12. M. I. Vinnik, Usp. Khim., 35, 1922 (1966). 13. Landolt-Bornstein Phys. Chem. Tabellen, Banden mit 3, J. Springer, Berlin (1936), p. 2145. 14. H. S. Harned and B. B. Owen, The Physical Chemistry of Electrolyte Solutions, Reinhold, New York (1950). QUENCHING OF DUROQUINONE BY AROMATIC DONORS OF ELECTRON AND HYDROGEN ATOM P. P. Levin, A. S. Tatikolov, and V. A. Kuz'min UDC 541.127:541.14.547.567 Triplet states of quinones react with electron donors through a charge transfer mechanism; the reaction rate constant increases with decreasing ionization potential of the donor [i, 2]. As a result of quenching by aromatic electron donors in low-polarity solvents, short-lived triplet exciplexes are formed [3, 4]. The structure of such exciplexes depends on the donor-- acceptor properties of the reactants and on the solvating power of the medium. The contribution of the state with complete charge transfer (CCT) to the structure of the triplet exciplexes will increase with increasing reduction potential of the quinone and with decreasing oxi- dation potential of the electron donor; the contribution also increases upon solvation by alcohols [5, 6]. In low-polarity solvents, the destruction of triplet exciplexes is accom- plished through a mechanism of radiationless intercombination conversion to the ground state [3, 4]; the rate constant for this process increases with decreasing energy level of the triplet exciplex [6, 7]. With an increase in the dielectric constant of the medium, there is now a second channel for destruction of the triplet exciplexes, i.e., dissociation into ion-radicals [5, 7]. The rate constant for dissociation of polar triplet exciplexes into ion-radicals is well described by the theory of diffusion-controlled dissociation of ion pairs in liquid solutions [5, 7, 8]. Triplet states of quinones will abstract a H atom from aromatic compounds containing N--H or O--H groups [3, 9, 10-12]. In the case of secondary aromatic amines, the transfer of the H atom takes place in a collision complex, and it competes with electron transfer [3, 9]. The outcome of the competition between these two reactions will depend on both the donor-- acceptor and acid--base properties of the reactants [i0]. The transfer of a H atom is apparent- ly accomplished by establishing an acid--base equilibrium in the radical pair formed after electron transfer, the same as in quenching by tertiary amines [13]. The kinetics and mechanism of photoreduction of quinones by aromatic donors of an electron and an H atom are also determined by the structure and kinetics of the destruction of the triplet exciplexes that are formed; however, the interrelation among the kinetics of triplet quenching, the nature of the radicals that are formed, and the properties of the exci- plexes have not been investigated adequately. In the present work, we applied pulse photolysis in a kinetic study of quenching of the triplet state of duroquinone by aromatic donors of an electron and an H atom in various sol- vents; we also investigated the nature of the intermediate species formed in the quenching. This quinone is of special interest, since it is the simplest model of quinones that are of biological importance. EXPERIMENTAL The absorption spectra and kinetics of the destruction of the intermediate products were investigated by means of a pulse photolysis unit [14]. The solutions were photoexcited by radiation in the 370-450 nm region. The solutions were pumped down to 10 -4 torr to remove 02. All measurements were performed at 20 • I~ The duroquinone (DQ) and the donors of the H atom (RH) were as follows: N,N-dimethyl-4-methoxyaniline (I); N,N-dimethylaniline (II); Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1005-1010, May, 1982. Original article submitted July 28~ 1981. 890 0568-5230/82/3105-0890507.50 9 1982 Plenum Publishing Corporation