Energy transfer from chemically linked rhodamine 101 to adsorbed methylene blue on microcrystalline cellulose particles

HERNÁN B. RODRÍGUEZ; ENRIQUE SAN ROMÁN

Kyoto

Simposio; XXIst IUPAC Symposium on Photochemistry; 2006

IUPAC

Rhodamine 101 (R101) was attached to the hydroxyl groups of microcrystalline cellulose particles by heterogeneous esterification using dicyclohexylcarbodiimide (DCC) as in situ activator of the dye carboxyl group. As a result of chemical linkage, absorption and emission spectra of R101 show a bathochromic shift of (10 ± 2) nm with respect to the dye adsorbed on the same support. Samples containing increasing amounts of absorbed methylene blue (MB) were studied by reflectance and fluorescence spectroscopy, with the aim of obtaining information about singlet energy transfer from R101 to MB. In the absence of MB, a true fluorescence quantum yield – devoid of reabsorption effects – of 0.80 ± 0.05 was observed for R101, which decreased steadily on adding MB. True quantum yields lower than one were also found in adsorbed R101 samples at high local concentrations. Both remission function and emission spectra were linear combinations of R101 and MB spectra up to a concentration of MB around 10-6 mol g-1. This shows that, while pure MB builds up dimeric species on cellulose even at 2 × 10-8 mol g-1, no evidence on MB aggregation is found in the presence of R101. This result shows also that neither heteroaggregates nor exciplexes are formed in the mixed system. Energy transfer efficiencies were calculated on grounds of a previously developed model (H.B. Rodríguez, A. Iriel and E. San Román, Photochem. Photobiol., in press) relying entirely on remission function and fluorescence data. Non radiative energy transfer efficiencies up to 60% were obtained both for thin and optically thick layers of particles and overall energy transfer efficiencies around 80% were obtained for thick layers when radiative energy transfer was also taken into account. While Förster behavior is obeyed at high MB concentrations, energy transfer rates higher than predicted by theory were obtained at low acceptor concentrations. This behavior is analyzed by Monte Carlo calculations and could be explained on grounds of energy migration among donor molecules, according to the high fluorescence quantum yield and local concentration of R101, invoking a non-random distribution of acceptors.