Regioisomerism Induced Circularly Polarized Luminescence Switching in Natural (+)-Camphor-Based Bis(pyrazolyl)pyridine Europium(III) Complexes

Chiral 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands containing camphor units have been used in their enantiopure, racemic, and meso forms to prepare luminescent europium(III) complexes. The bpp ligands have been fully characterized by single-crystal X-ray diffraction and chiroptical spectroscopy. For one of the enantiopure ligands, namely bis(4<i>S</i>,7<i>R</i>)camphor-2,2-bpp, the regioisomer bis(4<i>S</i>,7<i>R</i>)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 hexafluoroacetylacetonato ligands were used to complete the coordination sphere of the lanthanide. 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 noncentrosymmetric monoclinic <i>P</i>2₁ space group for the enantiopure compounds and the centrosymmetric monoclinic <i>P</i>2₁/<i>n</i> space group for the meso and the racemic ones. The metal center is ennea-coordinated within slightly distorted Muffin-type geometry. The three enantiopure complexes show strong circularly polarized luminescence activity, with anisotropy factors <i>g</i>_lum 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(4<i>S</i>,7<i>R</i>)camphor-2,2-bpp and bis(4<i>S</i>,7<i>R</i>)camphor-1,2-bpp ligands, yet having the same absolute configuration of the camphor units, thus allowing an unprecedented possibility of CPL switching.