Chiral lanthanide complexes in the history of circularly polarized luminescence: a brief summary

Circularly polarized luminescence (CPL) has attracted considerable attention owing to its potential applications in practical life. Although CPL can be generated using a linear polarizer and a quarter-wave plate combination, the generated light emerges by compromising the total output brightness. Therefore, the above-mentioned drawback has led to the development of original CPL emitters. In this perspective, the CPL-based research field continues to flourish, and many CPL emissive small organic molecules, supramolecular assemblies, nano-assemblies, and metal complexes have been explored. The dissymmetry factor (g_lum) is the measure of the extent of circular polarization of light. Chiral organic small molecules possess very small g_lum values. The chiral lanthanide (Ln) complexes are ideal molecular structures to depict the high luminescence dissymmetry factor (g_lum). These Ln complexes have forbidden electric-dipole and allowed magnetic dipole 4f–4f transitions, which make them weak emissive candidates. However, the rational design of the antenna ligands can effectively transfer the energy to Ln metal ions and lower the symmetry of the overall complex, making the electric dipole 4f–4f transition partly allowed. This mini-review article presents the basic principles and key factors for assessing the CPL activity in the chiral Ln complexes. It discusses small Ln complexes with various antenna ligands and distinct chirality induction from the chiral ligands to metal complexes. Furthermore, it briefly examines the rational ligand design and the future uses of chiral Ln metal complexes. Finally, different types of CPL spectrometers are elaborately discussed.