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 (POPs) were hydrothermally synthesized and the influence of keto-enol tautomerization on the CO₂ photoreduction potential has been rigorously investigated. Among the screened photocatalysts, POP-1 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 POP-1. 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.