Luminescence property modulation through structural modification of coordination polymers for ratiometric detection of mercuric ions in aqueous media

Abstract

To investigate the impact of structural modulation on luminescence behaviour, a systematic metal and co-ligand-based modification strategy has been studied for three isostructural mixed-ligand coordination polymers (CPs) derived from the π-conjugated ligand 2,2′-biquinoline-4,4′-dicarboxylic acid (H2bqdc). The first compound, {[Mn(bqdc)(azo)2(H2O)2] ·H2O} n (1) (azo =4,4′-azopyridine), was synthesized, characterized, and evaluated for its luminescence response toward Hg2⁺ detection, exhibiting a distinct quenching effect. Subsequently, a metal-based modification using Co(II) yielded the isostructural CP {[Co(bqdc)(azo)2(H2O)2].(H2O)0.5}n (2), allowing a comparison of the influence of metal centers on luminescence sensing behaviour. Both compounds 1 and 2 demonstrated luminescence quenching toward Hg2⁺, with compound 1 showing higher sensitivity (limit of detection, LOD = 12.55 μM) than compound 2 (LOD = 15.13 μM). Motivated by the similar response in these metal-based modification, next a co-ligand modification was performed by using 1,2-bis(4-pyridyl)ethylene (bpe) instead of azo to understand the impact of ligand replacement on the luminescence based sensing of Hg2⁺. The modified isostructural CP {[Mn(bqdc)(bpe)2(H2O)2]·H2O}n (3) exhibited a co-ligand dominated emission behavior, where the luminescence of bpe prevailed over that of bqdc, resulting in a ratiometric luminescence enhancement response toward Hg2⁺ ions with a lower LOD of 10.74 μM. PXRD and IR studies confirmed that the framework–analyte interactions as the primary reason for the observed variations in luminescence quenching or enhancement.