Linking the Chiral Expression of Conjugated Polymers to Their Aggregation Type

Abstract

Due to their strong absorption of visible light, chirality in chiral conjugated polymers (CPs) is typically expressed by electronic circular dichroism (ECD). Unfortunately, while many chiral CPs give rise to strong ECD upon stacking, predicting and reproducing the resulting behavior has proven to be extremely difficult. Here we show that the ECD of CPs is correlated to the known types I and II aggregation. It is demonstrated that type I aggregation results in an ECD signal with an additional monosignal Cotton effect, therefore yielding an asymmetrical spectrum. In contrast, in type II aggregation, the resulting ECD signal is symmetrical. This is determined for a model chiral CP, poly(3-(3,7-(S)-dimethyloctyl)thiophene) in two ways. Additionally, the actual process of both aggregation types is monitored by a combination of linear and nonlinear optical techniques. Finally, the molecular structure of various CPs is correlated to the aggregation type, providing a general relationship between the molecular structure of CPs and their ECD.