Probing Exciton Diffusion in Photosynthetic Complex Through Exciton-Exciton Annihilation

Photosynthesis, the cornerstone of plant bioenergy production, begins with the absorption of sunlight by antenna complexes and the subsequent transfer of excitons to the reaction center in Photosystem II (PSII). Efficient exciton transport is critical for optimizing light-harvesting and energy conversion, yet measuring this process is experimentally challenging due to the ultrafast (picosecond) timescales and structural complexity of photosynthetic systems. In this study, we investigated exciton diffusion dynamics in the C₂S₂-type PSII-LHCII supercomplex, a key light-harvesting structure in higher plants like spinach. Using transient absorption spectroscopy and exciton-exciton annihilation analysis, we determined an exciton diffusion length of 12 nm, indicating the ability of excitons to traverse the entire supercomplex. We further employed kinetic Monte Carlo simulations to model exciton trajectories, which revealed that excitons near the reaction center in subunits such as CP43 and CP47 transfer energy rapidly, within just a few picoseconds. In contrast, excitons in subunits like light-harvesting complex II (LHCII) exhibit longer dwelling times, which may be related to photoprotection mechanisms that prevent damage to the photosystem under intense sunlight. Our findings reveal how plants optimize light-harvesting and photoprotection mechanisms through precise control of exciton transport. This discovery deepens our understanding of the structural and functional relationships in photosynthetic supercomplexes, providing a framework for future studies on larger photosynthetic assemblies. Moreover, our research holds broad implications for improving natural photosynthesis, with potential applications in increasing crop productivity and advancing artificial photosynthetic systems for sustainable energy production.