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?

Abstract

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 S1 population ranges from ∼200 fs to over 7 ps, depending on the excitation and detection wavelengths. In contrast, a 200 fs Car S1→Chlorophyll (Chl) energy transfer component was the dominant time constant for our earlier two-photon fluorescence up-conversion measurements (Walla, P. J.; et al. J. Phys. Chem. B 2000, 104, 4799−4806). We also present the two-photon excitation (TPE) spectra of lutein and β-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 S1 fluorescence of lutein and β-carotene in solution. Different environments may affect the shape of the S1 spectrum significantly. To explain the blue shift of the TPE spectrum and the difference in the time constant...