title: Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer in natural and artificial light harvesters.
abstract: The initial energy transfer steps in photosynthesis occur 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 lead 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 states 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 the central role in the excited state population transfer to bacteriochlorophyll; resonance between the donor-acceptor energy gap and the vibrational ground state energies is the physical basis of the ultrafast energy transfer rates in these systems.
authors.affiliation: Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 121 16, Czech Republic.
authors.forename: Václav
authors.initials: V
authors.lastname: Perlík
authors.affiliation: Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 121 16, Czech Republic.
authors.forename: Joachim
authors.initials: J
authors.lastname: Seibt
authors.affiliation: Institute of Molecular Cell and System Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow Biomedical Research Centre, 120 University Place, Glasgow G12 8TA, Scotland.
authors.forename: Laura J
authors.initials: LJ
authors.lastname: Cranston
authors.affiliation: Institute of Molecular Cell and System Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow Biomedical Research Centre, 120 University Place, Glasgow G12 8TA, Scotland.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Cogdell
authors.affiliation: Photonics Institute, Vienna University of Technology, Gusshausstrasse 27, 1040 Vienna, Austria.
authors.forename: Craig N
authors.initials: CN
authors.lastname: Lincoln
authors.affiliation: Department of Physical Chemistry II, Ruhr-University Bochum, 44780 Bochum, Germany.
authors.forename: Janne
authors.initials: J
authors.lastname: Savolainen
authors.affiliation: Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 121 16, Czech Republic.
authors.forename: František
authors.initials: F
authors.lastname: Šanda
authors.affiliation: Institute of Physics, Faculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, Prague 121 16, Czech Republic.
authors.forename: Tomáš
authors.initials: T
authors.lastname: Mančal
authors.affiliation: Photonics Institute, Vienna University of Technology, Gusshausstrasse 27, 1040 Vienna, Austria.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Hauer
mesh_terms: D001429:Bacteriochlorophylls / Q000737:chemistry* / Q000378:metabolism; D002338:Carotenoids / Q000737:chemistry* / Q000378:metabolism; D002844:Chromatium / Q000737:chemistry* / Q000378:metabolism; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D013057:Spectrum Analysis
chemical_list: D001429:Bacteriochlorophylls; D045342:Light-Harvesting Protein Complexes; D002338:Carotenoids