Bacteriochlorins are characterized by intense near-infrared absorption bands (700-800 nm), which are of interest for diverse applications such as photodynamic therapy (PDT) of diseased tissues or dyes in solar cells. Although naturally-occurring bacteriochlorin derivatives are labile and this limits their applications, we synthesized photostable bacteriochlorins that can endure many cycles of photoinduced energy and electron transfer reactions without photodecomposition. The factors contributing to this photostability are discussed. In particular, it is shown that bacteriochlorins are prone to oxidation but this can be prevented by introducing electron-withdrawing substituents in the phenyl groups of tetraphenylbacteriochlorins. The oxidation potentials of tetraphenylbacteriochlorins increase when such substituents are present, and it is shown that in a series of structurally-related bacteriochlorins, chlorins and porphyrins, the photodecompositing quantum yields decrease as the oxidation potentials increase. The control of the oxidation potential is also relevant to determine the propensity of these photosensitizers to undergo energy or electron transfer to molecular oxygen.

Efficient energy transfer (Type II process) and electron transfer (Type I process) to molecular oxygen, with the consequent formation of reactive oxygen species, is critical to the success of PDT of solid tumors. We tuned the properties of tetraphenylbacteriochlorins to optimize a photosensitizer for PDT of cancer. The optimized photosensitizer, named redaporfin, proved very effective in the treatment of tumors implanted in animal models and is now in clinical trials for advanced head and neck cancer

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