Structure–Activity Relationships in D–π–A Covalent Organic Frameworks for Photocatalytic Water Splitting: Insights from DFT and TD-DFT Calculations

Covalent organic frameworks (COFs) are promising crystalline porous polymers for photocatalysis, yet their strong excitonic effects and rapid carrier recombination limit efficiency. However, strong excitonic effects and rapid electron–hole recombination remain key challenges. Herein, we employ density functional theory (DFT) and time-dependent density functional theory (TD-DFT) to systematically investigate the structure–activity relationships of three D–π–A-type COFs (COF-alkene, TapbBtt-COF, and TtaTpa-COF) for photocatalytic overall water splitting. Benchmarking identifies the M06L functional, SMD solvent model, and 6-311+G(2d,p) basis set as optimal. Our results reveal that molecular planarity, D–π–A configuration, and charge separation collectively govern performance. TtaTpa-COF exhibits the narrowest Eex (2.47 eV), longest absorption wavelength (502.15 nm), and lowest hole–electron overlap (0.51), enabling efficient carrier separation. For the hydrogen evolution reaction (HER), TtaTpa-COF shows the most favorable *H adsorption free energy (0.04 eV) and lowest LUMO level (−2.8 eV), yielding the highest activity. Notably, the D–π–A system governs active-site selectivity: COF-alkene favors the alkene-linked carbon, whereas the other two favor imine nitrogen. For the oxygen evolution reaction (OER), all follow the adsorbate evolution mechanism with *OOH formation as the rate-determining step. TtaTpa-COF exhibits the lowest limiting potential (4.33 eV), indicating superior water oxidation kinetics. This work establishes a clear structure–activity relationship linking D–π–A architecture to photocatalytic performance, providing a rational design framework for high-activity COF-based photocatalysts.