Photophysics of supported dyes: quantum yield determination

M. G. LAGORIO, A. IRIEL, L. E. DICELIO Y E. SAN ROMÁN

Cairo, Egipto

Congreso; 6th International Conference on Solar Energy and Applied Photochemistry; 2001

Several photochemical processes of environmental or biological relevance, i.e. the photosensitized or photocatalytical removal of pollutants and the destruction of cells induced by light, involve systems showing some kind of heterogeneity. Dye-to-dye and dye-to-environment interactions affect the photochemical response of heterogeneous systems. These interactions are evidenced not only by the partition equilibrium of the dye among the different phases or microphases and its eventual aggregation: they also strongly influence the photophysical properties of the dye. Therefore, the design of efficient photoactive materials requires an adequate selection of the dye - through its aggregation and acid-base properties, polarity and charge -, controlling at the same time intermolecular interactions through the selection of local concentrations, addition of spacers, etc.             The knowledge of quantum yields is essential to compare the photoactivity of different photosensitizers. Although this is obvious for homogeneous systems, the need of assessing quantum yields is often not taken into account when dealing with solid photosensitizers. The main reason is the difficulty of the determination when light dispersion has to be taken into account. Calculation has to be done in this case with the aid of some suitable dispersion model.             Various tools were developed in our laboratory for the calculation of fluorescence and singlet molecular oxygen quantum yields for supported dyes. They will be presented and compared with methods suggested by other authors.             A model was developed for the calculation of fluorescence quantum yields in opaque solid samples based on the Kubelka-Munk theory of diffuse reflectance taking into account reabsorption and reemission of fluorescence. Using this model it was demonstrated that monomers of aluminum tricarboxymonoamidephthalocyanine (AlTCPc) adsorbed on microcrystalline cellulose present the same fluorescence quantum yield as in solution.             Suitable standards are needed to calculate fluorescence quantum yields. Studies were performed to evaluate the validity of Rhodamine 101 as a standard for different matrices: microcrystalline cellulose, cellulose fibers, silica and silanized silica.             Singlet oxygen quantum yields of AlTCPc covalently linked to a series of solid matrices could be determined by the simultaneous measurement of diffuse reflectance and transmittance in aqueous suspension. The experimental setup did not require cells with special design as is needed by other methodologies. Samples must be dilute enough to assure that more than 70% of the incoming light is transmitted.             Results obtained by comparison of AlTCPc adsorbed or chemically linked to silanized silica show that chemical linkage improves photophysical activity yielding larger fluorescence and singlet oxygen quantum yields.