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After being largely ignored for many years, the simple structures o-xylylene (1) and isoindene (2) (also called o-quinodimethanes) and their derivatives are now under active investigation from theoretical, physical, physical organic, and synthetic organic viewpoints.The earliest report in which an o-xylylene structure was implicated was the reaction of , , ', '-tetrabromo-o-xylene with iodide ion to give 1,2-dibromobenzocyclobutene.1,2 This assignment was confirmed

The photoaddition of naphthalene and acrylonitrile at 313 nm and in hydroxy lie solvents to afford e«ífo-7-cyano-2,3-benzobicyclo[4.2.0]octa-2,4-diene (1), the mfo-8-isomer, and 1-and 2-naphthylpropionitrile (3 and 4) is investigated. The reactive state is naphthalene Si which is determined by kinetically relating quantum yields and fluorescence quenching dependences on acrylonitrile concentration. The fluorescence quenching is proposed to occur by charge-transfer exciplex formation. Good Stern-Volmer plots are obtained for quenching in acetonitrile and rert-butyl alcohol, and the rate constant for the latter is 14 X 107 M~1 sec-1. Fluorescence of indene and anthracene is also quenched, the rate for indene being diffusion controlled. The dilution plot of reciprocal quantum yield against reciprocal acrylonitrile concentration is linear if 2,3-dimethylbuta-l,3-diene is added to prevent triplet-sensitized decomposition of 1. It is proposed that the exciplex responsible for fluorescence quenching has a polar structure, and the substituted products 3 and 4 arise from protonation of the exciplex. The evidence is that (i) the fraction of substitution increases with medium polarity and (ii) reaction in deuteroxylated solvents gives 3 and 4 labeled in the methyl groups. It is believed that indene reacts similarly and some results for the latter are presented. The exciplex structure is discussed. A correlation exists between the energetics of electron transfer and exciplex behavior for some reactions of naphthalene and benzene derivatives and acrylonitrile. This correlation may be of useful predictive value. have been published.3•5-8(1) For preliminary reports of this work see (a) R. M. Bowman and

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

Publication costs assisted by The National Research Council of CanadaMethylenediphosphonic acid is monoclinic with space group p 2 1 / c and has lattice parameters a = 7.840(9), b = 5.494(3), c = 13.746(6) A'and p = 103.69(7)", with Z = 4. The structure was refined by fullmatrix least squares to a final R value of 0.032 with 1173 reflections measured with a Syntex automatic diffractometer. The P-C bonds are 1.790(3) and 1.794(3) A in length with a P-C-P angle of 117.2(1)". The P-0 bond lengths are 1.494(2) and 1.500(2) A while the average P-0(-H) bond length is 1.546 A.Each of the oxygen atoms involved in the former bonds is an acceptor for two hydrogen bonds.