Stable hydrogen-bonded organic frameworks for selective fluorescence detection of Al3+ and Fe3+ ions

Abstract

Four new hydrogen-bonded organic frameworks (HOFs) with the formulas {[H₄TCPE·bpa]·(CH₃OH·H₂O)}_n (1), {[H₄TCPE·bpe]·(CH₃OH·0.56H₂O)}_n (2), {[H₄TCPE·bpa]·CH₃OH}_n (3) and {[H₄TCPE·bpe]·(0.41CH₃OH·0.59CH₃CN)}_n (4) (H₄TCPE = tetrakis(4-carboxyphenyl)ethylene, bpa = 1,2-bis(4-pyridyl)ethane, bpe = 1,2-bis(4-pyridyl)ethylene) have been prepared under different conditions and structurally characterized. X-ray crystallographic analysis reveals that the hydrogen bond strengths of 3 and 4 synthesized by solvothermal method are much stronger than those of 1 and 2 prepared by the volatilization method. An obvious fluorescence shift was observed during the sensing process of 3 and 4 toward Al³⁺ and Fe³⁺ ions. For HOF 3, a red-shift emission (ca. 28 nm) occurs upon the addition of Al³⁺ ion. When Al³⁺ ion is added to HOF 4 in acetonitrile suspension, a red-shift emission (ca. 30 nm) as well as a great fluorescence enhancement happens. Both 3 and 4 exhibit strong fluorescence quenching with high selectivity and sensitivity toward Fe³⁺ ion. In addition, they display relatively good stabilities as well as reusability. Therefore, HOFs 3 and 4 are excellent fluorescent sensors toward Al³⁺ and Fe³⁺ ions.