Light- and Chemical-Doping-Induced Magnetic Behavior of Eu Molecular Systems

Variable temperature electron paramagnetic resonance (VT-EPR) was used to investigate the role of the environment and oxidation states of several coordinated Eu compounds. We find that while Eu(III) chelating complexes are diamagnetic, simple chemical reduction results in the formation of paramagnetic species. In agreement with the distorted D_3h symmetry of Eu molecular complexes investigated in this study, the EPR spectrum of reduced complexes showed axially symmetric signals (g_⊥ = 2.001 and g_∥ = 1.994) that were successfully simulated with two Eu isotopes with nuclear spin 5/2 (¹⁵¹Eu and ¹⁵³Eu with 48% and 52% natural abundance, respectively) and nuclear g-factors ¹⁵¹Eu/¹⁵³Eu = 2.27. Illumination of water-soluble complex Eu(dipic)₃ at 4 K led to the ligand-to-metal charge transfer (LMCT) that resulted in the formation of Eu(II) in a rhombic environment (g = 2.006, g = 1.995, g = 1.988). The existence of LMCT affects the luminescence of Eu(dipic)₃, and pre-reduction of the complex to Eu(II)(dipic)₃ reversibly reduces red luminescence with the appearance of a weak CT blue luminescence. Furthermore, encapsulation of a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shaped macrocycle, inhibited ligand-to-metal charge transfer, preventing the formation of Eu(II) upon illumination.