Intramolecularly hydrogen-bonded aromatic azo compound 1-cyclopropyldiazo-2-naphthol (CPDNO) exhibits complicated excited-state behaviors, e.g., wavelength-dependent photoinduced proton transfer and photoproducts. Its photochemistry differs from that of common aromatic azo compounds in which cis-trans photoisomerization is dominant. To rationalize the intriguing photochemistry of CPDNO at the atomic level, we have in this work employed the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods to explore the S0, S1, and S2 potential-energy profiles relevant to enol-keto proton transfer and isomerization reactions. It is found that the proton transfer along the bright diabatic (1)ππ* potential-energy profile is almost barrierless, quickly forming the fluorescent (1)ππ* keto minimum. In this process, the dark (1)nπ* state is populated via a (1)ππ*/(1)nπ* crossing point, but the proton transfer on this dark state is suppressed heavily as a result of a large barrier. In addition, two deactivation paths that decay the S1 enol and keto minima to the S0 state, respectively, were uncovered. For the former, it is exoenergetic and thereby thermodynamically favorable; for the latter, it is a little endothermic (ca. 5 kcal/mol). Both are energetically allowable concerning the available total energy. Finally, on the basis of the present results, the experimentally observed wavelength-dependent photoproducts were explained very well.

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.

In the present work, the combined electronic structure calculations and dynamics simulations have been performed to explore the decomposition reactions of 2H-azirine, 2-phenyl-2H-azirine, and 3-phenyl-2H-azirine in gas phase. Thermal cleavage of the C-C single bond yields nitrile ylides as products with a high barrier (>50.0 kcal mol(-1)). On the other hand, photochemical cleavage reactions starting from the (1)nπ* state of 2H-azirine and 3-phenyl-2H-azirine are ultrafast nonadiabatic processes (<100 fs), leading to nitrile ylides through the S1/S0 conical intersection. The fast formation of ylides in experiments was well reproduced by present dynamics simulations. For 3-phenyl-2H-azirine, population of the S1((1)ππ*) state is another decay pathway for the S2((1)nπ*) state. The C-N bond cleavage upon photoexcitation was usually considered to take place in T1 state. However, our calculations reveal that photocleavage of the C-N single bond for 2-phenyl-2H-azirine is very likely to take place in S1 state. The present work provides new insights into photocleavage mechanism of 2H-azirine and the related derivatives.

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