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)] x xH(2)O (Ln(3+) = Sm(3+), Eu(3+), Gd(3+), Tb(3+)), the first silicate possessing Ln(3+)-O-Ln(3+) dimers (inter-Ln distance ca. 3.9 A), i.e., two edge-sharing {LnO(6)} octahedra embedded in a crystalline matrix. It is totally unprecedented that in AV-24 Eu(3+)-O-Eu(3+) 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 Eu(3+) ion, and (ii) exhibit the unusually long (5)D(0) lifetime of 10.29 ms (12 K). In accord with the experimental evidence, a molecular orbital model shows that the Eu(3+)-O-Eu(3+) dimers are energetically more stable than the individual metal ions.