title: Impact of environmentally induced fluctuations on quantum mechanically mixed electronic and vibrational pigment states in photosynthetic energy transfer and 2D electronic spectra.
abstract: Recently, nuclear vibrational contribution signatures in two-dimensional (2D) electronic spectroscopy have attracted considerable interest, in particular as regards interpretation of the oscillatory transients observed in light-harvesting complexes. These transients have dephasing times that persist for much longer than theoretically predicted electronic coherence lifetime. As a plausible explanation for this long-lived spectral beating in 2D electronic spectra, quantum-mechanically mixed electronic and vibrational states (vibronic excitons) were proposed by Christensson et al. [J. Phys. Chem. B 116, 7449 (2012)] and have since been explored. In this work, we address a dimer which produces little beating of electronic origin in the absence of vibronic contributions, and examine the impact of protein-induced fluctuations upon electronic-vibrational quantum mixtures by calculating the electronic energy transfer dynamics and 2D electronic spectra in a numerically accurate manner. It is found that, at cryogenic temperatures, the electronic-vibrational quantum mixtures are rather robust, even under the influence of the fluctuations and despite the small Huang-Rhys factors of the Franck-Condon active vibrational modes. This results in long-lasting beating behavior of vibrational origin in the 2D electronic spectra. At physiological temperatures, however, the fluctuations eradicate the mixing, and hence, the beating in the 2D spectra disappears. Further, it is demonstrated that such electronic-vibrational quantum mixtures do not necessarily play a significant role in electronic energy transfer dynamics, despite contributing to the enhancement of long-lived quantum beating in 2D electronic spectra, contrary to speculations in recent publications.
authors.affiliation: Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki 444-8585, Japan.
authors.forename: Yuta
authors.initials: Y
authors.lastname: Fujihashi
authors.affiliation: Department of Chemistry, University of California, Berkeley and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Graham R
authors.initials: GR
authors.lastname: Fleming
authors.affiliation: Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki 444-8585, Japan.
authors.forename: Akihito
authors.initials: A
authors.lastname: Ishizaki
mesh_terms: D004583:Electrons; D004735:Energy Transfer; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism; D010788:Photosynthesis; D011789:Quantum Theory; D013057:Spectrum Analysis; D014732:Vibration
chemical_list: D045342:Light-Harvesting Protein Complexes