Institute of Physical and Theoretical Chemistry, Johann Wolfgang Goethe-University, Frankfurt am Main, Germany.
Phys Chem Chem Phys. 2011 Oct 14;13(38):17367-77. doi: 10.1039/c1cp21721f. Epub 2011 Sep 1.
Caged compounds are widely utilized for light-triggered control of biological and chemical reactions. In our study we investigated the photo-induced decarboxylation of all three constitutional isomers of nitrophenylacetate (NPA), which can be regarded as caged-CO(2). UV-pump/IR-probe spectroscopy was used to directly observe the nascent CO(2) in the region of 2340 cm(-1). Together with quantum chemical calculations the reaction models for all three components could be obtained. For meta- and para-NPA the main decarboxylation pathway proceeds via a triplet state with a lifetime of 0.2 ns. In the case of ortho-NPA the photodecarboxylation reaction is suppressed by an H(+)- or H˙-transfer reaction in the excited state as a result of the proximity of the nitro and acetate substituents. Nevertheless, the photodecarboxylation can be investigated due to the isolated spectral position of the CO(2) band. The analysis of the data reveals that a weak ultrafast release channel (<300 fs) represents the main photodecarboxylation reaction pathway for ortho-NPA. The detailed understanding of the molecular mechanisms of CO(2) uncaging should provide general guidelines for the design of systematically improved nitrobenzyl cages.
笼状化合物被广泛用于光触发控制生物和化学反应。在我们的研究中，我们研究了三种硝基苯乙酸酯（NPA）的结构异构体的光诱导脱羧反应，这可以被视为 CO2 的笼状化合物。我们使用 UV 泵浦/IR 探针光谱法直接观察到 2340cm-1 区域中的初生 CO2。结合量子化学计算，可以得到所有三种组分的反应模型。对于间位和对位 NPA，主要的脱羧途径通过三重态进行，寿命为 0.2ns。对于邻位 NPA，由于硝基和乙酸酯取代基的接近，激发态中的 H(+)或 H˙转移反应抑制了光解羧反应。然而，由于 CO2 带的光谱位置被隔离，仍然可以研究光解羧反应。数据分析表明，弱超快释放通道（<300fs）代表邻位 NPA 的主要光解羧反应途径。对 CO2 去笼化分子机制的详细了解应提供系统改进的硝基苄基笼的设计的一般指导原则。
