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

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 (gCPL) at the former band
of 1-R and 1-S were as large as −1.0 and
−0.8, respectively, while the gCPL of 3 at the 5D0 → 7F1 transition was relatively small (gCPL = −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.