(calculated distortion energies for 3 3 and 3 2 are, respectively, 3.1 and 1.7 kcal mol -1 ). This is corroborated with the fact that the transition state of the photodimerization of 3 exhibits a shorter C-C bond forming distance than what is the case for 2 (Fig. ), simultaneously as the hydrogens from the two dienes are further apart in the case of 3 (Fig. ). However, the reason for the lower reactivity of 1, which we observed experimentally, is not clear as its computed activation energy resembles that of 2. It may be a result of a slightly larger energy difference in the E(T1) of 1,1-dinaphthylmethanone sensitizer and 1 than between this sensitizer and 2.