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

TL;DR

This study demonstrates that vibronic coupling, involving carotenoid vibrations, explains the ultrafast energy transfer from carotenoids to bacteriochlorophyll in natural and artificial light-harvesting systems, surpassing traditional models.

Contribution

The paper introduces a vibronic model that accurately reproduces ultrafast energy transfer times, highlighting the role of vibrational ground states in the process.

Findings

Vibronic model matches experimental transfer times and spectra.

Vibrational ground states facilitate resonance for rapid energy transfer.

F"orster models overestimate coupling constants and fail to match all data.

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 F\"orster-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.