Hidden vibronic and excitonic structure and vibronic coherence transfer in the bacterial reaction center

Abstract

We report two-dimensional electronic spectroscopy (2DES) experiments on the bacterial reaction center (BRC) from purple bacteria, revealing hidden vibronic and excitonic structure. Through analysis of the coherent dynamics of the BRC, we identify multiple quasi-resonances between pigment vibrations and excitonic energy gaps, and vibronic coherence transfer processes that are typically neglected in standard models of photosynthetic energy transfer and charge separation. We support our assignment with control experiments on bacteriochlorophyll and simulations of the coherent dynamics using a reduced excitonic model of the BRC. We find that specific vibronic coherence processes can readily reveal weak exciton transitions. While the functional relevance of such processes is unclear, they provide a spectroscopic tool that uses vibrations as a window for observing excited state structure and dynamics elsewhere in the BRC via vibronic coupling. Vibronic coherence transfer reveals the upper exciton of the "special pair" that was weakly visible in previous 2DES experiments.

A Comprehensive Approach to Exciton Delocalization and Energy Transfer

“…RET rates are particularly large when the energy of the long-lived Kasha’s state of scriptD (green line), the states from where RET occurs, crosses states in the manifold relevant to the excited scriptA with either two or three vibrational excitations (red lines), the states that mainly receive the energy. A very clear energy resonance effect is observed affecting the RET rates and highlighting the prominent role of vibronic state alignment, fully in line with recent experimental results. , …”

“…A very clear energy resonance effect is observed affecting the RET rates and highlighting the prominent role of vibronic state alignment, fully in line with recent experimental results. 30,36 4.3. The RET-Exciton Crossover.…”

Multiple Quantum-Coherent Energy Transfer Pathways in Photosynthesis: Electronic-Vibrational Mixing within Photosystem II CP43/CP47 Antenna

“…3b), the low frequency 150 cm -1 map corresponding to the CP 675-682 displays minor but visible amplitude at the CP position, indicating that ET from the 675 to the 682 nm state may proceed via a vibronic coherent mechanism. The related monomeric Chl map at 170 cm -1 clearly deviates from the chair-type structure and from the structure observed for CP43/CP47, still, a virtually identical 2D map was observed recently for monomeric bacteriochlorophyll a at 77 K 50 (in isopropanol as used here). The 265 cm -1 map which corresponds to the CP 669-682 displays a complicated amplitude distribution.…”

“…In photosynthetic complexes, vibrational coherence is usually observed for vibrational frequencies that are higher in energy than the energy gaps between the electronic states. It is worth noting that for isolated Chl [43][44][45][46] (or bacteriochlorophyll [47][48][49][50] ) molecules in solution mainly vibrational coherences are observed.…”

Coherent excitation energy transfer in model photosynthetic reaction center: Effects of non-Markovian quantum environment

“…In recent years, the role of quantum coherence in the EET and ET processes of photosynthesis has got great interest. [1][2][3][4][5][6][7][8][9] The core complex is the main participant in the EET and ET processes of reaction center. In reality, it constitutes an open quantum system, which manifests an interplay between pigments, solar light and phonon baths.…”