Effects of the recognition sites of MOFs on turn-off fluorescence detection of Fe3+†

In this work, two fluorescent Cd(II)-based metal–organic frameworks (MOFs), named [CdL(dpa)]·2.5H₂O (1) and Cd₂L₂(2,2′-bpy)₂ (2) (H₂L = 5-[(dimethylamino)thioxomethoxy]-1,3-benzenedicarboxylic acid, dpa = 4,4′-dipyridylamine and 2,2′-bpy = 2,2′-bipyridine), were successfully exploited as fluorescent sensors for the detection of Fe³⁺ in an aqueous medium. Compound 1 was assembled with Cd²⁺, L²⁻ and dpa to construct a porous two-dimensional layer. The (dimethylamino)thioxomethoxy groups in the layer protrude into the adjacent layers to form an interdigitated motif. Compound 2 exhibited an infinite ladder-like chain with the (dimethylamino)thioxomethoxy groups hanging on the two sides of the chain. Fluorescence studies revealed that both 1 and 2 can effectively detect Fe³⁺ in H₂O through luminescence quenching (K_sv = 2.96 × 10⁴ M⁻¹ and LOD = 6.40 × 10⁻⁵ mM for 1; K_sv = 3.31 × 10⁴ M⁻¹ and LOD = 7.65 × 10⁻⁵ mM for 2). The synergistic competitive absorption and coordination interaction mechanism could explain the detection of Fe³⁺. Furthermore, the enlarged steric hindrance in compound 1 resulted in lower values of K_sv and LOD than those of compound 2, which impeded the coordination of Fe³⁺ with its N, O and S recognition sites.