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

• Crystallographic and spectroscopic site symmetry are discussed in relation to transition intensities. • The relationship between coordination type, number, geometry and spectra of Eu3+ complexes is shown in detail. • The factor primarily affecting hypersensitivity is ligand polarizability. • The hypersensitive ratio is not a marker of a certain site symmetry but can change according to site symmetry. • A new model is demonstrated to calculate the hypersensitive ratio of Eu3+, in agreement with experimental data. • Site symmetry, 0–0 energy level, and hypersensitivity ratio of 262 different europium complexes are tabulated. 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 5D 0 → 7F 1 transition (A), whereas in other cases, the red emission is due to the 5D 0 → 7F 2 transition (B), with a different mechanism. The intensity ratio B/A, which is commonly labelled R 2, has been termed the asymmetry ratio and has been used as a probe of europium site symmetry in a material. We demonstrate that R 2 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 R 2 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 R 2 are discussed. [ABSTRACT FROM AUTHOR]