A series of 9-substituted 10-methylacridinium ions (Acr -R) in which an electron-donor moiety (R) is directly linked with an electron-acceptor moiety (Acr ) at the 9-position was synthesized, and the photodynamics was fully investigated to determine the rate constants of photoinduced electron transfer (ET) and back electron transfer. The driving forces of photoinduced electron transfer and back electron transfer were determined by means of electrochemical and photophysical measurements. The dependence of the ET rate constants on driving force was well analyzed in the light of the Marcus theory of ET. The quantum yields of formation of the triplet ET states vary significantly, depending on the interaction between the donor (R) and acceptor (Acr ) moieties. Among the Acr -R examined, the 9-mesityl-10-methylacridinium ion (Acr -Mes) exhibits the best performance in terms of the lifetime of the triplet ET state and the quantum yield. Photoexcitation of Acr -Mes results in formation of the triplet ET state [ (Acr -Mes )], which has a long lifetime, a high energy (2.37 eV), and a high quantum yield (>75 %) in acetonitrile. The triplet ET state exhibits both the oxidizing and reducing activity of the Mes and Acr moieties, respectively.

Photocatalytic oxidation of iron(ii) complexes by dioxygen occurred using the organic photocatalysts, 9-mesityl-10-methylacridinium ions (Acr(+)-Mes) and 2-phenyl-4-(1-naphthyl) quinolinium ions (QuPh(+)-NA), in the presence of triflic acid in acetonitrile under visible light irradiation. The electron-transfer state of Acr(+)-Mes produced upon photoexcitation oxidized the iron(ii) complexes, whereas it reduced dioxygen with protons to produce iron(iii) complexes and H2O2.