Excitation mechanism in the photoisomerization of a surface-bound azobenzene derivative: Role of the metallic substrate
Excitation mechanism in the photoisomerization of a surface-bound azobenzene derivative: Role of the metallic substrate
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DOI：
10.1063/1.2997343
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发表时间：
2008-10-28
影响因子：
4.4
通讯作者：
Tegeder, Petra
中科院分区：
文献类型：
--
作者：
Hagen, Sebastian;Kate, Peter;Tegeder, Petra
采用双光子光电子能谱阐明了吸附在 Au(111) 上的分子开关四叔丁基偶氮苯 (TBA) 的电子结构和光致异构化的激发机制。我们的结果表明，金属表面的光激发和分子切换机制与自由分子的相应过程完全不同。与液相异构化中的直接（分子内）激发相反，表面结合的 TBA 的构象变化是由底物介导的电荷转移过程驱动的。我们发现，高于大约 2.2 eV 的阈值 h nu 的光激发会导致 Au d 带中的空穴形成，然后空穴转移到 TBA 的最高占据分子轨道。这种瞬时形成的正离子共振随后导致构象变化。光子能量依赖的光异构化横截面对于金属表面上吸附物的光化学反应表现出不寻常的形状。它显示出低于 h nu 约 2.2 eV 和高于 h nu 约 4.4 eV 的阈值行为。这些阈值分别对应于在 Au d 带中产生单个或多个热空穴所需的最小能量。这项研究为利用光控制与金属电极直接接触的分子开关的结构和功能提供了重要的新见解。 (C) 2008 年美国物理研究所。 [DOI：10.1063/1.2997343]
Two-photon photoemission spectroscopy is employed to elucidate the electronic structure and the excitation mechanism in the photoinduced isomerization of the molecular switch tetra-tert-butyl-azobenzene (TBA) adsorbed on Au(111). Our results demonstrate that the optical excitation and the mechanism of molecular switching at a metal surface is completely different compared to the corresponding process for the free molecule. In contrast to direct (intramolecular) excitation operative in the isomerization in the liquid phase, the conformational change in the surface-bound TBA is driven by a substrate-mediated charge transfer process. We find that photoexcitation above a threshold h nu approximate to 2.2 eV leads to hole formation in the Au d-band followed by a hole transfer to the highest occupied molecular orbital of TBA. This transiently formed positive ion resonance subsequently results in a conformational change. The photon energy dependent photoisomerization cross section exhibit an unusual shape for a photochemical reaction of an adsorbate on a metal surface. It shows a thresholdlike behavior below h nu approximate to 2.2 eV and above h nu approximate to 4.4 eV. These thresholds correspond to the minimum energy required to create single or multiple hot holes in the Au d-bands, respectively. This study provides important new insights into the use of light to control the structure and function of molecular switches in direct contact with metal electrodes. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2997343]