Naphthonitrile-Alkene Exciplexes. Comparison of Bimolecular and Bichromophoric Cases: Effects of Linking on Fluorescence and Photochemistry

Article abstract of DOI:10.1021/ja00381a023

The fluorescence quenching and photochemistry of 2-methyl-, 4-methyl-, and 6-methyl-1-naphthonitrile with tetramethylethylene have been studied in nonpolar solvents.Stern-Volmer constants (benzene solvent) are 10.1, 15.7, and 7.2 M^-1, respectively, showing that quenching is insensitive to the position of the methyl group.Exciplex emission was observed in each case, and exciplex lifetimes were measured by single photon counting techniques.The methylnaphthonitriles react with tetramethylethylene to give 1-cyano-7,7,8,8-tetramethyl-2,3-benzobicyclo<4.2.0>-2,4-octadiene derivatives, and limiting quantum yields were measured.In the case of 4-methyl-1-naphthonitrile, quenching with biacetyl showed that the exciplex is an intermediate in the cycloaddition reaction and no triplets are formed in the exciplex decay.Rates of exciplex collapse to cyclobutane products were derived.The bichromophoric molecules 1, 2, and 3, 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.Compounds 1, 2, and 3 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 internal cycloaddition with limiting quantum yields for the bimolecular cases shows that the three-atom chain greatly facilitates collapse of the exciplexes to products in these systems.Exciplex formation and decay are discussed in terms of current theory.The structure of one photoproduct, 7-cyano-5,6,6-trimethyl-8,9-benzo-3-oxatricyclo<5.4.0.0^1,5>-8,10-undecadiene (10), was determined by X-ray diffraction, and the structure is described.