UV-Vis absorption, luminescence, and X-ray photoelectron spectroscopic studies of rhodamine dyes adsorbed onto different pore size silicas

Sulforhodamine 101 and rhodamine 6G were adsorbed onto silicas with different pore sizes ranging from 22 to 150 a. Ground-state diffuse reflectance absorption spectra revealed the formation of different forms of adsorbed sulforhodamine 101 depending on concentration and on the pore size of the silica. For low loadings (0.001 to about 0.025 mu mol of dye/g of silica) the absorption spectra are broad, hypsochromically shifted in relation to the monomer spectra, and quite different from the ethanolic solution spectra. For high loadings (0.050 to about 0.20 mu mol g(-1)) they are similar to the solution spectra with a small shift of about 7 nm. For rhodamine 6G spectra are much more "solution type" in the entire range of concentration. under study (0.001 to about 0.20 mu mol g(-1)). The "weighed" fluorescence quantum yields (Sigma(i)f(i)phi(Fi)) determined for sulforhodamine 101 were 0.10 +/- 0.03, 0.35 +/- 0.05, and 0.50 +/- 0.10 for low loadings and for 25, 60, and 150 A silicas, respectively. For high loadings Sigma(i)f(i)phi(Fi) = 0.70 +/- 0.10. These values for Sigma(i)f(i)phi(Fi) can be compared with a value of about 0.70 obtained for rhodamine 6G in all silicas (low loadings) and a unitary phi(F) for high loadings. X-ray photoelectron spectroscopic studies present clear evidence for multiple occupation of the pores for all silicas under study. Surface concentration is quite independent of dye loading, although it changes from silica to silica. These studies indicate that sulforhodamine 101 forms nonplanar conformers in small pore size silicas as compared to large pore silica samples where the amount of conformers being formed is reduced. Rhodamine 6G samples exhibit very little conformer formation, but their phi(F) is still slightly dependent on pore size. Both rhodamines exhibit smaller fluorescence quantum yields when compared to the case of adsorption onto microcrystalline cellulose, this effect being more relevant in the sulforhodamine 101 case.