Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited‐State Proton Transfer and Deactivation via Conical Intersections |
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| Authors: | Pei‐Jie Guan Prof. Ganglong Cui Dr. Qiu Fang |
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| Affiliation: | Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 (China) |
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| Abstract: | We employed the complete active space self‐consistent field (CASSCF) and its multistate second‐order perturbation (MS‐CASPT2) methods to explore the photochemical mechanism of 2‐hydroxyazobenzene, the molecular scaffold of Sudan I and Orange II dyes. It was found that the excited‐state intramolecular proton transfer (ESIPT) along the bright diabatic 1ππ* state is barrierless and ultrafast. Along this diabatic 1ππ* relaxation path, the system can jump to the dark 1nπ* state via the 1ππ*/1nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay 1ππ* keto and 1nπ* enol species to the ground state via two energetically accessible S1/S0 conical intersections. Finally, we encountered an interesting phenomenon in the excited‐state hydrogen‐bonding strength: it is reinforced in the 1ππ* state, whereas it is reduced in the 1nπ* state. The present work sets the stage for understanding the photophysics and photochemistry of Sudan I–IV, Orange II, Ponceau 2R, Ponceau 4R, and azo violet. |
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| Keywords: | ab initio calculations conical intersection electronic structure excited state photochemistry |
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