In addition, it should not be possible to excite directly styrene because its triplet energy was reported to be 61.7 Kcal/mol, which is higher than that of the [Ir] catalyst (49.2 Kcal/mol). Furthermore, the triplet energy of a similar cyclopropyl phenyl ketone was calculated to be 71.4 Kcal/mol 20 in a previous study, implying an energy transfer to the cyclopropane is less likely. Further calculations will be needed to establish the triplet energy of carbonyl-substituted cyclopropanes, BCBs, and ethynylarenes. The unsuccessful reactions with alkynes, alkenes and BCBs bearing alkyl substituents (see Supporting Information) suggest that there might be an additional effect arising from the aromatic substituent. We hypothesized that there might be a - interaction between the electron poor aryl group on the cyclopropane and the aromatic substituent on the reaction partner, leading to a complex that would be then photoactivated. This hypothesis is supported by the fact that the cis diastereoisomer of 2s was observed as major product in the [3+2] cycloaddition, in contrast to previous studies, and by the regioselectivity observed for the [3+2] cycloaddition. In addition, the reaction became sluggish with an electron withdrawing group substituted alkyne (product 2q) or a cyclopropane having an electron donating group on the benzene ring (product 2e), in agreement with a less favored electron donor acceptor complex. These pre-formed complexes could have a lower triplet energy than single molecules. This type of activation was reported in previous studies. A similar reactivity pattern has, for example, been observed in the case of a [2+2] cycloaddition of styrene via energy transfer with Ir(ppy)3 as the photocatalyst. A mechanism via energy transfer to a pre-formed complex between carbonyl substituted cyclopropane and aromatic alkyne, alkene or BCBs leading to biradical formation and cyclization would be in agreement with these observations.