A new Cd(ii)-based MOF displaying flu topology as a highly sensitive and selective photoluminescent sensor for ferric and chromate ions

Abstract

Metal–organic frameworks (MOFs) are a class of multimodal three-dimensional materials that have been applied as luminescent sensors for ions and molecules existing in water bodies. Herein, a new Cd(II)-based MOF, [H₂bpd]₂[Cd₃(L)₂] (1) (bpd = bis(pyridin-4-ylmethyl)propane-1,3-diamine; H₅L = 3,5-di(2′,4′-dicarboxylphenyl)benzoic acid), was synthesized and characterized. Single crystal X-ray analysis revealed that 1 is anionic in nature and exhibits a flu-type topology with the point symbol (4¹²·6¹²·8⁴)(4⁶)₂. This MOF was used as a turn-off photoluminescent sensor to detect Fe³⁺ and CrO₄²⁻ by forming 1@Fe³⁺ and 1@CrO₄²⁻ complexes, respectively. In the case of the Fe³⁺ cation, 1 displayed a K_SV of 0.01763 and limit of detection (LOD) of 5.315 ppm, while for the chromate anion, it exhibited a K_SV of 0.07338 and limit of detection of 1.277 ppm. Interestingly, the emissive response of 1 was restored upon the addition of hydroquinone (hyd) and 4-hydroxybenzoic acid (hyd acid) (turn-on) due to the competitive formation of hyd@Fe³⁺ or hyd acid@CrO₄²⁻, respectively, with the concomitant release of 1. The sensing properties of 1 and the recovery of its emission response in the presence of hyd and hyd acid were evaluated with the help of Hirshfeld surface and density functional theory analyses, respectively. The recovery of the emission property of 1 on the addition of hyd and hyd acid over other antioxidants arises because of their larger E_HOMO and relatively less steric hindrance than those of the other oxidants employed in this study.