Interplay between Structural Hierarchy and Exciton Diffusion in Artificial Light Harvesting

Abstract: Unravelling the nature of energy transport in multi-chromophoric photosynthetic complexes is essential to extract valuable design blueprints for light-harvesting applications. Long-range exciton transport in such systems is facilitated by a combination of delocalized excitation wavefunctions (excitons) and remarkable exciton diffusivities. The unambiguous identification of the exciton transport, however, is intrinsically challenging due to the system's sheer complexity. Here we address this challenge by employing a novel spectroscopic lab-on-a-chip approach: A combination of ultrafast coherent two-dimensional spectroscopy and microfluidics working in tandem with theoretical modelling. This allowed us to unveil exciton transport throughout the entire hierarchical supramolecular structure of a double-walled artificial light-harvesting complex. We show that at low exciton densities, the outer layer acts as an antenna that supplies excitons to the inner tube, while under high excitation fluences it protects the inner tube from overburning. Our findings shed light on the excitonic trajectories across different sub-units of a multi-layered supramolecular structure and underpin the great potential of artificial light-harvesting complexes for directional excitation energy transport.