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 (gx
= 2.006, gy
= 1.995, gz
= 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.