Correspondence on "A Mitochondrion-Localized Two-Photon Photosensitizer Generating Carbon Radicals Against Hypoxic Tumors".

It is shown that data presented in a paper by Chao and co-workers do not support the formation of active "Carbon Radicals" as claimed according to the title. The assignments of observed ESR spectra and the mechanistic interpretation are severely flawed. Hence, other reactive intermediates must be responsible for the observed tumor-damaging effects.