The Eu(III) Ion as Luminescent Probe: Investigation of the Metal-Ion Sites in a Dicyclohexyl-18-crown-6 Complex

The metal ion sites of the 3:2 complex between europium nitrate and the A -isomer of dicyclohexyl-18-crown-6, Eu(NO₃)₂(DC18C6)]₂Eu(NO₃)₅], have been probed by high-resolution excitation and emission spectra at 296 and 77 K. The Eu(NO₃)₅]^2? anion gives rise to a luminescent spectrum dominated by the ⁵D₀→⁷F₂ transition. The crystal-field splitting of the ⁷F_j levels is close to that observed for (Phe₄As)₂Eu(NO₃)₅], pointing to a structurally similar pentakis(nitrato) species. The ⁵D₀←⁷F₀ excitation spectrum of the two crystallographically independent complex cations displays five maxima. A detailed analysis of the corresponding and selectively excited emission spectra leads to the following conclusions. Well differentiated spectra are assigned to different conformations of the complex cation, in which half of the ligand atoms, including O-atoms, Present large thermal motions. The other spectra are very similar and arise from slightly unequivalent Eu(NO₃)₂(DC18C6)]⁺ moieties differing in the conformation of their ethylene bridges. This dimonstrates the sensitivity of the Eu(III) ion as conformational probe in the solid state.