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)]<middle dot>2.5H(2)O (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 x 10(4) M-1 and LOD = 6.40 x 10(-5) mM for 1; K-sv = 3.31 x 10(4) M-1 and LOD = 7.65 x 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.