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

Sol-get 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 degreesC, doped with (a) and (b). In the glasses annealed above 500 degreesC, a structured broad band is found to be due to the ligand-to-metal charge transfer transition by generation of either [Cr(VI)O-4](2-) or [V(V)O-4](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)O-4](3-). Moreover, the coexistence of both Cr(III) and Cr(VI) ions is confirmed in some glasses (b) prepared tinder mild chemical conditions. (C) 2001 Elsevier Science B.V, All rights reserved.