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. The ESR spectra reported in a Research Article by Chao and co‐workers are not in agreement with reactive “carbon radicals” claimed to be produced by 730 nm irradiation of the iridium(III) complex Ir4‐red. The observed tumor‐damaging effects must be induced by other, unidentified reactive intermediates.