Exploiting Collective Effects to Direct Light Absorption in Natural and Artificial Light-harvesters

Title of dissertation: EXPLOITING COLLECTIVE EFFECTS TO DIRECT LIGHT ABSORPTION IN NATURAL AND ARTIFICIAL LIGHT-HARVESTERS Chris Schroeder, Doctor of Philosophy, 2016 Dissertation directed by: Professor Luis A. Orozco Joint Quantum Institute, University of Maryland Department of Physics and National Institute of Standards and Technology Photosynthesis – the conversion of sunlight to chemical energy – is fundamental for supporting life on our planet. Despite its importance, the physical principles that underpin the primary steps of photosynthesis, from photon absorption to electronic charge separation, remain to be understood in full. Electronic coherence within tightly-packed light-harvesting (LH) units or within individual reaction centers (RCs) has been recognized as an important ingredient for a complete understanding of the excitation energy transfer (EET) dynamics. However, the electronic coherence across units – RC and LH or LH and LH – has been consistently neglected as it does not play a significant role during these relatively slow transfer processes. Here, we turn our attention to the absorption process, which, as we will show, has a much shorter built-in timescale. We demonstrate that the – often overlooked – spatially extended but short-lived excitonic delocalization plays a relevant role in general photosynthetic systems. Most strikingly, we find that absorption intensity is, quite generally, redistributed from LH units to the RC, increasing the number of excitations which can effect charge separation without further transfer steps. A biomemetic nano-system is proposed which is predicted to funnel excitation to the RC-analogue, and hence is the first step towards exploiting these new design principles for efficient artificial light-harvesting. Exploiting Collective Effects to Direct Light Absorption in Natural and Artificial Light-Harvesters