Chalcogen-Substituted Fluorinated π-Conjugated Systems: DFT-Guided Insights Into Optoelectronic Properties, Luminescence Behavior, and Photovoltaic Performance.

Fluorine incorporation in organic molecules effectively modulates their electronic properties by lowering frontier molecular orbital energy levels due to its strong electron-withdrawing nature. This study employs DFT and TD-DFT based calculations to investigate fluorinated low-bandgap π-conjugated systems featuring a benzodifurandione core linked to oxindole units, with chalcogen variation (O, S, Se) in the central framework. Electronic structure analysis reveals a progressive redshift in absorption and photoluminescence spectra from O to Se, attributed to enhanced π-conjugation and increased atomic polarizability, reducing the HOMO-LUMO gap. M3 (X = Se) demonstrates the most redshifted optical properties, making it ideal for near-infrared (NIR) applications. Bulk heterojunction (BHJ) device assessments yield power conversion efficiencies (PCEs) of up to 7.00%, highlighting their potential in high-performance OSCs. Non-covalent interactions (NCIs), including hydrogen bonding and van der Waals forces, are characterized using Hirshfeld surface analysis, reduced density gradient (RDG) scatter plots, and the quantum theory of atoms in molecules (QTAIM), emphasizing their influence on molecular packing and stability. Electron localization function (ELF) and localized orbital locator (LOL) analyses further elucidate the balance of covalent and non-covalent interactions governing optoelectronic behavior. These findings provide fundamental design insights for next-generation fluorinated low-bandgap materials, advancing the development of high-efficiency OSCs.