【作者】 刘伟龙；

【导师】 苏文辉； 郑植仁；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 类胡萝卜素是自然界中最丰富的天然色素之一,广泛存在于包括人类在内的生物有机体内,在光合作用、预防人类疾病等方面起重要作用。人们采用稳态光谱、时间分辨超快光谱以及理论方法对类胡萝卜素的激发态能级结构及其动力学特性进行了广泛的研究,为探索类胡萝卜素的光物理和光化学功能的机制提供了重要信息。然而,这方面尚有一些不清楚的问题,如某些中间电子态或振动态是否存在、其弛豫过程如何,以及外界环境对各电子态和振动态能级及其弛豫过程有何影响等。这些问题导致人们还不能准确地了解类胡萝卜素在自然界中发挥其重要生物功能的物理机制。本文认为将高压极端条件与常规的光谱学手段相结合是解决上述问题的有效方法。由于高压条件可以放大分子间相互作用,所以研究高压条件下的稳态吸收和拉曼光谱,能够深入探讨环境因素对类胡萝卜素的电子态和振动态能级的影响;由于某些不同起源的光谱成分在高压下呈现不同的变化规律,所以进行高压条件下的时间分辨超快光谱研究会使某些常压条件下不易区分的过程变得易于区分和指认,从而有望澄清类胡萝卜素激发态动力学研究中的一些不易解决的问题。基于上述思想,本文选择常压条件下类胡萝卜素家族中研究成果最为丰富的β-胡萝卜素(β-carotene)为研究对象,研究了其高压稳态及飞秒时间分辨瞬态吸收光谱,同时进行了相应的理论分析。本文的研究成果揭示了影响类胡萝卜素稳态及超快光谱特征的内在和外在因素,提出了新的激发态能量弛豫路径,为深入探讨类胡萝卜素在自然界中发挥其生物功能的物理机制提供了重要依据。为了给高压实验结果的分析提供必要的基础,本文首先研究了常压条件下β-carotene吸收光谱的溶剂效应。实验测得了β-carotene在32种溶剂中的吸收光谱,用多模时域公式对光谱进行了分析。结果表明,β-carotene吸收光谱的0-0带能量和带宽受溶剂极化率影响较大,受溶剂极性影响较小,溶剂极性对带宽的影响比对0-0带能量的影响大得多。除了溶剂极性和极化率等参数之外,溶剂分子的大小和运动状态等微观因素也是影响吸收光谱的原因。随后进行了高压条件下β-carotene在正己烷和二硫化碳溶液中的稳态吸收和拉曼光谱研究。在二硫化碳溶液中,β-carotene吸收光谱随压力的红移和展宽程度都比正己烷溶液中的大,这是由于二硫化碳中溶质溶剂之间的色散相互作用对压力更敏感。为了解释不同溶剂中β-carotene分子S0→S2的跃迁偶极矩随压力变化趋势相反的实验结果,提出有效溶剂分子模型,这一模型进一步证明溶剂分子的大小、位置和取向等微观因素都影响类胡萝卜素在自然界中的捕光能力。通过比较两种溶剂中几个有代表性的拉曼振动模式的峰位随压力的变化关系,提出了键长缩短和电子振动耦合的竞争机制,证实C=C伸缩振动在S1→S0能量内转换过程中发挥了重要作用。拉曼光谱的实验结果还表明,高压条件下β-carotene分子结构发生了微小的扭转,这为分析瞬态吸收光谱的实验结果提供了必要的信息。采用Gaussian 03软件提供的密度函数理论方法研究了两端β-环扭转对β-carotene基态势能面和振动光谱的影响。计算发现,β-carotene分子中的C6-C7键很容易被扭转,这种扭转可以产生两个具有Ci对称性的稳定异构体—顺式(cis)结构和反式(trans)结构,trans→cis异构化只需要克服较低的势垒。虽然高压条件下两端β-环的微小扭转不会导致基态分子发生异构,但会使分子势能面趋于平坦。基于以上实验和理论分析结果,进行了高压条件下β-carotene在正己烷溶液中的瞬态吸收光谱研究。研制出了适合瞬态光谱实验的压机,搭建了高压条件下飞秒时间分辨泵浦-探测实验平台,在此平台上完成了β-carotene的瞬态吸收光谱实验,采用单值分解和全局拟合方法对实验数据进行了分析。比较各个光谱成分的能级位置及其动力学过程随压力的变化关系,发现β-carotene瞬态吸收光谱中的第二个成分对应的是S1态的cis→trans异构过程。提出S1→S0无辐射弛豫速率同时受到能隙和溶剂粘度的影响,这为人们认识天然色素-蛋白质复合物中类胡萝卜素的高效能量传递功能提供了重要参考。本论文将高压条件和飞秒时间分辨光谱技术相结合,首次实现了高压条件下整个白光谱段的瞬态光谱测量。这是研究超快动力学过程的新方法,为人们探讨深层次的物理和化学问题提供了全新的技术手段。更多还原

【Abstract】 Carotenoids are one of the most abundant pigments found in nature. They are present in most organisms including humans. Carotenoids play an important role in photosynthesis, protection against various diseases in humans, etc. The energy levels and dynamics of carotenoid excited states have been extensively investigated using steady-state and ultrafast time-resolved spectroscopy as well as theoretical analysis. These investigations have provided valuable information for elucidating the biological functions of carotenoids. However, lots of problems are still needed to be resolved, such as whether some intermediate electronic or vibrational states are really existent, how about their relaxation dynamics and environment dependence, and so on. Therefore, the physical mechanisms of the biological functions of carotenoids in nature have not been well and truly understood.The combination of high-pressure conditions and ordinary spectroscopic technique was proposed, in this dissertation, to clarify the aforementioned problems. Some intermolecular interactions can be amplified when being pressured, so the investigation on the steady-state spectroscopies under high pressure can reveal the enverimental effects on the electronic and vibrational levels. Some transient species can be correctely assigned under high pressure because of their different behavior when being pressed, so ultrafast spectroscopies under high pressure can be expected to clarify some puzzling problems in the ultrafast dynamics of carotenoids. For these reasons, the steady-state and femtosecond time-resolved transient absorption spectra ofβ-carotene, the most extensively investigated carotenoid at ambient condition, were measured under high pressure and theoretical analysis were also performed. This work illustrated the internal and external factors that affect the steady-state and ultrafast spectroscopies of carotenoids and proposed a new energy relaxation pathway, and therefore provided some novel insights for elucidating the biological functions of carotenoids in nature.In order to understand the results under high pressure, the solvent effect on the absorption spectra ofβ-carotene at ambient condition were firstly investigated. The absorption spectra in 32 solvents were measured and the time-domain formula was used to analyze the absorption spectra. The 0-0 band wavenumber and bandwidth depend mainly on polarizability and slightly on polarity of solvent, and polarity of solvents contributes much more to the bandwidth than to the wavenumber of 0-0 band. Besides the polarizability and polarity, other microcosmic factors, such as the size and movement actions of the solvent molecules, can also affect the absorption spectra. It is essential to take the microscopic characteristics of the solvent molecules into account in the investigation of the environment effect on carotenoids.Steady-state absorption and Raman spectra ofβ-carotene in hexane and CS2 solventions were investigated under high pressure. Both the red shift and broadening of the absorption spectra are stronger in CS2 than that in n-hexane because of the more sensitive pressure dependence of dispersive interactions in CS2. This was ascribed to the large polarizability and small size of CS2 molecule. The opposite pressure dependent behavior of the S0→S2 transition moment in these two solventions was explained with the effective solvent molecules model, which confirmed that the light-harvesting function of carotenoids can be influenced by the microcosmic factors of the solvent molecules, such as relative dimension, location and orientations. The diverse pressure dependences of several representatives Raman bands were explained using a competitive mechanism involving bond length changes and vibronic coupling. This model shows that the in-phase C=C stretching mode plays an essential role in the internal conversion from S1 to S0 states in carotenoids. It can also be concluded from the Raman spectra thatβ-carotene molecules have undergone a small structural torsion under high pressure. This conclusion offers us valuable information for analyzing the transient absorption spectra ofβ-carotene under high pressure.Density functional theory as implemented in the Gaussian 03 program package was used to investigate the effect ofβ-rings rotation on the potential energy surface and vibrational spectroscopic characteristics ofβ-carotene. It can be found from the calculation that C6-C7 bond ofβ-carotene molecule is easily to be twisted; two stable isomers (cis and trans) having Ci symmetry can be obtained by this rotation; the energy barrier for trans→cis isomerization is quite low. Although the small structural torsion ofβ-carotene molecule under high pressure can not result in the isomerization, it can make the potential energy surface more flat.The high-pressure transient absorption spectra ofβ-carotene in hexane solvention were finally investigated based upon the above experimental and theoretical analysis. We developed a new high-pressure cell that was suitable for the transient spectroscopy, built the femtosecond time-resolved transient absorption spectroscopic system under high pressure, and measured the high-pressure transient absorption spectra ofβ-carotene using this system. The time-resolved spectral data were analyzed by singular value decomposition followed by global fitting. Comparing the pressure dependences of the energy levels and the kinetics behavior of different spectral components, it can be concluded that cis→trans isomerization takes place at S1 state. The rate constant of the radiationless S1→S0 internal conversion process is affected by both the energy gap between S1 and S0 states and the viscosity of solvent. This conclusion can offer important insights into the efficient energy-transfer functions of carotenoids in natural pigment-protein complexes.This work realized the combination of high-pressure conditions and femtosecond time-resolved transient spectroscopic technique. The high-pressure transient absorption spectra ofβ-carotene in the whole white light region were measured for the first time. This is a new technique to investigate ultrafast processes, and therefore opens a fire-new approach to deeply explore physical and chemical issues.更多还原