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

Variabletemperature electron paramagnetic resonance (VT-EPR) wasused to investigate the role of the environment and oxidation statesof several coordinated Eu compounds. We find that while Eu(III) chelatingcomplexes are diamagnetic, simple chemical reduction results in theformation of paramagnetic species. In agreement with the distorted D (3h) symmetry of Eu molecular complexes investigatedin this study, the EPR spectrum of reduced complexes showed axiallysymmetric signals (g (& BOTTOM;) = 2.001 and g (& PAR;) = 1.994) that were successfully simulatedwith two Eu isotopes with nuclear spin 5/2 (Eu-151 and Eu-153 with 48% and 52% natural abundance, respectively) andnuclear g-factors Eu-151/Eu-153 = 2.27. Illumination of water-soluble complex Eu(dipic)(3) at 4 K led to the ligand-to-metal charge transfer (LMCT) that resultedin the formation of Eu(II) in a rhombic environment (g(x) = 2.006, g(y) = 1.995, g(z) = 1.988). The existence of LMCT affects theluminescence of Eu(dipic)(3), and pre-reduction of the complexto Eu(II)(dipic)(3) reversibly reduces red luminescence withthe appearance of a weak CT blue luminescence. Furthermore, encapsulationof a large portion of the dipic ligand with Cucurbit[7]uril, a pumpkin-shapedmacrocycle, inhibited ligand-to-metal charge transfer, preventingthe formation of Eu(II) upon illumination. By using EPR spectroscopy, the occurrenceof chemical andlight-induced charge transfer in Eu molecular complexes has been observed.This remarkable transformation changes nonmagnetic Eu(III) molecularcomplexes into a paramagnetic Eu(II) state. The presence of the excitedstate electron cloud in the LUMO, with the spin density around Eu,has been confirmed through TD-DFT calculations.