Computational Photochemistry of the Azobenzene Scaffold of Sudan I and Orange II Dyes: Excited-State Proton Transfer and Deactivation via Conical Intersections

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. No barriers! CASSCF and MS‐CASPT2 methods were used to study excited‐state proton transfer, photoisomerization, and excited‐state deactivation of 2‐hydroxyazobenzene in the lowest two excited singlet states, that is, 1nπ* and 1ππ*. It was found that excited‐state intramolecular proton transfer (ESIPT; see figure) along the 1ππ* potential energy profile is barrierless.