Contrasting impact of coordination polyhedra and site symmetry on the electronic energy levels in nine-coordinated Eu(iii) and Sm(iii) crystals structures determined from single crystal luminescence spectra

Lanthanide luminescence is characterised by "forbidden" 4f-4f transitions and a complicated electronic structure. Our understanding of trivalent lanthanide(III) ion luminescence is centered on Eu³⁺ because absorbing and emitting transitions in Eu³⁺ occur from a single electronic energy level. In Sm³⁺ both absorbing and emitting multiplets have a larger multiplicity. A band arising in transitions from the first emitting state multiplet to the ground state multiplet will have nine lines for a Sm³⁺ complex. In this study, high-resolution emission and excitation spectra were used to determine the electronic energy levels for the lowest multiplet and first emitting multiplet in four Sm³⁺ compounds with either tricapped trigonal prismatic TTP or capped square antiprismatic cSAP coordination polyhedra but different site symmetry. This was achieved by the use of Boltzmann distribution population analysis and experimentally determined transition probabilities from emission and excitation spectra. Using this analysis it was possible to show the effect of changing three oxygen atoms with three nitrogen atoms in the donor set for two compounds with the same coordination polyhedra and site symmetry. This work celebrates the 40^th anniversary of Kirby and Richardson's first report of [Eu(ODA)₃]^3- luminescence.