ULTRAVIOLET-VISIBLE AND FOURIER-TRANSFORM INFRARED DIFFUSE-REFLECTANCE STUDIES OF BENZOPHENONE AND FLUORENONE ADSORBED ONTO MICROCRYSTALLINE CELLULOSE

Benzophenone and fluorenone, which have a nonrigid and a rigid structu re, respectively, were used as probes to study the nature of the adsor ption process onto microcrystalline cellulose. Diffuse reflectance tec hniques were used in the UV-vis and infrared regions. Luminescence stu dies revealed that whenever fluorenone or benzophenone are entrapped i nto the natural polymer chains and in close contact with the substrate , a strong quenching effect exists for both probe's luminescence at ro om temperature. For fluorenone, the fluorescence quantum yields (Phi(F )) determined were about 0.10 when dichloromethane, cyclohexane, and b enzene (solvents which do not swell cellulose) were used for sample pr eparation, while for dioxane, acetone, ethanol, and methanol(solvents which efficiently swell cellulose) Phi(F) was approximately 0.01. Thes e values are about 1 order of magnitude higher than those obtained in solution, showing the importance of the rigid dry matrix in reducing t he nonradiative pathways of deactivation of the (pi,pi) fluorenone fi rst excited singlet state. Complementary, infrared studies showed that the carbonyl group of benzophenone is affected by entrapment (when th e solvents used induce the swelling of cellulose), whereas in fluoreno ne the same band is insensitive to the adsorption process, not allowin g the differentiation between entrapped molecules and surface crystall ites of this ketone. These observations implied that benzophenone is e ntrapped between the chains of the polymer forming hydrogen bonds betw een the carbonyl and the hydroxyl groups of the glycosidic chains, whi le the rigidity of fluorenone apparently restrains the ketone-substrat e interactions to the aromatic rings. Through the modifications observ ed in the carbonyl stretching band of benzophenone, it was possible to establish a swelling effect scale for the solvents, which is compared with previous results.