We have investigated the structure and photochemistry of carotenoids-containing chromoplast using two natural model systems displaying different properties – tomatoes and daffodil petals. Tomato chromoplasts contain lycopene crystalloids, which are responsible for red color whereas daffodil chromoplasts contain crystalloids of lutein/violaxanthin responsible for the yellow color. The absorption spectrum of lycopene crystalloids is significantly red-shifted respect to lycopene monomer while the absorption spectrum of lutein/violaxanthin crystalloids is not red-shifted respect to their monomeric forms. To understand this variation, we determined structural parameters using circular dichroisms and resonance Raman. The differences on the energetic pathways after photon absorption in tomato and daffodil chromoplasts were obtained by transient absorption. We observed that both samples generate long-lived triplet state by singlet exciton fission with remarkably different yield. Those results suggest that the organization of the carotenoids in the aggregates determines the singlet fission process and finally the generation long living triplet states. We created artificial carotenoids aggregates with different organizations to study the parameters controlling the generation of long living triplets by singlet fission. Theoretically, these systems could produce two triplet-excited states after absorbing one photon, which could be further used for energy generation or multi-electronic photo-catalysis.

Manuel Llansola Portoles is a CNRS researcher at the Institute for Integrative Biology of the Cell (Paris-Saclay University). His research aims at deciphering the fundamental principles of energy management of the photosynthetic apparatus by the use of advanced electronic and vibrational spectroscopies as well as transposing these principles within synthetic molecular system for improving artificial photosynthesis.