Luminescent\nMetal–Organic Framework with Outstanding\n“Turn-On” Hg<sup>2+</sup> Sensing Ability First Constructed\nby an AIE Ligand

Hg<sup>2+</sup> is highly toxic and can cause serious\nharm to the\nenvironment and humans. Thus, it is vital to develop efficient Hg<sup>2+</sup> sensors. In this work, a LMOF-based (LMOF = luminescent\nmetal–organic framework) “turn-on” Hg<sup>2+</sup> sensor (<b>1</b>) is first developed by an aggregation-induced\nemission (AIE) functional ligand. <b>1</b> with a formula of\n[Cd<sub>3</sub>(<b>L</b>)<sub>2</sub>(NDA)<sub>3</sub>(H<sub>2</sub>O)<sub>2</sub><b>·</b>(<b>L</b>)(MeOH)]<sub><i>n</i></sub> (<b>L</b> = 4,4′-((2,7-di-<i>tert</i>-butyl-9<i>H</i>-fluoren-9-ylidene)-methylene)dipyridine;\nH<sub>2</sub>NDA = 1,4-naphthalenedicarboxylic acid) exhibits a unique\n(3,4)-connected 2-D framework with 3-connected Cd1, 4-connected [Cd<sub>2</sub>(COO)<sub>3</sub>]<sup>+</sup> cluster, and <b>L</b> and NDA<sup>2–</sup> bridges. The fluorene rings of <b>L</b> in <b>1</b> stack in a back-to-back manner. <b>1</b> has excellent water, pH, and thermal stabilities and can\n“turn-on” detect Hg<sup>2+</sup> with ultrahigh sensitivity,\ngood selectivity, and recyclability in a water medium. Furthermore, <b>1</b> shows obvious naked-eye-visible emission change and an extremely\nfast sensing response toward Hg<sup>2+</sup> (<1 S) and can maintain\nits enhanced intensity over 5 min. The test paper is developed for\npractical “turn-on” Hg<sup>2+</sup> sensing application.\nThe “turn-on” Hg<sup>2+</sup> sensing mechanism of <b>1</b> is discussed in detail.