Fluorination‐Enforced Planarization for Enhanced Charge‐Transfer Stability in Cyclopentadithiophene–Benzothiadiazole Monomers

This study investigates three cyclopentadithiophene‐benzothiadiazole (CDT‐BT) monomers with progressive fluorination (A1: none, A2: mono‐, A3: di‐fluorinated) through density functional theory (DFT), time‐dependent DFT, and quantum topological analyses. Structural optimization demonstrates a marked decrease in donor–acceptor dihedral angles (5.48° in A1 to 0.53° in A3), evidencing fluorine's role in enforcing backbone planarity via stabilizing N⋯H and S⋯F contacts. Frontier orbital calculations reveal systematic stabilization of HOMO and LUMO energies (A1: −5.378/−2.725 eV; A3: −5.489/−2.837 eV), while maintaining a nearly constant gap (∼2.65 eV). Dipole moment variations in A2 (4.152 D) and A3 (3.991 D) highlight altered interfacial polarity. Simulated absorption spectra confirm that the HOMO‐LUMO transition dominates (>95%) with visible peaks at 543–548 nm, preserving optical activity. Charge population and NBO analyses indicate redistribution and delocalization across donor–acceptor units, whereas QTAIM, RDG‐NCI, ELF, LOL, and Hirshfeld descriptors highlight fluorine‐driven noncovalent interactions as critical to conformational rigidity and electronic stabilization. Collectively, these results establish fluorination as a precise molecular design strategy, enabling control over planarity, charge distribution, and energy alignment, thereby advancing CDT‐BT‐based polymers for organic photovoltaic and optoelectronic applications.