As a tomato ripens, its chloroplasts (which are colored green) transform into chromoplasts (which are colored red). Whereas chloroplasts carry out photosynthesis with the help of chlorophyll pigments, chromoplasts are not considered to have any metabolic function. Furthermore, they are thought to dissipate the excess light energy received thanks to their colored pigments based on carotenoids, which are also present in many other fruits, vegetables, flowers and algae.

In a previous study, the Joliot researchers identified a novel photophysical mechanism involving carotenoid (lycopene) aggregates extracted from mature tomato chromoplasts. In this scheme, the absorption of a solar photon promotes one electron from the ground state to a singlet-excited state, and then a "singlet exciton fission" process creates two triplet-excited states. This singlet fission is a singular quantum process that involves electronic spin couplings and is only observed in the lycopene aggregates, not in isolated molecules. Is this an exceptional phenomenon, or is it generic in chromoplasts?

To find out, the biologists decided to study aggregates of lutein and violaxanthin, two carotenoids present in the chromoplasts of daffodil (Narcissus pseudonarcissus L.). Time-resolved electron spectroscopy and Raman spectroscopy experiments unambiguously revealed that singlet fission occurs similarly in these aggregates, allowing the scientists for the first time to fully describe the singlet fission pathway of carotenoids in a flower.

This very fundamental biological result could inspire new research paths, especially for improving the quantum efficiency of photovoltaic cells. Indeed, these cells could produce, starting from the two triplet states, two electrons for every absorbed photon.

This work is the result of a collaboration between the CEA-Joliot, the Institut Jean-Pierre Bourgin (INRAE, AgroParistech, and the Université Paris Saclay) and Vilnius University (Lithuania).