Energy transfer among dyes on particulate solids

HERNÁN B. RODRÍGUEZ; ANALÍA IRIEL; ENRIQUE SAN ROMÁN

PHOTOCHEMISTRY AND PHOTOBIOLOGY

WILEY-BLACKWELL PUBLISHING, INC

Año: 2006 vol. 82 p. 200 - 207

0031-8655

Absorption and fluorescence properties of methylene blue (MB), a well known singlet molecular oxygen photosensitizer, and its mixtures with pheophorbide-a (Pheo) sorbed on microgranular cellulose are studied, with emphasis on radiative and non-radiative energy transfer from Pheo to MB. While pure MB builds up dimeric species on cellulose even at 2 x 10-8 mol g-1, addition of 2.05 x 10-7 mol g-1 Pheo largely inhibits aggregation up to nearly 10-6 mol g-1 MB. This fact and the change of the absorption spectrum of monomeric MB in the presence of Pheo reveal a strong interaction between both constituents. A model relying entirely on experimental data is developed, through which energy transfer efficiencies can be calculated for thin and thick layers of dye-loaded cellulose. At the largest concentration of MB assuring no dye aggregation, non-radiative energy transfer efficiencies reach a maximum value of nearly 40 %. This value is quite high, taking into account the low fluorescence quantum yield of Pheo, F = 0.21, and results from the existence of very high local concentrations of the acceptor within the supporting material. These results show that large energy transfer rates can exist at high concentrations, in a system devoid of any special molecular organization.