Organic photosensitizers are carbon-based molecules which are capable of photosensitizing. The earliest studied photosensitizers were aromatic hydrocarbons which absorbed light in the presence of oxygen to produce reactive oxygen species. These organic photosensitizers are made up of highly conjugated systems which promote electron delocalization. Due to their high conjugation, these systems have a smaller gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as well as a continuum of orbitals within the HOMO and LUMO. The smaller band gap and the continuum of orbitals in both the conduction band and the valence band allow for these materials to enter their triplet state more efficiently, making them better photosensitizers. Some notable organic photosensitizers which have been studied extensively include benzophenones, methylene blue, and rose Bengal.