Singlet - Singlet Energy Transfer in Heterogeneous Systems

M. MIRENDA, A. IRIEL, H. RODRÍGUEZ, M.G. LAGORIO AND E. SAN ROMÁN

Arizona, Tempe, EEUU

Congreso; 15th Inter-American Photochemical Society; 2004

Energy transfer in systems built-up by dyes attached to solid particles will be addressed with emphasis in the following aspects: a) modeling of inner filter effects in light dispersing media; b) absolute fluorescence quantum yields in heterogeneous systems; c) resonant energy transfer at high local dye concentrations; and d) antenna systems composed by arrays of donor and acceptor molecules.             Light scattering affects photophysical measurements in systems built up by particles or particle aggregates of micrometer size. Some time ago we developed a model to account for light scattering, molecular aggregation and radiative energy transfer. The model accounts for distortions in fluorescence spectra caused by fluorescence reabsorption and, especially, allows calculation of “single-particle” fluorescence quantum yields, provided a fluorescence reference in the solid phase is given. The lack of suitable references induced us to develop a methodology for the determination of absolute fluorescence quantum yields, based on total reflectance measurements for strongly fluorescent dyes.             We added recently further evidence on the validity of our theoretical approach and extended the methodology to account for multiple dye fluorescence and resonance energy transfer among different dyes. In this way, we were able to demonstrate fluorescent emission from dimers of rose Bengal (RB) adsorbed on microgranular cellulose. Comparison with previous results on RB adsorbed on charged nanoparticles,[1] shows that adsorption on cellulose, a neutral polar environment, represents an intermediate situation between adsorption on negatively and positively charged surfaces. Dimer formation, spectrum and fluorescence quantum yield can be correlated with the nature of the surface.             The need for fluorescence standards led us to investigate the properties of Rhodamine 101 on different supports. This dye, with unity quantum yield in solution, was studied on microcrystalline cellulose and silica gel. At the silica surface, FF = 0.75 after correction for reabsorption. For cellulose, FF goes from 0.6 to 0.9 as penetration of the dye into the matrix induced by swelling increases. This result is consistent with those of Mello et al.,[2] who observed using evanescent waves that the singlet lifetime of R101 decreases near the surface of a silica gel block, as compared with the value found in solution.             In systems consisting of two or more dyes at high acceptor concentrations, resonant and radiative energy transfer are competitive processes. Correction of spectra for reabsorption allows quantifying the resonant component with great accuracy. In this way, resonant energy transfer efficiencies can be quantitatively evaluated. Radiative and resonant energy transfer can be combined in order to concentrate excitation energy into specified regions of heterogeneous materials, provided that different dyes are arranged into a suitable structure. Evidence obtained so far may be used to develop appropriate strategies. [1] M.E. Daraio, and E. San Román. Helv. Chim. Acta, 2001, 84, 2601-2614 [2] A. J. de Mello, J. A. Elliot and G. Rumbles, J. Chem. Soc., Faraday Trans., 1997, 93, 4723-4731