On the theory of excitonic delocalization for robust vibronic dynamics in LH2

TL;DR

This paper explores how excitonic delocalization in LH2 enhances vibronic interactions and reduces dephasing, providing a theoretical basis for long-lasting oscillations observed in photosynthetic complexes.

Contribution

It introduces a theoretical framework demonstrating the synergy between excitonic delocalization and vibronic coupling in LH2, explaining persistent quantum coherence.

Findings

Excitonic delocalization correlates energy fluctuations and vibronic coupling.

Reduced excitonic dephasing rates due to delocalization.

A plausible explanation for long-lasting oscillations in photosynthesis.

Abstract

Nonlinear spectroscopy has revealed long-lasting oscillations in the optical response of a variety of photosynthetic complexes. Different theoretical models which involve the coherent coupling of electronic (excitonic) or electronic-vibrational (vibronic) degrees of freedom have been put forward to explain these observations. The ensuing debate concerning the relevance of either one or the other mechanism may have obscured their potential synergy. To illustrate this synergy, we quantify how the excitonic delocalization in the LH2 unit of Rhodopseudomonas Acidophila purple bacterium, leads to correlations of excitonic energy fluctuations, relevant coherent vibronic coupling and, importantly, a decrease in the excitonic dephasing rates. Combining these effects, we identify a feasible origin for the long-lasting oscillations observed in fluorescent traces from time-delayed two-pulse single molecule experiments performed on this photosynthetic complex.