X-Ray structural and photoluminescence spectroscopic investigation of the europium octamolybdate polymer Eu2(H2O)12[Mo8O27]·6H2O and intramolecular energy transfer in the crystalline lattice

Abstract

Photoluminescence and intramolecular energy-transfer properties of Eu₂(H₂O)₁₂[Mo₈O₂₇]·6H₂O, 1 have been determined in connection with the crystal structure. The compound has been prepared by treating K₂[MoO₄] with Eu(NO₃)₃ in aqueous solution at pH 3.0 and isolated in a crystalline form with a belt structure. A single-crystal X-ray analysis [triclinic, space group P, a= 10.105(3), b= 12.006(5), c= 9.365(4)Å, α= 122.59(3), β= 90.12(3), γ= 98.33(3)°, Z=1, Mo-Kα radiation, R= 0.063 for 4485 independent data with I > 3σ(I)] shows the cation to have the composition [Eu(H₂O)₆]³⁺. The latter is also bonded to one oxygen atom of each octahedron of an edge-sharing pair of MoO₆ octahedra in the Mo₈O₂₇ anion, which is isostructural to the condensation polymer of the octamolybdate through one common oxygen atom. In addition, Eu³⁺ is linked by one oxygen atom belonging to the MoO₆ octahedron in a neighbouring Mo₇O₂₇ unit, resulting in the formation of a tricapped trigonal-prismatic Eu(H₂O)₆O₃ group (average Eu–O bond length 2.48 Å). Luminescent transitions of both ⁵D₀→⁷F_J and ⁵D₁→⁷F_J for Eu³⁺ are observed upon photoexcitation of the O → Mo ligand-to-metal charge-transfer (l.m.c.t.) bands but the intramolecular energy transfer to the ⁵D₁ level takes place predominantly from the O → Mo l.m.c.t. states at energies higher than 3.6 eV (ca. 5.8 × 10^–19 J). The temperature dependence of the intramolecular energy transfer from the O → Mo l.m.c.t. states to Eu³⁺ indicates that the configuration of the Mo–O–Eu linkage (ca. 150°) allows hopping of a d¹ electron to the EuO₉ site due to thermally activated delocalization between MoO₆ and EuO₉ sites, which acts as a deactivation channel for the energy transfer from the O → Mo l.m.c.t. states to the emitting levels of Eu³⁺. Co-ordination of six aqua ligands leads to a low lifetime (0.17 ± 0.01ms) of the emitting state of ⁵D₀ with a resultant decrease in the total quantum yields of the emission, arising from vibronic coupling of the ⁵D₀ state with the vibrational states of high-frequency OH oscillators. The ⁷F₁, ⁷F₂, ⁷F₃, ⁷F₄, ⁵D₁ and ⁵D₂ crystal-field splittings for the C₁ site of Eu³⁺ are estimated on the basis of the high-resolution emission and excitation spectra at 77 K. Vibronic lines belonging to the ⁷F₀→⁵D₂ and ⁷F₁→⁶D₁ transitions are observed in the excitation spectrum.