Multidimensional Spectroscopy Development for the Study of Energy Materials

自然界经过数百万年发展而成的高效能量收集机制可以作为一种灵感和模型，在此基础上可以创造出新的合成分子结构。新的合成分子结构将支持能够收集阳光并将其转化为其他有用形式的可用或储存能量的节能设备。这是21世纪成功经济体需要解决的关键问题。自然光合作用和人工创造的分子结构的光吸收和发射光谱是研究其运行机制的关键观测资料。这样的光谱包含了分子激发态动力学的信息，分子如何受到内部运动和与周围环境的相互作用的影响。因此，关于能量如何动态传播（时间尺度和途径）以及能量如何用于化学反应等功能的信息可以通过研究光学光的吸收和发射来获得。然而，聚集的分子结构具有复杂的耦合，呈现出宽而拥挤的线性吸收光谱，从中不可能恢复任何详细的动力学信息。为了揭示广泛而拥挤的吸收光谱背后的物理机制，我将采用创新的二维电子光谱方法。在本实验中，相干激发和发射频率在光谱可见区域的相关性被测量，揭示了电子跃迁之间的耦合，否则在线性吸收和荧光光谱中被掩盖。打个比方，我们可以把分子光谱想象成管弦乐队发出的声音的频率分布。这里的任务是通过分析整个声音频率分布来识别演奏给定交响乐的所有乐器。当然，实际情况要复杂得多，因为分子之间的相互作用（耦合）和周围环境导致了光谱的变化。在管弦乐队/交响乐的类比中，一个给定的乐器会产生不同的音调（频率），因为它离另一个乐器更近或更远，或者因为房间更热或更冷。我将开发的工具允许这样的反卷积。我的兴趣集中在研究偶联卟啉纳米结构，它在仿生捕光分子结构方面具有巨大的潜力。这些纳米结构的一个显著特征是，它们在光吸收时表现出超快的激发离域，类似于在自然发生的光收集结构中观察到的。然而，目前尚不清楚这种快速的离域发生的速度和机制。利用超快二维电子能谱，我计划在这个基于卟啉的纳米级分子结构的新范围内，了解这一点和其他特征，以及能量转移和电荷传输的途径和时间尺度，直到量子力学水平。

The highly efficient energy collection machinery Nature developed over millions of years can serve as an inspiration and model upon which newly synthesized molecular structures can be created. New synthetic molecular structures will underpin energy efficient devices capable of harvesting and converting sunlight into other useful forms of readily available or stored energy. This is a key problem to be addressed by successful economies in the 21st century. The optical absorption and emission spectra of natural photosynthetic and artificially created molecular structures are the key observables into their operating mechanisms. Such spectra contain information about the molecular excited state dynamics, how molecules are affected by internal motions and by interactions with their surroundings. Therefore, information about how energy dynamically propagates (timescales and pathways) and how it is employed for functions such as chemical reactions can be obtained by studying optical light absorption and emission. However, aggregated molecular structures have complicated couplings presenting broad and congested linear absorption spectra from which it is impossible to recover any detailed dynamical information. In order to unravel the physical mechanisms underlying broad and congested absorption spectra I will deploy innovative two-dimensional electronic spectroscopy methods. In this experiment, the correlation of the coherent excitation and emission frequencies in the visible region of the spectrum is measured revealing couplings between electronic transitions that are otherwise obscured in the linear absorption and fluorescence spectra. As an analogy, one could imagine that the molecular spectra are like the frequency distribution of the sound coming from an orchestra. The task here would be to identify all the instruments performing a given symphony through an analysis of the overall sound frequency distribution. Of course, the reality is much more complex because molecules interact between them (coupling) and their surroundings leading to changes of their spectra. In the orchestra/symphony analogy, a given instrument would produce a different tone (frequency) because it is closer or further away to another instrument or because the room is warmer or colder. The tools I will develop permit such a deconvolution.My interest is focused on studying pi-conjugated porphyrin nanorings that show great potential as biomimetic light-harvesting molecular structures. One remarkable feature (among many others) of these nanorings is that they exhibit ultrafast excitation delocalization upon light absorption, similar to what is observed in naturally occurring light harvesting structures. However, it is not yet known how fast and what mechanism enables this fast delocalization to occur. Working with ultrafast two-dimensional electronic spectroscopy I plan to understand this and other features as well as the pathways and timescales of energy transfer and charge transport down to the quantum mechanical level, in this new range of porphyrin based nanoscale molecular structures.