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

Abstract

Lanthanide complexes based on the quinolinecarboxylate ligand 2-phenyl-4-quinolinecarboxylic acid (PQC), namely, Ln₂PQC₆ (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 Ln³⁺ centre exhibits distorted tricapped trigonal prism geometry. By the introduction of the PQC ligand, the europium sample Eu₂PQC₆ (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⋯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 Eu₂PQC₆ in depth. As an extended research, Eu₂PQC₆ 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.