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

Four new hydrogen-bonded organic frameworks (HOFs) with the formulas {[H4TCPE center dot bpa]center dot(CH3OH center dot H2O)}(n) (1), {[H4TCPE center dot bpe]center dot(CH3OH center dot 0.56H(2)O)}(n) (2), {[H4TCPE center dot bpa]center dot CH3OH}(n) (3) and {[H4TCPE center dot bpe]center dot(0.41CH(3)OH center dot 0.59CH(3)CN)}(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.