Chemical Quenching of Singlet Oxygen and Other Reactive Oxygen Species in Water: A Reliable Method for the Determination of Quantum Yields in Photochemical Processes?

The production of singlet oxygen in aqueous solutions by photosensitizers (PS) grafted on silica was evaluated using the widely-known singlet oxygen quencher, furfuryl alcohol (FFA). Although reliable data could be obtained when working with some of the water-soluble PS in homogeneous solution, strong diffusion effects prevented the measurement of accurate singlet oxygen quantum yields of the silica materials. Moreover, when electron-transfer reactions were possible and produced other reactive oxygen species (ROS), FFA oxidation did not result from singlet oxygen production but from electron transfer steps, as, for example, with TiO2 or with anthraquinone derivatives. Some literature data on singlet oxygen quantum yields with aqueous semiconductor suspensions in the presence of FFA have thus to be considered with care. Other issues were also raised with selective quenchers for other ROS such as hydroxyl radicals or superoxide radical anions and emphasize significant artefacts mainly arising from adsorption effects. The results obtained with anthraquinone derivatives confirmed its ubiquitous photochemical behavior and appeared as complex as that of the water-soluble anthraquinone-2-sulfonate AQ2S. In any case, the covalent grafting of PSs on silica allows tuning the oxidative properties of the material. It also makes possible the efficient production of singlet oxygen by water-insoluble PS, such as cyanoanthracene derivatives, in aqueous solutions.