Rational Design of Donor‐Acceptor Oligomers for Enhanced Charge Separation

ABSTRACT Intramolecular charge transfer (ICT) in donor‐acceptor (D‐A) oligomers is a fundamental mechanism for organic optoelectronics. However, symmetric D‐A‐D and A‐D‐A architectures are characterized by delocalized frontier molecular orbital (FMOs), which prevent efficient photoinduced charge separation. While asymmetric structures and halogen functionalization have been proposed as effective strategies to overcome these limitations, systematic guidelines for the rational design of efficient D‐A architectures are currently missing. To fill this gap, we present a comprehensive first‐principles investigation based on (time‐dependent) density functional theory of 22 dithieno[3,2‐b:2',3'‐d]pyrrole‐based oligomers, exploring the combined effect of architectural asymmetry and fluorination. Our analysis reveals the critical role of the functionalization: donor fluorination generally increases the bandgap, while acceptor fluorination maximizes the charge imbalance and promotes topological confinement and spatial segregation of the FMOs onto distinct molecular segments. The optical properties are critically influenced by the electronic characteristics, suggesting asymmetric, acceptor‐fluorinated oligomers as the best platforms for ICT, thanks to their strong charge‐transfer excitations possessing large oscillator strengths in the visible region. This work establishes clear, rational design rules for conjugated D‐A oligomers, demonstrating that the synergistic combination of structural asymmetry and site‐specific halogenation is essential for robust, directional ICT, paving the way for high‐performance organic semiconductors.