Synthesis of fused 2-(2′-hydroxyphenyl)benzoxazole derivatives: the impact of meta-/para-substitution on fluorescence and zinc binding

Abstract

A fused bis[2-(2′-hydroxyphenyl)benzoxazole] bis(HBO) 3 was synthesized via reaction of 4,6-dihydroxyisopthalic acid with 2-amnophenol in the presence polyphosphoric acid (PPA). In bis(HBO) 3, two benzoxazol-2-yl groups are connected via meta-phenylene, in comparison with para-phenylene in its isomer 5, in order to examine the effect of regiochemistry on optical properties. UV–vis and fluorescence spectra show that bis(HBO) 3 gave emission nearly exclusively from its keto tautomer, indicating more effective ‘excited-state intramolecular proton transfer (ESIPT)’ than mono HBO. Bis(HBO) 3 gave blue ESIPT fluorescence (λem≈480nm), while 5 gave orange-red emission at 597nm, showing a large effect of regiochemistry on emission. The effect was further evaluated through binding with Zn2+ cation, with 3 showing spectral blue shift upon zinc binding while 5 did not. In addition, 3 revealed ability to detect different stages of zinc binding, giving different spectral response to (3)2–Zn (with 2:1 ligand-to-metal ratio) and 3–Zn species (with 1:1 ligand-to-metal ratio).