Nature has achieved a remarkable quantum efficiencies in light-harvesting and energy transfer within the photosynthetic apparatus. Light-Harvesting Complex II (LHC II, figure 1) is one of the most abundant proteins on earth accounting for harvesting globally more than 50% of the light used for photosynthesis. One key of its function is an optimization over millions of years of relative distances and orientations of its pigments for very fast and effective inter-pigment energy transfer. In human made pigments aggregate distances are often either too large to enable efficient energy transfer or too small leading to losses in excitation energy due to processes such as concentration quenching.
Typically, energy transfer between pigments can be calculated accurately using their transition dipole moment. However, the optimized distances observed in light-harvesting complexes such as LHC II are so small that local interactions of the electronic wave functions become important. Therefore, an accurate calculation requires the consideration of transition densities ([1] and Krueger et.al. cited therein [2]). Figure 2 shows the calculation of the electronic interpigment couplings based on the transition density as well as dipole moment approximation using high resolution crystal structures that become recently increasingly available even for larger parts of the photosynthetic apparatus. The comparison demonstrates that below distances of ~2 nm calculations of couplings considering the entire transition densities is increasingly differing from the results observed using the dipole moment approximation. These are typical inter-pigment as observed in light harvesting complexes. We will use this theoretical frame work as well as our results observed from photosynthetic light-harvesting complexes to design artificial systems for harvesting diffusively scattered solar light that mimic natural systems in a technical efficient and robust way.
[1] A. Pieper, M. Hohgardt, M. Willich, D. A. Gacek, N. Hafi, D. Pfennig, A. Albrecht, P. J. Walla, "Biomimetic light-harvesting funnels for re-directioning of diffuse light", Nat. Commun., 9, 666 (2018).
[2] Jan S. Frähmcke and Peter J. Walla,"Coulombic couplings between pigments in the major light-harvesting complex LHC II calculated by the transition density cube method", Chemical Physics Letters 430, Issues 4-6, 397-403 2006).
[3] B.P. Krueger, G.D. Scholes, R. Jimenez and G.R. Fleming,"Electronic Excitation Transfer from Carotenoid to Bacteriochlorophyll in the Purple Bacterium Rhodopseudomonas acidophila", J. Phys. Chem. B 102, 12, 2284–2292 1998).