Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy

Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm^–1 frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm^–1 oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast

Coherent Oscillations in Chlorosome Elucidated by Two-Dimensional Electronic Spectroscopy

Chlorosomes are the most efficient photosynthetic light-harvesting complexes found in nature and consist of many bacteriochlorophyll (BChl) molecules self-assembled into supramolecular aggregates. Here we elucidate the presence and the origin of coherent oscillations in chlorosome at cryogenic temperature using 2D electronic spectroscopy. We observe coherent oscillations of multiple frequencies superimposed on the ultrafast amplitude decay of 2D spectra. Comparison of oscillatory features in the rephasing and nonrephasing 2D spectra suggests that an oscillation of 620 cm^–1 frequency arises from electronic coherence. However, this coherent oscillation can be enhanced by vibronic coupling with intermolecular vibrations of BChl aggregate, and thus it might originate from vibronic coherence rather than pure electronic coherence. Although the 620 cm^–1 oscillation dephases rapidly, the electronic (or vibronic) coherence may still take part in the initial step of energy transfer in chlorosome, which is comparably fast

Structural Dynamics of 1,2-Diiodoethane in Cyclohexane Probed by Picosecond X-ray Liquidography

We investigate the structural dynamics of iodine elimination reaction of 1,2-diiodoethane (C₂H₄I₂) in cyclohexane by applying time-resolved X-ray liquidography (TRXL). The TRXL technique combines structural sensitivity of X-ray diffraction and 100 ps time resolution of X-ray pulses from synchrotron and allows direct probing of transient structure of reacting molecules. From the analysis of time-dependent X-ray solution scattering patterns using global fitting based on DFT calculation and MD simulation, we elucidate the kinetics and structure of transient intermediates resulting from photodissociation of C₂H₄I₂. In particular, the effect of solvent on the reaction kinetics and pathways is examined by comparison with an earlier TRXL study on the same reaction in methanol. In cyclohexane, the C₂H₄I radical intermediate undergoes two branched reaction pathways, formation of C₂H₄I–I isomer and direct dissociation into C₂H₄ and I, while only isomer formation occurs in methanol. Also, the C₂H₄I–I isomer has a shorter lifetime in cyclohexane by an order of magnitude than in methanol. The difference in the reaction dynamics in the two solvents is accounted for by the difference in solvent polarity. In addition, we determine that the C₂H₄I radical has a bridged structure, not a classical structure, in cyclohexane