Morphology-Dependent Coherent Formation of Intrachain and Interchain States in Conjugated Copolymers

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Excited states with diverse configurations are observed simultaneously upon optical pumping in many donor–acceptor copolymers, acting as precursor states for charge generation or as trap states in organic optoelectronic devices. Nevertheless, their evolution on ultrafast time scales and correlations to structure dynamics remain elusive. In this study, we explore coherent electron–nuclear dynamics in the formation of excited states with different electronic configurations in a prototypical copolymer of naphthalene diimide–bithiophene using ultrafast transient absorption and 2-dimensional electronic spectroscopies, with temporal resolution better than 10 fs. By analyzing time-domain features of vibrational oscillations tagged on the dynamic traces of excited-state formation, we find that charge-transfer excitations (i.e., the precursor states of free charges) readily form within individual chains, exhibiting vibrational coherence transferred from initial hybrid excitation. However, trap-like excitations feature strong coupling to interchain vibrational modes, stabilized in locally ordered aggregation. The experimental and theoretical observations underscore the potential role of ultrafast structure planarization dynamics in regulating the formation of multiconfigurated excited states. This suggests a strategy to reduce losses of device performance caused by excited-state configurations through designing molecules and controlling morphologies to manipulate the interplay between electronic and vibrational degrees of freedom.