The Physical Basis for Long-lived Electronic Coherence in Photosynthetic Light Harvesting Systems

TL;DR

This paper identifies the physical factors enabling long-lived electronic coherence in photosynthetic systems, using an analytical model that matches experimental decoherence times and explains the underlying mechanisms.

Contribution

It introduces an analytically solvable model revealing key physical features responsible for long coherence lifetimes in photosynthesis.

Findings

Decoherence times of ~160 fs at 77 K and ~80 fs at 277 K for FMO

Oscillations persist for 600 fs at 77 K and 300 fs at 277 K

Good agreement with experimental data for PC645 at room temperature

Abstract

The physical basis for observed long-lived electronic coherence in photosynthetic light-harvesting systems is identified using an analytically soluble model. Three physical features are found to be responsible for their long coherence lifetimes: i) the small energy gap between excitonic states, ii) the small ratio of the energy gap to the coupling between excitonic states, and iii) the fact that the molecular characteristics place the system in an effective low temperature regime, even at ambient conditions. Using this approach, we obtain decoherence times for a dimer model with FMO parameters of $\approx$ 160 fs at 77 K and $\approx$ 80 fs at 277 K. As such, significant oscillations are found to persist for 600 fs and 300 fs, respectively, in accord with the experiment and with previous computations. Similar good agreement is found for PC645 at room temperature, with oscillations persisting for 400 fs. The analytic expressions obtained provide direct insight into the parameter dependence of the decoherence time scales.