Excited-state kinetics of the carotenoid S1 state in LHC II and two-photon excitation spectra of lutein and β-carotene in solution:: Efficient car S1→Chl electronic energy transfer via hot S1 states?

The excited-state dynamics of the carotenoids (Car) in light-harvesting complex II (LHC II) of Chlamydomonas reinhardtii were studied by transient absorption measurements. The decay of the Car S-1 population ranges from similar to200 fs to over 7 ps, depending on the excitation and detection wavelengths. In contrast, a 200 fs Car S-1-->Chlorophyll (Chl) energy transfer component was the dominant time constant for our earlier two-photon fluorescence up-conversion measurements (Walla, P. L; et al. J. Phys. Chem. B 2000, 104, 4799-4806). We also present the two-photon excitation (TPE) spectra of lutein and beta-carotene in solution and compare them with the TPE spectrum of LHC II. The TPE-spectrum of LHC II has an onset much further to the blue and a width that is narrower than expected from comparison to the S-1 fluorescence of lutein and beta-carotene in solution. Different environments may affect the shape of the S(1)spectrum significantly. To explain the blue shift of the TPE Spectrum and the difference in the time constants obtained from two-photon vs one-photon methods, we suggest that a major part of the Car S-1-->Chl electronic energy transfer (EET) is due to efficient EET from hot vibronic states of the Cars. We also suggest that the subpicosecond kinetics has a very broad distribution of EET time scales due to EET from hot states.