Physicochemical control of valence in luminescence of Cr(III) and V(III, IV) complexes embedded in xero–gel and sol–gel SiO2 glasses

Sol–gel silica glasses doped with transition metal complexes were prepared by the Sol–gel-method using complexes such as (a) Cr(III)(AA)3 (AA=acetylacetonato), (b) Cr(III) tris-nuclear basic acetate (Cr-trimer), (c) V(III)(AA)3, and (d) V(IV)O(AA)2. Luminescence, time-resolved luminescence, decay times, excitation and absorption spectra were measured at temperatures between 300 and 10 K. Sharp lines at 13,000 cm−1 due to single Cr(III) ions and exchange-coupled Cr(III) pairs are found in glasses annealed at temperatures below 150°C, doped with (a) and (b). In the glasses annealed above 500°C, a structured broad band is found to be due to the ligand-to-metal charge transfer transition by generation of either [Cr(VI)O4]2− or [V(V)O4]3− complex ions. The spectral shapes, lifetimes and vibronic structures in glasses doped with (c) and (d) are understood by assuming the following thermo-chemical processes by increase of annealing temperatures: (d)[V(III)(AA)3]→(c)[V(IV)O(AA)2]→[V(V)O4]3−. Moreover, the coexistence of both Cr(III) and Cr(VI) ions is confirmed in some glasses (b) prepared under mild chemical conditions.