Under the 9,10-dicyanoanthracene (DCA)-sensitized photoinduced electron-transfer (PET) conditions, 2,5-diaryl-3,3,4,4-tetradeuterio-1,5-hexadiene (d(4)-1) undergoes a degenerate Cope rearrangement to give a photostationary mixture of d(4)-1 and its 1,1,6,6-tetradeuterio analogue d(4)-1' in 52:48. This reaction involves 1,4-diaryl-2,2,3,3-tetradeuteriocyclohexane-1,4-diyl cation radical (d(4)-2(.+)), which can be captured by molecular oxygen as 1,4-diaryl-2,3-dioxabicyclo[2.2.2]octane (3). In contrast, nonphotoinduced electron-transfer (non-PET) reaction of d(4)-1 with cerium(IV) ammonium nitrate similarly forms d(4)-2(.+), but the degenerate Cope does not take place. This observed striking contrast between PET and non-PET was ascribed to the formation of 1,4-diaryl-2,2,3,3-tetradeuteriocyclohexane-1,4-diyl (d(4)-2) in the PET process by back-electron transfer from DCA(.-) to d(4)-2(.+). The results of Similar PET and non-PET reactions of 1,4-diphenyl-2,3-diazabicyclo [2.2.2] oct-2-ene (4c) and its 5,5,6,6-tetradeuterio analogue (d(4)-4c) support this conclusion. Photoacoustic calorimetry of the PET reaction of 2,5-diphenyl-1,5-hexadiene (Ic) suggests that 1,4-diphenylcyclohexane-1,4-diyl (2c) lies ca. 18.8 kcal/mol higher in energy than Ic and ca. 25.6 kcal/mol below the ion-radical pair [2c(.+)/DCA(.-)]. Deazetation of d(4)-4c demonstrates that d(4)-2c undergoes cleavage to d(4)-1c and d(4)-1c.'with bond-breaking kinetic isotope effects, k(d(4)-2c)(H)4/k(d(4)-2c)(D)4 = 1.11, completing a cation-radical cyclization-diradical cleavage mechanism.