Nature Chemistry, Published online: 18 August 2026; doi:10.1038/s41557-026-02233-3 Conventional energy transfer photocatalysis often requires matching triplet-state energies between catalysts and substrates, which can trigger polymerization side reactions through rapid diradical generation. Now it has been shown that heteroleptic copper(I) complexes enable endergonic triplet photosensitization of either alkenes or bicyclo[1.1.0]butanes, effectively suppressing alkene polymerization and broadening substrate scope.