Electron-vibrational coupling decreases trapping by low-energy states in photosynthesis

In photosynthetic light harvesting, states with energy well below that needed for charge separation can be found in abundance. They do not hinder the quantum efficiency of the primary processes; on the contrary, they can be highly functional, extending the absorption towards the red. Although many properties of these states are well described based on spectroscopic and theoretical studies, the physical mechanisms underlying their working are not known. Here we propose a mechanism which utilizes high-frequency vibrations of the photosynthetic pigments and the combined spatio-energetic aspect of the excitation dynamics. We present numerical calculations of the excitation dynamics in explicit electron-vibrational basis, with parameters based on photosynthetic complexes such as the Lhca4 complex of higher plants. The electron-vibrational states have two roles. For the trapped, low-energy excitation they provide a thermally populated ladder out of the trap. And for the high-energy excitation they provide local-bath states, effectively forming a bridge over the trap.