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-, CH3COO-, and H2PO4- 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 CH3COO-−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-, CH3COO-, and H2PO4- 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.