On the Molecular Origin of Photoinduced Dimerization of 3‐ and 4‐Phenylcoumarin

Due to their ability to undergo photocycloaddition and cycloreversion reactions under continuous irradiation, coumarins are well‐established building blocks for advanced photo‐responsive materials. However, fundamental structure–function relationships for the targeted design of coumarin‐based functional materials remain largely unexplored. Here, using transient absorption spectroscopy and quantum chemical calculations, we aim to bridge this gap by investigating 3‐ and 4‐phenyl‐7‐methoxycoumarin. Despite their structurally similar nature, a pronounced difference in their photoinduced behavior is observed: 3‐phenyl‐7‐methoxycoumarin follows a triplet formation pathway mediated by the phenyl moiety, while 4‐phenyl‐7‐methoxycoumarin returns to the electronic ground state on an ultrafast timescale via an S 1 /S 0 near‐degeneracy along the lactone C─O bond stretch within the coumarin frame, consistent with a conical intersection. Therefore, only 3‐phenyl‐7‐methoxycoumarin dimerizes in the long‐lived triplet state under continuous irradiation, making it a suitable building block for photoreactive functional materials, as alternative fast non‐radiative decay channels are suppressed.