The nature of coherences in the B820 bacteriochlorophyll dimer revealed by two-dimensional electronic spectroscopy

Abstract

Light-harvesting in photosynthesis is determined by the excitonic interactions in disordered antennae and the coupling of collective electronic excitations to fast nuclear motions, producing efficient energy transfer with a complicated interplay between exciton and vibrational coherences. Two-dimensional electronic spectroscopy (2DES) is a powerful tool to study the presence of these coherences in photosynthetic complexes. However, the unambiguous assignment of the nature of the observed coherences is still under debate. In this paper we apply 2DES to an excitonically coupled bacteriochlorophyll dimer, the B820 subunit of the light harvesting complex 1 (LH1-RC) of R. rubrum G9. Fourier analysis of the measured kinetics and modeling of the spectral responses in a complete basis of electronic and vibrational states allow us to distinguish between pure vibrational, mixed exciton-vibrational (vibronic), and predominantly exciton coherences. The mixed coherences have been found in a wide range of oscillation frequencies, whereas exciton coherences give the biggest contributions for the frequencies in the 400-550 cm(-1) range, corresponding to the exciton splitting energy of the B820 dimer. Significant exciton coherences are also present at higher frequencies, i.e., up to 800 cm(-1), which are determined by realizations of the disorder with a large energy gap between the two pigments (which increases the apparent value of the exciton splitting). Although the B820 dimer is a model system, the approach presented here represents a basis for further analyses of more complicated systems, providing a tool for studying the interplay between electronic and vibrational coherences in disordered photosynthetic antennae and reaction centres.

Coherence and population dynamics of chlorophyll excitations in FCP complex: Two-dimensional spectroscopy study

“…Coherence beatings can be analyzed by performing the Fourier transform of the time traces of the peaks. A very convenient way to represent the overall complexity of oscillations in the 2D spectrum is through the Fourier maps [36][37][38][39][40][41] . They are constructed by performing the Fourier transform of the each data point in the 2D spectrum over the delay time t 2 after subtraction of slow exponential decay contributions.…”

Probing environment fluctuations by two-dimensional electronic spectroscopy of molecular systems at temperatures below 5 K

“…However, n ≥ 2 is necessary for s h to be finite in Eq. (16 is usually observed that J(ω) → 0 as ω → 0 44 what is possible when n ≥ 3.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12] Probably, the milestone advancement of the technique was direct demonstration of electronic and vibrational, as well as mixed, quantum coherences via femtosecond beats of various diagonal and off-diagonal peaks in the 2D plots from 77 K up to room temperature. 1,5,7,[13][14][15][16][17][18] These results attained much interest and caused intensive debates on the origin and functional purpose of the oscillations: whether they arise from quantum excitation dynamics in bio-materials purposefully optimized by nature, or the oscillations record merely bond wiggling, which is natural and not necessarily a quantum phenomenon. 5,7,16,[19][20][21][22][23][24][25][26] However, it has been acknowledged that nuclear dynamics and its coupling to the electronic excitations leads to the important physical dynamic processes leading to energy photoconversion.…”

“…1,5,7,[13][14][15][16][17][18] These results attained much interest and caused intensive debates on the origin and functional purpose of the oscillations: whether they arise from quantum excitation dynamics in bio-materials purposefully optimized by nature, or the oscillations record merely bond wiggling, which is natural and not necessarily a quantum phenomenon. 5,7,16,[19][20][21][22][23][24][25][26] However, it has been acknowledged that nuclear dynamics and its coupling to the electronic excitations leads to the important physical dynamic processes leading to energy photoconversion. 21,[26][27][28][29] Thus, the most important questions are what is the cumulative nuclear response to the electronic excitations and whether there is a feedback.…”

A method for the direct measurement of electronic site populations in a molecular aggregate using two-dimensional electronic-vibrational spectroscopy

“…7,11,34,35 A number of synthetic and biological dimers which sample different parameter regimes have been described, which would serve as very interesting systems to test the practical application of this experiment. [58][59][60][61][62][63][64][65] It is important to note that there are a number of practical difficulties that might limit the general applicability of the approach described in this work. In particular, it crucially relies on the ability to resolve the spectral features corresponding to each exciton and to a vibration on each site in the 2DEV spectra.…”