Study of relaxation kinetics of excited states in carotenoids and energy transfer in photosynthesis

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Carotenoids play important roles in light-harvesting (LH) function of photosynthesis. Light energy is initially absorbed by carotenoid and transferred to bacteriochlorophyll with high efficiency. Ultrafast relaxation kinetics of carotenoids has been investigated systematically. Carotenoids with several conjugation lengths and different structures are prepared for the measurements. Their basic optical properties are measured by stationary absorption, fluorescence, and electroabsorption spectroscopies. Femtosecond time-resolved spectroscopy system has been improved and tunable excitation pulses in visible to near-infrared region are utilized. Femtosecond absorption, Raman, and fluorescence spectroscopies have been done under the same excitation condition to compare the results directly.Ultrafast phenomena in carotenoids are summarized as follows. At delay times of nearly equal to zero, nonlinear optical responses in a three-level system of the ground state (1^1A_g^-), the lowest opticall … More y allowed excited state (1^1B_u^+), and a highly excited state (n^1A_g^-) are observed in addition to the signal due to the resonant excitation of 1^1B_u^+. After the generation of 1^1B_u^+, the internal conversion to a lower dark state (2^1A_g^-) occurs with the time constant of 0.1〜0.2 ps. Another dark state (1^1B_u^-) is an intermediate state of the relaxation, but it cannot be time-resolved in this research. Redistribution and dissipation of excess vibrational energy take place simultaneously with the internal conversions. However, some vibrational modes have long lifetimes and 2^1A_g^-remains in vibrational excited levels longer than several picoseconds. The vibrational energy affect the relaxation kinetics and slows down the internal conversions.Preliminary measurement of the LH complex has been done and the experimental system is going to be refined for the study of the vibrational level in the LH complex.This research reveals the main relaxation kinetics of carotenoids and importance of the vibrational levels in the energy transfer from carotenoids to reaction center of the photosynthesis. Less