Carotenoids as a Shortcut for Chlorophyll Soret-to-Q Band Energy Flow

It is proposed that xanthophylls, and carotenoids in general, may assist in energy transfer from the chlorophyll Soret band to the Q band. Ground‐state (1Ag) and excited‐state (1Bu) optimizations of violaxanthin (Vx) and zeaxanthin (Zx) are performed in an environment mimicking the light‐harvesting complex II (LHCII), including the closest chlorophyll b molecule (Chl). Time‐dependent density functional theory (TD‐DFT, CAM‐B3LYP functional) is used in combination with a semi‐empirical description to obtain the excited‐state geometries, supported by additional DFT/multireference configuration interaction calculations, with and without point charges representing LHCII. In the ground state, Vx and Zx show similar properties. At the 1Bu minimum, the energy of the Zx 1Bu state is below the Chl Q band, in contrast to Vx. Both Vx and Zx may act as acceptors of Soret‐state energy; transfer to the Q band seems to be favored for Vx. These findings suggest that carotenoids may generally mediate Soret‐to‐Q energy flow in LHCII. A proposal for carotenoid function: Quantum‐chemical calculations indicate that xanthophylls may provide a depletion pathway for the Soret excitation energy of chlorophyll, which can then flow back to the chlorophyll Q band. Xanthophylls and, more generally, carotenoids seem to be capable of mediating and tuning this process.