Introducing Molecular Asymmetry in Copolymer Structures to Favor Herringbone-Like Aggregation and Tune the Chiral Expression

Abstract

Type I (comb-like) and type II (herringbone-like) aggregation in conjugated polymers has been thoroughly related to their intrinsic molecular structure, with parameters favoring strong π−π stacking leading to type I and parameters favoring van der Waals−London dispersion forces between the side chains yielding type II. Nonetheless, type II has mainly been investigated by disrupting the π−π stacking of type I. Here, we hypothesize that copolymer structures have a pronounced, positive effect on the ordering of type II aggregates, which becomes very evident in the chiral expression of the polymer if chiral monomers are incorporated. Therefore, two series of chiral copoly(3-alkyl)thiophenes with increasing asymmetry are studied. The difference in aggregation behavior, aggregating chiral building blocks combined with either aggregating (series 1) or nonaggregating achiral building blocks (series 2), is investigated along with the nature of the copolymer itself (block, tapered block, gradient, and random copolymer). This study identifies molecular asymmetry as an important tool to tune the process of aggregation and its influence on both the resulting aggregation type and chiral expression.