Photochemistry of Aromatic Compounds Involving Charge Transfer

Abstract

<p>Photoreaction of naphthalene with pyrrole in various solvents has been studied. The major products are 2-(l,4-dihydro-l-naphthyl)-pyrrole and 2-(1,2-dihydro-2-naphthyl)-pyrrole which together comprise 85% of the products mixture. Solvent dependence of both products ratio and quenching of naphthalene fluorescence have also been investigated. The evidence suggests that the reaction is initiated by charge transfer from pyrrole to the lowest excited singlet naphthalene followed by a proton shift from the N-position of pyrrole to a negative charge center in the naphthalene ring. Using N-deuteriopyrrole in the reaction, it was found that deuterium is incorporated at the naphthalene ring in the products. Further evidence to support the proton shift mechanism is that N-methylpyrrole quenches naphthalene fluorescence as efficiently as pyrrole, but the reaction does not take place.</p> <p>The photoreaction of l-cyanonaphthalene with tetramethylethylene in methanol and benzene have been studied. Substitution products were found in methanol solution but a cyclo-adduct was formed in benzene. Tetramethylethylene quenches fluorescence of l-cyanonaphthalene both in methanol and benzene.</p> <p>In contrast, 2-cyanonaphthalene only reacts with tetramethylethylene in methanol to give substitution products with one mole of methanol incorporated. The failure of reaction in benzene is consistent with the absence of fluorescence quenching in that solvent; in methanol, quenching occurs at the diffusion controlled limit.</p>