PHOTOCHEMISTRY ON SURFACES - FLUORESCENCE EMISSION QUANTUM YIELD EVALUATION OF DYES ADSORBED ON MICROCRYSTALLINE CELLULOSE

A simple method to determine the fluorescence quantum yield (phi(F)) of dyes adsorbed on microcrystalline cellulose is presented. The method is based on corrected fluorescence emission spectra and can easily be applied provided the energy profile of excitation is accurately determined. The quantitative determination of phi(F) is based on the ratio of the slopes of curves which correlate the fluorescence intensity and the absorbed light for both standard and unknown samples. The evaluation of the absorbed light is done by determining the reflectance R through the use of an integrating sphere. The remission function can then be determined as a function of wavelength in the two cases. Rhodamine 101 (R101), a rigid molecule with unitary phi(F), was used as a standard compound to determine the phi(F) values for rhodamine 6G (R6G) and auramine O (AURO), all dyes adsorbed on cellulose, and we obtained 1.02 +/- 0.03 and 0.14 +/- 0.01, respectively. Strong aggregation was detected for the two rhodamine dyes in the ground state, which we assign to dimer formation known to influence the fluorescence emission with respect to absorbed intensity. AURO does not aggregate when adsorbed on microcrystalline cellulose for loadings up to 10-mu-mol g-1. The intensity of the emission depends on concentration in all cases, but no emission was detected from the aggregated forms of the two rhodamines. The ground-state diffuse reflectance study for R101 and R6G enables us to determine the equilibrium constants for dimer formation of these two rhodamines adsorbed on cellulose which are K(D) = 0.4 x 10(6) and 1.3 x 10(6) mol g-1, respectively. The results obtained for phi(F) of these dyes show that the method is sensitive, making it possible to determine emissions from samples ranging from 1 nmol of the dye per gram of cellulose up to 10-mu-mol g-1. The determination of quantum yields can be obtained with an accuracy > 3%.