Towards opto-structural parameters to enhance the circularly polarized luminescence brightness of Eu^III -diketone complexes with chiral auxiliary ligands

Circularly polarized luminescence (CPL) from lanthanide (Ln(III)) complexes has aroused extensive attention in the last decade owing to its potential use in displays or as anti-counterfeiting devices, for instance. However, the design of Ln(III) complexes matching high luminescence dissymmetry factor and CPL brightness still limits their real application. To help deal with this challenging task, presented herein is a series of [Eu(hfa)(3)(chiral)] complexes, (chiral = S-Bn-pybox = 2,6-bis((S)-4-benzyl-4,5-dihydrooxazole-2-yl)pyridine, S-Ph-pyox = (S)-4-phenyl-2-(pyridin-2-yl)-4,5-dihydrooxazole, or (R)-Cl-(S)-Ph-pzox = N-((R)-2-cloro-1-phenyletyl)-3-((S)-4-phenyl-4,5-dihydrooxazole-2-yl)pyrazine-2-carboxamide; hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedione), aiming to understand through opto-structural correlations how the overall emission quantum yield (phi(L)(Ln)), dissymmetry factor (g(lum)), as well as CPL brightness can be finely tuned by changing the electronic structure and symmetry of the Eu-III coordination polyhedron. [(Eu(hfa)(3)(S)-Bn-pybox)] displayed the largest |g(lum)| values (0.36 in dichloromethane) for the D-5(0) -> F-7(1) transition and the highest CPL brightness reported so far (465 M-1 cm(-1) in dichloromethane) for the D-5(0) -> F-7(2) transition. By combining the experimental photoluminescence and computational data, we were able to show that the bright CPL is a consequence of the quite high emission quantum yield in dichloromethane (41.4%), the large branching ratio of the D-5(0) -> F-7(2) transition, and the increase of the magnetic dipole momentum contribution to the Eu-III transitions owing to the low Eu-III local microsymmetry. Thus, our investigation helps to provide further guidance on how the electronic structure of the complexes and the Eu-III local microsymmetry can play synergistically to improve the CPL brightness of Eu-III complexes.