pH-Dependent Luminescence of Self-Assembly Tb³⁺ Complexes with Photosensitizing Units

Luminescent metal complexes in aqueous solution have been attracting substantial interest as chemical and biological sensors. In this study, aiming to develop lanthanide complexes that exhibit bright luminescence in aqueous media, we designed and synthesized new amphiphilic ligands based on ethylenediamine with two aromatic groups and two cholesteryl groups, which act as photosensitizers and hydrophobic moieties, respectively. In particular, the Tb³⁺ complex of a pyridyl-substituted ligand showed an intense green emission that stems from the efficient energy transfer from the excited pyridyl groups to the Tb center in ethanol. In aqueous media, the Tb³⁺ complexes self-assembled into nanoparticles with diameters of around 40 nm, which show pH-dependent luminescence in the pH range of 6.0-8.5 ascribable to protonation of the coordinated pyridyl groups. These findings demonstrate the potential of aggregates of photosensitized lanthanide complexes as sensors under weakly alkaline conditions.