The mechanisms of a bifunctional fluorescent probe for detecting fluoride and sulfite based on excited-state intramolecular proton transfer and intramolecular charge transfer

Abstract

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F−) and sulfite (SO32–) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F− ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623\u2009nm, which agrees with the observed experimental value (634\u2009nm) after adding F−. Therefore, the fluorescence red-shift (from 498\u2009 to 634\u2009nm) of HBT-FS observed in experiment after adding F− is caused by ESIPT. The SO32– ion is added to the C5 site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404\u2009nm. The experimentally measured fluorescence at 371\u2009nm after adding SO32– is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO32–, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371\u2009nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F− and SO32–.