Photochemistry of organic molecules in microscopic reactors

The photochemistry of dibenzyl ketone and its derivatives has been employed to investigate the ability of zeolite molecular sieves to modify and to control the reaction channels available to organic molecules adsorbed on the internal and external zeolite surfaces. It is found that the observed photochemistry is very sensitive to the size/shape characteristics of the substrate ketones and of the pores and internal void space of the zeolites. Although unprecedented reactions of ketones have been found to be induced by absorption of the ketones on the zeolite surfaces, the reactions are consistent with expectations based on the topological structure of the zeolite surfaces and on the mechanism of ketone photolysis in homogeneous solution. ZEOLITE MOLECULAR SIEVES. DESIGNER MICROSCOPIC REACTORS Zeolites (fron the Greek words zeo, "to boil" and lithos "stone")1 are synthetic o natural minerals that often expel water so violently when heated that they appear to boil. Classical zeolites are crystalline aluxainosilicates whose internal porous structure contain channels and/or cages filled with exchangeable cations and which may also be filled with adsorbed water. The framework composition of zeolites consists of cations, aluminum, silicon, and oxygen. The framework constitution of zeolites consists of tetrahedral Al atoms and tetrahedral Si atoms linked by the sharing of 0 atoms (Figure 1). The porous structure of zeoM,(AlO2)(SiO2)mH2O COMPOSI110N Figure 1. Composition and constitution of a /0N O\ /°NO ,O'i' classical aluiainosilicate zeolite framework. Al Si Si Si CONSTITUTION The subscript x refers to the number of Al j: f j j J . atoms (negative charges) in the framework; the "0 C) 0 9 ( Q ( tj'.rs subscript y refers to the number of Si atoms in the framework;and the subscript n refers to HYDROPHILIC HYDROPHOBIC HYDROPHIUC the charge of cation M. lites results from the framework configuration, i.e., the three-dimensional geometric network of Al04 and 5i04 tetrahedra. The zeolite frameworks which are obtained from natural or synthetic preparations contain pores, channels, cages, and interconnected voids. These void spaces and the internal zeolite surface occur in periodic fashion because of the crystalline nature of the framework. The combination of the topology of the internal void space and the chemical characteristics of the internal framework structure provide chemists with "designer microscopic reactors" in which chemical reactions can be performed. It can be imagined that the size and shape of these microscopic reactors, in conjunction with the peculiarities of molecular diffusion in periodically repeating void spaces will result in unusual characteristics of chemical reactions which are performed on molecules adsorbed on zeolites. Indeed, the ability of zeolites to selectivity adsorb molecules based on size/shape selectivity rules has led to their designation as "molecular sieves". As a result of this "sieving" characteristic, the usual domination of substrate molecular structure in determining the course of chemical reactions might be replaced in certain circumstances by a domination of environmental structure in determining the course of chemical reactions for reactions conducted on zeolite nolecular sieves. In this report we show how these ideas can be given experimental realization by the judicious selection of zeolites, substrates, and photoreactions. Before describing the actual systems investigated, we shall review briefly some characteristics of important classes of zeolites whose internal surface and internal void space will drive the cheist's imagination in the selection of substrates and photoreactions. Then we shall review a photoreaction whose outstanding generality, mechanistic characteristics, engineering versatility, and convenience of execution and analysis will further define the selection of substrates for study.