Luminescent sensing profiles based on anionresponsive lanthanide(III) quinolinecarboxylate materials: solid-state structures, photophysical properties, and anionic species recognition

Lanthanide complexes based on the quinolinecarboxylate ligand 2-phenyl-4-quinolinecarboxylic acid (PQC), namely, Ln(2)PQC(6) (Ln = La3+, Pr3+, Nd3+, Sm3+ and Eu3+) were synthesized by a convenient solvothermal reaction, and the related crystal structures were determined. These complexes are isostructural and feature a discrete dinuclear moiety, in which solvent molecules act as coligands and the Ln(3+) centre exhibits distorted tricapped trigonal prism geometry. By the introduction of the PQC ligand, the europium sample Eu(2)PQC(6) (5) showing intense red emission in the solid state and solution, with both the replaceable coordination environment and potential hydrogen-bonding-accepting site, can be explored as a luminescent probe for sensing various anions. Among the tested ions (F-, Cl-, Br-, I-, NO3-, ClO4-, SCN-, HCO3-, HSO4- and H2PO4-), HSO4- and H2PO4- possess a strong response in the Eu3+ emitting behaviour leading to a unique quenching with a low detection limit of 15.3 and 8.3 nM, respectively, and a quick response time of less than 30 s. The different recognition mechanisms for two anions, including O-H center dot center dot N hydrogen bonding and metal coordination modes have been proposed, which are supported by UV-vis, fluorescence spectroscopy, nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) measurements. In addition, density functional theory (DFT) calculations were carried out to verify the two recognition mechanisms of Eu(2)PQC(6) in depth. As an extended research, Eu(2)PQC(6) interaction with the nucleoside phosphates adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine monophosphate (AMP) in mixed aqueous solution was investigated, which can act as a special luminescent sensor for all three phosphates. The strong quenching by ADP (99.5%), has been ascribed to the strong bonding affinity (log K-a = 5.99) based on electrostatic attractions and steric effects.