Exploring electronic-vibrational coupling in chlorophylls and photosynthetic complexes by polarization-controlled 2D electronic spectroscopy*

Electronic-vibrational (vibronic) coupling has been suggested to play an important role in energy transfer and charge separation processes in photosynthesis. It is, however, highly elusive phenomenon to investigate. To this end we have employed polarization-controlled 2DES together with advanced Fourier analysis [1]. We use double-crossed polarization scheme, which extracts signals generated by excitation of coherences, involving transitions with different orientation of dipole moments. It allows for studying electronic and vibronically mixed coherences, and therefore enables to directly detect the presence of vibronic mixing (coupling).

In one study we investigated chlorophyll c molecule, where we found a clear evidence of mixing of the two lowest electronic states, Qx and Qy via vibronic coupling. Interestingly, we discovered at least two vibrational modes that are involved in the coupling. Since this type of coupling is expected to be rather general, vibronic mixing is expected in all chlorophyll-type molecules.

In another study we revisited coherence dynamics in the FMO complex at 77 K [2]. Applying the same experimental method and analysis techniques, and aided by theory, we find a very rich picture of the coherence signals. We determined that all long-lived coherences have clearly vibrational origin. While electronic coherences are also observed, they dephase on the ~100 fs time scale. Importantly, we further observe that specific vibrational coherences are excited via vibronically coupled excitonic transitions.

Finding ubiquitous vibronic coupling in photosynthetic pigments and complexes that contain them rises a question if its presence is the signature of smart Nature’s design, or an unavoidable consequence of the photophysical properties of the porphyrin-type molecules.

[1] Paleček, D. et al., Sci. Adv. 2017, 3 (9), e1603141.
[2] Thyrhaug, E. et al., Nat. Chem. 2018, 10, 780.