Unraveling Structure‐Property Relationships in Benzothiadiazole‐Based Covalent Organic Frameworks for Enhanced Optoelectronic Properties: A Density Functional Theory/Time‐Dependent Density Functional Theory Study

Covalent organic frameworks (COFs) exhibit significant potential in photocatalysis. Nevertheless, the relationship between their structural units and optoelectronic properties remains largely unclear. In this study, it systematically investigates the optoelectronic characteristics of benzothiadiazole-based COFs (BT-COFs) using density functional theory (DFT) and time-dependent DFT (TD-DFT). It analyzes 14 BT-COFs with diverse donor-acceptor (D-A) structures to elucidate their charge transfer characteristics and electron-hole separation efficiencies. This study's results demonstrate that halogen substitution (F, Cl) promotes charge separation by localizing holes on the BT units and reducing the overlap between electrons and holes. Introducing appropriate π-conjugated units can narrow the energy gap and facilitate red-shifts in the UV-visible absorption spectrum. However, excessive conjugation in BT-COFs can disrupt planarity and hinder charge transport. Notably, CTF-M2, with its asymmetric structure, exhibits minimal electron-hole overlap (S_r = 0.08) and achieves a charge separation as high as 99.9%. Furthermore, the PPy-COFs show superior charge transfer capacity (CT: 0.9-0.97 e), which can be attributed to the unique conjugated structure of the PPy units. This study establishes a crucial theoretical foundation for selecting new D-A units based on BT-COFs and provides valuable guidance for designing photocatalytic materials.