Solution thermodynamics, photophysical, and DFT studies of La(III), and Er(III) chelates with a multifunctional preorganized tripod ligand

Abstract

The interactions of a novel quinoline-based N₃O₆ polydentate chelator, CYTOM5OX, towards La³⁺ and Er³⁺ have been examined by solution thermodynamics, coordination behavior, photophysical, and DFT studies. The influence of pH on the complexation behavior has been explored by adopting pH-metry, electronic absorption, and emission spectroscopic techniques. The potentiometric and spectrophotometric studies established six pKa values for CYTOM5OX between 2.8–10. Lanthanide ions form thermodynamically stable complexes with the ligand, giving high formation constants log β₁₁₀ = 30–35 and pLn = 20 − 24. Irrespective of the lanthanides used, the monometallic Ln(LH) is the main species formed at neutral pH. The principal coordination sphere contains two and three aqua molecules for La(CYTOM5OX) and Er(CYTOM5OX), respectively, according to time-resolved photoluminescence experiments in an H₂O and D₂O solvent at a pH of 7.0. Both absorption and emission studies of these compounds show significant spectral changes. The probe showed simultaneous sensing of La³⁺ and Er³⁺ ions in the visible range under neutral pH, exhibiting a green fluorescence. To validate the experimental findings, the ground state and excited state theoretical spectra of the species have been computed from DFT-optimized structures using the GGA (BLYP-D3) functional and the PM7-Sparkle energy-minimized geometries at CIS excitation levels using the ZINDO/s approach.