Fluorescence sensing of anions based on inhibition of excited-state intramolecular proton transfer

Condensation of 2-(2′-aminophenyl)benzoxazole with p-toluenesulfonyl chloride and phenyl isocyanate yields two new anion sensors (TABO and PUBO), which can undergo excited-state intramolecular proton transfer (ESIPT) upon excitation. For the acid receptor TABO, the ESIPT process can be readily disturbed by basic anions such as F, CHCOO, and HPO by deprotonating the sulfonamide unit, whereas in the case of PUBO, a good hydrogen-bonding donor, the ESIPT process is inhibited either by the fluoride-induced deprotonation of the urea unit or by the formation of a strong CHCOO-urea intermolecular hydrogen bond complex, and these two types of inhibition mechanisms consequently result in different ratiometric responses. But other anions with less hydrogen-bonding acceptor abilities cannot inhibit the ESIPT, Interestingly, the different inhibition abilities of F, CH COO, and HPO produce different spectral behaviors in PUBO, so this new sensor successfully distinguishes the subtle difference in these three anionic substrates of similar basicity and surface charge density. © 2007 American Chemical Society.