Photoinduced electron-transfer (PET) reactions of 1, 4-diphenyl-2, 3-diazabicyclo [2.2. 2] oct- 2-ene 1 result in a quantitative formation of 2, 5-diphenylhexa-1, 5-diene 2, in sharp contrast to the results of non-PET reactions and showing the importance of a back electron-transfer (BET) process in PET reactions.