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

• DOI: 10.1016/j.tetlet.2016.06.098
• Publication Date: 2016-06-24T00:08:04Z
• Authors: Chathura S. Abeyw...; Yi Pang
• Keywords: Benzoxazole

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 ( λ em ≈ 480 nm), while 5 gave orange-red emission at 597 nm, showing a large effect of regiochemistry on emission. The effect was further evaluated through binding with Zn 2+ 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).