Dual visible and near-infrared luminescence in mononuclear macrocyclic erbium(iii) complexes via ligand and metal centred excitation

Considering the structural design of some of the scarce molecular-based Er-centred emitters in the literature, we explored the optical properties of three ErIII hexaazamacrocyclic complexes, namely Er-EDA (1), Er-OPDA(2) and Er-DAP(3). The macrocyclic ligands in these complexes differ in the lateral spacers, and are derived from 2,6-pyridine-dicarbaldehyde and ethylenediamine (EDA), ortho-phenylenediamine (OPDA) or 1,3-diaminopropane (DAP). Upon ligand-centred excitation, the bluish-green and green emissions of the ErIII ion were detected only for the complexes containing macrocycles with aliphatic spacers (1 and 3), which evidenced that these ligands can sensitize the ErIII luminescence. On the other hand, the ligand derived from the aromatic diamine (2) does not sensitize the ErIII luminescence. Energy transfer mechanisms, temperature sensing, CIE coordinates and CCT values were analyzed. Besides the excitation in the ligands, the erbium-centred excitation at 980 nm allowed the detection, in all cases, of bluish-green, green and red up-converted emissions, and also the downshifted NIR emission. The possible mechanisms involved in these transitions were described and analyzed according to the available data.