Photoluminescence and Coordination Behaviour of Lanthanide Complexes of Tris (Aminomethyl)Ethane-5-Oxine in Aqueous Solution

Photophysical properties of a multidentate tripodal ligand, 5,5′-(2-(((8-hydroxyquinolin-5-yl) methylamino)methyl)-2-methylpropane-1,3-diyl) bis (azanediyl)bis (methylene)diquinolin-8-ol, (TAME5OX), with La³⁺ and Er³⁺ ions have been examined for photonics applications. The change in behavior in electronic spectra of these complexes reveals the use of TAME5OX as a sensitive optical pH based sensor to detect Ln³⁺ ions whereas indication of strong green fluorescence allows simultaneous sensing within the visible region in competitive medium. The intense fluorescence intermittently gets quenched under acidic and basic conditions due to photoinduced intramolecular electron transfer from the excited 8-hydroxyquinoline (8-HQ) moiety to the metal ion. This renders these compounds the OFF-ON-OFF type of pH-dependent fluorescent sensor. The thermodynamic stability and coordination behaviour of the chelator with the said lanthanide ions have also been probed by potentiometric, UV ? visible and fluorescence spectrophotometric method. TAME5OX forms protonated complex Ln (H₄L)]⁴⁺ below pH ~4.0 which sequentially deprotonates through one proton process with increase of pH. The stability constants of neutral complexes have been determined to be in the range log β₁₁₀ = 32–34 and pLn in the range of 14–20, indicating TAME5OX is a good synthetic lanthanide chelator. Theoretical spectra were also calculated by ZINDO/s methodology at single excitations (CIS) level on PM7 as sparkle energy-minimized geometries.