Schaper Klaus, Etinski Mihajlo, Fleig Timo
Group for Organic Photochemistry, Institute for Organic Chemistry and Macromolecular Chemistry, Heinrich-Heine-University, Düsseldorf, Germany.
Photochem Photobiol. 2009 Sep-Oct;85(5):1075-81. doi: 10.1111/j.1751-1097.2009.00560.x. Epub 2009 Apr 6.
The photochemistry of caged compounds of the o-nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol.84 162-171] that we found long-lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5-methylendioxy-2-nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o-nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5-methylendioxy-2-nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro-group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o-nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.
过去已对邻硝基苄基型笼形化合物的光化学进行了深入研究。然而，即便在最近仍发现了新的副反应。早些时候，我们报道过[Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol.84 162 - 171]，对于带有4,5 - 亚甲基二氧基 - 2 - 硝基苄基笼形基团且吸收峰红移的化合物，我们发现了长寿命三重态，但其不会导致产物生成。在此，我们报道一些理论研究，这些研究解释了这些化合物的特殊行为。这些研究表明，与邻硝基苄基化合物本身相比，这些化合物吸收峰的红移并非由于相关态的能量发生了移动，而是由于各自跃迁的振子强度发生了显著变化。4,5 - 亚甲基二氧基 - 2 - 硝基苄基化合物三重态缺乏反应活性可归因于态切换。在三重态系综中，最低态是一个无反应活性的电荷转移态，而单重态系综中的最低态是硝基中的一个有反应活性的局域激发态。从这些结果我们得出结论，基于邻硝基苄基笼形基团创建吸收峰向更长波长方向更强烈红移的笼形化合物衍生物很可能是不可能的。
