Diffuse reflectance laser photolytic studies of naphthalene, biphenyl and some aromatic hydrocarbons adsorbed in the cavities of faujasitic zeolites

Abstract

Time-resolved absorption and emission characteristics of naphthalene, biphenyl and a few other aromatic molecules adsorbed in the cavities of zeolites NaX and NaY have been investigated as a function of sample loading. At low loadings, biphotonic ionization was observed to generate both the radical cations of guest aromatic species and trapped electrons in the zeolite frameworks. Different types of trapped electrons were found depending on the hydration levels of the zeolite, i.e. Na₄³⁺ in dehydrated zeolites and a mixture of Na₃²⁺ and Na₂⁺ in hydrated zeolites. As the loading was increased, increased formation of the radical anions of the guest species occurred with a significant decrease in the yield of trapped electrons. It was firmly established that radical anions are not produced from trapped electrons and neutral molecules on µs timescales. We propose, therefore, that they are produced from neutral molecules by scavenging photo-ejected electrons before they are trapped. This mechanism of anion formation is characteristic of zeolites in which acceptor molecules with restricted diffusional motions are concentrated in the vicinity of photo-ionized molecules in cage networks. The dimer cation of naphthalene was also found to be formed photochemically in the zeolite cage networks at high loadings. A specific effect of zeolite cages which facilitates the photochemical reactions of guest organic molecules is described.