The triplet energies, E(T1), of the conjugated dienes 1 -7 and the photosensitizer 1,1dinaphthylmethanone at (U)B3LYP-D3/6-311+G(d,p) level are listed in Table . Since we utilized a mixture of Z-and E-isomers in the experiments with 5 and 6, we computed E(T1) of all the isomers present. The E(T1) of 1,1-dinaphthylmethanone (52.2 kcal mol -1 ) is slightly higher than those of the dienes studied herein, making it a suitable sensitizer in our studies. The triplet lifetime of 1,1-dinaphthylmethanone is 300 ns, which is long enough for molecular collisions and energy transfer to take place. All dimerization reactions were run using neat dienes, except for 1,3-butadiene (1) which was run dilute in hexane, i.e., the form in which it is commercially available. Photoirradiations of dienes 2 -7 with a 0.1 mol% photosensitizer loading (λ = 365 nm, t = 24 h) gave the dimer yields reported in Table . Considering that the photosensitizer 1,1-dinaphthylmethanone is kept throughout this study, and for an applications point of view, we base our comparisons on the yields of the dimer products and not on the quantum yields. The formation of dimers in all irradiated samples could be confirmed through GC-MS analyses (Fig. -S4, ESI). Trace amounts of trimers were found for 7 and 2 (Fig. , ESI).