not only to mimic the biological helices but also to realize their functions. Although numerous synthetic polymers that fold into a single-handed helix have been reported, double-stranded helical polymers are almost unavailable except for a few oligomers. In addition, the exact structures of most helical polymers remain obscure. Therefore, the development of a conceptually new method for constructing double-stranded helical polymers and a reliable method for unambiguously determining the helical structures are important and urgent challenges in this area. In this Account, we describe the recent advances in the synthesis, structures, and functions of single- and double-stranded helical polymers from our group and others and provide a brief historical overview of synthetic helical polymers. We found unique macromolecules that fold into a preferred-handed helix through noncovalent bonding interactions with specific chiral guests. During the noncovalent helicity induction process, these guest molecules significantly amplified chirality in a dynamic helical polymer. During the intensive exploration of the helicity induction mechanism, we observed an unusual macromolecular helical memory in dynamic helical polymers. Furthermore, we found that rigid-rod helical poly(phenylacetylene)s and poly(phenyl isocyanide)s showing a cholesteric or smectic liquid crystal self-assemble to form two-dimensional crystals with a controlled helical conformation on solid substrates upon exposure to solvent vapors. We visualized their helical structures including the helical pitch and handedness by atomic force microscopy (AFM). We propose a modular strategy to construct complementary double helices by employing chiral amidinium-carboxylate salt bridges with m-terphenyl backbones. The double-stranded helical structures were characterized by circular dichroism in solution and X-ray diffraction of the crystals or the direct AFM observations. Serendipitously, we found that oligoresorcinols self-assemble into well-defined double helices resulting from interstrand aromatic stacking in water. These oligoresorcinols bound cyclic and linear oligosaccharides in water to form rotaxanes and hetero-double helices, respectively. The examples presented in this Account demonstrate the notable progress in the synthesis and structural determination of helical polymers including single- and double-stranded helices. Not only do we better understand the principle underlying the generation of helical conformations, but we have also used the knowledge of these unique helical structures to develop novel helical polymers with specific functions.

Hierarchical Amplification of Macromolecular Helicity of Dynamic Helical Poly(phenylacetylene)s Composed of Chiral and Achiral Phenylacetylenes in Dilute Solution, Liquid Crystal, and Two-Dimensional Crystal

Optically active poly(phenylacetylene) copolymers consisting of optically active and achiral phenylacetylenes bearing L-alanine decyl esters (1L) and 2-aminoisobutylic acid decyl esters (Aib) as the pendant groups (poly(1L(m)-co-Aib(n))) with various compositions were synthesized by the copolymerization of the optically active 1L with achiral Aib using a rhodium catalyst, and their chiral amplification of the macromolecular helicity in a dilute solution, a lyotropic liquid crystalline (LC) state, and a two-dimensional (2D) crystal on the substrate was investigated by measuring the circular dichroism of the copolymers, mesoscopic cholesteric twist in the LC state (cholesteric helical pitch), and high-resolution atomic force microscopy (AFM) images of the self-assembled 2D helix-bundles of the copolymer chains. We found that the macromolecular helicity of poly(1L(m)-co-Aib(n))s could be hierarchically amplified in the order of the dilute solution, LC state, and 2D crystal. In sharp contrast, almost no chiral amplification of the macromolecular helicity was observed for the homopolymer mixtures of 1L and Aib in the LC state and 2D crystal on graphite.

A mechanistic insight into the organocatalytic properties of imidazolium-based ionic liquids and a positive co-solvent effect on cellulose modification reactions in an ionic liquid

Detailed insights into the organocatalytic properties of imidazolium-based ionic liquids (Im-ILs) for transesterification of cellulose with isopropenyl acetate (IPA) are presented. According to model transesterification reactions and their computational analysis, acetate anions of Im-ILs play an essential role in the promotion of the reactions. Mechanistic considerations in the optimization of the protocol of IL-catalyzed transesterification reactions have enabled a significant improvement in reaction conditions and a positive co-solvent effect for cellulose modifications in an imidazolium acetate ionic liquid.

Chiral fluorescent sensors based on cellulose derivatives bearing terthienyl pendants

Chiral fluorescent sensors were designed and synthesized from naturally occurring optically active cellulose, in which a terthienyl moiety was used as a fluorescent signaling unit, and their chiral recognition abilities were investigated on the basis of an enantioselective fluorescence response to aromatic nitro compounds as quenchers. A cellulose phenylcarbamate derivative (Ce-1b) exhibited an apparent enantioselectivity for various types of aromatic nitro compounds containing either central or axial chirality. Considering the fact that a corresponding monosaccharide derivative showed almost no enantioselectivity, the chiral recognition ability of the cellulose-based fluorescence sensor is attributed to its regular higher-order structures, probably a one-handed helical conformation. The resolution ability of Ce-1b as a chiral stationary phase for high-performance liquid chromatography was basically higher than that of the previously reported cellulose tris(phenylcarbamate), revealing that the terthienyl-based pendant provides attractive chiral recognition sites along the helical backbone as well as the fluorescent functionality.

A helical array of pendant fullerenes on a helical poly(phenylacetylene) induced by non-covalent chiral interactionsElectronic Supplementary Information (ESI) available: Full synthetic and analytical details and UV-vis, CD, IR and NMR spectra of the copolymers. See http://www.rsc.org/suppdata/cc/b3/b312511d/

Novel [60]fullerene-based poly(phenylacetylene)s prepared by the copolymerization of achiral phenylacetylenes bearing a C(60) or crown ether pendant form a one-handed helix upon complexation with L- and D-alanine, yielding a helical array of the pendant fullerenes with a predominant screw-sense along the polymer backbone.

Synthesis and chiroptical properties of a π-conjugated polymer containing glucose-linked biphenyl units in the main chain capable of folding into a helical conformation

An optically active π-conjugated polymer (poly-9) containing glucose-linked biphenyl units in the main chain was synthesized through the copolymerization of 5,5′-diiodo-2,2′-bithiophene and a new diethynyl compound, whose molecular design has taken inspiration from naturally occurring ellagitannins. The chiroptical properties of poly-9 were investigated in solution and the solid state. The absorption and circular dichroism spectra of poly-9 were clearly changed depending on the solvents due to the conformational alteration within a single polymer chain. Because a corresponding model molecule and polymer did not show marked solvent-dependent spectral changes, the backbone conformation of poly-9 is considered to be capable of interconverting between random-coil and preferred-handed helical states in response to the exterior environment. We also found that poly-9 exhibited efficient circularly polarized luminescence with a green color, whose dissymmetry factor reached 1.9 × 10−2 when the backbone was folded into the helical conformation in the film state.

Helical Polyacetylenes Induced via Noncovalent Chiral Interactions and Their Applications as Chiral Materials

Construction of predominantly one-handed helical polyacetylenes with a desired helix sense utilizing noncovalent chiral interactions with nonracemic chiral guest compounds based on a supramolecular approach is described. As with the conventional dynamic helical polymers possessing optically active pendant groups covalently bonded to the polymer chains, this noncovalent helicity induction system can show significant chiral amplification phenomena, in which the chiral information of the nonracemic guests can transfer with high cooperativity through noncovalent bonding interactions to induce an almost single-handed helical conformation in the polymer backbone. An intriguing “memory effect” of the induced macromolecular helicity is observed for some polyacetylenes, which means that the helical conformations induced in dynamic helical polyacetylene can be transformed into metastable static ones by tuning their helix-inversion barriers. Potential applications of helical polyacetylenes with controlled helix sense constructed by the “noncovalent helicity induction and/or memory effect” as chiral materials are also described.

Chiral stationary phases consisting of π-conjugated polymers bearing glucose-linked biphenyl units: reversible switching of resolution abilities based on a coil-to-helix transition

A series of optically active π-conjugated polymers (poly-2–poly-5) consisting of alternating thieno[3,4-b]thiophene and glucose-linked biphenyl units were synthesized through copolymerizations by Sonogashira–Hagihara cross-coupling. Absorption and circular dichroism spectroscopy and theoretical calculations revealed that poly-2 undergoes a conformational transition between the random-coil and helix in both solution and the solid state in response to the external solvent environment (chloroform and acetonitrile). Coated-type chiral stationary phases (CSPs) for high-performance liquid chromatography were prepared from both the random-coil and helical poly-2, and the influence of the polymer backbone structure on the chiral recognition ability was evaluated. The two CSPs showed somewhat complementary resolution abilities and the kinds of resolved racemates were dependent on the backbone conformation. In addition, an immobilized-type CSP with universal solvent durability was also prepared through copper-catalyzed click cyclization between an alkyne-appended poly-2 analogue and an azide-functionalized silica gel. The resulting CSP displayed repeatable switching of the chiral recognition ability based on a coil-to-helix transition of the polymer backbone by alternate column treatment with common organic solvents, such as chloroform and acetonitrile.

Achieving high ON/OFF ratio and good stability in organic nonvolatile resistive memory devices with polyisocyanide bearing oligothiophene

Polyisocyanide bearing oligothiophene pendants were used as an active layer in a nonvolatile resistive memory device. The current density current–voltage characteristics of the device showed nonvolatile memory behavior. The device exhibited a large ON/OFF ratio of 107 and a retention time of >4 × 104 under a read voltage of 0.5 V. The log J–V1/2 curves exhibit a good linear relationship in the OFF state, which suggests that the conduction mechanism in the OFF state obeys the thermionic emission model. In contrast, in the ON state, the current slope was 1.2 and the activation energy of ON state was ~6 meV. These conductive features in the ON state could be attributed to charge conduction through filamentary pathways.