Toward highly efficient long-lived excited state generation in crowded constrained environments

HERNAN B. RODRIGUEZ; ISABEL FERREIRA MACHADO; SILVIA E. BRASLAVSKY; LUIS FILIPE VIEIRA FERREIRA; ENRIQUE SAN ROMAN

Cordoba

Encuentro; XI ELAFOT; 2012

UNRC

Application of dye-modified materials (nanoparticles, microparticles, thin films) in the fields of photosensitization and photocatalysis requires high dye loadings in order to maximize light absorption. However, under these conditions the proximity of dye molecules leads to the formation of molecular aggregates and statistical traps, which dissipate the excitation energy and lower the population of useful excited states. For that reason, it has been argued that molecular organization is essential to avoid energy wasting. However, using laser-induced optoacoustic spectroscopy (LIOAS), it has been found that xanthene dyes rose bengal and erythrosin B show concentration independent triplet quantum yields when adsorbed on microcrystalline cellulose, though molecular aggregation leads to significant fluorescence quenching in the same concentration range [1]. Furthermore, using reflectance, steady-state fluorescence and laser-induced time resolved spectroscopies and applying lifetime distribution and bilinear regression analyses, it was demonstrated that eosin Y and phloxine B triplets have concentration independent decays in the same environment [2,3]. Further LIOAS experiments confirmed that both dyes have nearly constant triplet quantum yields. Dye-to-dye interactions cannot be excluded because changes in absorption spectra, fluorescence red-shifts and fluorescence quenching were observed on increasing dye concentration. In particular, weak exciton interactions between close-lying molecules explain the formation of statistical traps for singlet excitation. Exciton interactions depend on the magnitude of the monomer transition moment, being much greater for the singlet state. Accordingly, the interaction of close-lying monomers accelerates singlet state radiationless deactivation, whereas triplet excited molecules behave nearly as isolated monomers. On the other side, the independence of triplet quantum yields on concentration can be explained on grounds of compensation between the singlet state deactivation routes. Hypotheses accounting for this behavior are presented. The effect of site heterogeneity and dye-support interactions in the properties of dyes embedded into microcrystalline cellulose is also discussed. Finally, the results are compared with literature data and the relevance of high triplet quantum yields and lifetimes is analyzed from the point of view of practical applications. However, using laser-induced optoacoustic spectroscopy (LIOAS), it has been found that xanthene dyes rose bengal and erythrosin B show concentration independent triplet quantum yields when adsorbed on microcrystalline cellulose, though molecular aggregation leads to significant fluorescence quenching in the same concentration range [1]. Furthermore, using reflectance, steady-state fluorescence and laser-induced time resolved spectroscopies and applying lifetime distribution and bilinear regression analyses, it was demonstrated that eosin Y and phloxine B triplets have concentration independent decays in the same environment [2,3]. Further LIOAS experiments confirmed that both dyes have nearly constant triplet quantum yields. Dye-to-dye interactions cannot be excluded because changes in absorption spectra, fluorescence red-shifts and fluorescence quenching were observed on increasing dye concentration. In particular, weak exciton interactions between close-lying molecules explain the formation of statistical traps for singlet excitation. Exciton interactions depend on the magnitude of the monomer transition moment, being much greater for the singlet state. Accordingly, the interaction of close-lying monomers accelerates singlet state radiationless deactivation, whereas triplet excited molecules behave nearly as isolated monomers. On the other side, the independence of triplet quantum yields on concentration can be explained on grounds of compensation between the singlet state deactivation routes. Hypotheses accounting for this behavior are presented. The effect of site heterogeneity and dye-support interactions in the properties of dyes embedded into microcrystalline cellulose is also discussed. Finally, the results are compared with literature data and the relevance of high triplet quantum yields and lifetimes is analyzed from the point of view of practical applications. Acknowledgements: Part of this work was a result of a scientific and technological cooperation program between MINCyT (Argentina) and FCT (Portugal), project PO/09/32. Funding from FCT, CONICET, UBA and ANPCyT is also acknowledged. Part of this work was a result of a scientific and technological cooperation program between MINCyT (Argentina) and FCT (Portugal), project PO/09/32. Funding from FCT, CONICET, UBA and ANPCyT is also acknowledged. References: [1] E. P. Tomasini, S. E. Braslavsky, E. San Román. Photochem. Photobiol. Sci. 11, 2012, 1010?1017. [2] H. B. Rodríguez, E. San Román, P. Duarte, I. Ferreira Machado, L. F. Vieira Ferreira. Photochem. Photobiol., in press. [3] P. Duarte, D. P. Ferreira, I. Ferreira Machado, L. F. Vieira Ferreira, H. B. Rodríguez, E. San Román. Molecules 17, 2012, 1602-1616. Photochem. Photobiol. Sci. 11, 2012, 1010?1017. [2] H. B. Rodríguez, E. San Román, P. Duarte, I. Ferreira Machado, L. F. Vieira Ferreira. Photochem. Photobiol., in press. [3] P. Duarte, D. P. Ferreira, I. Ferreira Machado, L. F. Vieira Ferreira, H. B. Rodríguez, E. San Román. Molecules 17, 2012, 1602-1616. Photochem. Photobiol., in press. [3] P. Duarte, D. P. Ferreira, I. Ferreira Machado, L. F. Vieira Ferreira, H. B. Rodríguez, E. San Román. Molecules 17, 2012, 1602-1616. Molecules 17, 2012, 1602-1616.