Zeolites as Templates for Preparation of Large-Ring Compounds:  Intramolecular Photocycloaddition of Diaryl Compounds

The fluorescence spectra and photocycloadditions of poly(ethylene glycol) labeled at the chain terminal with 2-naphthyl groups (N−P_n−N) or one 9-anthryl and one 1-naphthyl groups (N−P_n−A) and polymethylene end-labeled with two 2-naphthyl groups (N−M_n−N) or two 9-anthryl groups (A−M_n−A) included in NaY zeolite have been investigated. At loading levels less than 1 guest molecule per 10 supercages of zeolite, N−P_n−N, N−M_n−N, and A−M_n−A exclusively exhibit intramolecular excimer emission. Selective excitation of the naphthyl moiety in N−P_n−A mainly results in emission from the anthryl chromophore and intramolecular exciplex fluorescence. Irradiation of the above compounds leads to formation of intramolecular photocyclomers to the exclusion of intermolecular products. These results are explained in terms of the compartmentalization of the guest molecules in the supercages of NaY zeolite. Thus, this work demonstrates the utility of micropores of zeolites to promote the formation of large-ring compounds in synthetically useful quantities. Of particular interest is the enhanced formation of the cross-photocyclomers of anthracene and naphthalene derivatives (a process not observed in homogeneous solution) in high yields.