Photochemistry of Naphthonitrile-Olefin Systerns

Abstract

<p>The photochemistry of 1- and 2-naphthonitrile with the olefins tetramethylethylene and 1,2-dimethylcyclopentene has been studied in a variety of solvents. In hexane, benzene, and 1,2-dimethoxyethane the major products in the reaction with tetramethylethylene are the cyclobutanes 1-cyano-7,7,8,8-tetramethyl-2,3-benzobicyclo[4.2.0]octa-2,4-diene (from 1-naphthonitrile) and 6-cyano-7,7,8,8-tetramethyl-2,3-benzo-bicyclo[4.2.0]octa-2,4-diene (from 2-naphthonitrile). Reaction with 1,2-dimethylcyclopentene gives cyclobutanes with analogous structures, in genzene and dimethoxyethane. In high polarity solvents, products of photoreduction are formed. 2-Naphthonitrile and tetramethylethylene upon irradiation in 2,2,2-trifluoroethanol give 1-(2'-trifluoroethoxy-1' ,1' ,2' ,2-tetramethyl)-3-cyano-1,4-dihydronaphthalene and 3,4-dihydro-2-naphtho-nitrile. Reactions in other high polarity solvents give complex mixtures of products. Stern-Volmer quenching of 2-naphthonitrile fluorescence by tetramethylethylene increases dramatically as solvent polarity increases from hexane to methanol. The Stern-Volmer constants are 0.50 M⁻¹ and 148 M⁻1 for 2-naphthonitrile fluorescence quenching in hexane and methanol, respectively. Similar, although less dramatic, behaviour is observed for 1-naphthonitrile fluorescence quenching by tetramethylethylene. Lifetimes of the luminescent exciplexes formed from 1-naphthonitrile and the olefins have been measured in the lower polarity solvents. With both teframethyl-ethylene and 1,2-dimethylcyclopente, the exciplex lifetimes increase, level off, and then decrease with increasing solvent polarity. For the 1-naphthonitrile-tetramethylethylene exciplex, the measured lifetimes are 3.75, 10.4, 12.6, 12.7, and 4.7 ns in hexane, benzene, dimethoxyethane, tert-butanol, and iso-propanol, respectively. The photochemistry and photophysics of the naphthonitrile-olfein systems are discussed in terms of a changing quenching mechanism with increasing solvents polarity. It is proposed that quenching in non-polar solvents involves exciplex formation, while in polar media electron transfer occurs. The results are discussed within the context of models for exciplex formation and decay introduced by Weller and Michl.</p>