Maximizing the Photocatalytic Performance of Covalent Organic Frameworks by Decoupling Charge Separation and Recombination via Guest π-Conjugation

Covalent organic frameworks (COFs) have emerged as promising candidates for solar-driven photocatalysis; however, simultaneously achieving ultrafast electron–hole separation and suppressed charge recombination remains challenging. Here, we demonstrated a theory-guided design of high-performance host–guest COFs by tuning π-conjugation of embedded guest molecules. Combining many-body nonadiabatic molecular dynamics simulations based on linear response time-dependent density functional theory with transient spectroscopy and photocatalytic experiments, we elucidate the microscopic mechanisms governing excited-state dynamics in these systems. Our simulations suggest that increasing guest π-conjugation enables subpicosecond hole transfer from the COF host to the guest molecules by lowering the acceptor excitation energy and enhancing the density of acceptor states. Simultaneously, charge recombination is strongly suppressed through two synergistic effects. First, highly π-conjugated guests enhance low-frequency structural fluctuations and excited-state energetic disorder, thereby accelerating quantum decoherence. Second, increased guest π-conjugation promotes spatial separation of electrons and holes, while stronger instantaneous structural fluctuations under ambient conditions further localize charge carriers. These effects reduce electron–hole overlap and nonadiabatic coupling and markedly slow recombination relative to less conjugated systems. Femtosecond pump–probe spectroscopy supports the predicted guest-dependent differences in early excited-state dynamics. Guided by these mechanistic insights, we demonstrate experimentally that COFs incorporating highly conjugated guest molecules exhibit superior photocatalytic performance, achieving up to 97.38% conversion of 5-hydroxymethylfurfural with excellent product selectivity. The work establishes a molecular-level understanding of how guest properties govern excited-state dynamics in host–guest COFs and provides general design principles for developing high-performance photocatalysts.