Harnessing Enol-Keto Tautomerism in Pyrene-Based Covalent Organic Framework toward Optical Sensing

Covalent organic frameworks (COFs) are emerging as promising tautomeric platforms for solid optical sensing owing to structure tunability and inherent porosity. Herein, a novel hydroxyl-functionalized pyrene-based COF is designed and synthesized through a solvothermal method. In comparison to the nonhydroxylated COF analogue, TFFPy-COF-OH undergoes enol-keto tautomerism on exposure to water, leading to an ultrafast, visually, and reversible color change from yellow to red. More interestingly, the addition of trace organic solvents to the aqueous suspension of TFFPy-COF-OH triggers a pronounced luminescence enhancement, while the fluorescence remains remarkably stable even after 50% (v/v) water is introduced into the organic dispersion, demonstrating exceptional resistance to aqueous quenching. This phenomenon was attributed to competitive adsorption behavior in TFFPy-COF-OH, where it preferentially bound to organic molecules in a water/organic solvent mixture, leading to a conversion toward enol form and resulting in stronger fluorescence. In addition, TFFPy-COF-OH shows excellent fluorescence sensitivity for THF detection in water, with a calculated LOD of 0.014%. We believe that our results provide new insights into the optical applications of enol-keto tautomerism in the formation of COFs.