Photoreduction of Oxoisoaporphines by Amines: Laser Flash and Steady-State Photolysis, Pulse Radiolysis, and TD-DFT Studies

Abstract

Photoreduction of oxoisoaporphine (OIA) (1-aza-benzo-[de]anthracen-7-one) and its 5-methoxy (5-MeO-OIA) derivative by selected amines (two non-alpha-hydrogen-donating amines (1,4-diaza[2.2.2]-bicyclooctane (DABCO) and 2,2,6,6-tetramethylpiperidine (TMP)) and three alpha-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-MeO-OIA are characterized by intense absorption maxima located at lambda(max) = 450 nm and lifetimes of 34.7 +/- 0.5 and 44.6 +/- 0.4 micros, 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 (approximately 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 lambda(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 alpha-hydrogen-donating amines these species undergo protonation that leads to the formation of neutral hydrogenated radicals A1H(*)/A2H(*) 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.

Photoreduction of Azaoxoisoaporphines by Amines: Laser Flash and Steady‐State Photolysis and Pulse Radiolysis Studies

“…The 1 H-NMR study of the photoreduction of AOIA and OIA has provided the identity of the metastable photoproducts hydrogenated at the carbonyl O-atom (AOH À ). Their formation rationalizes the observed isotopic H/D exchange and supports the stepwise photoreduction mechanism elaborated earlier for 2,3-dihydro-oxoisoaporphines (2,3-dh-OIA) (21)(22)(23) and OIA (24).…”

“…The E T values of 3 A1 and 3 A2 were found to be more than 10 kJ mol À1 less than the E T values measured for triplet states derived from OIA (24). The stabilization of 3 A1 and 3 A2 with respect to the triplet states of oxoisoaporphines ( 3 OIA) can be attributed to the presence of the second N-atom in the AOIA molecules.…”

“…Solutions were subsequently purged for at least 30 min. per 200 mL of sample with the desired gas before pulse irradiation (24,26,27).…”

“…A detailed description of the evaluation of photoconsumption quantum yields of AOIA has been given elsewhere (24). The variation in concentration of AOIA was evaluated from the absorbances measured at k = 376 and 380 nm for 3-AOIA and 2-Me-3-AOIA respectively (24). The photon flux was determined by using Aberchrome ® 540 (33).…”

“…The photoreduction mechanism proceeds through a stepwise electron-proton-electron transfer process (24). The first step is an excitation of the ground-state of 2,3-dh-OIA and OIA (A) to the excited singlet state ( 1 A) (Eq.…”