Chapter 4: Production of Singlet Oxygen by Direct Photoactivation of Molecular Oxygen

In this chapter, we will discuss singlet oxygen direct production schemes that imply only natural transition of molecular oxygen that we name direct photoactivation. Our purpose will be to highlight that despite its low efficiency compared to nondirect production methods (ca. 10⁴–10⁵ times less efficient that photosensitized production), direct photoactivation can be seen as a powerful tool to study the chemistry and biology of singlet oxygen. First, a brief history of the discovery of the molecular oxygen electronic transitions under consideration in this chapter is presented. We will briefly review pioneering studies that have allowed a better understanding of peculiar spectroscopic properties of O₂. Secondly, we discuss recent works that focus on singlet oxygen direct photoactivation in liquid solvents at room temperature and atmospheric pressure. These studies have led to estimation of absorption cross sections of molecular oxygen bands in homogeneous liquids as well as in heterogeneous solutions. Finally, we will discuss singlet oxygen direct photoactivation in biological systems as an alternative to traditional photodynamic therapy. We highlight potential outcomes of this production scheme for both therapeutic use and to study cellular oxidative stress response.