Fotosensitizers

Fotosensitizers (also spelled photosensitizers) are molecules that, upon absorption of light, undergo electronic excitation and transfer energy or electrons to other species, initiating photochemical or photobiological reactions. They are characterized by absorption in a relevant spectral region, efficient intersystem crossing to a reactive triplet state, and the ability to generate reactive oxygen species (ROS) such as singlet oxygen or free radicals.

Common classes include porphyrins and chlorins, phthalocyanines, phenothiazinium dyes (e.g., methylene blue), xanthene dyes (e.g., rose bengal), and various synthetic organic chromophores and metal complexes. Performance is evaluated by absorption spectrum, singlet oxygen quantum yield, photostability, and chemical compatibility with the intended environment.

Mechanistically, fotosensitizers operate via energy transfer to molecular oxygen (Type II photochemistry) producing singlet oxygen, or via electron transfer (Type I) producing radical species. These pathways underpin applications in photodynamic therapy (PDT) for cancer and infections, photochemical synthesis, environmental photodegradation of pollutants, dye-sensitized solar cells, and optical sensing.

Limitations include limited tissue penetration of activating light, potential phototoxicity to non-target tissues, and the need for selective delivery. Research directions focus on improving selectivity and stability through targeted delivery systems, formulation in nanoparticles, wavelength-shifting strategies to use deeper-penetrating light, and designing agents with tailored photophysical properties.