Influence of Keto–Enol Tautomerism in Regulating CO₂ Photoreduction Activity in Porous Organic Porphyrinic Photopolymers

Photoassisted CO₂ reduction employing a metal-free system is both challenging and fascinating. In our study, we present a structural engineering strategy to tune the potential energy barrier, which, in turn, affects the photoreduction ability. A series of porphyrin-based porous organic polymers (<b>POPs</b>) were hydrothermally synthesized and the influence of keto-enol tautomerization on the CO₂ photoreduction potential has been rigorously investigated. Among the screened photocatalysts, <b>POP-1</b> demonstrated the highest CO₂/CO conversion efficacy, producing 518 μmol g^-1 h^-1 of CO selectively under light illumination for 2 h. Density Functional Theory computational investigations concretely highlighted the reaction mechanistic pathway supporting the CO₂ conversion reaction. Additionally, the electron density mapping underpinned the thermodynamic energy barrier requirements for the progress of the reaction and elucidated the reason for the enhanced photocatalytic activity seen in <b>POP-1</b>. In situ Fourier-Transform Infrared spectroscopy was carried out for real-time investigations to understand the synergistic reaction dynamics and unlock the generation of key reaction intermediates during the CO₂ reduction reaction process. Additionally, ultrafast transient absorption spectroscopy plays a vital role in understanding the surface interaction dynamics of our designed catalysts. Overall, this straightforward modulation strategy not only enhances CO₂ reduction performance but also contributes toward presenting a crisp and concrete understanding of the structure-property relationship, opening up the possibilities for the development of artificial photocatalysts. The results introduce a strategy for photocatalytic CO₂ reduction using an efficient, stable, and recyclable metal-free photocatalytic system.