Observation of the S1 state of spheroidene in LH2 by two-photon fluorescence excitation

The two-photon fluorescence excitation spectrum of LH2, measured by directly exciting the carotenoid S1 transition and monitoring fluorescence from the bacteriochlorophyll, provides the first direct experimental verification of carotenoid S1 to bacteriochlorophyll energy transfer. It also provides an estimate of the in situ spheroidene S1 transition energy of 13 900±150 cm−1, slightly red-shifted from solution estimates. The relative abilities of the carotenoids spheroidene (10 conjugated double bonds) and rhodopin glucoside (11 conjugated double bonds) to transfer energy via their S1 states are discussed in terms of spectral overlap factors and competing processes such as S1→S0 internal conversion.