Fluorescence of europium(III) trapped in silica gel-glass as a probe for cation binding and for changes in cage symmetry during gel dehydration

The fluorescence of Eu(III) is a sensitive probe for the gel-glass transformation. Silica gel-glass containing Eu(III) was prepared by hydrolysis of tetramethoxysilane. The total fluorescence of EU(III) from the ⁵D_o electronic state exhibits a gradual increase and a relative increase of intensity of the hypersensitive transition ⁵D_o → ⁷F₂ as a function of time and temperature of dehydration of the gel. This behavior is explained by formation of a chemical bond between Eu(III) and the oxygens of the silicate group whereby Eu(III) acquires a symmetry similar to that in a regular glass obtained by melting the appropriate oxides. The degeneracy of the ⁷F₁ and ⁷F₂ states is removed in low-symmetry sites in the glass as reflected by the half-width of the emission bands.