An unusual pathway of excitation energy deactivation in carotenoids: Singlet-to-triplet conversion on an ultrafast timescale in a photosynthetic antenna

Carotenoids are important biomolecules that are ubiquitous in nature and fi nd widespread application in medicine. In photosynthesis, they have a large role in light harvesting (LH) and photoprotection. They exert their LH fun ction by donating their excited singlet state to nearby (bacterio)chlorophy ll molecules. In photosynthetic bacteria, the efficiency of this energy tra nsfer process can be as low as 30%. Here, we present evidence that an unusu al pathway of excited state relaxation in carotenoids underlies this poor L H function, by which carotenoid triplet states are generated directly from carotenoid singlet states. This pathway, operative on a femtosecond and pic osecond timescale, involves an intermediate state, which we identify as a n ew, hitherto uncharacterized carotenoid singlet excited state. In LH comple x-bound carotenoids, this state is the precursor on the reaction pathway to the triplet state, whereas in extracted carotenoids in solution, this stat e returns to the singlet ground state without forming any triplets. We disc uss the possible identity of this excited state and argue that fission of t he singlet state into a pair of triplet states on individual carotenoid mol ecules constitutes the mechanism by which the triplets are generated. This is, to our knowledge, the first ever direct observation of a singlet-to-tri plet conversion process on an ultrafast timescale in a photosynthetic anten na.