Coherent Chemistry: Ultrabroadband Two-dimensional Electronic Spectroscopy

分子系统中的光驱动反应是地球上生命存在和成功延续的核心。光合作用最终通过将太阳能转化为化学能来支持地球上的所有生命，而人工太阳能转换、光伏设备和光催化是取代依赖化石燃料的发电，从而改善全球变暖影响的基本技术。这导致了在理解和最终控制激发态反应方面的激烈研究活动。在这项工作中，我们解决了利用激光的独特性质，特别是相干来调节化学反应性的长期梦想。我们开发的工具将独立于这一总体目标，对激发态化学的性质产生迄今最清晰、最详细的见解。所有光化学机制和动力学研究的主要工具是“闪光光解”技术。现在称为瞬时吸收(TA)的方法，当与现代激光技术相结合时，能够达到低于10ps的时间分辨率，并可用于测量从紫外线到中红外甚至更远的瞬时光谱。现在TA有许多变体，但第一个真正新颖的扩展是在本世纪初随着二维电子光谱学(2DES)的发展而出现的。2DES的基本特征是它允许输入(激励)能量与输出(发射、产物吸收、受激发射)信号相关联。因此，在波数x处的激发导致在波数y处的输出将具有x：y交叉峰。此外，时间演化产生关于作为时间函数的初态和终态之间的耦合的性质的信息。在我们的例子中，交叉峰可能显示相干耦合，这反过来又暗示了控制的可能性。这种2D光谱在磁共振研究中非常常见，揭示了自旋-自旋耦合。然而，尽管这些研究揭示了非常详细的结构信息，但激发能太低，不能影响化学转化。化学的目的不仅是解释分子行为，也是为了改变它。电子激发中隐含的能量足以引发化学反应，通过将2DES应用到光驱动的反应中，我们将对激发态反应动力学的性质提供独特、新的和详细的见解。2DES的优势已经在电子能量转移的重要情况下得到证明，它提供了对光合作用中光收集等异常快速的能量转移的途径和机制的详细见解。将2DES扩展到化学变化的情况下的挑战是，激发和产物信号在能量上相距很远，需要非常大的相干带宽(数百太赫兹)来同时激发和探测反应系统。这就需要新的激光光源、新的测量方法和新的理论。在这个项目中，讨论了每个方面，总体目标是提供对一些最重要的模型反应的光化学动力学的最详细的洞察，例如对电池化学至关重要的电子和质子转移反应。这些测量将提供明确的答案，即相干性是否在光化学中起着可观察到的作用，因此可以被用来改变速率和机制。即使相干性最终被证明不是这些反应中的关键因素，我们也将获得对反应性的前所未有的洞察，时间分辨率仅为几飞秒，这是最快的核运动的时间尺度。

Light driven reactions in molecular systems are central to the existence of life on earth and to its successful continuation. Photosynthesis ultimately supports all life on the planet through the conversion of solar to chemical energy, while artificial solar energy conversion, photovoltaic devices and photocatalysis are fundamental technologies in replacing fossil fuel dependent power generation, and thus ameliorating the effects of global warming. This has led to intense research activity in understanding and ultimately controlling excited state reactions. In this work we address the long standing dream of tuning chemical reactivity using the unique properties of laser light, specifically coherence. The tools that we develop will, independently of this overarching objective, yield the clearest and most detailed insight yet into the nature of excited state chemistry.The workhorse for all investigations of photochemical mechanism and dynamics is the technique of 'flash photolysis'. Now called transient absorption (TA) the method is, when combined with modern laser technology, capable of sub 10 fs time resolution, and can be used to measure transient spectra from the UV to the mid IR, and beyond. There are now many variants of TA, but the first truly novel extension came about in the early years of this century with the development of two-dimensional electronic spectroscopy (2DES). The essential feature of 2DES is that it allows a correlation of the input (excitation) energy with the output (emission, product absorption, stimulated emission) signal. Thus an excitation at wavenumber x leading to an output at wavenumber y will have a x:y cross peak. Further the temporal evolution yields information on the nature of the coupling between the initial and final state as a function of time. In our case the cross peak may reveal coherent coupling, which in-turn suggests the possibility of control. Such 2D spectra are very familiar from magnetic resonance studies, revealing spin-spin coupling. However, while these studies reveal exquisitely detailed structural information, the excitation energies are too low to affect chemical transformations. The aim of chemistry is not only to interpret molecular behaviour, but also to change it. The energy implicit in electronic excitation is sufficient to initiate chemical reactions, and by applying 2DES to light driven reactions we will provide unique, new and detailed insights into the nature of excited state reactive dynamics.The advantages of 2DES have already been demonstrated for the important case of electronic energy transfer, where it has provided detailed insight into the pathway and mechanism of the exceptionally fast energy transfer underlying - for example - light harvesting in photosynthesis. The challenge in extending 2DES to the case of chemical change is that the excitation and product signals are energetically far apart, requiring an exceptionally large coherent bandwidth (several hundred THz) to simultaneously excite and probe the reactive system. This necessitates new laser sources, new measurement methodologies and new theory. In this project each aspect is addressed, with the overall objective being to provide the most detailed insight yet into the photochemical dynamics of some of the most important model reactions, such as the electron and proton transfer reactions central to the chemistry of the cell. These measurements will provide unambiguous answers as to whether or not coherence plays an observable role in photochemistry, and can therefore be exploited to modify rates and mechanisms. Even if coherence turns out not to be a key player in these reactions, we will have obtained unprecedented insights into reactivity, with time resolution of only a few fs, the timescale of the fastest nuclear motions.