Photophysics of Eosin Y Absorbed on Microcrystalline Cellulose

HERNAN B. RODRIGUEZ; PAULO DUARTE; ISABEL FERREIRA MACHADO; LUIS FILIPE VIEIRA FERREIRA; ENRIQUE SAN ROMAN

Mendoza

Conferencia; 21st I-APS Winter Conference; 2011

Inter-American Photochemical Society

The photophysical behavior of Eosin Y absorbed on microcrystalline cellulose particles was studied by steady state and time resolved diffuse reflectance and emission spectroscopies. Emission measurements include phosphorescence and direct and delayed fluorescence. Solid samples, spanning two orders of magnitude in dye concentration, were prepared by evaporation of the solvent from suspensions of weighed amounts of cellulose in ethanol containing known concentrations of the dye. Measurements were performed in previously dried thin and optically thick layers.   Diffuse reflectance spectra show some deviations pointing to the occurrence of ground state interactions at the highest concentrations. Thick layer fluorescence spectra show the typical effects of emission reabsorption by the dye. Interactions between excited and ground state molecules are also observed as a fluorescence red shift on increasing the dye concentration in thin layers. Fluorescence quantum yield measurements, presently underway, show after correction by reabsorption a significant decrease at the highest concentrations, attributed to long range energy transfer and trapping. The same reason causes shortening of fluorescence lifetimes as concentration increases. . Phosphorescence and delayed fluorescence were measured at room temperature. No degassing was needed as the cellulose environment protects the triplet state from oxygen quenching. A complex decay independent of sample concentration was found. Phosphorescence spectra show a pronounced blue shift with time. A unique spectrum is obtained for all samples at all times after shifting spectra to achieve a common maximum.   Delayed fluorescence spectra are sample-dependent because of light reabsorption. Decays are also complex: bimodal lifetime distributions similar to those found for phosphorescence decays, except for the weight factors of the long and short time contributions are obtained. Spectra are somewhat affected by emission from the supporting material and the phosphorescence blue tail. Results are consistent with thermally activated delayed fluorescence. However, low temperature experiments will be performed in the near future to ascertain the possible participation of triplet-triplet annihilation.  Results are interpreted in terms of site heterogeneity: long lived, more energetic triplet states coexist with short lived, less energetic ones. Lifetime distribution analysis is consistent with the long lived states corresponding to dye molecules tightly entrapped within cellulose chains. Short lived states result from dye molecules in more flexible environments. Experimental results and qualitative and quantitative arguments leading to the present conclusions will be presented.