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 constants obtained from two-photon vs one-photon methods, we suggest that a major part of the Car S1→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.