Photo-induced charge and energy transport: from single molecules to disordered materials

My research interests center on the flow of energy and electric charge in, and between, molecules. Chemical energy and electric charge flow during fundamental chemical reaction events, such as when a molecule changes shape, or transfers an electron to one of its neighbors. These processes are especially important when a molecular system interacts with light; after the light energy is absorbed the system must then find some way to “relax”. Describing these sorts of processes from a theoretical perspective requires the development and application of simulation methods that lie at the interface of statistical mechanics and quantum dynamics. Combining these areas is essential to accurately capture these fundamental chemical steps in many forefront problems ranging from solar energy conversion and chemical catalysis to biological sensing and signaling. Molecular simulations of this type can help uncover the underlying mechanisms that lie at the heart of many energy conversion problems, and the ultimate goal of my research program is to extend and apply these approaches to study light-initiated charge and energy transport. In general, studying charge and energy transfer problems in molecular systems is an essential step to engineering new systems that can be manipulated in order to achieve desired outcomes. For example, our current understanding of how naturally occurring systems, such as plants and bacteria, harvest and harness energy from the sun lacks the detail necessary to provide a sufficient set of “design principles” for the development of improved synthetic light-harvesting systems that can perform similar functions. The challenge inherent in these problems is that the behavior of the quantum system is determined by interactions with its environment. These interactions can span many natural time and length scales, and the environment can be disordered. Bridging length and time-scales in molecular simulation is a long-standing holy grail' problem in the research community. Indeed, emerging computational infrastructures continue to push the boundaries in terms of the systems and processes that can be treated. However, these computational speed-ups are far out-paced by the inherent exponential scaling of the equations of quantum mechanics. For this reason, new dynamics algorithms are currently needed in order to access the relevant length and time-scales in, for example, the fundamental processes at play in a solar cell or a light-harvesting bacterium. These insights into how molecular systems and their environments can be tuned to alter their functionality will lead to a deeper understanding of how to leverage molecular-level design principles to aid in the development of improved solar energy conversion materials, molecular electronics, and catalytic systems. Hence, the impact of this work will have importance in many other areas of chemistry, in physics, and in materials science.