Long-range effects of chirality in aromatic poly(isocyanide)s

Optically active helical vinylbiphenyl polymers with reversible thermally induced stereomutation

A series of novel chiral vinylbiphenyl monomers, (+)-2-[(S)-alkoxycarbonyl]-5-(4′-hexyloxyphenyl)styrene (S-(+)-I-Mm, m = 0, 1, 2, 3)/(−)-2-[(R)-sec-butyloxycarbonyl]-5-(4′-hexyloxyphenyl)styrene (R-(−)-I-M0), were designed and synthesized to search for new building blocks of optically active helical polymers.

Impact of a minority enantiomer on the polymerization of alanine-based isocyanides with an oligothiophene pendant

A synthetic polymer requiring a long-range homochiral sequence (>50 repeating units) to fold into a helical conformation has been prepared.

Two-fold odd-even effect in self-assembled nanowires from oligopeptide-polymer-substituted perylene bisimides

Organic nanowires are important building blocks for nanoscopic organic electronic devices. In order to ensure efficient charge transport through such nanowires, it is important to understand in detail the molecular parameters that guide self-assembly of π-conjugated molecules into one-dimensional stacks with optimal constructive π-π overlap. Here, we investigated the subtle relationship between molecular structure and supramolecular arrangement of the chromophores in self-assembled nanowires prepared from perylene bisimides with oligopeptide-polymer side chains. We observed a "two-fold" odd-even effect in circular dichroism spectra of these derivatives, depending on both the number of l-alanine units in the oligopeptide segments and length of the alkylene spacer between chromophore and oligopeptide substituents. Our results indicate that there is a complex interplay between the translation of molecular chirality into supramolecular helicity and the molecules' inherent propensity for well-defined one-dimensional aggregation into β-sheet-like superstructures in the presence of a central chromophore. Strong excitonic coupling as expressed by the appearance of hypsochromically and bathochromically shifted UV-vis absorptions and strong CD signals was systematically observed for molecules with an odd number of l-alanines in the side chains. The latter derivatives gave rise to nanowires with a significantly higher electron mobility. Our results, hence, provide an important design rule for self-assembled organic nanowires.

Probing the limits of the majority-rules principle in a dynamic supramolecular polymer

By systematic variation of the chemical structure of benzene-1,3,5-tricarboxamide (BTA) derivatives, the effect of chemical structure on the amplification of chirality was studied and quantified. In combination with temperature-dependent amplification experiments, the limits of the majority-rules principle were also investigated. For all BTA derivatives a high, constant helix reversal penalty was determined, which is related to the intermolecular hydrogen bonds that are present in all studied derivatives. For asymmetrically substituted BTA derivatives an odd-even effect was found in the degree of chiral amplification when changing the position of the stereogenic center with respect to the amide functionality. It was found that the mismatch penalty could be directly related to the number of stereocenters present in the molecules. Increasing this number from one to three resulted in an increase in this energy penalty while leaving the helix reversal penalty unaffected. For the majority-rules principle this implies that a single stereocenter present in the molecule contains sufficient chiral information at the molecular level to result in a chirally amplified state at the supramolecular level. Further evidence that the mismatch penalty is directly related to the number of stereocenters was obtained from mixed majority-rules experiments where two BTA derivatives with different numbers of stereocenters with opposite stereoconfiguration were studied in a majority-rules experiment. Finally, the ultimate limits of chiral amplification for the majority-rules principle were investigated, revealing that, given a certain helix reversal penalty, there is an optimum to which the mismatch penalty can be reduced while also enhancing the degree of chiral amplification. Temperature-dependent majority-rules experiments could indeed confirm these simulations. These findings show the relevance of both energy penalties when trying to enhance the degree of chiral amplification for the majority-rules principle in a one-dimensional helical supramolecular polymer.

Asymmetrically substituted benzene-1,3,5-tricarboxamides: self-assembly and odd-even effects in the solid state and in dilute solution

Molecular organization: Chiral benzene tricarboxamides with methyl substituents at defined positions self-assemble into supramolecular polymers of preferred helicity by three-fold alpha-helical-type hydrogen bonding. The odd-even effect is operative and all derivatives are liquid crystalline showing a Col(ho) phase (see figure).Asymmetric benzene-1,3,5-tricarboxamides (aBTAs) comprising two n-octyl and one chiral methyl-alkyl side chain were synthesised and characterised. The influence of the position and the configuration of the chiral methyl group (methyl at the alpha, beta or gamma position) in the aliphatic side chains on the liquid-crystalline properties and the aggregation behaviour of the aBTAs was systematically studied and compared to symmetrical benzene-1,3,5-tricarboxamides (sBTAs). Solid-state characterisation (polarised optical microscopy, IR spectroscopy, X-ray diffraction and differential scanning calorimetry) revealed that all aBTAs show threefold, alpha-helical-type intermolecular hydrogen bonding between neighbouring molecules and exhibit a columnar hexagonal organisation from room temperature to well above 200 degrees C. Moving the chiral methyl group closer to the amide group stabilises the liquid-crystalline state, as evidenced by a higher clearing temperature and corresponding enthalpy. The self-assembly of dilute solutions of the aBTAs in methylcyclohexane ( approximately 10(-5) mol L(-1)) was investigated with circular dichroism (CD) spectroscopy. The sign of the Cotton effect demonstrated a pronounced odd-even effect, whereas the value of the molar ellipticity, Deltaepsilon, in the aBTAs was independent of the position of the methyl group. Subsequent temperature-dependent CD measurements showed that the aggregation of all aBTAs can quantitatively be described by the nucleation-growth model and that the stability of the aggregates increases when the chiral methyl group is closer to the amide moiety. The results presented herein illustrate that even small changes in the molecular structure of substituted benzene-1,3,5-tri-carboxamides affect their solid-state properties and their self-assembly behaviour in dilute solutions.

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.

Helicity induction and memory of the macromolecular helicity in a polyacetylene bearing a biphenyl pendant

A novel, cis-transoidal poly-(phenylacetylene) bearing a carboxybiphenyl group as the pendant (poly-1) was prepared by polymerization of (4'-ethoxycarbonyl-4-biphenylyl)acetylene with a rhodium catalyst followed by hydrolysis of the ester groups. Upon complexation with various chiral amines and amino alcohols in dimethyl sulfoxide (DMSO), the polymer exhibited characteristic induced circular dichroism (ICD) in the UV/Vis region due to the predominantly one-handed helix formation of the polymer backbone as well as an excess of a single-handed, axially twisted conformation of the pendant biphenyl group. Poly-1 complexed with (R)-2-amino-1-propanol showed unique time-dependent inversion of the macromolecular helicity. Furthermore, the preferred-handed helical conformation of poly-1 induced by a chiral amine was further "memorized" after the chiral amine was replaced with achiral 2-aminoethanol or n-butylamine in DMSO. In sharp contrast to the previously reported memory in poly((4-carboxyphenyl)acetylene), the present helicity memory of poly-1 was accompanied by memory of the twisted biphenyl chirality in the pendants. Unprecedentedly, the helicity memory of poly-1 with achiral 2-aminoethanol was found to occur simultaneously with inversion of the axial chirality of the biphenyl groups followed by memory of the inverted biphenyl chirality, thus showing a significant change in the CD spectral pattern.

X-Ray Spectroscopic and Diffraction Study of the Structure of the Active Species in the NiII-Catalyzed Polymerization of Isocyanides

The structure of the active complex in the Ni-catalyzed polymerization of isocyanides to give polyisocyanides is investigated. It is shown by X-ray absorption spectroscopy (XAS), including EXAFS (extended X-ray absorption fine structure) and XANES (X-ray absorption near edge structure), and single-crystal X-ray diffraction, to contain a carbene-like ligand. This is the first structural characterization of a crucial intermediate in the so-called merry-go-round mechanism for Ni-catalyzed isocyanide polymerization.