Solvent Effects on the Formation and Decay of Ionic Intermediates for the Photoreduction of Anthraquinone by Triethylamine in Ethanol, Toluene, and Acetonitrile at Room Temperature

Article abstract of DOI:10.1246/bcsj.65.3141

In toluene and ethanol containing triethylamine (TEA), a reaction of the lowest excited triplet state <³AQ(T₁)> of anthraquinone (AQ) with ground-state TEA gave rise to the formation of an exciplex <³(AQ-TEA)^*> which converted into a contact ion pair between the AQ radical anion (AQ^-.) and the TEA radical cation (TEA^+.).By an intramolecular proton transfer, this contact ion pair yielded anthrasemiquinone radical followed by the formation of 9,10-anthracenediol (AQH2); in ethanol, the monoanion (AQH^-) of AQH2 was also produced.In ethanol TEA, moreover, not only the decay rate constant of ³(AQ-TEA)^* but also the quantum yield for the photoreduction of AQ were affected by the change of TEA concentration.This was interpreted in terms of the existence of a rapid interconversion between ³(AQ-TEA)^* and a triplex <³(AQ-TEA2)^*> of ³AQ(T₁) with ground-state TEA.In contrast to the exciplex formation in ethanol and toluene, free AQ-anion radical (and TEA-cation radical) and ³(AQ-TEA)^* 3(AQ-TEA)^* or the contact (or solvent-separated) ion pair dissocited into the radical ions (AQ-anion radical and TEA-cation radical) followed by their second-order reaction yielding finally AQH2 and AQH^-.