Ultraviolet visible absorption, luminescence, and X-ray photoelectron spectroscopic studies of a rhodamine dye covalently bound to microcrystalline cellulose

Rhodamine B isothiocyanate was adsorbed onto microcrystalline cellulose by two different methods: deposition from ethanolic and aqueous solutions followed by solvent evaporation and also from aqueous solutions in equilibrium with the powdered solid and following a dyeing protocol. After the above-mentioned samples were carefully washed, the fluorescence quantum yields (phi(F)) determined were about 0.40 +/- 0.03 and 0.28 +/- 0.03 for ethanol and water, respectively, solvents that efficiently swell cellulose, when these two solvents were used for sample preparation with the first method, while for dyed samples phi(F) is only 0.10 +/- 0.05. These values for phi(F) can be compared with 0.70 +/- 0.03 obtained for rhodamine B entrapped in the polymer chains of microcrystalline cellulose. X-ray photoelectron spectroscopic studies present evidence for a smaller positive charge density on the nitrogen atom for dyed samples when compared with the adsorbed ones. This is compatible with nitrogen atoms, which do not participate in the conjugated system. These findings indicate that rhodamine B has different conformers in dyed samples as compared to adsorbed samples. In the former case, the chemical bond, anchoring the dye to microcrystalline cellulose, leads to nonplanar conformers with smaller phi(F) and fluorescence lifetime (tau(F)) values. In the latter case, planar conformers predominate, with the consequent increase of both lifetime and fluorescent quantum yield.