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Diffuse reflectance and laser-induced techniques were used to access photochemical and photophysical processes of benzil in solid supports, namely p-tert-butylcalix[n]arenes with n = 4, 6, and 8. A comparative study was performed using these results and those obtained with another electronically inert support, silicalite, which is a hydrophobic zeolite. In the latter substrate, ground-state benzil has the two carbonyl groups in an s-trans planar conformation while in the calixarenes a distribution of conformers exists, largely dominated by skew conformations where the carbonyl groups are twisted one to the other. In all substrates, room-temperature phosphorescence was obtained in air-equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for benzil/p-tert-butylcalix[6]arene, showing that this host cavity well accommodates benzil, enhancing its room-temperature phosphorescence. p-tert-Butylcalix[6] and [8]arene molecules provide larger hydrophobic cavities than silicalite, and inclusion complexes are formed with these hosts and benzil as guest; p-tert-butylcalix[4]arene does not include benzil. This probe is deposited outside the calix[41 cavity, in the form of microcrystals. Triplet-triplet absorption of benzil was detected in all cases and is predominant in the silicalite channel inclusion case. Benzil ketyl radical formation occurs with inclusion in calix[6]arene and calix[8]arene. In the three cases, benzoyl radical was detected at long times (in the millisecond time scale). Product analysis and identification clearly show that the main detected degradation photoproducts in all substrates are benzoyl radical derivatives. Calix[6] and [8]arenes are able to supply hydrogen atoms that allow also another reaction, the reduction to benzoin through benzil ketyl radical formation.

Diffuse reflectance and laser-induced techniques were used to access photochemical and photophysical processes in solid phases, namely calix[ n ]arenes ( n is the number of phenolic units in the ring), organic substances which are capable of forming inclusion complexes with several neutral organic substances and metal ions. We have used p - tert -butylcalix[4], p - tert -butylcalix[6] and p - tert -butylcalix[8]arenes as solid matrixes and benzophenone as probe. A comparative study is presented here, mainly using the results obtained with the calix[6]arene as host and those obtained with two other electronically inert supports: microcrystalline cellulose and silicalite, a hydrophobic zeolite. In all substrates, room temperature phosphorescence was obtained in air equilibrated samples. The decay times vary greatly and the largest lifetime was obtained for silicalite, where benzophenone is included into hydrophobic channels. Calix[6]arene and cellulose provide full protection against oxygen quenching while silicalite only protects the guest molecule partially. Calixarene molecules provide larger hydrophobic cavities than silicalite and also a better selectivity towards the guest size. This selectivity does not exist in the microcrystalline cellulose case. FTIR absorption spectra show that the distortion from planarity in the benzophenone molecule is larger in silicalite than in calix[6]arene, while in cellulose the distortion is slight. In spite of this, benzophenone exhibits the highest phosphorescence emission quantum yield in the case of silicalite. Benzophenone ketyl radical formation occurs with entrapment in cellulose and also with inclusion in calix[6]arene and calix[8]arene while in silicalite only triplet–triplet absorption is detected.

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.