Photochemical and Photophysical Studies within Zeolites

Abstract

In this article, we illustrate how one can utilize a zeolite matrix to control the photophysical and photochemical behavior of guest molecules included in them. In the first part, the emphasis is placed on the cation and on a single zeolite, faujasite (X and Y). Photophysical properties of naphthalene and other aromatic guest molecules included in X-type faujasite zeolites (M⁺X, M = Li, Na, K, Rb, Cs, Tl) have been investigated. As expected for an external heavy-atom-perturbed excited state, both singlet- and triplet-excited-state lifetimes and emission efficiencies depend upon the identity and accessibility of the cation present within the zeolite supercage. The power of the heavy-atom-cation effect in zeolites has been demonstrated by recording phosphorescence from several olefins whose phosphorescence has not previously been recorded. The second section brings out an inherent feature of a field at its infancy-serendipity. Surprisingly, radical ions of organic molecules can be generated and stabilized within zeolites by a simple procedure. This study has been expanded to include oligomers of thiophenes and α,ω-diphenylpolyenes which serve as models for conducting polymers. This is followed by a presentation wherein the importance of the relative size of the host cavity to that of the guest to achieve maximum selectivity in a photoreaction is highlighted. Concepts developed in this section with faujasite and pentasil (ZSM-5 and ZSM-11) as models are believed to be general and applicable to other organized media.