Mapping Pathways in Photo-Catalytic Cycles using Ultrafast Spectroscopy

催化剂广泛用于生产各种日常应用化学品的反应中，包括药物和聚合物等先进材料。它们提高了产品形成的速率，并且它们的使用可以避免苛刻的工艺条件，如高温。被可见光激活的光催化剂正引起人们的关注，因为发光二极管（LED）等廉价光源可用于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。尽管最近快速发展的光催化循环量身定制，进行特定的化学转化，相对较少的努力一直致力于了解的方式，其中的光催化剂的工作（其作用机制）和性能的光催化剂，应优化更高的效率。拟议的研究将对催化循环中涉及的活性物质及其寿命进行详细观察，在有利的情况下，将通过回收初始形式的催化剂来观察整个催化循环中从启动到终止的每一个步骤。通常对应于飞秒到皮秒的间隔（小于十亿分之一秒）。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，化学反应序列中的各个步骤可以在一组测量中观察到。紫外光谱特别能提供有关激发电子态的活性中间体的信息，而红外光谱则提供有关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台超快激光器，一台位于布里斯托大学，另一台位于卢瑟福阿普尔顿实验室（RAL）。布里斯托激光器将作为主力系统，分析反应中间体，并研究从开始到1.3纳秒的反应。最有趣的系统将使用RAL的激光系统进行研究，该系统具有独特的能力，可以在单组测量中观察11个数量级的时间（从100飞秒到10毫秒）。凭借这一卓越的能力，我们将首次捕获光催化循环从开始到结束的每一步。然后可以解释每个步骤发生的速率，以确定光催化剂、反应性基底和周围溶剂的哪些性质对于确定反应效率是最重要的。有了这种类型的新见解，我们将为重要的化学反应设计新型光催化循环，例如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其性能。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，例如用于医学应用，将有一个扩展的调色板反应在他们的处置。这种更大的化学控制也将开辟新的可以合成的分子类别。化学和制药行业依靠化学合成来制造新产品，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过程，并将提高他们对如何通过用廉价光源照明来激活流动的化学品样品来开发新过程的理解。

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.