Excited-State Kinetics of the Carotenoid S_1 State in LHC II and Two-Photon Excitation Spectra of Lutein and β-Carotene in Solution: Efficient Car S_1 →Chl Electronic Energy Transfer via Hot S_1 States?

摘要

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 ～200 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-photo 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 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 S_1 fluorescence of lutein and β-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 not states.