Europium(III) coordination chemistry: structure, spectra and hypersensitivity

The red or orange emission spectra of tripositive europium have led to thousands of publications and to phosphor applications. The orange emission mainly arises from the ⁵D₀ → ⁷F₁ transition (A), whereas in other cases, the red emission is due to the ⁵D₀ → ⁷F₂ transition (B), with a different mechanism. The intensity ratio B/A, which is commonly labelled R2, has been termed the asymmetry ratio and has been used as a probe of europium site symmetry in a material. We demonstrate that R2 is not an exclusive indicator of site symmetry, although it does vary if there is change in the structural symmetry of the coordination environment. We propose the new term ‘hypersensitivity ratio’ to replace ‘asymmetry ratio’. The concepts of crystallographic and spectroscopic site symmetry are reviewed, together with the theoretical basis of the electronic spectra of Eu³⁺. Structures of europium complexes with coordination numbers between 6 and 12 are illustrated and previous interpretations of hypersensitivity are discussed. A tabulation of spectroscopic and crystallographic data is given and analyzed for 262 europium compounds. Moreover, new calculations of the ratio R2 are presented for selected examples from the table, and these possess good agreement with the experimental data. Other factors which can affect the value of R2 are discussed.