Scheme 1.

Simplified scheme of photo-oxidation pathways. The absorption of light can lift a sensitizer (sen) from its ground (⁰sen) to a higher excited singlet (¹sen) state. The ¹sen state gets further converted to a triplet (³sen) state via intersystem crossing (ISC). In this state, the sen transfers either an electron (e⁻) or energy to a suitable substrate via the type I or II reaction pathways resulting in the formation of radicals and radial ions (type I) or singlet oxygen (¹O₂) (type II). In subsequent reactions a broad variety of different oxidation products are created.