Electron transfer from ascorbic acid to various phenothiazine radicals

The reaction of electron transfer from ascorbic acid to N-alkylphenothiazine radical cations was studied in the pH range 0 to 7.2. The radical cations were produced by oxidation with Br2- or Cl2- using the pulse radiolysis technique. The radical cations are reduced by the deprotonated form of ascorbic acid in the pH range 2.2 to 7.2. The product of this reaction has a spectrum which is identical with that of the ascorbate radical. It is concluded that, if a charge-transfer complex is formed upon the reaction of ascorbic acid with the phenothiazine radical cations, its lifetime is shorter than 7 x 10-8s. It is suggested that even at high acidities no long lived complex is formed and it is proposed that at very low pH the thermodynamically stable species in this reaction is the phenothiazine radical cation rather than the ascorbic acid radical. It is shown experimentally that at 1 M HCl the ascorbic acid radical oxidizes N-alkylphenothiazine to give the radical cation of the latter in reverse to the direction of the electron transfer at higher pH. 8 figures, 2 tables