Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States.
J Am Chem Soc. 2011 Oct 5;133(39):15384-96. doi: 10.1021/ja200652r. Epub 2011 Sep 13.
Transient anisotropy measurements are reported as a new spectroscopic tool for mechanistic characterization of photoinduced charge-transfer and energy-transfer self-exchange reactions at molecule-semiconductor interfaces. An anisotropic molecular subpopulation was generated by photoselection of randomly oriented Ru(II)-polypyridyl compounds, anchored to mesoscopic nanocrystalline TiO(2) or ZrO(2) thin films, with linearly polarized light. Subsequent characterization of the photoinduced dichromism change by visible absorption and photoluminescence spectroscopies on the nano- to millisecond time scale enabled the direct comparison of competitive processes: excited-state decay vs self-exchange energy transfer, or interfacial charge recombination vs self-exchange hole transfer. Self-exchange energy transfer was found to be many orders-of-magnitude faster than hole transfer at the sensitized TiO(2) interfaces; for Ru(dtb)(2)(dcb)(2), where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine and dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, anchored to TiO(2), the energy-transfer correlation time was θ(ent) = 3.3 μs while the average hole-transfer correlation time was <θ(h+)> = 110 ms, under identical experimental conditions. Monte Carlo simulations successfully modeled the anisotropy decays associated with lateral energy transfer. These mesoscopic, nanocrystalline TiO(2) thin films developed for regenerative solar cells thus function as active "antennae", harvesting sunlight and transferring energy across their surface. For the dicationic sensitizer, Ru(dtb)(2)(dcb)(2), anisotropy changes indicative of self-exchange hole transfer were observed only when ions were present in the acetonitrile solution. In contrast, evidence for lateral hole transfer was observed in neat acetonitrile for a neutral sensitizer, cis-Ru(dnb)(dcb)(NCS)(2), where dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine, anchored to TiO(2). The results indicate that mechanistic models of interfacial charge recombination between electrons in TiO(2) and oxidized sensitizers must take into account diffusion of the injected electron and the oxidized sensitizer. The phenomena presented herein represent two means of concentrating potential energy derived from visible light that could be used to funnel energy to molecular catalysts for multiple-charge-transfer reactions, such as the generation of solar fuels.
瞬态各向异性测量被报道为一种新的光谱工具，用于在分子-半导体界面处对光诱导电荷转移和能量转移自交换反应进行机理表征。通过用光选择性地照射随机取向的 Ru(II)-多吡啶化合物，可以在纳米晶 TiO(2)或 ZrO(2)薄膜上产生各向异性的分子亚群，这些化合物通过线性偏振光固定在薄膜上。随后，通过在纳秒至毫秒时间尺度上的可见吸收和光致发光光谱对光致双折射变化进行后续表征，从而可以直接比较竞争过程：激发态衰减与自交换能量转移，或界面电荷复合与自交换空穴转移。结果发现，在敏化的 TiO(2)界面上，自交换能量转移比空穴转移快几个数量级；对于Ru(dtb)(2)(dcb)(2)，其中 dtb 是 4,4'-(C(CH(3))(3))(2)-2,2'-联吡啶，dcb 是 4,4'-(COOH)(2)-2,2'-联吡啶，锚定在 TiO(2)上，能量转移相关时间为θ(ent)=3.3 μs，而平均空穴转移相关时间为<θ(h+)>=110 ms，在相同的实验条件下。蒙特卡罗模拟成功地模拟了与横向能量转移相关的各向异性衰减。这些为再生太阳能电池开发的介观、纳米晶 TiO(2)薄膜因此可以作为活性“天线”，在其表面收集阳光并传递能量。对于二价阳离子敏化剂Ru(dtb)(2)(dcb)(2)，只有在乙腈溶液中存在离子时，才观察到各向异性变化，表明发生了自交换空穴转移。相比之下，对于中性敏化剂 cis-Ru(dnb)(dcb)(NCS)(2)，在没有离子的情况下，在纯乙腈中观察到了侧向空穴转移的证据，其中 dnb 是 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-联吡啶，锚定在 TiO(2)上。结果表明，界面电子与氧化敏化剂之间电荷复合的机理模型必须考虑到注入电子和氧化敏化剂的扩散。本文所介绍的现象代表了两种集中可见光衍生势能的方法，可以将能量集中到用于多电荷转移反应的分子催化剂上，例如太阳能燃料的生成。
