Low lying carotenoid "dark" singlet states in light-harvesting complexes revealed by multi-photon fluorescence excitation spectroscopy

Carotenoids serve important functions in photosynthetic organisms: as structural components of pigment-protein complexes, accessory light-harvesting pigments and photoprotective agents. Elucidation of the role(s) of the lowest excited singlet states of carotenoids, in particular, of the ("optically dark") state 2A_g ^-(S₁) in light harvesting and excess energy dissipation is a field of intense current research interest. Two-photon absorption (TPA) of carotenoids detected by (bacterio)chlorophyll, (B)Chl, fluorescence (following excitation energy transfer, EET) can be used to localize the (optically "dark") 2A_g ^-(S₁) state of relevant carotenoids in photosynthetic pigment-protein complexes. The fluorescence excitation spectra of several bacterial and plant light-harvesting complexes have been measured upon short-pulse (100 fs) laser excitation in the presumed 2A_g ^- energy range of the bound carotenoids.