Ultrafast Spectroscopy Reveals Significant Differences in LH2 Exciton Mobility at Cryogenic and Ambient Temperatures

Spectroscopic studies of energy transport through the photosynthetic apparatus have been crucial to expanding our understanding of biological energy conversion. Correlating spectroscopic information to the electronic structure and function in these complex systems remains highly challenging, however. While cryogenic experimental conditions help in improving the effective spectral resolution and sample stability, the observed fine-grained dynamics do not necessarily reflect in vivo functionality. To address this issue, we target the temperature dependence of energy migration in light-harvesting complex 2 of purple bacteria. Temperature- and polarization-controlled two-dimensional electronic spectroscopy reveal rapid exciton immobilization at low temperatures, while intensity-dependent experiments allow identification of transport barriers. We find that exciton trapping, dominating the dynamics at 80 K, becomes negligible above 150 K, implying that observations at cryogenic temperatures do not always directly reflect biological function. We additionally find that considerable care and explicit modeling may be necessary for correct interpretation of multiexciton experiments.