Electronic strain effect on Eu(III) complexes for enhanced circularly polarized luminescence

Luminescent Eu(iii) complexes with point-chiral phosphine oxide ligands, [Eu(hfa)(3)((R,R)-B2QPO)] (hfa: hexafluoroacetylacetonato, B2QPO: 2,3-bis(tert-butylmethylphosphine oxide)quinoxaline) and [Eu(hfa)(3)((R)-B3QPO)] (B3QPO: 2-tert-butylmethylphosphine oxide-3-(di-tert-butylphosphineoxide)quinoxaline), are reported for the investigation of the electronic strain effect on the coordination sphere. Single crystal X-ray crystallography reveals the strong structural strain of the hfa ligands in [Eu(hfa)(3)((R,R)-B2QPO)]. The emission quantum yields of [Eu(hfa)(3)((R,R)-B2QPO)] in solution (55%) and solid (63%) are comparable to those of previously reported bright luminescent Eu(iii) complexes. The chiroptical properties of [Eu(hfa)(3)((R,R)-B2QPO)] and [Eu(hfa)(3)((R)-B3QPO)] were characterized using circular dichroism (CD) and circularly polarized luminescence (CPL) spectra. The dissymmetry factor of [Eu(hfa)(3)((R,R)-B2QPO)] was estimated to be 0.08. The chiroptical phenomena of the Eu(iii) complexes are closely related to their structural (geometry) and electronic (LMCT: ligand-to-metal charge transfer) strains.