Photoreduction of electron-deficient azaarenes by di- and trialkylamines

Abstract

The photophysical and photochemical properties of several electron-deficient azaarenes were studied by pulsed (λ_exc = 308 nm) and steady-state photolysis. The triplet state characteristics of 3,5,6-triphenyl-1,2,4-triazine (1), 3-phenyl-1,2,4-benzotriazine (2), 3-phenyl-1,2,4-phenanthro[9,10-e]triazine (3) and tetraphenylpyrimidine (4) and, for comparison, of phenazine in several solvents are described. The quantum yield of triplet formation is substantial for 1–4 in solution at room temperature, whereas the fluorescence is negligible. Phosphorescence in glassy media at −196 °C and efficient formation of singlet molecular oxygen at 25 °C were detected. The triplet state of 1–3 is quenched by triethylamine (TEA) and other tertiary amines; the rate constant for quenching by TEA is k_q = (0.9–1.5) × 10⁸ M⁻¹ s⁻¹ in acetonitrile and slightly smaller in cyclohexane. Quenching the triplet state of 1–3 by 1,4-diazabicyclo[2.2.2]octane (DABCO) is more efficient and k_q is only slightly lower than the diffusion-controlled limit. The secondary transient of 1–3 in the presence of DABCO is ascribed to the radical anion, whereas with TEA or diethylamine (DEA) H-adduct radicals with maxima around 400 nm were observed. These radicals and the corresponding Et₂N–˙CHMe radical subsequently undergo a slow termination reaction yielding the corresponding dihydroarenes. The mechanisms of the two subsequent reduction reactions are discussed. The dihydroheterocycles are thermally converted back to the azaarenes on addition of oxygen.