Triplet cyclohexadiene 3 7 has C2 symmetry so that the H-C1-C2-H and H-C3-C4-H dihedral angles are both 9.3°. In contrast, As seen in Figure , the activation barriers become higher and the dimerization steps become less exergonic as the ring size increases. Yet, the activation energy for the first dimerization step of 7 is even lower than those of 1 and 2. In addition, the longer T1 lifetime contributes for the more rapid dimerization of 7 as it leads to a higher concentration of 3 7 than of other 1,3-dienes in their T1 states. By assuming that every encounter between the T1 photosensitizer and 7 is successful for the energy transfer, and that these encounters are diffusion controlled (see the ESI), we estimate the concentrations of T1 state species as: [ 2). Thus, the concentration of 3 7 is three to nearly fifty times higher than the ones calculated for the two larger cyclic dienes and for isoprene.