Energy transfer among dyes on particulate solids

H.B. RODRÍGUEZ; A. IRIEL; E. SAN ROMÁN

PHOTOCHEMISTRY AND PHOTOBIOLOGY

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 ´ 10-8 mol g-1, addition of 2.05 ´ 10-7 mol g-1 Pheo largely inhibits aggregation up to nearly 10-6 mol g-1 MB. At the same time, the absorption spectrum of monomeric MB in the presence of Pheo differs from the spectrum in pure cellulose. Both effects reveal a strong influence of Pheo on the medium properties. 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 high local concentrations of the acceptor within the supporting material. These results show that large energy transfer rates can exist in a system devoid of any special molecular organization.