Modulating the Structure, Luminescence, and Sensing Performance of Photochromic Coordination Polymers through Terminal Ligands

Fe3+ and Hg2+ pose serious health risks, which necessitate dual-response sensors to monitor them. Herein, two new coordination polymers (CPs) [Cd(L)(1-NA)OH]n (1) and [Cd(L)(9-AC)2EtOH]n (2) were synthesized using the 9,10-bis(di(pyridin-4-yl)methylene)-9,10-dihydroanthracene (L) bridging ligand and 1-NA/9-AC (1-Naphthoic acid/9-Anthracenecarboxylic acid) terminal ligands. 1-NA and 9-AC have different molecular sizes, which lead to distinct chain skeletons, π–π stacking, and porosities in 1 and 2. Three-connected L and [Cd(1-NA)OH] units link with each other to form a 1-D zigzag chain in 1, while L single bridges connect with [Cd(9-AC)2EtOH] units to generate a 1-D linear chain in 2. Notably, photochromism-induced luminescence enables a dual-response sensing performance for 1 and 2. 1 exhibits “turn-on” and blueshifted Fe3+ sensing, which is the first instance reported among all CP-based Fe3+ sensors, while 2 has no obvious Fe3+ sensing ability. The sensing mechanism involves an Fe3+-triggered transformation from ligand-to-ligand charge transfer to a localized excitation process. Furthermore, 1′ and 2′ (1 and 2 after photochromism) can detect Hg2+ via a “turn-off” mechanism through a ligand-to-metal charge transfer process. 1′ exhibits more sensitive Hg2+ sensing performance than 2′. Specifically, 1′ achieves a remarkably low limit of detection (LOD: 9.16 nM). This work reveals the crucial role of terminal ligands in CP-based sensors.