https://doi.org/10.1351/goldbook.S05497
Chemical transformation of one molecular entity caused by photoexcitation of a sensitizer, which undergoes temporary covalent bond formation with the molecular entity (M). 
Notes:
- This mechanism is the basis of the Type I photooxygenation.
- The reactive molecular entity could be ground-state molecular dioxygen, $\ce{O2}$, in which case energy transfer may occur producing singlet molecular oxygen, this being the basis of the Type II photooxygenation.