Symmetry breaking of rare earth Eu(iii) complexes with an achiral aromatic ligand: achieving strong emission and ultra-narrowband circularly polarized luminescence

Abstract

Narrowband circularly polarized luminescence (CPL) plays a critical role in fabricating high color purity displays but is still in its infancy. Given the unique f–f transition of electrons in the 4f orbital, rare earth complexes generally possess very narrowband emission. However, most of the chiral Eu(III) complexes exhibit weak emission due to the limited light harvesting (absorption) ability of stereoscopic but non-aromatic ligands. Herein, achiral 1-(2-naphthoyl)-3,3,3-trifluoroacetone (NTA), including π-conjugated naphthalene and β-diketone, as a coordinating moiety is employed to coordinate with Eu(III) to achieve the metal–organic complex EuNTA. Benefiting from its immense absorption (ε = 88 500 M⁻¹ cm⁻¹), the luminescence quantum yield of EuNTA is as high as 91%. Due to the strong intermolecular π–π and C–H⋯π interactions, purely chiral aggregates of symmetry breaking-induced Λ-EuNTA and Δ-EuNTA have been successfully obtained through crystallization with spontaneous resolution, resulting in highly asymmetric CPL with a dissymmetry factor of 10⁻². Impressively, Λ/Δ-EuNTA exhibits extremely narrowband CPL with a full width at half maximum of mere 5 nm. Such an achiral π-conjugated ligand-induced symmetry breaking strategy can achieve both strong emission and ultra-narrowband CPL, which is anticipated to put forward a new route toward the further utilization of rare earth complexes in advanced display technology.