The complex architecture and remarkable energy efficiency of light-harvesting complexes in photosynthetic bacteria or plants have intrigued scientists for decades. Besides being one of life's most fundamental processes, light harvesting has been investigated with a goal to design improved photovoltaics during our current energy crisis. A crucial question has concerned the role of quantum effects in photosynthetic energy transfer. However, the large size e.g., of the LH2 complex in Rsp. molischianum bacteria comprising 24 coupled bacteriocholorophyll (BChl) molecules and thousands of vibrations (of pigments and the surrounding protein), had prevented exact simulations due to the exponential scaling of quantum mechanics, and the intermediate energy scales in these systems had rendered perturbative methods inaccurate.

Using recent developments in real-time path integral methods, we performed the first, numerically exact simulations of nonadiabatic energy transfer through the entire LH2 complex of 24 states and 1200 spectroscopic normal modes (50 per BChl), at room temperature. Our simulations yield a microscopic view of how the energy absorbed through a Franck-Condon excitation of the outer (B800) ring, transfers to the inner (B850) ring over 1 ps, in excellent agreement with previous experiments. We use an excitonic initial condition (optically excited state in the B00 band) and find a two-timescale nature of energy relaxation emerging from exciton-vibration mixing. Interestingly, the weak inter-ring exciton couplings lead to efficient pathways through a manifold of vibrationally dressed states. Using a local initial excitation of a B800 molecule, we further illustrate how the architecture of the two-ring arrangement and the fluctuations of nuclear vibrational motion together enable the efficiency of nature's energy trapping process.

Efficient inter-ring energy transfer in the bacterial LH2 complex.

Nancy Makri

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