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

Covalent organic frameworks (COFs) have attracted extensive attention as promising photocatalysts for CO2 reduction. However, there are few reports on the relationship between keto-enol tautomerism and COFs photocatalyst activity. Three different beta-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 mu mol g(-1) of CO and CH4 respectively within 10 h. This work provides an explanation of the relationship between keto-enol tautomerism and photocatalytic activity.