On the formation of aggregates in silica-rhodamine 6G type II hybrids

The spectroscopic features of dye aggregates in organic-inorganic Rhodamine 6G-silica hybrids are investigated in sol-gel prepared type II bulk and thick film samples in the 10 -4-10 -3 mol l -1 concentration range. No aggregates are observed in film samples irrespective of the dye concentration and the spectroscopic features are ascribed to fluorescent monomers. On the contrary, by means of excitation and emission fluorescence measurements and the analysis of fluorescence decay kinetics the formation of fluorescent aggregates is reported in bulk samples and the geometry of the formed aggregates at the silica surface is discussed on the single exciton theory framework. Beside the distribution of a single emitting specie observed in bulk samples and individuated as oblique J dimers, the analysis of the excitation spectra indicate also the presence of "dark" monomer units. An energy transfer mechanism is hypothesized to explain the interaction of the monomer-dimer pair.