Two-photon fluorescence excitation spectroscopy of photosynthetic pigments and pigment-protein complexes

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 S₁ 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 S₁ state to the Chl or BChl S₁ state. The carotenoid S₀ to S₁-transition is two-photon allowed. Consequently, peaks in the two-photon fluorescence (TPF) excitation spectra of LHCs have usually been ascribed to this transition.