Cui Ganglong, Guan Pei-Jie, Fang Wei-Hai
Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education, College of Chemistry, Beijing Normal University , Beijing 100875, China.
J Phys Chem A. 2014 Jul 3;118(26):4732-9. doi: 10.1021/jp503865y. Epub 2014 Jun 18.
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.
分子内氢键芳香偶氮化合物1-环丙基重氮-2-萘酚（CPDNO）表现出复杂的激发态行为，例如波长依赖的光诱导质子转移和光产物。其光化学与以顺反光异构化为主的普通芳香偶氮化合物不同。为了在原子水平上合理解释CPDNO引人入胜的光化学，我们在这项工作中采用了完全活性空间自洽场（CASSCF）及其二阶微扰（CASPT2）方法，以探索与烯醇-酮质子转移和异构化反应相关的S0、S1和S2势能面。结果发现，沿着明亮的非绝热(1)ππ势能面的质子转移几乎没有势垒，迅速形成荧光(1)ππ酮最低点。在此过程中，暗(1)nπ态通过(1)ππ/(1)nπ*交叉点被占据，但由于势垒较大，该暗态上的质子转移受到严重抑制。此外，还发现了两条分别将S1烯醇和酮最低点衰减到S0态的失活路径。对于前者，它是放能的，因此在热力学上是有利的；对于后者，它有点吸热（约5千卡/摩尔）。就可用的总能量而言，两者在能量上都是允许的。最后，根据目前的结果，很好地解释了实验观察到的波长依赖的光产物。
