ELECTRON-TRANSFER REACTIONS INVOLVING ACRIDINE AND RELATED COMPOUNDS

Acridine is rapidly reduced by esol- (k = 3 × 1010 M-1 s-1) and by (CH3)2ĊO- (k = 3 × 109 M-1 s-1 in water and 1.4 × 109 M-1 s-1 in i-PrOH). Reduction by (CH3)2ĊOH and CO2- is rapid and efficient only when the acridine is protonated (pKa = 5.6). With neutral acridine, however, the slow reduction is accompanied by addition of these radicals to acridine. Both forms of acridine are reduced by pyridinyl radicals, although the acid more rapidly than the neutral form. In all these one-electron reductions the anion radical of acridine is produced, which in aqueous and alcoholic solutions undergoes rapid protonation on the nitrogen to form a neutral radical, with a pKa apparently above 14. Related heterocyclic compounds, such as 1,10-phenanthroline and phenazine, behave in a similar fashion. The radicals produced by reduction of acridine and other heterocycles can transfer an electron to 9,10-anthraquinone in nonaqueous media or to anthraquinonesulfonate in aqueous solutions (k ∼ 109 M-1 s-1). The spectra of the resulting semiquinone radicals were found, in all cases examined, to be identical with those obtained by direct reduction of the quinone under similar conditions without uthe intermediacy of the heterocycle. This is in contrast with previous findings with a porphyrin-anthraquinone system. It is concluded that, despite the previous suggestion that an intermediate complex is formed upon electron transfer from tetraphenylporphyrin radical to anthraquinone, no such complex appears to be formed in the electron transfer reactions involving acridine and related heterocycles. © 1979 American Chemical Society.