Pac C, Miyamoto I, Masaki Y, Furusho S, Yanagida S, Ohno T, Yoshimura A
Department of Chemical Process Engineering, Faculty of Engineering, Osaka University, Japan.
Photochem Photobiol. 1990 Nov;52(5):973-9. doi: 10.1111/j.1751-1097.1990.tb01813.x.
The photosensitized monomerization of the cyclobutane dimers of 1,3-dimethylthymine by p-chloranil was investigated by means of steady-state irradiation and laser-flash photolysis. Quantum yields for the monomerization are 0.34 for the cis,syn dimer, 0.39 for the trans,syn dimer, and much less than 10(-2) for the cis,anti isomer. Formation of the chloranil anion radical associated with quenching of triplet chloranil by the dimers demonstrates that electron transfer from dimers to triplet chloranil occurs to initiate the monomerization. Kinetic analysis suggested that the syn-dimer cation radicals undergo the ring cleavage at greater than or equal to 10(9) s-1 before escaping from the solvent cage, while the reactivity of the anti-dimer cation radical is very low. The different reactivities of the syn and anti dimer cation radicals are discussed in terms of through-bond coupling between the n orbitals of N(1) and N(1') involving the cyclobutane-ring sigma orbitals. In the cases of the syn-dimers, the sensitizer-dimer ion-radical pairs undergo the rapid geminate recombination that works as a major energy dissipating channel responsible for the lower-than-unity quantum yields. It has been found that the presence of Mg(ClO4)2 at 0.1 M enhances approximately 1.5 times either the monomerization of the syn dimers or the formation of the chloranil anion radical. A laser-flash photolysis study shows that Mg2+ forms a complex with either the triplet or the anion radical of chloranil. The net salt effects are attributed to the retardation of the rapid geminate recombination by the participation of Mg2+ in the sensitizer-dimer ion-radical pairs.
通过稳态辐照和激光闪光光解研究了对氯苯醌引发的1,3 - 二甲基胸腺嘧啶环丁烷二聚体的光敏单聚反应。顺式，顺式二聚体的单聚量子产率为0.34，反式，顺式二聚体为0.39，而顺式，反式异构体的量子产率远低于10^(-2)。二聚体与三重态对氯苯醌猝灭相关联的对氯苯醌阴离子自由基的形成表明，从二聚体到三重态对氯苯醌发生电子转移以引发单聚反应。动力学分析表明，顺式二聚体阳离子自由基在从溶剂笼逃逸之前以大于或等于10^(9) s^(-1)的速率发生环裂解，而反式二聚体阳离子自由基的反应活性非常低。根据涉及环丁烷环σ轨道的N(1)和N(1')的n轨道之间的键间耦合，讨论了顺式和反式二聚体阳离子自由基的不同反应活性。在顺式二聚体的情况下，敏化剂 - 二聚体离子自由基对经历快速的双分子复合，这是导致量子产率低于1的主要能量耗散通道。已发现0.1 M的Mg(ClO4)2的存在使顺式二聚体的单聚或对氯苯醌阴离子自由基的形成增强约1.5倍。激光闪光光解研究表明，Mg2+与对氯苯醌的三重态或阴离子自由基形成络合物。净盐效应归因于Mg2+参与敏化剂 - 二聚体离子自由基对而导致的快速双分子复合的延迟。
