Two-Dimensional Surface Chirality Control by Solvent-Induced Helicity Inversion of a Helical Polyacetylene on Graphite

Abstract

We report the direct evidence for the macromolecular helicity inversion of a helical poly(phenylacetylene) bearing l- or d-alanine pendants with a long alkyl chain in different solvents by atomic force microscopy observations of the diastereomeric helical structures. The diastereomeric helical poly(phenylacetylene)s induced in polar and nonpolar solvents self-assembled into ordered, two-dimensional helix bundles with controlled molecular packing, helical pitch, and handedness on graphite upon exposure of each solvent. The macromolecular helicity deposited on graphite from a polar solvent further inverted to the opposite handedness by exposure to a specific nonpolar solvent, and these changes in the surface chirality based on the inversion of helicity could be visualized by atomic force microscopy with molecular resolution, and the results were quantified by X-ray diffraction of the oriented liquid crystalline, diastereomeric helical polymer films.

Chiral nanostructure in polymers under different deposition conditions observed using atomic force microscopy of monolayers: poly(phenylacetylene)s as a case study

“…58,61 Interestingly, poly-(R)-10, adopts a cis-cisoidal configuration at the polyene, resulting in a helical structure that has three residues per turn (3/1), and rotates (internal helix) in the same sense than the external helix formed by the pendants. This result differs from the opposite sense found by Yashima et al [52][53][54][55] in their cis-transoidal PPAs.…”

“…So, poly-(D)-8 could invert its helical structure depending on the polarity of the solvent used for the formation of the 2D-crystals onto the AFM substrate (HOPG, Figure 5). 54 These results were in agreement with the effects observed by CD in solution. Thus, a first positive Cotton effect was observed when the polymer was dissolved in benzene (right-handed pendant helical array determined by AFM, cis-transoidal backbone).…”

“…[52][53][54] The sample for AFM was prepared by spin coating a dilute solution of the PPA onto the solid substrate (mica or HOPG) and kept under solvent atmosphere overnight in order to favour the selfassembly of the polymer chains. 53 Using this procedure, Figure 4a).…”

Synthesis and structure determination of helical polymers

“…Direct evidence for inversion of the macromolecular helicity of D-19 in different polar and nonpolar solvents has also been visualized for the first time by AFM with molecular resolution on exposure to each solvent vapor. 84 Regular 2D crystal formation on substrates after annealing in organic solvent vapors may be required for high-resolution AFM, 43 as the 2D crystals of polymer chains have the advantage of being sufficiently robust to resist being damaged by the tip during high-resolution AFM measurements; moreover, the flat surface of 2D crystals allows optimizing AFM settings for high-resolution imaging.…”

The transformation of chiral signals into macroscopic properties of materials using chirality-responsive polymers

“…In their subsequent work, the authors separated single-handed helices from racemic helices through facile treatment with acetone. 29 In 2006, Sakurai et al 30 reported a solvent-mediated helix transition. The helical structure of poly(phenylacetylene) underwent a pH-mediated transformation The pH value in aqueous media can easily be adjusted, which provides a convenient method for mediating the chirality of materials.…”