Crystal structure, spectroscopy and crystal field analysis of substituted 1,10-phenanthroline-europium complexes

Crystal structures and vibrational and electronic spectroscopic data are reported for europium(III) complexes with 5(or 4)-R-1,10-phenanthroline (phen) ligands (R = chloro, methyl, nitro, amino). All complexes comprise a 10-coordinate Eu³⁺ ion with three chelating nitrato anions and two bidentate phen ligands. The crystal structures are of three distinct types with space groups C2/c, P2₁/n and P1̄. The room-temperature emission spectra are dominated by the forced electric dipole ⁵D₀→⁷F₂ emission of Eu³⁺. The excitation spectra show that the triplet donor state of the nitro-substituted complex has a lower energy than the other complexes and the lower luminescence quantum efficiency of this complex is rationalized. The emission spectra recorded at 10 K have been compared with those of the corresponding Eu³⁺-doped yttrium complexes using different excitation lines as a criterion of sample purity. The spectrum of the unsubstituted phen complex is more clearly resolved than those of the other complexes and this has been attributed to disorder resulting in inhomogeneous broadening. A detailed crystal field analysis is given for the phen complex from the fitting of 24 crystal field energy levels. A comparison with other complexes has been made through derived second-rank crystal field strengths. No simple relationships have been derived from comparisons of energy levels, spectral line positions, or second-rank crystal field strengths with crystallographic data or Taft constants.