Synthesis of a Bis[2-(2’-hydroxyphenyl)benzoxazole]pyridinium Derivative: The Fluoride-Induced Large Spectral Shift for Ratiometric Response

(E)-4-(3,5-Bis(benzo[d]oxazol-2-yl)-2,6-dihydroxystyryl)-1-ethylpyridin-1-ium iodide (3) with extended conjugation was synthesized by coupling a styryl substituent into the bis(HBO) 1 structure. Probe 3 exhibited bright red emission (λem ≈ 600 nm) with a large Stokes shift (Δλ ≈ 210 nm), attributing to strong ICT enhanced by ESIPT. The role of the styryl substituent was further evaluated with the aid of low temperature fluorescence. Probe 3 exhibited excellent selectivity towards fluoride anions in solution by generating a near infrared emission peak at λem ≈ 720 nm. Probe 3 was found to be a useful tool for quantifying fluoride content in solution by both absorbance and emission measurements. The fluoride binding of 3 was assumed to occur via strong intermolecular hydrogen bonding with the F− anion (K = 3413). The association stoichiometry for the 3-F complex was found to be (1 : 1) according to the Benesi-Hildebrand method.