Hydroxyl-enabled dipole engineering in metal-free COFs for highly selective solar CO₂-to-CO conversion

Photocatalytic CO₂ reduction reaction (pCO₂RR) holds considerable promise for solar-to-fuel conversion, however, simultaneously optimizing activity, selectivity, and stability in covalent organic frameworks (COFs) remains a fundamental challenge. Although metal-free COFs offer structural robustness and readily tunable CO₂—interactive sites via heteroatom engineering, their charge-transport efficiency is often constrained by strong exciton binding and fast carrier recombination. Herein, a hydroxyl-functionalized porphyrin-based COF (Por-BDA-2OH) was rationally constructed, in which hydroxyl incorporation established in-plane O-H···N = C interactions, amplified the ground-state dipole moment (μ_g), and reinforced the internal electric field (IEF), thus accelerating photoinduced charge separation and prolonging the lifetime of photogenerated carriers. In parallel, such hydrogen-bonding interactions also strengthened CO₂ adsorption/activation and broadened light harvesting. As a result, Por-BDA-2OH delivered outstanding pCO₂RR performance, achieving a CO evolution rate of 91.5 μmol·h⁻¹·g⁻¹, superior to most reported metal-free COF photocatalysts. Overall, this work identifies hydrogen-bond-assisted dipole engineering as an effective molecular strategy to improve photoexciton separation dynamics in metal-free COFs, thereby offering a broadly applicable design principle to coordinate the intrinsic activity–selectivity–stability relationship in metal-free COF photocatalysts, with broader implications for the design of molecularly precise light-harvesting systems.