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

类胡萝卜素在光合作用的光收集（LH）功能中起着重要作用。光能最初被类胡萝卜素吸收，然后高效地转移到细菌叶绿素中。对类胡萝卜素的超快弛豫动力学进行了系统的研究。为测量准备了几种共轭长度和不同结构的类胡萝卜素。它们的基本光学性质是通过固定吸收、荧光和电吸收光谱来测量的。改进了飞秒时间分辨光谱系统，利用了可见光至近红外波段的可调谐激发脉冲。在相同的激发条件下进行了飞秒吸收光谱、拉曼光谱和荧光光谱，直接比较了结果。类胡萝卜素的超快现象总结如下。在延迟时间接近于零的情况下，除了1^1B_u^+的共振激励信号外，还观察到基态（1^1A_g^-），最低光学参量允许激发态（1^1B_u^+）和高激发态（n^1A_g^-）的三能级系统的非线性光学响应。在1^1B_u^+产生后，以0.1 ~ 0.2 ps的时间常数发生向较低暗态（2^1A_g^-）的内部转换，另一个暗态（1^1B_u^-）是弛豫的中间态，但本研究无法对其进行时间分辨。多余振动能量的重新分配和耗散与内部转换同时发生。然而，一些振动模式具有很长的寿命，并且2^1A_g^-在振动激发能级中保持的时间超过几皮秒。振动能影响弛豫动力学，减缓内部转化。LH配合物的初步测量已经完成，实验系统将进一步完善，以研究LH配合物的振动水平。本研究揭示了类胡萝卜素的主要松弛动力学和振动水平在类胡萝卜素向光合作用反应中心的能量传递中的重要性。少

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