Dissipation Pathways in a Photosynthetic Complex

Determining how energy
flows within and between molecules is crucial
for understanding chemical reactions, material properties, and even
vital processes such as photosynthesis. While the general principles
of energy transfer are well established, elucidating the specific
molecular pathways by which energy is funneled remains challenging,
as it requires tracking energy flow in complex molecular environments.
Here, we demonstrate how photon excitation energy is partially dissipated
in the light-harvesting Fenna–Matthews–Olson (FMO) complex,
mediating the excitation energy transfer from light-harvesting chlorosomes
to the photosynthetic reaction center in green sulfur bacteria. Specifically,
we isolate the contribution of the protein and specific vibrational
modes of the pigment molecules to the energy dynamics. For this, we
introduce an efficient computational implementation of a recently
proposed theory of dissipation pathways for open quantum systems,
based on second-order perturbation theory in the electronic couplings.
Using it and a state-of-the-art FMO model with highly structured and
chromophore-specific spectral densities, we demonstrate that energy
dissipation is dominated by low-frequency modes (<800 cm–1) as their energy range is near-resonance with the energy gaps between
electronic states of the pigments. We identify the most important
modes for dissipation to be in-plane breathing modes (∼200
cm–1) of the bacteriochlorophylls in the complex.
Conversely, far-detuned intramolecular vibrations with higher frequencies
(>800 cm–1) play no role in dissipation. Interestingly,
the FMO complex first needs to borrow energy from the environment
to release excess photonic energy, indicating that the energy exchange
between the system and thermal environment is not strictly unidirectional
in time but involves a transient thermally activated step. Beyond
their fundamental value, these insights can guide the development
of artificial light-harvesting devices and, more broadly, engineer
environments for chemical and quantum control tasks.