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.

Excitation Energy Transfer and Trapping in Dye‐Loaded Solid Particles

“…Dye aggregation was recognized as one of the most important factors impairing photoactivity. 13 Self-aggregation can be controlled by the simultaneous attachment of a second dye, as it has been demonstrated for methylene blue (MB) coadsorbed with pheophorbide-a (Pheo) 14 or in the presence of chemically attached rhodamine 101 (R101). 15 In both cases, MB aggregation is reduced in comparison with the same dye adsorbed separately on the same support.…”

“…The occurrence of concentrationdependent Stokes shifts, resulting in a displacement to higher wavelengths of the fluorescence maximum as the dye concentration increases, once spectra are corrected for inner filter effects, while the absorption spectrum remains unchanged, is a common observation in these systems. [10][11][12]14,15 The effect is noticed even at average intermolecular distances in excess of 10 nm. This behavior may be ascribed to a gradual perturbation of the excited-state microenvironment with the increase of the dye concentration due to the interaction of the excited state with ground-state neighbor molecules.…”

“…This behavior may be ascribed to a gradual perturbation of the excited-state microenvironment with the increase of the dye concentration due to the interaction of the excited state with ground-state neighbor molecules. 18 In two-dye systems, nonradiative (Förster) energy transfer leads to donor excited-state deactivation followed by acceptor excitation 14,15 rather than to energy trapping by molecular aggregates or statistical traps. It was demonstrated that very high energy transfer efficiencies can be achieved without any particular molecular order.…”

“…In general, single-dye fluorescence quantum yields, once corrected for dye aggregation and inner filter effects, decrease steadily with dye concentration. 12,14 Concentration quenching may take place even when no spectroscopic evidence on aggregation is found. 12 The trapping effect of dimeric species has been demonstrated for MB adsorbed on cellulose microparticles (average size 20 μm), after calculation of the dimerization equilibrium constant, K = 225 nm 2 , from spectroscopic data (units reflect the fact that surface concentrations are expressed in molecules per nm 2 ; in more common units, K = 1.36 × 10 10 dm 2 mol −1 ).…”

“…8,9 Energy transfer processes have been less studied in these conditions. During the past years, several methods and models have been developed in our laboratory to account for reabsorption and reemission of fluorescence, 10-13 singlet-singlet nonradiative energy transfer (NRET), 14,15 and calculation of absolute fluorescence quantum yields. 16 Systems composed of single dyes or pairs of dyes with substantial spectral overlap (donor-acceptor systems) were studied.…”