Journal of Inclusion Phenomena and Molecular Recognition in Chemistry 21: 299-324, 1995. 299 91995 KluwerAcademic Publishers. Printed in the Netherlands. Alkali-Metal Clusters as Prototypes for Electron Solvation in Zeolites NICK P. BLAKE and GALEN D. STUCKY 1. Introduction In this section we describe, in some detail, work conducted on alkali-metal clusters in zeolites. Almost without exception, the clusters that we will discuss cannot exist in the gas-phase. As we shall demonstrate, the role of the zeolite is to provide an electrostatic containment field which gives rise to certain ionic sites within the zeolite. Alkali-metal ions which, of course, play the role of counter-ion in the aluminosilicate zeolites, only take on the character of a cluster through introduction of an alkali atom or electron to the zeolite. Under these conditions, the electron (either the valence electron of the alkali atom or the introduced electron) is solvated by the electron trap afforded by these counter-ions. The ions relax around the electron and the result is an alkali-metal cluster. The concept is more than merely notional, since many of the physical properties (EPR and absorption cross-section) of the associated electron can be explained in terms of an alkali-metal cluster. Further, circumstantial evidence that the valence electron of an introduced alkali-atom is auto-ionized comes from the fact that XRD reveals that the nucleus takes up one of the ionic sites in the zeolite. There are a number of reasons why such systems are interesting: 1) Firstly, these clusters, when isolated, are color centers. They may be produced by exposure of the zeolite to ~-rays or UV light and can be bleached by exposure to a high-intensity white-light source. This cathodochromic and photochromic behavior has, in the past, found applications in CRTs and read/write devices [1]. 2) Secondly, at higher cluster concentrations, aggregation of clusters leads to new types of materials. These are known to possess different electronic properties, most notably evident in the modified EPR signal and optical absorption spectrum. All of these changes point to the fact that the clusters are coupled and, as such, are highly suitable candidates for the study of metal-insulator transitions. The unique, channel-like structure of the zeolite ensures a highly directional coupling of neighboring clusters found only in these periodic porous structures. The magnetic and electronic properties of these coupled clusters are perhaps the most exciting aspects of the present alkali-metal cluster work. 3) Thirdly, the formation of alkali-metal clusters in photo-induced charge-transfer involving guests within zeolite hosts provides a mechanism for the stabilization of charge-transfer processes. Typically, charge-transfer processes occur at energies