Synthesis of polyisocyanides bearing oligothiophene pendants: higher-order structural control through pendant framework design

Abstract

L-Alanine-based polyisocyanides bearing oligothiophene (OT) pendants (poly-3a and poly-3b) were synthesized and the influence of the OT pendants on the chiroptical properties and helix-forming abilities of the polymers was investigated. Poly-3a bearing a quinquethiophene unit with two n-hexyl chains successfully formed a preferred-handed helical conformation by simultaneous interactions of the OT-pendant π–π stacking and hydrogen bonding of the amide groups. In contrast, poly-3b, which possesses a branched alkyl chain on behalf of the terminal n-hexyl chain of poly-3a, did not form a higher-ordered helical conformation. The results suggest that the structure of the alkyl chains positioned far from the polymer backbones remotely and sensitively affected the ability of the polymer to form a helical conformation. We also found that the structure of the OT pendants significantly affect the molecular weight as observed by AFM measurements. The polyisocyanide poly-3c bearing a bithiophene unit was additionally synthesized to examine the influence of the number of thiophene rings within the OT pendants on the higher-order structure formation. L-Alanine-based polyisocyanides bearing oligothiophene (OT) pendants were synthesized and the influence of the OT pendants on the helix-forming abilities of the polymers was investigated. The polyisocyanide possessing quinquethiophene units with two n-hexyl chains readily formed a right-handed helical conformation with a helical array of the OT pendants biased by the chirality of the L-alanine residues. This study revealed that the simultaneous interactions of the π–π stacking and the hydrogen bonding between the pendants are required to arrange the OT units into a helically twisted manner along the backbone.