Tracking coordination environment and optoelectronic structure of Eu3+ and Sm3+ sites via X-ray absorption spectroscopy and X-ray excited optical luminescence

Abstract

In this work, the photophysical characteristics of the [Eu(tta)3(H2O)2] and [Sm(dbm)3(phen)] complexes, tta: 3-thenoyltrifluoroacetonate, dbm: dibenzoylmethane, and phen: phenanthroline were explored through X-ray excited optical luminescence (XEOL) in correlation with the local electronic/chemical structure evidences from the element-specific X-ray absorption fine structure (XAFS) analysis. The monochromatic hard X-ray photon at a synchrotron light source has added advantageous tunability with desirable penetration depth, avoiding optical photons scattering contrary to the optical LASER to unravel the relationship of the luminescence to the electronic structure and coordination environment of RE3+ (RE: Eu and Sm) site in compounds by tuning the photon energy of monochromatic X-ray to the L-edge of the rare earth. Accordingly, the XEOL spectra of both the rare earth complexes measured under excitations with tunable Eu3+ and Sm3+ L-edge monochromatic X-ray photon energy manifested considerably higher emission intensity from the hypersensitive 5D0→7F2 and 4G5/2→6H9/2 forced electric dipole (FED) transitions of Eu3+ and Sm3+ ions, respectively. This result suggested the non-centrosymmetric RE3+ site (lower symmetry), validating through quantitative XAFS analyses, which confirm the triclinic and monoclinic local structure for the [Eu(tta)3(H2O)2] and [Sm(dbm)3(phen)] complexes, respectively.