Electronic Energy Transfer in scattering media

A. IRIEL AND E. SAN ROMÁN

La Plata, Argentina

Congreso; VIII Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2004

In materials composed by dyes attached to solid particles at high local concentrations, corresponding to dye-to-dye mean distances as small as 10 Å, energy transfer can occur both by a radiative and a non-radiative, dipolar mechanism. The radiative mechanism, operating at long distances, can be accounted for by modeling, so as to reveal the importance of dipolar energy transfer. In the present study, 3-sulfonamidopropionic acid, succinimidyl ester (Dapoxyl®, labs = 388 nm; lem = 554 nm) is used as the donor (D), to transfer its excitation energy to 5-(and-6)-carboxynaphthofluorescein (labs = 625 nm; lem = 680 nm), acting as the acceptor (A), on the surface of microgranular cellulose particles (mean size » 20 mm). Considering that the overlap between emission and absorption spectra is negligible for D, extension of models for energy transfer in light scattering media developed in our laboratory[1],[2] yields following expression for the overall emission spectrum: where a0i is the fraction of light absorbed by each species, Fi their fluorescence quantum yields, fi their fluorescence spectra, I0 the excitation intensity, Rt the total reflectance of the sample, g is a correction factor depending exclusively on remission functions and Pij are overlap integrals. In the above expression, e is the efficiency of dipolar energy transfer, which may be readily calculated, as all remaining parameters in the above expression can be accessed experimentally. Pure and mixed samples ranging from 2 × 10-8 to 2 × 10-6 mol D or A / g cellulose were studied. Both D and A have the same remission function spectra in pure and mixed samples and the emission spectra obtained for the mixtures are linear combinations of those of the pure components. From the last spectra, we were able to calculate dipolar energy transfer efficiencies ranging from zero at low acceptor concentrations to more than 50 % as concentration increases. [1] M.G. Lagorio, L.E. Dicelio, M.I. Litter, E. San Román, J. Chem. Soc. Faraday Trans.,1998, 94, 419-425 [2] H. Rodríguez, M.G. Lagorio, E. San Román, Photochem. Photobiol. Sci., 2004, 3, in press