Two-Dimensional Helix-Bundle Formation of a Dynamic Helical Poly(phenylacetylene) with Achiral Pendant Groups on Graphite

摘要

The design and synthesis of helical polymers with a controlled helix sense have been attracting great interest because of the wide variety of possible applications in materials science, chemical sensing, separation of enantiomers, and enantiose-lective catalysis. The detailed structural characterizations of the helical polymers are essential to explore the relationships between their structures and properties and to further develop more sophisticated chiral materials. However, the determination of helical structures including helical pitch and handedness still remains very difficult. Conventional spectroscopic methods in dilute solution are not straightforward, and X-ray analysis in the solid state is a laborious task; these methods may not provide unambiguous helical structural information, and in particular, the helical sense. The direct observation of the helical polymers by atomic force microscopy (AFM) on solid substrates should be one of the most promising methods to settle this problem and has been extensively studied. Recently, we reported that rigid rodlike helical poly(phenylacetylene)s bearing L- or D-alanine residues with a long n-decyl chain as the pendants (poly-L-Ala in Figure 1a) hierarchically self-assembled on highly oriented pyrolytic graphite (HOPG) upon exposure to organic solvent vapors, such as benzene; flat poly(phenylacetylene) monolayers losing helical structures epitaxially formed on the basal plane of the graphite, on which helical poly(phenylacetylene)s further self-assembled into chiral two-dimensional (2D) helix-bundles with a controlled helicity. High-resolution AFM revealed their helical conformations in the 2D crystals and enabled us to determine the molecular packing, helical pitch, and handedness. Herein we show that an optically inactive but dynamically racemic helical poly(phenylacet-ylene) bearing achiral a-aminoisobutyric acid (Aib) residues with the same n-decyl chain as the pendants (poly-Aib in Figure 1 a) also crystallizes under an organic solvent vapor atmosphere on HOPG, resulting in 2D helix-bundle formation (Figure 1c). Poly-Aib no longer has any stereogenic centers, but likely consists of an equal mixture of interconvertible right- and left-handed helical segments separated by rarely occurring helical reversals in solution (Figure 1c, left), as do polyisocyanates and polysilanes, and therefore, poly-Aib in dynamic equilibrium is a chiral or dynamically racemic helical polymer. We could successfully visualize such enantiomeric right- and left-handed helical segments and the helical reversals of poly-Aib on HOPG by high-resolution AFM, and the results were quantified by X-ray diffraction of the oriented liquid crystalline (LC) poly-Aib film.