Time-resolved near-infrared emission spectroscopy was used to quantify the singlet oxygen yields. Figure shows representative 1 O2 emission decay traces for 6MP, Aza, and the phenalenone standard, while Tables and collect the determined singlet oxygen quantum yields. Singlet oxygen quantum yields of 0.19 and 0.11 were determined for 6MP at pH 5.5 and 7.5 under air-saturated conditions, respectively. Under oxygen-saturated conditions, the singlet oxygen quantum yields increase to 0.29 and 0.16 at pH 5.5 and 7.5, respectively. In the case of Aza, singlet oxygen quantum yields of <0.01 and 0.001 were determined at pH 7.5 and in acetonitrile, respectively. The 10-fold lower yield of Aza compared to 6MP evidence that the covalent attachment of a nitroimidazole group significantly affects its photochemistry.