Photoreduction of oxoisoaporphines by amines: Laser flash and steady-state photolysis, pulse radiolysis, and TD-DFT studies

Photoreduction of oxoisoaporphine (OIA) (l-aza-benzo-[de]anthracen-7-one) and its 5-methoxy (5-MeOOIA) derivative by selected amines (two non-a-hydrogen-donating amines (l,4-diaza[2.2.2]-bicyclooctane (DABCO) and 2,2,6,6-tetramethylpiperidine (TMP)) and three a-hydrogen-donating amines (triethylamine (TEA), diethylmethylamine (DEMA), and dimethylethylamine (DMEA))) has been studied in deaerated neat acetonitrile solutions using laser flash and steady-state photolysis. The triplet excited states of OIA and 5-MeOOIA are characterized by intense absorption maxima located at λ_max = 450 nm and lifetimes of 34.7 ± 0.5 and 44.6 ± 0.4 μs, respectively. In the presence of tertiary amines, both triplets are quenched with a rate constant that varies from the near diffusion limit (>10 ⁹ M^-1 s^-1) to a rather low value (∼10 ⁷ M^-1 s^-1) and shows the expected dependence on the reduction potential for one-electron-transfer reactions. The transient absorption spectra observed after quenching of the respective triplet states are characterized by distinct absorption maxima located at λ_max = 480 and 490 nm (for OIA and 5-MeO-OIA, respectively) and accompanied by broad shoulders in the range of 510-560 nm. They were assigned to either solvent-separated radical ion pairs and/or isolated radical anions. In the presence of a-hydrogen-donating amines these species undergo protonation that leads to the formation of neutral hydrogenated radicals AlHVA2H with two possible sites of protonation, N and O atoms. Pulse radiolysis and molecular modeling together with TD-DFT calculations were used to support the conclusions about the origin of transients.