Harnessing Keto‐Enol Tautomerism to Modulate β‐Ketoenamine‐based Covalent Organic Frameworks for Visible‐Light‐Driven CO2 Reduction

Abstract Covalent organic frameworks (COFs) have attracted extensive attention as promising photocatalysts for CO 2 reduction. However, there are few reports on the relationship between keto‐enol tautomerism and COFs photocatalyst activity. Three different β ‐ketoenamine‐based COFs were prepared by varying the proportion of hydroxyl units in the aldehyde precursor, in which only the HCOF‐2 shows the structural features of the reversible keto‐enol tautomerism. Through density functional theory (DFT) calculations and experiments, the effects of keto‐enol tautomerism on the light absorption intensity, band position, band gap, and carrier separation of these COFs were investigated. The reversible keto‐enol tautomerism contributes to narrowing the band gap and promoting the light absorption capacity. In addition, the reversible keto‐enol tautomerism leads to molecular polarization, which promotes the separation and transmission of carriers within the material. As a result, HCOF‐2 exhibits optimal photocatalytic activity, yielding 309 and 96 μmol g −1 of CO and CH 4 respectively within 10 h. This work provides an explanation of the relationship between keto‐enol tautomerism and photocatalytic activity.