Luminescence of Zn-Yb dinuclear Schiff base complex: Enhanced NIR emission by modification with larger conjugated light-harvesting moieties

The fundamental challenge for the highly efficient NIR-luminescence of lanthanides is to identify the suitable chromophores as sensitizer. Herein, the first Salen-type Schiff base ligand modified by conjugated fluorene has been designed to enhance the near-infrared emission of the ytterbium complex. The photophysical properties reveal the potential of the Schiff base dyes as an efficient and strong visible light sensitizer for the lanthanide-based NIR emitters. The modified rigid Schiff base ligand is noted to encapsulate the Ln3+ ions as it prevents the coordination of the solvent molecules, thus, tending to quench the luminescence phenomenon. Due to the intermolecular π-π interactions, the excited state energy levels of ligands are observed to reduce, thus, leading to the highly efficient energy transfer from the triplet state of ligands to the Yb3+ ions. In addition, the shifting of the absorption wavelength of the complex towards low energy (450 nm) makes it suitable for biomedical applications. Further, the ytterbium complex exhibits the highest intrinsic quantum yields (3.68%) among the Salen-type ytterbium complexes.