The use of the ring strain of cyclopropanes is well established in synthetic chemistry to enable C-C bond cleavage and generate reactive intermediates for further functionalization. In addition to activated Donor -Acceptor systems, which have been widely studied in the past decades, less reactive acceptor-only cyclopropanes could also be recently functionalized through the reductive activation of a carbonyl group on the ring (Scheme 1A). Due to the high redox potential of most carbonyl-substituted cyclopropanes, strong reductants were often required such as Sm, the combination of Ti and Mn 10 or more recently boron and pyridine derivatives. Photoredox catalysis could be also applied, yet the coordination with a Lewis acidic transition metal is crucial to facilitate the reduction as reported by Yoon and Meggers. As an alternative to reductive approaches, energy transfer catalysis allows the direct activation of substrates into a biradical excited state under mild conditions. In particular, energy transfer catalysis has been used to activate carbonyl cyclopropanes, but it has been mostly utilized to promote stereocenter epimerization due to the fast recombination rate of the biradical. To the best of our knowledge, energy-transfer catalysis has never been exploited to promote annulation reaction of cyclopropanes with  or  bonds as partners, although care has to be taken to distinguish it from photoredox catalysis, as both pathways can be promoted by Lewis or Brønsted acid coordination to the carbonyl group. In this study, we report the light activation of carbonyl substituted cyclopropanes for [3 + 2] cycloaddition with both alkynes and olefins in absence of any reducing or coordinating agent (Scheme 1B). Furthermore, with the addition of diphenylphosphate as a mild Brønsted acid co-catalyst, the protocol could be extended to a [3 + 2] cycloaddition with bicyclo[1.1.0]butanes (BCBs), yielding highly functionalized bicyclo[3.1.1]heptanes (BCHs). BCHs have been recently investigated as bioisosteres for meta-substituted arenes (Scheme 1C) as part of a resurging interest in non-aromatic benzene bioisosteres. The synthesis of BCHs is however challenging. Most recent studies focus on functionalizing [3.1.1]propellane as a pre-formed skeleton of BCHs. However, the poor stability of [3.1.1]propellane along with difficult handling and storage still remains a limitation for applications. In contrast, bicyclo[1.1.0]butane derivatives are stable at room temperature and easy to handle. The synthesis of BCBs is now well developed due to the recent focus of interest in their use as building blocks, especially in photocatalytic processes. While we were working on this project, Molander and co-workers reported the first annulation of cyclopropyl amines (CPAs) and BCBs to give amino BCHs via oxidative activation. Interestingly, this work is highly complementary to our approach, both from the substitution pattern (amine donors vs carbonyl acceptors) and the mode of activation (electron-transfer vs homolytic cleavage). We started our investigation with phenylcarbonyl cyclopropane 1a with two methyl groups on the ring as a model substrate due to a higher reactivity compared to non-substituted ones, as reported in previous studies (Table ). To challenge our approach, we chose phenyl acetylene (3a) as a radical trap under visible light irradiation due to small number of reports on this type of transformation compared to olefin partners. Based on previous study of Brown and co-workers on carbonyl-substituted BCBs, thioxanthone (PC1) was chosen as energy transfer catalyst for the reaction. However, no cycloaddition product was obtained with PC1 (entry 1). An almost quantitative amount of ring-opening methoxylation product 2a1 was observed. It was likely due to competitive oxidation of the radical intermediate at the tertiary carbon. Nevertheless, we were pleased to observe the formation of product 2a in 42% NMR yield using isopropyl-substituted thioxanthone (PC2) as photocatalyst (entry 2). In parallel to these experiments, we also tried acridinium-based photocatalyst PC3 (entry 3). However, poor yield was observed, and the starting material 1a was decomposed. Carbazole-based photocatalyst PC4 failed to give any product and the starting material was recovered (entry 4). In contrast, the iridium-based photocatalyst PC5 gave 2a in 77% NMR yield (entry 5). A lower yield was obtained in other solvents such as acetonitrile (entry 6, see Supporting Information for further details). An attempt to reduce the amount of trapping reagent to 2 equivalents resulted in a drop of the yield to 52% (entry 7) due to the competition with intramolecular cyclization to give 2a2. This phenomenon was described in a previous report by Procter and co-workers. In order to reduce the amount of phenyl acetylene used, we chose substrate 1b bearing diester groups to inhibit the oxidation reaction occurring at the tertiary carbon. In fact, the stoichiometry of phenylacetylene (3a) could be successfully diminished to only 1.5 equivalents to give 79% of product 2b (entries 9-11). Reaction conditions: 0.1 mmol 1 (1 equiv.), one Kessil lamp (440 nm, 40 W). a 1 H NMR yields were determined with CH2Br2 as an internal standard. Isolated yield after chromatography is given in brackets. b A 427 nm Kessil lamp was used.