Nona-Coordinated Chiral Eu(III) Complexes with Stereoselective Ligand–Ligand Noncovalent Interactions for Enhanced Circularly Polarized Luminescence

Abstract

Circularly polarized luminescence (CPL) of chiral Eu(III) complexes with nona- and octa-coordinated structures, [Eu(R/S-iPr-Pybox)(D-facam)₃] (1-R/1-S; R/S-iPr-Pybox, 2,6-bis(4R/4S-isopropyl-2-oxazolin-2-yl)pyridine; D-facam, 3-trifluoroacetyl-d-camphor), [Eu(S,S-Me-Ph-Pybox)(D-facam)₃] (2-SS; S,S-Me-Ph-Pybox, 2,6-bis(4S-methyl-5S-phenyl-2-oxazolin-2-yl)pyridine), and [Eu(Phen)(D-facam)₃] (3; Phen, 1,10-phenanthroline) are reported, and their structural features are discussed on the basis of X-ray crystallographic analyses. These chiral Eu(III) complexes showed relatively intense photoluminescence due to their ⁵D₀ → ⁷F₁ (magnetic-dipole) and ⁵D₀ → ⁷F₂ (electric-dipole) transition. The dissymmetry factors of CPL (g_CPL) at the former band of 1-R and 1-S were as large as −1.0 and −0.8, respectively, while the g_CPL of 3 at the ⁵D₀ → ⁷F₁ transition was relatively small (g_CPL = −0.46). X-ray crystallographic data indicated specific ligand–ligand hydrogen bonding in these compounds which was expected to stabilize their chiral structures even in solution phase. CPL properties of 1-R and 1-S were discussed in terms of transition nature of lanthanide luminescence.