Unraveling the nature of coherent beatings in chlorosomes

Abstract

Coherent two-dimensional (2D) spectroscopy at 80 K was used to study chlorosomes isolated from green sulfur bacterium Chlorobaculum tepidum. Two distinct processes in the evolution of the 2D spectrum are observed. The first being exciton diffusion, seen in the change of the spectral shape occurring on a 100-fs timescale, and the second being vibrational coherences, realized through coherent beatings with frequencies of 91 and 145 cm(-1) that are dephased during the first 1.2 ps. The distribution of the oscillation amplitude in the 2D spectra is independent of the evolution of the 2D spectral shape. This implies that the diffusion energy transfer process does not transfer coherences within the chlorosome. Remarkably, the oscillatory pattern observed in the negative regions of the 2D spectrum (dominated by the excited state absorption) is a mirror image of the oscillations found in the positive part (originating from the stimulated emission and ground state bleach). This observation is surprising since it is expected that coherences in the electronic ground and excited states are generated with the same probability and the latter dephase faster in the presence of fast diffusion. Moreover, the relative amplitude of coherent beatings is rather high compared to non-oscillatory signal despite the reported low values of the Huang-Rhys factors. The origin of these effects is discussed in terms of the vibronic and Herzberg-Teller couplings.

Quantum biology revisited

“…In between these limiting cases arises the general situation of superpositions of states with mixed vibrational-electronic character, which defines vibronic coherence-a field of significant current interest (71). As both electronic (72) and purely vibrational coherences (73)(74)(75) modulate ultrafast spectra in the form of periodic oscillations, the need to distinguish between them is obvious. As discussed previously (76,77), the assignment of long-lived small-amplitude QBs in several photosynthetic systems to interexciton coherences based solely on their frequencies and sometimes phase (78-80) is insufficient.…”

Contrasting Modes of Self-Assembly and Hydrogen-Bonding Heterogeneity in Chlorosomes of Chlorobaculum tepidum

“…Currently, it is unclear how the hierarchy of structural and dynamical properties enables an amazing quantum efficiency for transfer within and between tubes, and the dominant mechanisms are strongly debated. 3 , 4 …”

Exciton Transport in Molecular Aggregates – From Natural Antennas to Synthetic Chromophore Systems

“…In the meantime, all components of the green-sulphur bacterial photosynthetic apparatus have been investigated, i.e., the chlorosome antenna, [193,194] the FMO complex, [141,195,196] the baseplate, [197] and the reaction center. [198] With improved signal-to-noise ratios, it has even become possible to measure the whole photosynthetic apparatus at once, revealing the energy transfer pathways through all components.…”