【作者】 王英惠；

【导师】 杨延强；

【作者基本信息】 哈尔滨工业大学 ， 光学， 2009， 博士

【摘要】 20世纪,人们合成了一系列具有光电特性的有机材料,称为有机光电材料。同无机半导体材料相比,它兼有柔软性、容易制备等优点,已成为材料、物理、化学等领域的热点课题。人们对有机光电材料的合成以有机光电器件的制备开展了大量的研究。有机光电材料从结构上分为有机分子和聚合物两大类。其中,聚合物材料因成膜简单、电输运性能优良、光电转换效率高等优点而引起人们广泛的关注。然而,在器件工作过程中,有机聚合物的很多物理过程和化学问题还不十分清楚,相关的基本理论也不够完善,比如激发态能量的弛豫过程、化学键振动相位关系的耗散和转移以及光致诱导荧光猝灭的机制等等,这些问题制约了有机共轭聚合物的发展和应用。近年来,超快激光光谱技术发展迅速,并成为研究材料光物理过程和光化学问题的有力工具。在本课题中实现了三种超快非线性激光光谱系统,并开展了有机共轭聚合物的光物理过程和光化学问题的研究。聚乙烯咔唑(PVK)和聚(2-甲氧基-5-(2’-乙基己氧基)-1,4-对苯乙炔)(MEH-PPV)是两种具有光电特性的的共轭聚合物。利用上述超快光谱系统,我们研究了它们化学键振动相位关系的耗散、激发态振动相干特性以及光致诱导荧光猝灭等问题。首先,搭建超快多元相干反斯托克斯拉曼散射(CARS)光谱系统,测量基态乙醇分子C-H键的振动动态过程,验证了该激光光谱系统的可靠性。研究发现,在乙醇分子内振动相干关系可从“CH3”基团传输到“CH2”基团,所需时间大约90fs,传输速率约1670m/s,服从“化学键”传输机制。利用该系统研究了有机共轭聚合物PVK在基态时的C-H化学键伸缩振动模的动态过程。探测发现,PVK的C-H化学键伸缩振动模在3000cm?1左右(这与红外光谱测量结果一致),在其时间分辨CARS信号中出现了拍频现象。研究表明,时间分辨CARS信号包含三种成份:PVK中的电子在外加光场调制下的相位变化过程,C-H伸缩振动模的失相过程和C-H伸缩振动模之间的相干耦合过程。通过数据拟合得知,C-H伸缩振动模的失相时间大约是740fs,拍频的周期大约是170fs。其次,利用超快多元光子回波(PE)光谱系统,研究了MEH-PPV的激发态振动相干特性。系统的信号包含自由感应衰减信号(FID)和光子回波(PE)信号,其光谱中包含多个劈裂的特征峰(源自聚合物化学键的振动)。“峰移”(Peak shift)测量表明MEH-PPV激发态具有“相位重构能力”(Rephasing capability),且“相位重构能力”随聚合物激发态的上升而增强。调节光谱系统中激光脉冲之间的时序,获得了时间分辨光子回波信号。研究表明,时间分辨光子回波信号包含主要三种成份:化学键的振动失相过程、化学键的振动能量弛豫过程和量子拍频现象。通过数据拟合得知,相干振动的失相时间约为一百飞秒,而振动能量的弛豫寿命在一个皮秒左右。量子拍频现象源自于主链上化学键的振动相干耦合。最后,通过宽带瞬态光栅光谱系统,探测MEH-PPV光致诱导荧光猝灭的动态过程。研究发现,主链和侧链上的化学键在激光的诱导下因与空气中的氧反应,而发生了断裂。其中主链上C-H键的断裂影响了激子在共轭基团之间的传输,而共轭基团上碳原子之间化学键(包括C-C和C=C)的断裂降低了激子的产量,二者都能降低MEH-PPV的荧光量子效率。侧链上化学键损伤后的产物可认为是PPV类的新型衍生物,其电子结构虽与MEH-PPV相似,但还是略有不同,故其荧光光谱形状和峰位同MEH-PPV相比都有一定的改变。数据分析表明,各个化学键的损伤动态过程和损伤速率与分子能态的关系密切。本课题搭建了多种超快非线性激光光谱系统,并利用上述光谱系统开展了有机共轭聚合物PVK和MEH-PPV的振动相干特性的研究,获得了一系列新颖的有意义的结论,有利于今后深入开展这个领域的相关工作。更多还原

【Abstract】 In the 20th century, people synthesized a series of organic materials with photoelectric properties, which are called organic photoelectric material. In comparison with inorganic semiconductor materials, they have many merits such as soft, easy preparation etc, and become hot spot in the domain of materials, physics, and chemistry. People extensively investigate synthesis of organic photoelectric materials and preparation of organic photoelectric equipments. Organic photoelectric materials can be divided into organic molecules and organic polymer according to their configuration. Organic photoelectric polymer attracts extensive attention because of its easy film preparation, excellent electron transport property and high photoelectric conversion efficiency. However, during the work process of equipment many physical process and chemical problem of organic photoelectric polymer is not clear; some basic theories are not perfect, such as energy relaxation process of excited state, dissipation and transfer of chemical bond’s vibration phase and mechanism of fluorescence quenching due to photoinduced oxidation damage, etc. These problems restrict improvement and application of organic photoelectric polymer.Recently, ultrafast nonlinear laser spectroscopy technique has been improved greatly and become a facilitative tool for investigating the photophysical processes and photochemical problem. In this work, we build three kinds of ultrafast nonlinear spectroscopic systems and investigate photophysical and photochemical properties of organic photoelectric polymer. Polyvinyl carbazole (PVK) and poly[2-methoxy-5-(2′-ethylhexoxy)]-1,4-phenylene vinylene (MEH-PPV) are two typical organic conjugated polymer with photoelectric property. With these spectroscopic systems, vibrational phase dissipation, energy relaxation and photodamage of PVK and MEH-PPV have been detailed investigated.At first, ultrafast multiplex coherent anti-stokes Raman scattering (CARS) spectroscopic system is built, vibrational kinetic process of C-H bonds of ethanol in gournd state is detected, and the reliability of this spectroscopic system is verified. Vibrational coherence would transfer from“CH3”group to“CH2”group, the transfer time is estimated about 90 fs, the velocity is estimated about 1670 m/s and its process follows the mechanism of“bond transfer”. By using this system, the kinetic process of stretching vibrational mode of C-H bonds of PVK in ground state is detected. It is found that the C-H stretching vibrational modes are located at about 3000 cm?1, and the beating phenomenon occurs in their time-resolved CARS signal. Investigation suggests that time-resolved CARS signal consists of three components: the variation of electronic phase in PVK coming from the modulation of external light field, dephasing process of C-H stretching vibration and vibrational coherent coupling process of C-H stretching vibration. Titting results give out a vibration dephasing time of about 740 fs and a beating period of about 170 fs.Secondly, the vibrational coherent characteristic of MEH-PPV at excited state is investigated by using ultrafast multiplex photon echo spectroscopic system. The signal consists of free induced decay (FID) and photo echo (PE) signals. Their spectral line-shape encloses multiple splitting peaks (which come from the vibration of chemical bonds). Peak shift measurement suggests that MEH-PPV own rephrasing capability, which enhances with the excited state of polymer. By modulating temporal sequence among the laser pulses in this system, time-resolved photon echo signal is obtained. It is found that time-resolved photon echo signal consists of three components: vibratioin dephasing process and vibratioinal energy relaxation process of chemical bonds, and quantum beating phenomenon. Fitting results show that dephasing time of coherent vibration is about 100 fs, lifetime of vibrational energy relaxation is about 1 ps and quantum beating phenomenon comes from vibrational coherent coupling of bonds in major chain.At last, the kinetic process of photoluminescence quenching caused by the photon-induced oxidation is detected by using broadband transient grating spectroscopic system. Investigation shows that chemical bonds in the side and major chains react with oxygen in the atmosphere, which accounts for the breakup of chemical bonds. The breakup of C-H bonds in major chain influences the exciton transport among conjugated groups, and the damage of bonds between carbon atoms in conjugated groups (C=C/C-C bonds) leads to the decreasing of exciton yield. Both of them can decrease the fluorescence quantum yields of MEH-PPV. The product of side chain breakup is considered to be a new derivative of PPV. Its electronic structure is similar to that of MEH-PPV, but still has some difference, therefore the line-shape and peak of fluorescence are changed compared with MEH-PPV. Analysis of data suggests that the damage dynamic process and the damage velocities of chemical bonds have close relationship with the molecular energy state.With investigation above, we further investigate vibrational coherent characteristic of organic conjugated polymer and obtain a series of meaningful conclusion, which will be helpful for further research work in this field.更多还原