Heterogeneity-Resolved Ultrafast Transient Absorption Spectroscopy of Single Supramolecular Light-Harvesting Antennas

Abstract

Photosynthetic light harvesting proceeds rapidly and robustly through precisely arranged pigment molecules. However, the molecular arrangements are not uniform because they exhibit structural heterogeneity among individual complexes and across spatial regions, and undergo dynamic fluctuations. Such static and dynamic disorder can substantially perturb excitation dynamics. Here, we report a highly sensitive transient absorption microscope that integrates single-objective absorption microscopy, balanced detection, and lock-in amplification, enabling quantitative analysis of heterogeneity and temporal fluctuations in excitation dynamics. By analyzing individual chlorophyll-derivative aggregates mimicking photosynthetic light-harvesting antennas, we demonstrated that two kinetic components with nearly identical time constants can be resolved based on differences in their time-constant distributions. We further quantified the photophysical properties of each component, including absorbance change, fluorescence intensity, fluorescence efficiency, and fluorescence peak intensity ratio. These results establish an analytical framework for excitation dynamics that leverages not only the mean values of time constants but also their distribution profiles.