Luminescent Cerium(III) Complexes with Poly(mercaptoimidazolyl)borate: A New Emitter Based on S-Coordinating Ligands

Lanthanide ions are hard Lewis acids and usually form weak bonds with soft sulfur donors, which result in the supposed instability of their complexes with S-coordinating ligands. Compared with luminescent lanthanide complexes with hard nitrogen or oxygen donor ligands, the development of luminescent lanthanide complexes based on sulfur-donor ligands currently lags behind. In this work, two types of poly(mercaptoimidazolyl)borate ligands, one is tridentate and the other is tetradentate, were used for the design and synthesis of two series of novel S-coordinating cerium(III) (Ce(III)) complexes, which were found to be not only luminescent but also with moderate to surprisingly good stability in air. By tuning the ligand type and substituents, blue- and yellow-green-emitting Ce(III) complexes with different emission mechanisms of d-f transition, delayed d-f transition, and ligand-centered fluorescence/phosphorescence were obtained, among which the highest photoluminescence quantum yield (PLQY) was 97%, and the most stable one can still maintain 74% of the initial PLQY even after exposure to air for 1 month. Furthermore, we investigated the degradation mechanism of these complexes in air, revealing the oxidation of low-valence sulfur(II) to high-valence sulfur-containing S=O bonds. This work shows the potential of S-coordinating ligands in luminescent Ce(III) complexes and provides new perspectives for designing Ce(III) complexes with soft donors.