Taken together, the results indicate that 1,3-cyclohexadiene and isoprene are the most suitable substrates in the photodimerizations. If one plots the activation energies of the first dimerization steps of 1 -9 against the corresponding reaction energies one sees that 7 and 8 are outliers (Fig. ). With 7 and 8 included, an R 2 value of 0.32 is obtained for a linear regression, whereas a value 0.79 is found without these compounds. Dienes 7 and 8 have a lower activation energy than predicted from the fit, a fact that should stem from their localized 1,4-diradical characters.