The influence of linker substitution on the fluorescence responsive sensing of isostructural coordination polymers: visual turn-on, ratiometric, and turn-off sensing in water

Abstract

Four isostructural and isomorphous coordination polymers, {[Zn₂(OH)(NO₂-1,4-bdc)_1.5(Cz-3,6-bpy)]·2H₂O}_n (1), {[Zn₂(OH)(NO₂-1,4-bdc)_1.5(Cz-Pr-3,6-bpy)]·0.5H₂O}_n (2), {[Zn₂(OH)(Br-1,4-bdc)_1.5(Cz-3,6-bpy)]·H₂O}_n (3), and {[Zn₂(OH)(Br-1,4-bdc)_1.5(Cz-Pr-3,6-bpy)]·0.5H₂O}_n (4), where NO₂-1,4-bdc = 2-nitrobenzene-1,4-dicarboxylate, Br-1,4-bdc = 2-bromobenzene-1,4-dicarboxylate, Cz-3,6-bpy = 3,6-bis(pyridin-4-yl)-9H-carbazole, and Cz-Pr-3,6-bpy = 3,6-bis(pyridin-4-yl)-9-n-propyl-9H-carbazole, have been hydro(solvo)thermally synthesized. Compounds 1–4 have similar two-dimensional layer structures simplified to rhombus grid-like sql nets in topology. Both nitro-substituted 1 and 2 are fluorescence silent, behaving as visual fluorescence turn-on sensors for the detection of Fe³⁺, Cr³⁺, and Al³⁺ in water. Both bromo-substituted 3 and 4 emit strong cyan fluorescence and can be treated as visually ratiometric fluorescent sensors for the detection of Fe³⁺, Cr³⁺, and Al³⁺ and visual fluorescence turn-off sensors for the detection of CrO₄²⁻ and Cr₂O₇²⁻ in water, with high selectivity and sensitivity. The work clearly shows that the substituent (nitro and bromo) on the benzene-1,4-dicarboxylate linker has a great impact not only on essential luminescence properties but also the fluorescence responsive performances of coordination polymers.