The photophysical and photochemical properties of several electron-deficient azaarenes were studied by pulsed (λexcâ =â 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 1â 4 in solution at room temperature, whereas the fluorescence is negligible. Phosphorescence in glassy media at â 196â °C and efficient formation of singlet molecular oxygen at 25â °C were detected. The triplet state of 1â 3 is quenched by triethylamine (TEA) and other tertiary amines; the rate constant for quenching by TEA is kqâ =â (0.9â 1.5)â à â 10⁸ M^{â 1} s^{â 1} in acetonitrile and slightly smaller in cyclohexane. Quenching the triplet state of 1â 3 by 1,4-diazabicyclo[2.2.2]octane (DABCO) is more efficient and kq is only slightly lower than the diffusion-controlled limit. The secondary transient of 1â 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 Et2Nâ .rad;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.