Theoretical characterization of excitation energy transfer in chlorosome light-harvesting antennae from green sulfur bacteria

TL;DR

This paper presents a theoretical model of excitation energy transfer in chlorosomes, revealing rapid initial coherence followed by slower incoherent transfer, aligning with experimental data.

Contribution

It introduces a combined stochastic, molecular dynamics, and electronic structure approach to model EET in chlorosomes, providing detailed insights into energy transfer mechanisms.

Findings

Coherent transfer lasts about 50 fs post-excitation.

Incoherent transfer occurs over 1 ps to tens of ps.

Simulation results match experimental time-resolved spectroscopy data.

Abstract

Chlorosomes are the largest and most efficient natural light-harvesting antenna systems. They contain thousands of pigment molecules - bacteriochlorophylls (BChls)- that are organized into supramolecular aggregates and form a very efficient network for excitonic energy migration. Here, we present a theoretical study of excitation energy transfer (EET) in the chlorosome based on experimental evidence of the molecular assembly. Our model for the exciton dynamics throughout the antenna combines a stochastic time propagation of the excitonic wave function with molecular dynamics simulations of supramolecular structure, and electronic structure calculations of the excited states. The simulation results reveal a detailed picture of the EET in the chlorosome. Coherent energy transfer is significant only for the first 50 fs after the initial excitation, and the wavelike motion of the exciton is completely damped at 100 fs. Characteristic time constants of incoherent energy transfer, subsequently, vary from 1 ps to several tens of ps. We assign the time scales of the EET to specific physical processes by comparing our results with the data obtained from time-resolved spectroscopy experiments.