Steric Factors in 1-Naphthonitrile-Olefin Exciplex Formation

Abstract

<p>The fluorescence quenching and photochemistry of 2-methyl-, 4-methyl- and 6-methyl-1-naphthonitrile with tetramethylethylene have been studied in benzene and ethyl acetate. Stern-Volmer constants (benzene solvent) are 10.1, 15.7 and 9.0 M‾¹, respectively, showing that quenching is insensitive to position of the methyl group. Exciplex emission was observed in each case, and exciplex lifetimes were measured by the technique of time-correlated single photon counting. The methyl-1-naphthonitriles react with tetramethylethylene to give 1-cyano-7,7,8,8-tetramethyl-2,3-benzobicyclo[4.2.0]octa-2,4-diene derivatives, and limiting quantum yields were determined. Quenching with biacetyl showed that the exciplex is an interm0ediate in the cycloaddition reaction involving 4-methyl-1-naphthonitrile. Also, no triplets were formed from the decay of the exciplex. The decay rate constants for deactivation of the exciplex were measured. The bichromophoric molecules 2,3-dimethyl-2-butenyl-1-cyano-2-methylnaphthyl ether, 2,3-dimethyl-2-butenyl 1-cyano-4-methylnaphthyl ether and 2,3-dimethyl-2-butenyl 1-cyano-6-methylnaphthyl ether, in which the chromophores are linked by a three atom chain, were synthesized. All three show strong quenching of the monomer (naphthonitrile) fluorescence and weak exciplex emission. The three bichromophoric compounds also react on irradiation giving internal cycloaddition products. The structures of the latter were determined, and the structure work is described. Comparison of quantum yields for intra-molecular cycloaddition with limiting quantum yields for the bimolecular cases, indicate that the three-atom chain greatly expedites collapse of the exciplexes to products in these systems. The results are discussed within the context of the model for exciplex formation and decay introduced by Michl.</p>