Properties of singlet oxygen

Singlet oxygen (1O2) is the first electronic excited state of molecular oxygen (O2). Considered as a distinct chemical entity, its electronic configuration is responsible for its unusual thermodynamic stability but remarkably enhanced reactivity towards other molecules in the singlet state. 1O2 photophysical properties, such as its main pathways of physical deactivation (radiative and nonradiative) and their associated kinetic constants, have gained increased consideration over the last decades. These photophysical processes were first studied in homogeneous solvents, but as 1O2 became relevant as a possible oxidizing agent in polymer sciences or as a photocytotoxic species in photodynamic therapy, the scientific community made an enormous effort to elucidate these processes in such heterogeneous systems. In this chapter, we focus on the 1O2 photophysical properties, reviewing the most relevant observations in homogeneous as well as in heterogeneous microenvironments such as polymers, nanoparticles or cellular systems.