Ultrafast Photochemical Dynamics in Complex Environments

阳光为地球上的许多重要过程提供动力，例如植物的生长和大气中污染物的净化。紫外线(UV)和可见光波长的太阳光被分子吸收，分子利用获得的能量驱动化学反应，这个过程更广为人知的是光化学。这种对光的吸收改变了电子在分子中的分布方式，进而影响连接原子和决定分子结构的化学键。因此，光化学是启动结构和化学变化的有效方法，并且有可能比其他激活反应的方法更可持续，无论是通过加热还是通过使用含有稀有和昂贵元素的催化剂。大自然在许多方面利用了光化学的好处，包括视觉、光合作用和光形态发生(植物生长对光的反应)。人类技术越来越多地利用太阳能，例如在太阳能电池中发电，或者将水分解成氧气和氢作为化石燃料的替代品。在光化学反应中，结构变化以非常快的时间尺度发生。最初的电子重组发生在不到1000万亿分之一秒的时间内，也就是众所周知的飞秒。这个时间尺度比我们日常经验中的任何时间都短得多：一秒内的飞秒与3000万年中的几秒一样多。当电子改变其在分子中的排列时，一些化学键减弱或断裂，分子开始改变形状。这些与组成原子运动相对应的结构变化被称为核动力学(因为原子核运动)，并且由于原子核的质量要大得多，所以比电子的运动慢得多。然而，这些结构变化可能发生在数十或数百飞秒的时间尺度上--即所谓的“超快”时间尺度。现代实验技术使用产生几十飞秒长的光脉冲的激光，使我们能够在发生时观察这些核动力学，从而提供关于分子吸收光时如何反应的非凡洞察力。对分子复杂动力学的精确计算机模拟现在也变得可行，但由于电子和原子核在运动时的量子力学行为，使得模拟变得困难。在这个项目中，我们将结合尖端的实验和计算研究方法来揭示分子如何经历被光吸收激活的化学变化。我们将应用的一系列补充方法提供了前所未有的洞察力。发生的变化很大程度上受到分子周围环境的影响，例如生物系统中的液体溶剂(例如水)或蛋白质。这种环境可以限制分子的运动，并可以耗尽吸收的光提供的能量，将其作为热量耗散。我们将探索一系列不同的环境如何影响不同类型分子的光化学路径，并测量注入的能量流出环境的速度。我们将利用这一新知识来解决两个更重要的主要问题。第一个问题是一种名为UVR8的蛋白质如何调节植物对阳光的反应方式，例如通过幼苗生长或开花。第二部分阐述了含有有机分子的气溶胶颗粒在地球大气中的生长方式，从而导致云的形成(进而影响地球气候)、城市空气质量下降，以及吸入颗粒对人类健康的有害影响。许多其他领域的研究将进一步受益，包括太阳能转换和精细化学品、制药、农用化学品和聚合物的可持续合成。

Sunlight powers many vital processes on Earth such as the growth of plants and the cleansing of pollutants from the atmosphere. Ultraviolet (UV) and visible wavelengths of the sunlight are absorbed by molecules which use the energy gained to drive chemical reactions, a process known more generally as photochemistry. This absorption of light changes the way the electrons are distributed within molecules, in turn affecting the chemical bonds which connect the atoms and determine the structure of the molecule. Photochemistry is therefore an effective way to initiate structural and chemical change and has the potential to be more sustainable than alternative ways to activate reactions, either by heating or by using a catalyst containing scarce and expensive elements. Nature has harnessed the benefits of photochemistry in many ways, including vision, photosynthesis and photomorphogenesis (the response of plant growth to light). Human technology is increasingly exploiting the energy of sunlight, for example to generate electricity in solar cells or to split water into oxygen and hydrogen for use as alternatives to fossil fuels.In a photochemical reaction, structural changes occur on very fast timescales. The initial electronic reorganization occurs in less than a thousand trillionth of a second, known as a femtosecond. This timescale is far shorter than anything in our everyday experiences: there are as many femtoseconds in a second as there are seconds in 30 million years. As the electrons change their arrangements in a molecule, some of the chemical bonds weaken or break and the molecule starts to change shape. These structural changes corresponding to movement of the constituent atoms are known as the nuclear dynamics (because the atomic nuclei move) and are slower than the motions of the electrons because of the much larger masses of the nuclei. Nevertheless, these structural changes can take place on timescales of tens or hundreds of femtoseconds - the so-called "ultrafast" timescale. Modern experimental techniques using lasers that generate pulses of light a few tens of femtoseconds long allow us to observe these nuclear dynamics as they happen, thereby providing extraordinary insights about how molecules respond when they absorb light. Accurate computer simulations of the complex dynamics of the molecules are now also becoming feasible but are made difficult by the quantum mechanical behaviour of the electrons and the nuclei as they move. In this programme, we will combine cutting-edge experimental and computational research methods to unravel how molecules undergo chemical changes activated by absorption of light. The array of complementary methods we will apply offers unprecedented insights. The changes that occur are heavily influenced by the environment surrounding a molecule, such as a liquid solvent (e.g. water) or a protein in a biological system. This environment can restrict the motions of the molecule and can drain away the energy provided by the absorbed light, dissipating it as heat. We will explore how a range of different environments influence photochemical pathways for different types of molecules, and we will measure how quickly the injected energy flows out to the surroundings. We will use this new knowledge to tackle two major questions of wider importance. The first concerns how a protein called UVR8 regulates the way that plants respond to sunlight, for example by seedling growth or flowering. The second addresses the way that aerosol particles containing organic molecules grow in the Earth's atmosphere, with consequences for the formation of clouds (in turn affecting the Earth's climate), reduction in air quality in cities, and deleterious effects on human health by particle inhalation. There will be further benefits to many other fields of research including solar energy conversion and sustainable synthesis of fine chemicals, pharmaceuticals, agrochemicals, and polymers.