Conformation-Dictated Aggregation Photophysics in Isoindigo-Based Copolymers

The aggregation of polymer chains directly influences the morphology of thin films used in optoelectronic devices. Therefore, understanding the relationship between the backbone conformation and aggregation of conjugated polymers is essential for ensuring optimal electronic performance. In this work, we study the effect of the backbone conformation on the aggregation photophysics of isoindigo-based copolymers, namely, bithiophene-isoindigo (P2TI) and thienothiophene-spaced bithiophene-isoindigo (P2TITT). The latter was systematically tuned by inserting thieno[3,2-b]thiophene (TT) into the former. Modification of the backbone by inserting TT was found to affect the planarity due to reduced steric hindrance between the donor and acceptor units. This reduced steric hindrance was further evidenced by the difference in the oscillator strength of the first excited-state transition, identified as an intramolecular charge transfer transition in time-dependent density functional theory (TD/DFT) calculations. Temperature-dependent photoluminescence (PL) of the two polymers was well reproduced using two Franck–Condon progressions, indicating the formation of both H- and J-type aggregates. This was supported by the presence of two emission lifetimes obtained from time-resolved fluorescence measurements. The evolution of the first two vibronic peaks with temperature clearly showed a stronger interchain interaction in P2TITT.