Sensing mechanism for a fluoride chemosensor: invalidity of excited-state proton transfer mechanism

Our density functional theory (DFT)/time-dependent DFT calculations for the fluoride anion sensor, 5,7-dibromo-8-tert-butyldimethylsilyloxy-2-methylquinoline (DBM), suggested a different sensing mechanism from the experimentally proposed one (Chem. Commun., 2011, 47, 7098). Instead of the formation of fluoride-hydrogen-bond complex (DBMOHF) and excited-state proton transfer mechanism, the theoretical results predicted a sensing mechanism based on desilylation reaction and intramolecular charge transfer (ICT). The fluoride anion reacted with DBM and formed an anion (DBMO), with the ICT causing a red shift in the absorbance and emission spectra of the latter. The calculated vertical excitation energies in the ground and first excited states of both DBM and DBMO, as well as the calculated (1)H NMR spectra, significantly reproduced the experimental measurements, providing additional proofs for our proposed sensing mechanism for DBM.