Chiral amplification in double-stranded helical polymers through chiral and achiral amidinium-carboxylate salt bridges

Abstract

A series of m-terphenyl-based random copolymers of chiral and achiral amidines, and their complementary homopolymers of achiral carboxylic acids were prepared by the copolymerization of a p-diiodobenzene derivative, with the diethynyl monomers containing a chiral or achiral amidine group and a carboxyl group using the Sonogashira coupling reaction. The obtained chiral/achiral amidine copolymers assembled into a double-stranded helical structure upon complexation with the complementary achiral homopolymer of carboxylic acids through interstrand amidiniumcarboxylate salt bridges. The complexes exhibited characteristic induced cotton effects in the π-conjugated main-chain chromophore regions, indicating that the interstrand duplexes possess a preferred-handed double-helical structure. The effect of the chiral and achiral amidine contents on the amplification of the helical chirality (the sergeants and soldiers effect) during the interstrand double-helix formation was investigated by comparing the cotton effect patterns and intensities of the duplexes with those of the corresponding all-chiral amidine-based double-helical polymer. © 2012 The Society of Polymer Science, Japan (SPSJ) All rights reserved.