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 (S₁ /2¹A_g⁻) for one-photon excitation from the electronic ground state (S₀ /1¹A_g⁻) of relevant carotenoids, the relative energetic position of the carotenoids S₁ state cannot be readily investigated by conventional spectroscopic techniques. In several light-harvesting complexes (LHCs) this state, however, is assumed to be involved in excitation energy transfer to adjacent Chl or bacteriochlorophyll (BChl) molecules. This is based on a supposed close energetic proximity of the carotenoid S₁ state to the Chl or BChl S₁ (Q ) state. The carotenoid S₀ to S₁-transition is two-photon allowed. Consequently, peaks in the two-photon fluorescence (TPF) excitation spectra of LHCs (detected by Chl- or BChl-fluorescence) have usually been ascribed to this transition [1].

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