Photophysics of mixed-ligand polypyridyl ruthenium(II) complexes immobilised in silica sol-gel monoliths

A series of compounds [Ru(bpy)(3-x)(dpp)(x)](2+) (bpy = 2,2'-bipyridyl, dpp = 4,7-diphenyl-1,10-phenanthroline) have been synthesised and physically immobilised in silica sol-gel monoliths. Transient resonance-Raman studies show that for the immobilised mixed-ligand complexes, as in solution, the emitting ((MLCT)-M-3) state is dpp based. The resonance-Raman evidence also suggests that the structure of this state is the same in both environments. The emission lifetimes of the immobilised complexes cannot be analysed as single exponential decays and the values obtained are dependent on the initial pH of the sdl-gel. The temperature-dependent emission behaviour of the compounds is substantially changed upon immobilisation in the sol-gel. Both the variation of the emission lifetime and energy are significantly different from that observed for the solution-based species. For the bpy containing complexes no evidence is obtained for the population of the photoactive ligand held state, whereas for the complex [Ru(dpp)(3)](2+) population of this state is observed. The relevance of these observations for the design of optical sensors for oxygen is discussed.