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

## TL;DR

This study uses ultrafast spectroscopy to show that energy transport in a photosynthetic complex behaves differently at very low versus normal temperatures.

## Contribution

The paper reveals that cryogenic conditions can obscure biological relevance in energy transport studies of LH2 complexes.

## Key findings

- Exciton trapping dominates at 80 K but becomes negligible above 150 K.
- Low-temperature experiments may not reflect in vivo biological function.
- Multiexciton experiments require careful modeling for accurate interpretation.

## Abstract

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.