title: Energy transfer among dyes on particulate solids.
abstract: 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 nonradiative energy transfer from Pheo to MB. Although 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. 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, nonradiative 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, Phi = 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.
authors.affiliation: INQUIMAE/DQIAyQF, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina.
authors.forename: Hernán B
authors.initials: HB
authors.lastname: Rodríguez
authors.forename: Analía
authors.initials: A
authors.lastname: Iriel
authors.forename: Enrique
authors.initials: E
authors.lastname: San Román
mesh_terms: D002482:Cellulose; D002734:Chlorophyll / Q000031:analogs & derivatives* / Q000737:chemistry; D004396:Coloring Agents / Q000737:chemistry*; D004735:Energy Transfer; D008751:Methylene Blue; D013053:Spectrophotometry
chemical_list: D004396:Coloring Agents; D002734:Chlorophyll; D002482:Cellulose; C032623:pheophorbide a; D008751:Methylene Blue