Exciton transport in molecular aggregates imaged by ultrafast microscopy
Libai Huang, Purdue University
Long-range exciton transport is a key challenge in achieving efficient solar energy harvesting in both organic solar cells and photosynthetic systems. Exciton transport is not well understood in the intermediate regime where many molecular aggregates and photosynthetic antennas belong. Excitons are delocalized over part of the system and the exciton delocalization (coherent) length is defined by the competition between intermolecular coupling strength and disorder and transport has mixed coherent and incoherent characteristics. Within each delocalized segment the excitation energy propagates ballistically. However, there currently lacks an experimental tool to directly characterize exciton transport in space and in time to elucidate mechanisms. Here we report a direct visualization of exciton diffusion in tubular molecular aggregates by transient absorption microscopy with ~ 200 fs time resolution and ~ 50 nm spatial precision. These measurements provided exciton diffusion constants of 3−6 cm2−1 for these aggregates, which were 3−5 times higher than a theoretical lower bound for pure incoherent hopping. Simulation showed that the states crucial for exciton transport are delocalized over < 10 molecules, suggesting that coherent effects play a role despite relatively weak delocalization.