Jorge Moreira

Diffuse reflectance studies of β-phenylpropiophenone and benzophenone inclusion complexes with calix[4], [6] and [8]arenesDedicated to Professor Frank Wilkinson on the occasion of his retirement.

The formation of inclusion complexes of β-phenylpropiophenone and benzophenone with p-tert-butylcalix[4], [6] and [8]arenes (powdered solid samples) was studied with the use of diffuse reflectance techniques. Experimental evidence was obtained for inclusion in all cases. The benzophenone n → π* absorption band exhibits hypsochromic shifts with an increase of the calixarene ring aperture, providing evidence for an increase in cavity polarity. β-Phenylpropiophenone exhibits strong room temperature phosphorescence in contrast to its solution behaviour, and this is a clear evidence for the inclusion of this probe within the calixarene molecules. Calix[4]arene and calix[8]arene provide a more rigid environment to β-phenylpropiophenone whereas calix[6]arene appears to be more flexible. Transient absorption spectra of inclusion complexes with benzophenone (1 : 1 mol : mol, excited at 355 nm) show major triplet formation in the calix[4]arene case, while the ketyl radical of benzophenone is formed in the calix[6]arene case and is predominant in the calix[8]arene case. Phenoxyl radical transient absorption also becomes evident in the last two cases. For β-phenylpropiophenone (1 : 1 mol : mol) phenoxyl radicals are formed but no evidence was found for triplet or ketyl radical formation of this ketone in the calix[4]arene case. However, strong support for the ketyl radical of the aryl alkyl ketone was found for calix[6]arene and calix[8]arene inclusion complexes.

Surface photochemistry: Photodegradation study of pyrene adsorbed onto microcrystalline cellulose and silica

Ground-state diffuse reflectance, time resolved laser-induced luminescence, diffuse reflectance laser flash-photolysis transient absorption and chromatographic techniques were used to elucidate the pho- todegradation processes of pyrene adsorbed onto microcrystalline cellulose and silica. Ground-state diffuse reflectance showed that on both substrates low concentrations display absorption of pyrene monomers. At high concentrations spectral changes attributed to aggregate formation were observed. Laser induced fluo- rescence showed that pyrene onto microcrystalline cellulose mainly presents fluorescence from monomers, while for silica, excimer-like emission was observed from low surface loadings (≥ 0.5 µmol g −1 ). Transient absorption and photodegradation studies were performed at concentrations where mainly monomers exist. On silica, pyrene presents transient absorption from its radical cation. On microcrystalline cellulose both radical cation, radical anion and pyrene triplet-triplet absorption were detected. Irradiation followed by chro- matographic analysis showed that pyrene decomposes on both substrates. For pyrene on microcrystalline cellulose 1-hydroxypyrene was the main identified photoproduct since in the absence of oxygen further ox- idation of 1-hydroxypyrene was very slow. For pyrene on silica photodegradation was very efficient. Almost no 1-hydroxypyrene was detected since in the presence of oxygen it is quickly oxidized to other photooxida- tion products. On both substrates, pyrene radical cation is the intermediate leading to photoproducts and oxygen it is not involved in its formation.