AbstractThe photooxygenation of 3,4‐dihydro‐2H‐pyran sensitized by cyanoanthracenes (9,10‐dicyanoanthracene and 9‐cyanoanthracene) in different solvents (CH3CN, CH3Cl2,C6H6and CCl4) has been studied in this paper. The products, product distribution as well as solvent isotope effect are the same as those in the reaction of singlet oxygen. Fluorescence quenching, exciplex formation and free energy change also reveal that electron transfer occurs between sensitizer‐excited singlet state and substrate and then singlet oxygen is formed subsequently as the reactive intermediate in