The sensing mechanism of a flavone-based ESIPT fluorescent chemodosimeter for selective recognition towards fluoride:...

The sensing mechanism of 3-hydroxyflavone-based (3-HF) fluorescent chemodosimeter 3-triisopropylsilylflavone (3-TPSF) for detecting fluoride (F^-) has been theoretically investigated. The calculated Laplacian bond order confirms that the Si-O bond of 3-TPSF is the reaction site of F^-. The free energy barrier of 18.33 kcal mol^-1 indicates that F-triggered desilylation reaction can occur and then form the anionic state of 3-HF (3-HF^-) with a fluorescence peak at 545 nm. 3-HF^- captures H⁺ of the mixed aqueous medium to be transformed into 3-HF with an intramolecular hydrogen bond (O₁-H⋯O₂). The energy barrier of 1.86 kcal mol^-1 in the S₁ state obtained from the constructed potential energy curves confirms that the excited state intramolecular proton transfer (ESIPT) in 3-HF occurs to form a tautomer structure, which produces a long-wavelength emission of 549 nm. The fluorescence emitted from both 3-HF^- and 3-HF agrees with the experimental value of 530 nm appearing after adding F^-. Charge transfer analyses indicate that the extent of intramolecular charge transfer in 3-HF^- is more intense than that of 3-TPSF, which induces a large Stokes shift of 180 nm. Therefore, the sensing mechanism is attributed to the combination of a large charge transfer feature and ESIPT that are caused by desilylation reaction. The significant fluorescence change makes 3-TPSF a chemodosimeter for detecting F^-.