A Theoretical Investigation Into Energy Transfer In Photosynthetic Open Quantum Systems

TL;DR

This paper investigates energy transfer in photosynthetic systems, comparing exact and approximate methods, and finds that incoherent models can reliably predict transfer dynamics, highlighting robustness to environmental changes.

Contribution

It demonstrates that the inexpensive Foerster theory accurately predicts energy transfer rates despite neglecting quantum coherence, and explores environmental effects on transfer robustness.

Findings

Foerster theory predicts transfer rates well despite ignoring coherence.

Energy transfer is robust to environmental disorder.

Exact and approximate methods agree on key transfer dynamics.

Abstract

This thesis looks at the electronic energy transfer in the Fenna-Matthews-Olson complex, in which evidence of long-lived coherence has been observed in 2-dimensional infrared experiments. I use three techniques: the numerically exact Hierarchical Equations of Motion, and the perturbative Redfield and Foerster theories, the latter of which ignores quantum coherence in the transfer. Both of the approximate methods perform very well - and while oscillations in site populations (a hallmark of coherence) are present in the exact transfer dynamics and absent in the dynamics of Foerster theory, the latter gives a reasonable prediction of transfer rates and steady-state populations, despite being incoherent - suggesting that coherence is not vital for the dynamics of transfer. Since Foerster theory is very inexpensive to run and performs so well, I then apply it to calculate the effects of static disorder in bacteriochlorophyll site energies and of a more structured spectral density. Ultimately, the energy transfer in the complex is found to be very robust to changes in its environment, which is advantageous for its biological function.