PHOTOCHEMISTRY ON SURFACES - MATRIX-ISOLATION MECHANISMS STUDY OF INTERACTIONS OF BENZOPHENONE ADSORBED ON MICROCRYSTALLINE CELLULOSE INVESTIGATED BY DIFFUSE-REFLECTANCE AND LUMINESCENCE TECHNIQUES

The swelling of microcrystalline cellulose by the use of polar protic solvents such as ethanol or methanol enables the penetration of benzop henone into submicroscopic pores of the natural polymer, while solvent s such as benzene or dichloromethane do not open the polymer chains, t hus not producing any entrapped benzophenone. Ground-state diffuse ref lectance studies revealed a dramatic blue shift, in the 350-nm absorpt ion of benzophenone in the former case, in accordance with a strong in teraction of the hydroxyl groups of cellulose with the ketone. Diffuse reflectance laser flash photolysis studies of benzophenone adsorbed o n microcrystalline cellulose showed, in cases where benzophenone is en trapped in the polymer chain, the formation of a transient which decay s nonexponentially and exhibits a maximum absorption at about 530 nm, assigned to triplet benzophenone. After ca. 25 mu s, this transient ge nerates another species with an absorption maximum at 545 nm. We assig ned this new species to the diphenylketyl radical. In all cases where the solvent does not swell-cellulose, a different behavior was observe d typical for benzophenone microcrystals triplet decay. The ketyl radi cal formation is greatly reduced in this case. Triplet benzophenone de cays by complex kinetics and lives about 10 mu s when adsorbed onto mi crocrystalline cellulose, while the ketyl radical, when formed, lives 1 order of magnitude longer than the triplet. Samples which exhibit a high yield of ketyl radical formation also have a smaller phosphoresce nce emission in accordance with the fact that large amount of triplet molecules were consumed in the process of hydrogen abstraction from th e matrix, involving hydrogens linked to carbons bearing a hydroxyl gro up.