Sulfone‐Functionalized Covalent Heptazine Polymeric Networks for Selective CO ₂ Photoreduction in Water: Mechanistic Insights into Sacrificial Donor‐Dependent Selectivity

Abstract The urgent need to mitigate CO 2 emissions and develop sustainable energy alternatives has driven extensive research into photocatalytic CO 2 reduction. In this study, a rationally designed sulfone‐functionalized covalent heptazine polymeric networks (HSF) that achieves highly efficient and selective CO 2 photoreduction in pure water is reported. The introduction of sulfone units enhances photogenerates charge separation, optimizes CO 2 adsorption, and improves protonation properties, leading to a remarkable CO production rate in pure water with >99% selectivity under visible light irradiation. An exceptionally high apparent quantum yield (AQY) of 32.13% at λ = 400 nm has been achieved‐ the highest ever reported to date for any metal‐free photocatalyst. Intriguingly, by introducing triethylamine (TEA) as a sacrificial electron donor, the product selectivity shifts from CO to CH 4 , achieving 80% CH 4 selectivity. Mechanistic insights from in situ transient absorption spectroscopy and density functional theory (DFT) calculations reveal that the sulfone moiety plays a pivotal role in facilitating charge carrier dynamics and modulating the adsorption configuration of CO 2 . Furthermore, it shows that TEA not only acts as a hole scavenger but also functions as a proton transfer agent, enabling efficient protonation and the stepwise reduction of *CO to CH 4 .