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捕光功能中起着重要作用。光能最初被类胡萝卜素吸收，然后高效地转化为细菌叶绿素。系统地研究了类胡萝卜素的超快松弛动力学。制备了几种共轭长度和不同结构的类胡萝卜素用于测量。用静态吸收光谱、荧光光谱和电吸收光谱测量了它们的基本光学性质。改进了飞秒时间分辨光谱系统，采用了可调谐的可见光到近红外的激发脉冲。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的直接比较。类胡萝卜素的超快现象概述如下。在几乎等于零的延迟时间下，基态三能级系统(1^1a_g^-)的非线性光学响应达到最低的光学…除了1^1B_u^+的共振激发信号外，还观察到了较多的y允许激发态(1^1B_u^+)和高激发态(n^1a_g^-)。在1^1B_u^+产生后，内转换到较低的暗态(2^1a_g^-)，时间常数为0.1~0.2ps。另一个暗态(1^1B_u^-)是驰豫的中间态，但在本研究中它不能被时间分辨。过剩振动能量的重新分配和耗散与内部转换同时发生。然而，一些振动模的寿命很长，2^1a_g^-保持在振动激发能级上的时间长于几皮秒。振动能影响类胡萝卜素的驰豫动力学，使内部转换变慢。初步测量了类胡萝卜素复合体的振动能级，并对实验系统进行了改进。本研究揭示了类胡萝卜素的主要弛豫动力学，以及振动能级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。较少

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