Fluorescence turn off–on mechanism of selective chemosensor for hydrogen sulfide: A theoretical perspective.

• The non-fluorescence of the FLVN-OCN is ascribed to obvious charge transfer. • The fluorescence enhancement of the HPO is the result of ESIPT mechanism. • The electronic spectra measured experimentally is ascribed to the HPO-PT. • The PECs indicate that the proton transfer process of HPO occurs in the S 1 state. The photophysical and photochemical properties of the novel sensor 3-(cyanooxy)-hydroxyflavone (FLVN-OCN) and 3-hydroxyflavone (HPO) generated by the reaction with hydrogen sulfide (H 2 S) have been investigated theoretically. Analysis of structure parameters and infrared vibrational spectra indicate that hydrogen bond is strengthened in the first (S 1) excited state, which is beneficial to the excited-state intramolecular proton transfer (ESIPT). The strength of hydrogen bond is determined by the reduced density gradient model. The calculated absorption and emission spectra of the tautomer (HPO-PT) are located at 412.5 and 536.8 nm, respectively, which are in good agreement with the experimental values (425 and 525 nm). The results show that the measured spectra experimentally are ascribed to HPO-PT configuration. Combined with the frontier molecular orbital and hole-electron analysis, the electron density of the product HPO transfers from the oxygen atom on the hydroxyl group to the adjacent oxygen atom, which affects the fluorescence properties, resulting in significant fluorescent changes with the addition of H 2 S. The obtained energy barrier of the S 1 state (4.22 kcal/mol) is lower than that of the ground (S 0) state (14.58 kcal/mol) through the potential energy curves (PECs) of HPO, which further indicates that the ESIPT process is more likely to occur in the S 1 state. The work explains the detection mechanism for H 2 S of the fluorescent probe FLVN-OCN and provides a reference for the further study of this probe. [ABSTRACT FROM AUTHOR]