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

Photoreduction of 7H-benzo[e]perimidin-7-one (3-AOIA, A1) and its 2-methyl derivative (2-Me-3-AOIA, A2) by non-H-donating amines (1,4-diazabicyclo[2.2.2]octane [DABCO]; 2,2,6,6-tetramethylpiperidine [TMP]), and a hydrogen-donating amine (triethylamine [TEA]), has been studied in deaerated neat acetonitrile solutions using laser flash photolysis (LFP) and steady-state photolysis. The triplet excited states of A1 and A2 were characterized by a strong absorption band with (max)=440nm and lifetimes of 20 and 27s respectively. In the presence of tertiary amines, both triplet excited states were quenched with rate constants close to the diffusional limit (k(q) ranged between 10(9) and 10(10)M(-1)s(-1)). The transient absorption spectra observed after quenching with DABCO and TMP were characterized by maxima located at 460nm and broad shoulders in the range of 500-600nm. These transient species are attributed to solvent-separated radical ion pairs and/or to isolated radical anions. In the presence of TEA, these transients undergo proton transfer, leading to the neutral hydrogenated radicals, protonated over the N1- and O-atoms. Transient absorption spectra of these transients were characterized by maxima located at 400 and 520nm and 430nm respectively. Additional support for these spectral assignments was provided by pulse radiolysis (PR) experiments in acetonitrile and 2-propanol solutions.