Abstract
Chlorophylls (Chls) and carotenoids are light-harvesting pigments and essential structural components of photosynthetic pigment-protein complexes. Due to the optically forbidden character of the lowest excited singlet state for one-photon excitation from the electronic ground state of relevant carotenoids, the relative energetic position of the carotenoids S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> state cannot be readily investigated by conventional spectroscopic techniques. In several light-harvesting complexes this state, however, is assumed to be involved in excitation energy transfer to adjacent Chl or bacteriochlorophy ll molecules. This is based on a supposed close energetic proximity of the carotenoid S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> state to the Chl or BChl S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> state. The carotenoid S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> to S <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> -transition is two-photon allowed. Consequently, peaks in the two-photon fluorescence (TPF) excitation spectra of LHCs have usually been ascribed to this transition.
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