CONTROVERSY SURROUNDING THE EXCITED STATES OF CAROTENOIDS…
Carotenoids, the pigments responsible for the orange and yellow colors of plants, play a critical role in harvesting photons during photosynthesis and also provide effective photoprotection of the photosynthetic apparatus. Despite decades of intensive research, our understanding of carotenoid photophysics, and consequently of the light-induced processes in carotenoid-containing systems, remains incomplete. Scientists have long relied on a simplified three-level electronic model (S₀, S₁, S₂) to understand the role of carotenoids in photosynthesis. However, this traditional framework could not account for all the complex, ultrafast transient signals observed experimentally, suggesting the existence of additional electronic states, known as dark states because they are not visible in absorption, whose number and nature have been the subject of considerable debate.
…SOLVED USING RAMAN SPECTROSCOPY
By developing an original instrument capable of measuring a stimulated Raman spectroscopy signal within approximately one hundred femtoseconds under resonant conditions, the authors were able to selectively observe, by tuning these resonance conditions, the vibrational fingerprints of three distinct dark electronic states within the carotenoid electronic-state manifold and to determine their nature. In doing so, they propose a more precise framework than the original three-level model, which already provides a better understanding of the energy-transfer processes involving carotenoids during the early stages of photosynthesis and may, in particular, help explain the role of these molecules in photoprotection.
These results provide an unprecedentedly precise resolution to years of controversy surrounding the role of carotenoids in photosynthesis. They also shed new light on the photophysics of these essential molecules, while establishing a relevant spectroscopic framework for characterizing their multiple roles.
Summary adapted from the I2BC news
Artistic representation of excited states in carotenoids © MJ
Llansola-Portoles / CEA-CNRS-Université Paris-Saclay
Contact : Manuel Llansola-Portoles manuel.llansola@i2bc.paris-saclay.fr
- Carotenoids are non-chlorophyll pigments stored in chromoplasts (organelles found in plant cells), giving fruits and flowers their yellow, orange, or red hues.
- The simplified three-state model of carotenoids describes the key electronic energy levels involved in light absorption and ultrafast relaxation : the ground state S₀, the dark state S₁, and the bright state S₂. However, this model could not account for the network of optically dark excited states.
- The dark excited states of carotenoids are electronic energy levels that are inactive in absorption and non-radiative, and are essential for understanding carotenoid photophysics. Their main representatives are the S₁ state, theoretically predicted in 1987, and an intermediate S* state, proposed twenty years later.
The femtosecond stimulated resonance Raman spectroscopy platform, which tracks transient species with structural specificity and femtosecond temporal resolution, provides access to structural information (vibrational modes) on transient excited states generated by photon absorption.
By exploiting resonance, this setup makes it possible to selectively isolate the signal from a given transient state and monitor its dynamics with sub-picosecond temporal resolution, making it a measurement platform that is currently unique worldwide. © I2BC