Naphthonitrile-alkene exciplexes. Comparison of bimolecular and bichromophoric cases: effects of linking on fluorescence and photochemistry

Abstract

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-', 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-l-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[S.4.O.O1~5]-8,lO-undecadiene (lo), was determined by X-ray diffraction. and the structure is described.Exciplex formation was proposed some time ago by Hammond and co-workers2 to account for the quenching of fluorescence of aromatic compounds by dienes.2 The observation of exciplex fluorescence from 1 -naphthonitrile and electron-rich alkenes by Taylor3 showed that exciplexes are indeed formed in fluorescence quenching by ethylene derivatives. A kinetic analysis of the 1-naphthonitrile-olefin system has been carried o~t .~,~ Exciplex formation was found to be reversible, and rate constants for the ( I ) (a) Department

The photocycloaddition of naphthoates with acetylacetone; dramatic acid promotions of product quantum yields

“…rich olefins to give cyclobutane and azetine products (1,8). The published investigations reveal important aspects of the mechanisms, in which the intermediacy of an exciplex has been implied (8-1 1).…”

Photoaddition and Photocyclization Processes of Aromatic Compounds

“…(7) Such studies again emphasize the important effect of solvent on the course and selectivity of the addition process. Adducts 17 and 18 are the sole products from 2,3-dimethylbut-2-ene with 1-and 2-naphthon- (45,46) It is also important to note that the cyclobutane compounds are photolabile and undergo decomposition unless Pyrex apparatus is employed. Use of Corex filtered radiation along with high reactant conversions yields 2-azobutadienes and azetines (cf.…”

“…(67) Nevertheless the type of compound accessible by this one-step route would be difficult to obtain following even a multistep thermal pathway, and in particular cases, the photoprocess does display good selectivity or may be modified to do so. The potential complexity of the reaction mixture is typified by the products from the irradiation of benzene and buta-1,3-diene which comprise the meta 1,2-and cis-and trans-para-I,4-adducts 45,46, and 47, respectively, as well as the substitution product l-phenylbut-2-ene. (67) Under (46) the experimental conditions, the trans para 1,4-adduct 47 undergoes both thermal dimerization and further reaction with the diene to give 48 (49) respectively.…”

Photochemistry of Arenes—Reloaded

“…[15] Why have the meta cycloadducts been overlooked so far? McCullough and co-workers [16] would have discovered them 25 years ago, if they had interpreted the facile cycloreversion of the ortho adducts correctly. However, the Mizuno group realised this fact and used it for the formation of the meta cycloadducts.…”