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 S-1 transition and monitoring fluorescence from the bacteriochlorophyll, provides the first direct experimental verification of carotenoid S-1 to bacteriochlorophyll energy transfer. It also provides an estimate of the in situ spheroidene S-1 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 S-1 states are discussed in terms of spectral overlap factors and competing processes such as S-1 --> S-0 internal conversion. (C) 1999 Elsevier Science B.V. All rights reserved.