Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-Phenyl)-4-Oxo-but-2-Enoate Ester Derivatives.
Comparison of the Photochemistry of Acyclic and Cyclic 4-(4-Methoxy-Phenyl)-4-Oxo-but-2-Enoate Ester Derivatives.
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DOI：
10.1021/acs.jpca.0c04319
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发表时间：
2020-08
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通讯作者：
S. K. Sarkar;Ranaweera A. A. Upul Ranaweera-Ranaweera-A.-A.-Upul-Ranaweera-2126045335;Rajkumar Merugu;Nayera Abdelaziz;J. Robinson;Heidi A Day;J. Krause;A. Gudmundsdottir
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作者：
S. K. Sarkar;Ranaweera A. A. Upul Ranaweera-Ranaweera-A.-A.-Upul-Ranaweera-2126045335;Rajkumar Merugu;Nayera Abdelaziz;J. Robinson;Heidi A Day;J. Krause;A. Gudmundsdottir
为了阐明三重激发态表面上简单烯烃的顺反异构化机理，比较了具有作为内置三重态敏化剂的对甲氧基苯乙酮部分的无环和环状乙烯基酮（分别为1和2）的光化学。当照射时，酮1产生其顺式异构体，而酮2不产生任何光产物。酮1的激光闪光光解产生λmax ~ 400 nm（τ ~ 125 ns）的瞬态光谱。这种瞬态被分配给1的第一个三重激发态（T1），它可能衰变形成三重双自由基（1BR），其寿命比三重酮短。相比之下，2的激光闪光光解揭示了两个瞬变（τ ~ 20和440 ns），分别归属于2和三重态双自由基2BR的T1。密度泛函理论（DFT）的计算支持的三重激发态和双自由基的中间体照射后形成的酮1和2的表征，并允许比较的双自由基的中间体的物理性质。由于2BR中的双自由基中心通过共轭稳定，2BR比1BR更刚性。因此，2BR的较长寿命可归因于较低效率的系统间穿越到基态。
To clarify the cis-trans isomerization mechanism of simple alkenes on the triplet excited state surface, the photochemistry of acyclic and cyclic vinylketones with a p-methoxyacetophenone moiety as a built-in triplet sensitizer (1 and 2, respectively) was compared. When irradiated, ketone 1 produces its cis-isomer whereas ketone 2 does not yield any photoproducts. Laser flash photolysis of ketone 1 yields a transient spectrum with λmax ~ 400 nm (τ ~ 125 ns). This transient is assigned to the first triplet excited state (T1) of 1, which presumably decays to form a triplet biradical (1BR) that is shorter lived than the triplet ketone. In comparison, laser flash photolysis of 2 reveals two transients (τ ~ 20 and 440 ns), which are assigned to T1 of 2 and triplet biradical 2BR, respectively. Density functional theory (DFT) calculations support the characterization of the triplet excited states and the biradical intermediates formed upon irradiation of ketones 1 and 2, and allow a comparison of the physical properties of the biradical intermediates. As the biradical centers in 2BR are stabilized by conjugation, 2BR is more rigid than 1BR. Therefore, the longer lifetime of 2BR can be attributed to less efficient intersystem crossing to the ground state.