Ultrafast Triplet Generation and its Sensitization Drives Efficient Photoisomerization of Tetra-cis-lycopene to All-trans-lycopene.

Lycopene biosynthesis in photosynthetic organisms controls the metabolic flux of reaction center carotenoids like β-carotene and lutein through the geometric four-step isomerization of 7,9,9',7'-tetra-cis-lycopene (prolycopene) to its all-trans form. In plants and cyanobacteria, a redox-controlled flavoenzyme carotenoid isomerase catalyzes the prolycopene isomerization although its functional loss inside the chloroplast can be rescued by light. In order to address the chloroplast-specificity and efficiency of the light-induced isomerization reaction, we need to critically understand the excited state dynamics of prolycopene and the nature of electronic states that lead to the isomerization. Using broadband femtosecond transient absorption spectroscopy, we observe ∼610 fs rise of the long-lived triplet state from the photoexcited S2 with a quantum yield of ∼0.19. The triplet state eventually triggers the first C═C bond isomerization at the symmetric 9 or 9' position on the tetra-cis backbone to yield the tri-cis product with 15% quantum efficiency. However, direct sensitization of the photoreactive triplet state via meso-tetraphenyl porphyrin sensitizer under steady state illumination leads to an efficient production of all-trans-lycopene with 58% quantum yield. Our work implies that chlorophyll-enriched chloroplasts should form an optimized photoreaction vessel for prolycopene isomerization, and synthetic utilization of such cis-carotenoids can lead to efficient triplet harvesting photon-conversion devices.