Theoretical exploration of the 4π-photocyclization mechanism of α-tropone derivatives†

In this study, we employed ab initio methods to investigate the 4π-photocyclization process of substituted α-tropones. Our findings suggest that the initial stages of this reaction occur from low-energy excited states such as S₂ and S₃, as the barriers in higher-energy excited states are prohibitively high. Additionally, we observed that the presence of acid catalysts or substitution of tropones with electron-donating groups reduces the energy barriers. Regarding the formation of bicyclo[3.2.0]hepta-3,6-dien-2-one derivatives, both T₁/S₀ and S₁/S₀ crossings were identified as plausible pathways for this deactivation process. Specifically, electron-donating groups on the tropones promote degeneracy between S₁ and S₀ states during the return to the ground state. Moreover, T₁/S₀ crossing systems were consistently observed across all investigated scenarios.