Intraband dynamics and exciton trapping in the LH2 complex of Rhodopseudomonas acidophila

Over the last several decades, the light-harvesting protein complexes of purple bacteria have been among the most popular model systems for energy transport in excitonic systems in the weak and intermediate intermolecular coupling regime. Despite this extensive body of scientific work, significant questions regarding the excitonic states and the photo-induced dynamics remain. Here, we address the low-temperature electronic structure and excitation dynamics in the light-harvesting complex 2 of Rhodopseudomonas acidophila by two-dimensional electronic spectroscopy. We find that, although at cryogenic temperature energy relaxation is very rapid, exciton mobility is limited over a significant range of excitation energies. This points to the presence of a sub-200 fs, spatially local energy-relaxation mechanism and suggests that local trapping might contribute substantially more in cryogenic experiments than under physiological conditions where the thermal energy is comparable to or larger than the static disorder.

Automated summary

The purple bacteria light-harvesting protein complexes have been popular models for energy transport, but questions remain regarding excitonic states and photo-induced dynamics. A study using two-dimensional electronic spectroscopy on Rhodopseudomonas acidophila's light-harvesting complex 2 found rapid energy relaxation at cryogenic temperatures, but limited exciton mobility at certain excitation energies, suggesting potential local trapping effects especially in cryogenic conditions.