Potentialities of diffuse reflectance laser-induced techniques in solid phase: A comparative study of benzophenone inclusion within p- tert-butylcalixarenes, silicalite and microcrystalline cellulose

Abstract

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.