Conformation-Dictated\nAggregation Photophysics in\nIsoindigo-Based Copolymers

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