Voltammetric behavior and electronic molecular structures of several azulenylketones

The formation of the radical and/or ion species of five azulenylketones, 1-benzoylazulene 4, 1-benzoyl-5-isopropyl-3,8-dimethylazulene 5, di-1-azulenyl ketone 6, 3,8-dimethyl-5-isopropyl-1-azulenyl 1-azulenyl ketone 7, bis(3,8-dimethyl-5-isopropyl-1-azulenyl) ketone 8, was studied with cyclic voltammetry. The cathodic peak potentials of the formation of the anion radicals of 4, 6 and 7 almost coincided with each other and those of 5 and 8 also coincided with each other. The cathodic peak potentials of the formation of the dianions of 6, 7 and 8 were nearly equal to those of the corresponding azulenes, azulene 1 and guaiazulene 2, respectively. All the peak potentials of the azulene compounds measured shifted negatively when the azulene rings were substituted by alkyl groups. These results and the energy calculation with MOPAC led to the conclusions (A) that the two azulene rings are not coplanar for the anion radicals and dianions of the di-1-azulenylketones; (B) that the anion radical generated at the carbonyl group is electronically stabilized by one azulene ring having lower LUMO or one of the rings when two azulene rings are identical; (C) that the formation of the dianion, the one-electron injection into the anion radical, occurs at another azulene ring; and (D) that the electronic inductive effect of alkyl groups on the azulene ring destabilizes the anion radical and dianion species of the ketones and stabilizes the cation species of the ketones qualitatively.