Type I and Type II Photosensitized Oxidation Reactions: Guidelines and Mechanistic Pathways

Abstract Here, 10 guidelines are presented for a standardized definition of type I and type II photosensitized oxidation reactions. Because of varied notions of reactions mediated by photosensitizers, a checklist of recommendations is provided for their definitions. Type I and type II photoreactions are oxygen‐dependent and involve unstable species such as the initial formation of radical cation or neutral radicals from the substrates and/or singlet oxygen ( 1 O 2 1 ∆ g ) by energy transfer to molecular oxygen. In addition, superoxide anion radical ( ) can be generated by a charge‐transfer reaction involving O 2 or more likely indirectly as the result of O 2 ‐mediated oxidation of the radical anion of type I photosensitizers. In subsequent reactions, may add and/or reduce a few highly oxidizing radicals that arise from the deprotonation of the radical cations of key biological targets. can also undergo dismutation into H 2 O 2 , the precursor of the highly reactive hydroxyl radical ( ) that may induce delayed oxidation reactions in cells. In the second part, several examples of type I and type II photosensitized oxidation reactions are provided to illustrate the complexity and the diversity of the degradation pathways of mostly relevant biomolecules upon one‐electron oxidation and singlet oxygen reactions.