Two new Cd(II)-based coordination polymers derived from flexible linker: Highly sensitive and selective "turn off-on" fluorescent sensors for ferric and dichromate ions

Coordination polymers (CPs) are peculiar multi-dimensional materials that find utility as luminescent sensors for ions and molecules existing in wastewater discharge. Herein, two new Cd(II)-based CPs with formula [Cd(L) (phen)]center dot 0.5H(2)O (1) and [Cd(L)(2,2 '-bipy)] (2) (H2L = 3,3'-((1,2-phenylenebis(methylene))bis(oxy))dibenzoic acid, phen = 1,10-phenanthroline and 2,2 '-bipy = 2,2 '-bipyridine) have been synthesized and characterized. The single crystal X-ray analysis for 1 reveals that Cd(II) centre adopts an octahedral geometry and is coordinated to four carboxyl O atoms from three L2- ligands and two N atoms from phen molecules. While in CP 2, the Cd(II) centre is seven coordinated, which is completed by five carboxylate O atoms from two distinct L2- ligands and two N atoms from the 2,2-bipy ligand. Both 1 and 2 have been used as turn-off fluorescence sensor for selective and sensitive detection of Fe3+ and Cr2O72-. The CP 1 exhibited superior selectivity and sensitivity than 2 with limit of detection (LOD) 1.732 x 10(-5) M and 5.996 x 10(-6) M against Fe3+ and Cr2O72- respectively. Interestingly, the emissive behaviour of 1 get restored on adding 2-hydroxybenzoic acid (2-hyd acid) (turn on) due to the competitive formation of 2-hyd acid@Fe3+ or 2-hyd acid@Cr2O72- and concomitant release of 1. The fluorescence recovery selectivity of 2-hyd acid over other antioxidants has been attributed to the synergistic effect of high E HOMO value and a small space steric hindrance. The sensing properties of the MOF have been addressed with the help of Hirshfeld surface and density functional theory analyses.