ULTRAVIOLET-VISIBLE ABSORPTION, LUMINESCENCE, AND X-RAY PHOTOELECTRONSPECTROSCOPIC STUDIES OF A RHODAMINE DYE COVALENTLY BOUND TO MICROCRYSTALLINE CELLULOSE

Rhodamine B isothiocyanate was adsorbed onto microcrystalline cellulos e by two different methods: deposition from ethanolic and aqueous solu tions 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 fluores cence quantum yields (phi(F)) determined were about 0.40 +/- 0.03 and 0.28 +/- 0.03 for ethanol and water, respectively, solvents that effic iently swell cellulose, when these two solvents were used for sample p reparation with the first method, while for dyed samples phi(F) is onl y 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 micr ocrystalline cellulose. X-ray photoelectron spectroscopic studies pres ent evidence for a smaller positive charge density on the nitrogen ato m for dyed samples when compared with the adsorbed ones. This is compa tible with nitrogen atoms, which do not participate in the conjugated system. These findings indicate that rhodamine B has different conform ers in dyed samples as compared to adsorbed samples. In the former cas e, the chemical bond, anchoring the dye to microcrystalline cellulose, leads to nonplanar conformers with smaller phi(F) and fluorescence li fetime (tau(F)) values. In the latter case, planar conformers predomin ate, with the consequent increase of both lifetime and fluorescent qua ntum yield.