Ultrafast Photochemical Dynamics in Complex Environments

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.