Towards opto-structural parameters to enhance the circularly polarized luminescence brightness of EuIII β-diketone complexes with chiral auxiliary ligands

Abstract

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 (ϕ^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 ⁵D₀ → ⁷F₁ transition and the highest CPL brightness reported so far (465 M⁻¹ cm⁻¹ in dichloromethane) for the ⁵D₀ → ⁷F₂ 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 ⁵D₀ → ⁷F₂ 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.