Circularly polarized luminescence modulation in europium(III) complexes based on camphor-fused bis(pyrazolyl)pyridine ligands

Chiral 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands containing camphor units fused to the pyrazole rings have been used in their enantiopure, racemic and meso forms to prepare luminescent europium(III) complexes. The ligands, for which the connection between pyrazoles and pyridine is through the N2 nitrogen atom, have been fully characterized by single crystal X-ray diffraction and chiroptical spectroscopy. Alternatively, for one of the enantiopure ligands, namely bis(4S,7R)camphor-2,2-bpp, the regioisomer bis(4S,7R)camphor-1,2-bpp, containing one pyrazole connected through the N1 nitrogen atom, has been obtained and used to coordinate the europium(III) ion. Ancillary negative hexafluoroacetylacetonato (hfac) ligands were used to complete the coordination sphere of the lanthanide and to ensure the charge compensation. The complexes with the 2,2-bpp ligands have been structurally characterized by single crystal X-ray diffraction. They are isostructural in spite of the non-centrosymmetric monoclinic P21 space group for the enantiopure compounds and the centrosymmetric monoclinic P21/n space group for the meso and the racemic ones. The metal centre is ennea-coordinated within slightly distorted Muffin type geometry. The three enantiopure complexes show strong circularly polarized luminescence (CPL) activity in solution and in the solid state, with anisotropy factors glum reaching up ±0.2 for the 7F1 ⟵ 5D0 magnetic dipole sensitive transition. Most remarkably, the polarization of the emission is inverted for the two complexes bearing the regioisomeric bis(4S,7R)camphor-2,2-bpp and bis(4S,7R)camphor-1,2-bpp ligands, yet having the same absolute configuration of the camphor units, thus allowing an unprecedented possibility of CPL switching.