Molecule-Like Eu3+-Dimers Embedded in an Extended System Exhibit Unique Photoluminescence Properties

Much is known about the photoluminescence of lanthanide-containing systems, particularly amorphous silicates or organic-inorganic hybrids and crystalline metal-organic frameworks. Comparatively, stoichiometric microporous Ln-silicates are poorly studied. Here, we report the exceptional photoluminescence of microporous AV-24, K-7[Ln(3)Si(12)O(32))center dot xH(2)O (Ln(3+) = Sm3+, Eu3+, Gd3+, Tb3+), the first silicate possessing Ln(3+)-O-Ln(3+) dimers (inter-Ln distance ca. 3.9 angstrom), i.e., two edge-sharing {LnO(6)} octahedra embedded in a crystalline matrix. It is totally unprecedented that in AV-24 Eu3+-O-Eu3+ dimers behave like discrete entities, i.e., molecules: they (i) have a unique emission signature, with pseudopoint group symmetry (C-i), different from the symmetry (C-1) of each individual constituent Eu3+ ion, and (ii) exhibit the unusually long D-5(0) lifetime of 10.29 ms (12 K). In accord with the experimental evidence, a molecular orbital model shows that the Eu3+-O-Eu3+ dimers are energetically more stable than the individual metal ions.