Chapter 6: Reference Photosensitizers for the Production of Singlet Oxygen

Determination of the quantum yield of singlet molecular oxygen production (Φ_Δ) by a photosensitizer is often based on relative methods (¹O₂ reactivity or phosphorescence). The experimental results obtained for a sensitizer of unknown Φ_Δ (sample) are compared with those of a well-known standard or reference sensitizer of preferably high and accurately determined Φ_Δ under appropriate experimental conditions. This chapter discusses the efficiency of ¹O₂ production on the basis of intrinsic or photosensitizer-dependent factors (nature and configuration of the excited state, lifetime, quenching ability by O₂ and photostability) and extrinsic or medium-dependent factors (oxygen concentration, solvent viscosity and polarity, photosensitizer aggregation state, temperature and pressure). Analysis of these factors determines the general features required for any reference ¹O₂ photosensitizer. Phenalenone, an aromatic ketone, fulfills all these requirements and consequently can be termed as the universal reference compound for Φ_Δ determination. Besides phenalenone, other aromatic ketones, heterocycles, carbon nanoforms or coordination compounds with accurately determined Φ_Δ values can also be used as references for ¹O₂ production, not only in solution but in the solid phase as well (organic/inorganic materials such as dye-doped monoliths, films, or micro-/nanofibers and particles). Values of Φ_Δ and other photophysical data for ¹O₂ generation by reference systems are collected in this chapter.