**1. Introduction**

Nobel metal clusters confined in restricted environment of zeolite possess remarkable absorption and emission properties, large Stokes shifts and, with a few exceptions, exceptionally high external quantum efficiencies (EQE's) which are of paramount importance in various processes and applications [1–10]. In the last years, a converging view is that the origin of their optical properties resides in their molecular-like characteristics as a result of a strong quantum confinement leading to discrete energy levels. However, these intriguing effects appear to depend not only on confinement but also on size, structure, and hydration level, charge state of the cluster and host-guest interactions. Electrostatic interactions between zeolite cavity and confined metal nanoparticles govern the photophysical properties of these materials. Metal clusters self-assembled in the well-defined cavities of aluminosilicate crystalline framework of zeolites possess the most fascinating optical properties because of the complex interaction that the clusters develop with the zeolite framework. This is probably the reason that fundamental research on such materials has been less attractive, perhaps because of the difficulties encountered in rigorous determination of the exact nature of these interactions. A number of questions are frequently asked. What is the nature of the electronic transitions and especially of the long-lived emitting species? Is there a charge transfer between zeolite framework and metal cluster involved in the excited state dynamics? Is the luminescence originating from recombination of electrons trapped or simply from species which are not strongly coupled to the zeolite structure? Is an intersystem crossing occurring upon excitation which

indicates a forbidden transition/relaxation and thus the long lifetime of the luminescent electronic state? What is the physics that determines which state decays radiatively and can we map the excited state dynamics?

This chapter critically reviews the studies related to the structural and photophysical properties of metal clusters within zeolites matrices and summarizes the progress made in understanding the host-guest interactions. The goal is to provide useful insight into the nature of such interactions, methods and experiments used in identifying the excited state dynamics and the reaction mechanisms leading to the emitting species. Although a number of excellent research articles have been published in the last years they only cover rather specific areas like organic photochemistry, confinement, charge transfer, theoretical modeling or photostimulated luminescence [11–15]. The chapter is organized in three sections relevant to the interplay cluster-framework and seen from a mechanistic point of view that is further supported by various theoretical and experimental based studies like DFT, diffraction or time-resolved luminescence spectroscopy. The first part presents in short the structure and chemical properties of zeolites which is then followed by the progress in understanding the formation and structure of the metal clusters stabilized in the zeolite cavities, pores and channels. The last part sheds light onto the electronic properties and the origin of intense luminescence and how these depend on the interplay between cluster and framework. Especially interesting are the combined experimental and computational approaches used to elucidate the structures and electronic transition of clusters inside the cavity. Particular emphasis is then placed on various debated mechanisms as models to address the quantized electronic interaction which can lead to new insights into their optical, luminescence, crystal habit, metal-core, ligand-shell, and environmental properties [16].
