title: Computational photochemistry of the azobenzene scaffold of Sudan I and Orange II dyes: excited-state proton transfer and deactivation via conical intersections.
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 (1) ππ* state is barrierless and ultrafast. Along this diabatic (1) ππ* relaxation path, the system can jump to the dark (1) nπ* state via the (1) ππ*/(1) nπ* crossing point. However, ESIPT in this dark state is largely inhibited owing to a sizeable barrier. We also found two deactivation channels that decay (1) ππ* keto and (1) nπ* 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 (1) ππ* state, whereas it is reduced in the (1) nπ* 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.
authors.affiliation: Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing 100875 (China).
authors.forename: Pei-Jie
authors.initials: PJ
authors.lastname: Guan
authors.forename: Ganglong
authors.initials: G
authors.lastname: Cui
authors.forename: Qiu
authors.initials: Q
authors.lastname: Fang
mesh_terms: D001391:Azo Compounds / Q000737:chemistry*; D001557:Benzenesulfonates / Q000737:chemistry*; D004396:Coloring Agents / Q000737:chemistry*; D015394:Molecular Structure; D009284:Naphthols / Q000737:chemistry*; D055668:Photochemical Processes; D011522:Protons; D011789:Quantum Theory
chemical_list: D001391:Azo Compounds; D001557:Benzenesulfonates; D004396:Coloring Agents; D009284:Naphthols; D011522:Protons; C024336:1-phenylazo-2-naphthol; C009850:azobenzene; C014722:2-naphthol orange
keywords: ab initio calculations; conical intersection; electronic structure; excited state; photochemistry