Mapping Pathways in Photo-Catalytic Cycles using Ultrafast Spectroscopy

催化剂广泛用于各种日常应用的化学反应，包括药品和聚合物等先进材料。它们提高了产品形成的速度，并且它们的使用可以避免苛刻的工艺条件，如高温。由可见光激活的光催化剂正引起人们的关注，因为像发光二极管（led）这样的廉价光源可以用来驱动有用的化学反应。人们对用更可持续的有机化合物取代含有过渡金属的光催化剂也越来越感兴趣。尽管近年来为进行特定化学转化而量身定制的光催化循环发展迅速，但对光催化剂的工作方式（其作用机制）和光催化剂的性质（应优化以提高效率）的理解却相对较少。拟议的研究将详细观察催化循环中涉及的活性物质及其寿命，在有利的情况下，将通过恢复催化剂的起始形式来观察整个催化循环从开始到结束的每一步。反应中间体的生产和去除的时间尺度很短，通常对应于飞秒到皮秒的间隔（不到十亿分之一秒）。在这项研究中使用的超快激光器能够产生足够短的紫外线和红外光脉冲，以拍摄这些瞬态物质浓度变化的快照。因此，一系列化学反应的各个步骤可以在一组测量中观察到。紫外光谱对激发态的活化中间体提供了特别的信息，而红外光谱提供了在任何特定时间存在的不同分子和自由基的具体信息。这些前所未有的研究将使用两个超快激光器，一个位于布里斯托尔大学，另一个位于卢瑟福阿普尔顿实验室（RAL）。布里斯托尔激光将作为主力系统，分析反应中间体并研究从开始到1.3纳秒的反应。然后将使用RAL的激光系统研究最有趣的系统，该系统具有在单组测量中观察11个数量级时间（从100飞秒到10毫秒）的反应的独特能力。凭借这种卓越的能力，我们将首次捕捉到光催化循环从头到尾的每一步。每个步骤发生的速率可以被解释，以确定光催化剂、反应底物和周围溶剂的哪些性质对决定反应的效率最重要。有了这种类型的新见解，我们将为重要的化学反应设计新的光催化循环，例如那些在碳原子之间形成新键的反应（有机分子的基本结构特征），并使用有机化学家采用的方法测试它们的性能。好处将是广泛的。有机化学家设计更有效的途径来选择目标分子，例如用于药物应用，将有一个扩展的反应调色板可供他们使用。这种更强的化学控制也将开辟可合成的新分子类别。化学和制药工业依靠化学合成来创造新产品，如药物或具有精确定制特定应用特性的先进材料。他们将利用所获得的知识来改进现有的工业流程，并将提高他们对如何通过廉价光源照明激活流动的化学样品来开发新流程的理解。

Catalysts are widely used in reactions which produce chemicals for a variety of everyday applications including pharmaceuticals and advanced materials such as polymers. They enhance the rates at which the products form, and their use can avoid harsh process conditions such as high temperatures. Photocatalysts that are activated by visible light are attracting attention because cheap light sources such as light emitting diodes (LEDs) can be used to drive useful chemical reactions. There is also growing interest in replacing photocatalysts containing transition metals with more sustainable organic compounds. Despite the recent and rapid development of photocatalytic cycles tailored to carry out specific chemical transformations, relatively little effort has been devoted to understanding the ways in which the photocatalysts work (their mechanisms of action) and the properties of the photocatalysts which should be optimized for greater efficiency. The proposed research will make detailed observations of the reactive species involved in catalytic cycles and their lifetimes, and in favourable cases will aim to observe every step in a full catalytic cycle from its initiation to its termination by recovery of the catalyst in its starting form.The timescales for production and removal of the reactive intermediates are short, typically corresponding to femtosecond to picosecond intervals (less than one billionth of a second). The ultrafast lasers to be used in this research are capable of generating pulses of ultraviolet and infrared light short enough to take snapshots of the changing concentrations of these transient species. Consequently, the individual steps in a sequence of chemical reactions can be observed in a single set of measurements. Ultraviolet spectra are particularly informative about activated intermediates in excited electronic states, whereas infrared spectra provide specific information about the different molecules and radicals present at any particular time.These unprecedented studies will use two ultrafast lasers, one located at the University of Bristol and the other at the Rutherford Appleton Laboratory (RAL). The Bristol laser will act as the workhorse system, profiling reaction intermediates and studying reactions up to times of 1.3 nanoseconds from initiation. The most interesting systems will then be studied using a laser system at RAL which has the unique capability to observe reactions over 11 orders of magnitude of time (from 100 femtoseconds to 10 milliseconds) in single sets of measurements. With this remarkable capability, we will capture every step in a photocatalytic cycle from start to finish for the first time. The rates at which each step occurs can then be interpreted to determine which properties of the photocatalyst, reactive substrate and surrounding solvent are most important for determining the efficiency of the reaction. Armed with new insights of this type, we will design novel photocatalytic cycles for important chemical reactions, such as those that form new bonds between carbon atoms (an essential structural feature of organic molecules), and test their performance using the methods adopted by organic chemists. The benefits will be widespread. Organic chemists designing more efficient pathways to chosen target molecules, for example for medicinal applications, will have an extended palette of reactions at their disposal. This greater chemical control will also open up new classes of molecule that can be synthesized. The chemical and pharmaceutical industries rely on chemical synthesis to create new products such as drugs or advanced materials with properties tailored precisely to specific applications. They will draw upon the knowledge gained to refine existing industrial processes, and will also improve their understanding of how to develop new processes by activation of flowing samples of chemicals by illumination with cheap light sources.