The Electrochemical Reduction of 9,10-Diphenylanthracene

Abstract

The electroreduction of 9,lO-diphenylanthracene (DPA) in dimethylformamide solutions was studied by polarography, cyclic voltammetry, chronopotentiometry, and coulometry. The results generally confirmed the mechanism proposed for the reduction of aromatic hydrocarbons based on polarographic experiments. Coulometric experiments demonstrated that with sufficient care in solution preparation 1 Faraday per mole of DPA was consumed and fairly stable solutions of DPAcould be obtained. The protonation of DPAby hydroquinone was also studied. The half-wave potential for the reduction of DPA to DPAwas shown to be equal to that calculated by molecular orbital theory, using an angle between the 9and 10-phenyl groups and the anthracene nucleus based on electron spin resonance measurements. An explanation of the pronounced streaming which occurs at mercury electrodes during oxidation of DPAis given. he electrochemistry of aromatic hydrocarbons in T aprotic solvents has been the subject of a number of studies. These investigations are of interest because the radical ions which are formed at the electrode may undergo further chemical or electrode reactions. Further interest lies in the correlation between the polarographic half-wave potentials for the reduction of the hydrocarbon (R) to its radical anion (R-.) with electron affinities, ultraviolet absorption frequencies, or parameters calculated by molecular orbital (MO) theory.*r3 Although the results of dc and ac polarography fit very well with assumed mechanisms and the known chemistry of aromatic hydrocarbons (e.g., reduction with alkali metals), very few studies have been made using some of the more recent electrochemical techniques such as cyclic voltammetry and chronopotentiometry. Neither have electrochemical studies been made of the radical anions themselves. Furthermore, the coulometric reduction of these compounds at the first reduction plateau gave evidence of a twoelectron r e d ~ c t i o n , ~ which was ascribed to the reaction of Rwith the solvent (dimethylformamide) producing R H . , which was reduced in a second one-electron step at these potentials. This finding seemed at variance with the known stability of R , as observed through optical and electron spin resonance (esr) studies. Finally, although theoretical considerations lead to the predication that the protonated radical (RH. ) should reduce at less cathodic potentials than R, no direct attempts have been made to obtain R H and study its electrochemistry. The aim of this investigation was to study the electrochemical reduction of an aromatic hydrocarbon by a variety of electrochemical techniques, in order to confirm and extend the previously described mechanisms. The choice of 9,lO-diphenylanthracene (DPA) was based on interest in this substance in chemiluminescence experiment^.^ Furthermore, a comparison of elec(1) To whom correspondence and request for reprints should be directed. (2) This work has been reviewed in (a) G. J. Hoijtink, Ric. Sci., Suppl., 30, 217 (1960); (b) G. J. Hoijtink, Rec. Trav. Chim., 76, 885 (1957); (c) M. E. Peover in “Electroanalytical Chemistry. A Series of Monographs on Recent Advances,” Vol. 11, A. J. Bard, Ed., Marcel Dekker, Inc., New York, N. Y., in preparation. (3) (a) A. Maccol, Nafure, 163, 178 (1949); (b) F. A. Matsen, J. Chem. Phys., 24, 602 (1956); (c) G. J. Hoijtink, Rec. Trau. Chim., 74, 1525 (1955). (4) D. E. G. Austen, P. H. Given, D. J. E. Ingram, and M. E. Peover, Nature, 182, 1784 (1958). trochemical and esr parameters with those calculated by MO theory can provide information about the extent of nonplanarity of DPA. The electrochemical reduction of DPA and oxidation of DPAin the absence of proton donors will be described first. Then the effect of proton donors will be discussed. Finally a correlation of the observed halfwave potentials and those predicted by MO theory will be given and an explanation of the stirring effect which occurs during the electrooxidation of anion radicals will be presented.