A highly stable Zn coordination polymer exhibiting pH-dependent fluorescence and as a visually ratiometric and on–off fluorescent sensor

Abstract

Hydro(solvo)thermal reactions of Zn(NO₃)₂·6H₂O, naphthalene-1,4-dicarboxylic acid (1,4-H₂ndc), and 3,6-bis(pyridin-4-yl)-9H-carbazole (Cz-3,6-bpy) afforded a coordination polymer [Zn₂(OH)(1,4-ndc)_1.5(Cz-3,6-bpy)]·2H₂O (1), which has a complicated two-dimensional layer structure that can be simplified as a topological grid-like sql net. 1 showed high thermal (ca. 350 °C) and chemical stability as well as good acid and base resistance (3 ≤ pH ≤ 12). In addition, 1 emitted intense solid-state fluorescence and exhibited solvent- and pH-dependent fluorescence in the suspension phase. In particular, there is a linear relationship between the fluorescence intensity and the pH value of the aqueous solution ranging from 4–7, making 1 a potential pH probe. Moreover, 1 can also be utilized as a multi-responsive fluorescent sensor for the detection of various analytes. Addition of Fe³⁺, Al³⁺, and Cr³⁺ into the H₂O suspension of 1 caused visually discernible fluorescence color change and, meanwhile, slight intensity reduction by Fe³⁺, making it a good ratiometric fluorescent sensor. On the other hand, the fluorescence of 1 was effectively quenched by Cr(VI) oxyanions involving CrO₄²⁻ and Cr₂O₇²⁻ in H₂O and by nitroaromatic compounds involving nitrobenzene (NB), 1,4-dinitrobenzene (1,4-DNB), 4-nitrophenol (4-NP), and 4-nitrotoluene (4-NT) in N,N′-dimethylacetamide (DMAc), making it an excellent on–off fluorescent sensor. The possible sensing mechanisms have been studied.