Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters

Abstract

The initial energy transfer in photosynthesis occurs between the light-harvesting pigments and on ultrafast timescales. We analyze the carotenoid to bacteriochlorophyll energy transfer in LH2 Marichromatium purpuratum as well as in an artificial light-harvesting dyad system by using transient grating and two-dimensional electronic spectroscopy with 10 fs time resolution. We find that Förster-type models reproduce the experimentally observed 60 fs transfer times, but overestimate coupling constants, which leads to a disagreement with both linear absorption and electronic 2D-spectra. We show that a vibronic model, which treats carotenoid vibrations on both electronic ground and excited state as part of the system's Hamiltonian, reproduces all measured quantities. Importantly, the vibronic model presented here can explain the fast energy transfer rates with only moderate coupling constants, which are in agreement with structure based calculations. Counterintuitively, the vibrational levels on the carotenoid electronic ground state play a central role in the excited state population transfer to bacteriochlorophyll as the resonance between the donor-acceptor energy gap and vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems.

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Cited by in corpus (7)

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• Local protein solvation drives direct down-conversion in phycobiliprotein PC645 via incoherent vibronic transport
• Hidden Vibronic and Excitonic Structure and Vibronic Coherence Transfer in the Bacterial Reaction Center
• Ultrafast Energy Transfer with Competing Channels: Non-equilibrium Foerster and Modified Redfield Theories
• Simulations of coherent nonlinear optical response of molecular vibronic dimers
• Simulation of ab initio optical absorption spectrum of β-carotene with fully resolved S_{0} and S_{2} vibrational normal modes
• Magnetic pulses enable multidimensional optical spectroscopy of dark states