Optically Active, Amphiphilic Poly(meta-phenylene ethynylene)s: Synthesis, Hydrogen-Bonding Enforced Helix Stability, and Direct AFM Observation of Their Helical Structures

Abstract

Optically active, amphiphilic poly(meta-phenylene ethynylene)s (PPEa) bearing L- or D-alanine-derived oligo(ethylene glycol) side chains connected to the backbone via amide linkages were prepared by microwave-assisted polycondensation. PPEa's exhibited an intense Cotton effect in the π-conjugated main-chain chromophore regions in various polar and nonpolar organic solvents due to a predominantly one-handed helical conformation stabilized by an intramolecular hydrogen-bonding network between the amide groups of the pendants. The stable helical structure was retained in the bulk and led to supramolecular column formation from stacked helices in oriented polymer films as evidenced by X-ray diffraction. Atomic force microscopy was used to directly visualize the helical structures of the polymers in two-dimensional crystalline layers with molecular resolution, and, for the first time, their absolute helical senses could unambiguously be determined.

π-Conjugated polymer-layered structures: synthesis and self-assembly

“…A variety of assembled structures can be formed through covalent linkages between π-conjugated units (Figure 1b). Flexible linkers enable intramolecular folding, and the resulting folded molecules can adopt unique morphologies such as fibers [20][21][22] and two-dimensional nanosheets. 23 Conjugated linkers extended the π-conjugation length and the conjugated polymers can form nanofibers through their strong π-π interactions.…”

Evidence of Entropy-Driven Bistability through 15N NMR Analysis of a Temperature- and Solvent-Induced, Chiroptical Switching Polycarbodiimide

“…18−26 Throughout this time, novel, helical macromolecular scaffolds have emerged including systems such as oligio- 27 and poly(m-phenylene ethynylenes), 28,29 polyisocyanates, 30,31 polyisocyanides, 32−34 poly-(phenyl acetylenes), 35,36 poly(quinaxoline-2,3-dials), 37,38 and polycarbodiimides 39 to name a few. Polycarbodiimides, a relatively young class of helical macromolecules, have been an intense area of interest in the Novak group since the discovery of the controlled "living" Ti(IV)-mediated polymerization of carbodiimides in 1994 due to their attractive properties such as liquid crystallinity, 40,41 high helical inversion barrier, 42 and chiroptical switching.…”

A Facile Synthetic Route to Amphiphilic Poly(Meta-Phenylene Ethynylene) and Poly(Meta-Phenylene Ethynylene)-Block-Polyisocyanide Using a Single Catalyst

“…Yashima and coworkers previously reported that poly(meta-phenylene ethynylene)s bearing L-alanine-derived oligo(ethylene glycol) side chains ((S)-PPEa) had a preferred-handed helical conformation induced by the covalent bonding of chiral alanine pendants, even in CHCl3 [38]. In CHCl3, the helical conformation of (S)-PPEa is likely to be stabilized by intramolecular hydrogen bonds between nonadjacent amide pendants [38].…”