Ligand Engineering of Luminescent Coordination Polymers Achieving Sensing Effect Transformation

The transformation of the sensing effect is first studied by ligand engineering. Hg²⁺ causes serious harm. Therefore, developing rich Hg²⁺ sensors is very essential. Three distinct π-conjugated functional ligands 1-Naphthoic acid (1-NA), 9-Anthracenecarboxylic acid (9-AC), and 1-Pyrenecarboxylic Acid (1-PA) are deliberately selected to construct three new L-based luminescent coordination polymers (LCPs) [Cd(L)₂(1-NA)₂]_n (1), [Cd(L)(9-AC)₂(H₂O)]_n (2), and [Cd(L)(1-PA)₂]_n (3) (L = 4,4'-((9h-fluoren-9-yl)-methylene)-dipyridine). 1-3 exhibit distinct chain skeletons, luminescence, and Hg²⁺ sensing effects. 1 or 2 shows a 1-D sawtooth or linear chain, which is formed by L bridges linking with [Cd(L)(1-NA)₂] or [Cd(9-AC)₂(H₂O)] units, while 3 is a 1-D cluster chain skeleton, constructed by two kinds of 1-PA bridges linking with [Cd(L)]²⁺ units. Luminescence of 1 is originated from the 1-NA-L LLCT process, while the emission of 2 or 3 is based on that of 9-AC or 1-PA. Particularly, they exhibit distinct Hg²⁺ sensing effects. 1 can detect Hg²⁺ through fluorescence enhancement and blue-shift, which is first found in all LCP-based Hg²⁺ sensors; 2 only shows a negative Hg²⁺ sensing ability; 3 can detect Hg²⁺ through fluorescence quenching. The detection limits of 1 and 3 toward Hg²⁺ are 340 and 120 nM, respectively. The mechanisms of distinct sensing effects are explored. This work reveals the crucial role of ligand engineering in LCP-based sensors.