Sensitizers (denoted "Sens") are compounds, such as fluorescein dyes, methylene blue, and polycyclic aromatic hydrocarbons, which are able to absorb electromagnetic radiation (usually in the visible range of the spectrum) and eventually transfer that energy to molecular oxygen or the substrate of photooxygenation process. Many sensitizers, both naturally occurring and synthetic, rely on extensive aromatic systems to absorb light in the visible spectrum.   When sensitizers are excited by light, they reach a singlet state, Sens*. This singlet is then converted into a triplet state (which is more stable), Sens*, via intersystem crossing. The Sens* is what reacts with either the substrate or O in the three types of photooxygenation reactions.