Dual Memory of Enantiomeric Helices in Poly(phenylacetylene)s Induced by a Single Enantiomer through Helix Inversion and Dual Storage of the Enantiomeric Helicity Memories

A series of stereoregular poly(phenylacetylene)s carrying prochiral phosphonic acid monoesters as the pendant groups (monophenyl ester: poly-1a; monoethyl ester: poly-1b; monoisopropyl ester: poly-1c) were synthesized. The polymers formed a preferred-handed helical conformation upon complexation with nonracemic amines, whose helix senses were inverted by temperature or solvent composition. The resulting diastereomeric right- and left-handed helices induced by chiral amines were retained or memorized after the chiral amines were completely replaced with achiral diamines, thus producing the corresponding enantiomeric helices. The helicity memory, however, is temporary and will be gradually lost due to its inherently dynamic nature. In order to permanently store or save the helicity memory, the phosphorus residues of the polymers were methyl esterified with diazomethane, which proceeded in an enantioselective way to generate a chiral center at the phosphorus residues with optical activity due to chirality transfer from the induced or memorized helical chirality of the polymer backbones, which enabled the storage of both the right- and left-handed macromolecular helices induced by the single enantiomer of the chiral amines. The enantioselectivity was significantly affected by the structures of the pendant phosphonate groups, and poly-1c bearing a monoisopropyl ester group showed the highest enantioselectivity, which further induced one of the helices of the polymer main chain with significant amplification of the helical chirality.