Resonance energy transfer between dye molecules in hybrid films of a layered silicate, including the effect of dye concentration thereon

Rhodamine 6G (R6G) and Oxazine 4 (Ox4), considered a "Forster Resonance Energy Transfer" (FRET) pair due to the significant overlap between the fluorescence of R6G (as energy donor, ED) and the absorption band of Ox4 (as energy acceptor, EA), were intercalated into an expandable layered silicate, saponite (Sap). Several films with different dyes/Sap loadings were prepared with the aim of studying the energy transfer process between both dyes at variable concentrations in the solids. The films were characterized by spectral methods in the visible spectral range. A theoretical model of FRET efficiency was proposed for hybrid solid materials, built around the construction of probability density functions of the intermolecular distances in the solids. The theoretical results were compared with FRET efficiencies determined experimentally using steady-state and time-resolved fluorescence spectroscopy. Considering the very high sensitivity of the FRET efficiency to the intermolecular distances between ED and EA, the theoretical model could predict experimental data relatively well only for low dye loadings (< 1% of cation exchange capacity). The main reasons for the failure of the model at higher dye loadings were dye aggregation and the occurrence of ED species which did not participate in FRET.