6-Hydroxy-3-oxo-3H-xanthene-9-carboxylic acid is introduced as the first transition-metal-free carbon monoxide releasing molecule activated by visible light (photoCORM). This water-soluble fluorescein analogue releases carbon monoxide in both water and methanol upon irradiation at 500 nm. When selectively irradiated in the presence of hemoglobin (Hb) under physiological conditions, released CO is quantitatively trapped to form carboxyhemoglobin (COHb). The reaction progress can be accurately monitored by characteristic absorption and emission properties of the reactants and products.
