Structure and Efficiency in Bacterial Photosynthetic Light Harvesting

Abstract

Photosynthetic organisms use networks of chromophores to absorb and deliver solar energy to reaction centres. We present a detailed model of the light-harvesting complexes in purple bacteria, including explicit interaction with sunlight; radiative and non-radiative energy loss; and dephasing and thermalizing effects of coupling to a vibrational bath. We capture the effect of slow vibrations by introducing time-dependent disorder. Our model describes the experimentally observed high efficiency of light harvesting, despite the absence of long-range quantum coherence. The one-exciton part of the quantum state fluctuates continuously, but remains highly mixed at all times.These results suggest a relatively minor role for structure in determining efficiency. We build hypothetical models with randomly arranged chromophores, but still observe high efficiency when nearest-neighbour distances are comparable to those in nature. This helps explain the high transport efficiency in organisms with widely differing antenna structures, and suggests new design criteria for artificial light-harvesting devices.

Efficient frequency-selective single-photon antennas based on a bio-inspired nano-scale atomic ring design with 9-fold symmetry

“…Here we show that in the weak driving regime such a ring structure can also behave like a parabolic mirror [57,58] concentrating the light at its focus and thus strongly enhancing the coupling of an incoming plane wave photon to the additional central emitter, where it can be absorbed, creating an effective absorption cross section way beyond that for a single free space atom. Surprisingly we find that a nano-ring of N = 9 dipoles, as it appears in many common biological light harvesting complexes, called LHC2 [59][60][61][62][63], exhibits superior performance compared to other antenna atom numbers.…”

“…In contrast to pure scattering or light extinction often used (e.g. [59,69]), our definition of the absorption efficiency σ abs accounts for both, the probability of scattering a photon by the system and its subsequent transfer to the auxiliary impurity state. Note that this cross section still can exceed the resonant single emitter scattering cross-section σ = 6π/k 2 0 .…”

“…Hence the system can be regarded as a minimalist simulator of natural light-harvesting complexes present e.g. in purple bacteria that share a similar structure and optical properties [59,63,64].…”

Reliable transition properties from excited-state mean-field calculations

“…An important application of this second property is in describing transport of excitons through a network of chromophores, as is seen in the early stages of photosynthesis, as well as synthetic analogues, such as organic polymer light-emitting diodes 1 and chromophores hosted on DNA scaffolds. [2][3][4] These systems are often simulated using a Frenkel exciton Hamiltonian, [5][6][7][8]…”

Do photosynthetic complexes use quantum coherence to increase their efficiency? Probably not

“…Many theoretical (and experimental) works have addressed these questions, yet the question in the title remains largely unanswered. One reason is that while experiments are performed in vitro with coherent (pulsed) light, natural systems operate under very different conditions, namely, continuous incoherent excitation (18,(20)(21)(22)(23)(24), and observing coherence under natural conditions is a very challenging task. That and more, it is hard to make the connection between the observed experimental findings and the energy transfer efficiency, which is related to the total rate at which energy can flow from the antenna to the reaction center (two ingredients that are essentially absent in the experiments).…”