共轭聚合物的合成、自组装行为及其光电性质研究

【作者】 马谆；

【导师】 黄维；

【作者基本信息】 复旦大学 ， 高分子化学与物理， 2007， 博士

【摘要】 本论文的研究内容主要涉及共轭聚合物光电材料的合成、光电性质及其在表面自组装行为的研究。上世纪九十年代以来，共轭聚合物光电材料的研究已经成为当今高分子科学的一个研究热点。共轭聚合物光电材料在有机发光二极管、场效应晶体管、太阳能电池、生物与化学传感等领域的应用研究方兴未艾。各种不同结构、不同发光波长的共轭聚合物正在被源源不断地开发出来。其中，超支化结构由于其合成相对简单，且具有某些优良的性能，正在被逐步地引入到共轭聚合物体系中。同时将低能级的发光染料共聚到高能级的共轭聚合物上，是进行发光波长调节，甚至实现单一聚合物链白光发射的有效手段。这时体系内的能量转移过程对于发光波长和荧光效率都有着决定性的影响。由于当前的能量转移研究全部集中于线性链结构，所以，我们就对超支化共轭聚合物内的能量转移进行了系统的研究。另一方面，共轭聚合物/寡聚物是迄今为止最有前途的光电分子器件的备选材料。而这些器件的效能与共轭分子的结构、纯度、材料形貌以及超分子结构有着密切的关系。已有的经验证明，在有机光电材料的研究上，只有将有机合成所得到的宏观化学结构与微观尺度的分子空间分布联系起来才有望实现基于分子水平的器件性能提升，并实现用分子结构的设计来调控最终材料性能的目标。因此，我们设计、合成了带有功能性基团的共轭聚合物和寡聚物，用扫描探针显微镜对它们在石墨表面的自组装结构进行了详细的研究，对可能的影响因素进行了综合的归纳、总结。对共轭聚合物在界面组装行为的研究，为进一步设计、合成π共轭聚合物以及它们在纳米尺度的可控构筑提供了经验，也为开发有机光电分子器件奠定了一定的基础。本论文的工作就是在文献调研的基础上，结合本课题组以往工作的基础和项目申请的实际情况，对共轭聚合物的光物理性质及其在界面的自组装行为进行了系统的研究，主要分为三个部分，分别简要叙述如下：第一部分中，我们将不同含量的支化单元(间位三取代苯)引入到芴和苯噻唑的共聚物中，以此构建支化度分别为5％，15％及40％的超支化共聚物。这些超支化聚合物有较好的热稳定性，其分解温度随着支化度的上升而升高。退火后光谱的红移值及峰形变化较小也说明了超支化结构有助于提高分子的热稳定性。另外，我们还特别研究了这些超支化聚合物中的能量转移过程及其与聚合物荧光效率之间的关系。其中，低能级的共聚单体2，1，3—苯噻唑在体系中充当激子陷阱，芴单元受激发而产生的激子会通过链内或(和)链间能量转移的方式传递到苯噻唑单元。由此产生的一个结果便是在浓溶液和固态膜条件下，从芴单元发出的蓝光被完全猝灭，只呈现对应于苯噻哗单元的绿光发射。但是在稀浓液和亚浓浓液中，这类超支化聚合物的能量转移效率随着溶液的浓度和支化度会发生很大的变化。实验数据表明随着支化度的上升，聚合物的紫外和荧光光谱都呈现蓝移的趋势；这是由于支化单元的引入打断了线性链原有的共轭链长。另外，在本体系中溶液临界浓度被用于衡量能量转移效率，其数值随着支化度的升高而增大。这说明了链内的能量转移在聚合物的亚浓溶液中起着能量转移的主要作用。然而在薄膜态，由于分子间的距离变得非常小，所以只出现低能级的520 nm左右的绿光发射峰。循环伏安法显示，在本体系中，超支化结构会阻碍电荷的注入，这有可能会降低相应器件的量子效率。所有这些结果表明了高度支化的结构能有效地阻止链内和链间的能量转移，其还显著地影响了固态中能量传递的过程，进而影响薄膜态的荧光效率。第二部分是本课题组与北京国家纳米科学中心的合作研究项目。我们合成了系列的共轭聚合物/寡聚物，并利用扫描探针显微镜对它们在界面自组装所形成的高度有序的、稳定的超分子结构进行了表征，探究了影响共轭分子自组装行为的各种作用因素。主要包括两方面内容：第四章，我们合成了一系列具有相同主链不同侧链的聚苯炔(PPE)共轭聚合物，并在高定向热解石墨(HOPG)和辛基苯溶液的两相界面，采用STM在分子尺度观察了共轭聚合物链的二维组装结构及其链折叠的情况。系统研究了各种相互作用(分子间、分子与基底间)以及分子链的刚性对于超分子有序结构的影响。STM观察的结果证明了侧链的结构、取代密度及其相互作用的强弱不仅会影响聚合物在表面的超分子结构，还对主链的折叠产生决定性的作用。通过与已有文献报道的聚烷基噻酚石墨表面组装行为的比较，揭示了聚合物主链的刚性在其折叠方式和折叠程度方面也起着重要的作用。另外，通过STM测量分子链的长度，我们计算得出了相应聚合物的分子量和聚合度，并将结果与凝胶渗透色谱的数据进行了比较分析。发现由STM测得的分子量分布很好地符合Schulz-Zimm分布，也进一步证实了测量和计算结果的真实性、有效性；亦表明STM对于共轭聚合物在基底表面组装结构的研究中可以发挥不可替代的作用。第五章，我们合成了一系列的端基分别为一、二、三个羧酸的星状寡聚芴。通过羧酸基团，我们将氢键作用引入到了星状分子的各条臂上，而这种结构和已被广泛研究的苯三酸(trimestic acid)体系形成了类似的结构：这也是启发我们设计这个分子结构的原因。从较简单的氢键作用——羧酸基团开始，研究如何利用氢键作用诱导共轭光电分子在表面的有序组装。在普通的大气条件下，在辛基苯或辛酸和高定向热裂解石墨所形成的两相界面上用STM观察了这些星状寡聚芴分子的组装形态。我们发现烷基链的长度和羧酸端基的数目对于自组装单层膜(SAM)的形成有着至关重要的作用。由于氢键间的相互作用、样品和石墨基底间的π-π作用以及长链烷基的稳定化作用，使得每个带有正十二烷侧链的星状寡聚芴都能形成稳定的分子组装结构；而与此相对应的是，带有丁基侧链的星状寡聚芴则无法形成稳定的组装结构。另外，氢键数目的不同也产生了完全不同的组装结构，特别的是对于三羧酸星状寡聚芴，在辛基苯/石墨界面上没有能独立形成苯三酸式的六边形蜂窝状结构，而是需要加入客体分子以帮助这种结构的稳定。同时，三羧酸星状寡聚芴却能在辛酸与HOPG的两相界面上自身形成蜂窝状的稳定结构。所以对这两种情况，我们也分别进行了STM研究。最后一部分内容是本人在曾经担任实验室基质辅助激光解吸/离子化飞行时间质谱(MALDI-TOF MS)仪器助管近两年时间内，在大量的样品测试过程中，发现共轭聚合物在该质谱检测手段下表现出的一些特殊现象，并通过另外的辅助实验分析将这些现象与共轭聚合物的相关内在性质联系在一起，给予了理论的解释，并该章节中进行总结。更多还原

【Abstract】 The research areas of this dissertation are focused on syntheses, photophysical characterization and self-assembly behaviors of light-emitting conjugated polymers.Since the 1990’s,π-conjugated polymers have attracted many interests owing to their wide-ranging application potentials in the development of organic light-emitting diodes, field effect transistors, solar cells, chem-/biosensors. A variety of conjugated polymers with different structures, different light-emitting wavelength are being developed. Due to the simple preparation process and some advantages in properties, hyperbranched structures have been introduced into conjugated polymers. Meanwhile, the copolymerization of lower band-gap monomers with higher band-gap chromophores is one of the most promising methods to tune the light wavelength and even realize white light-emitting from a single polymer chain. In this system, energy transfer process has remarkable influences on the light-emitting colors and fluorescence efficiency. However, nearly all present researches are focused on energy transfer in linear conjugated polymer backbones, therefore, we designed to make a systematic research on energy transfer in hyperbranched conjugated polymers.On the other hand, conjugated polymers and oligomers are the most promising candidates for the materials of opto-electronic molecular devices. And the structure, purity, morphology and supramolecular orientation of conjugated molecules show dominant effects on the properties of devices. The experience demonstrates that the properties of molecular devices will be further improved if the relation between macroscopic chemical structures and microscopic molecular ordering is established. And so the target of tuning material properties by molecular design will be realized. Therefore, we designed, synthesized a series of conjugated polymers and oligomers with functional groups and employed scanning probe microscopy to observe their self-assembly structures on the graphite surface. It provides information for the further design, synthesis and controlled nano-sized fabrication ofπconjugated polymers and builds a solid base for the future development of organic opto-electronic molecular devices.In this dissertation, based on the previous reports and the research experience of our group, we did the systematic research on photophysical properties of conjugated polymers and their self-assembly behaviors at the interface, including three major aspects: In the first project, we introduced different contents of branching units （1,3,5-substituted benzene rings） into the poly（fluorene-co-benzothiazole） to construct the hyperbranched copolymers with branching degrees ranging from 5% to 40%. These highly branched polymers possessed good thermal stability and their decompostion temperatures increased with the increase of branching degrees. The spectra of these polymers did not show obvious changes after annealing them at 200℃in air, which also demonstrated highly branched structures led to better thermal stability. Furthermore, the energy transfer properties and their correlation with PL efficiencies of hyperbranched conjugated copolymers were studied. The narrow band gap comonomer 2,1,3-benzothiadiazole acts as a powerful exciton trap which allows efficient energy transfer of the exciton from fluorene segment to benzothiazole unit in the copolymers. As a result, the blue emission from the fluorene segment is completely quenched in concentrated solutions and in the solid state. But with the change of the solution concentration and the branching degrees; the efficiency of energy transfer of these hyperbranched polymers varied in solutions. Our experiment data demonstrated that the absorption and emission peaks of these polymers are blue-shifted gradually with the increase of branching degrees, which suggested that introduction of branching units interrupted the linearπ-system to some extent and reduced the effective conjugation length. Meanwhile, the critical concentration C* was used to scale the energy transfer properties in this system and its value increased with branching degrees. This indicates that intrachain interaction play a major role in the energy transfer in dilute solution and especially in these hyperbranched polymers with high branching degrees. However, as films, because of very close distance among all chromophores, only low band gap units （benzothiazole） emit luminescence around 520 nm. Cyclic voltammetry showed that hyperbranched structure in this system hindered the charge injection, which might result in low quantum efficiency for device application when. highly branching degrees. All these results demonstrated that highly branched structure would effectively impede the intra- and interchain energy migration, especially in solutions; and remarkably influence the energy transfer process in the solid state.The second project of this dissertation is the National Science Foundation supported research project cooperated with National Center for Nanoscience and Technology, China. A series of conjugated polymer and oligomers were synthesized and the highly ordered and stable supramolecular structures of these molecules at the liquid-solid interface were recorded by STM. We also summarized some effects which controlled their self-assembly behaviors.In chapter four, a series of PPEs with identical conjugated backbone but different side chains were synthesized and their two-dimensional assembly structure and chain foldings were studied by STM. The supramolecular fabrication influenced by the interactions between molecules, between molecules and substrate and the rigidity of polymer chains, were systematically discussed. The STM results demonstrated that the concentration and structure of side chains not only affected the supramolecular order on the interface, but also the chain folding in the adsorbed layer. The comparison of the chain folding of these rigid-rod polymers with that of the more flexible P3ATs illustrated that besides the interaction between polymer chains or the interaction between the side chains the rigidity of the conjugated backbone also plays an important role on the molecular assembling and chain folding at the interface. The degree of polymerization of these polymers are directly determined by STM and compared with that obtained from conventional gel permeation chromatography measurements. The statistics of the polymer contour length from the submolecularly resolved STM images shows satisfactory agreements with the theoretically expected Schulz-Zimm distribution, demonstrating the reliability of STM methods and the great importance of STM in the research of surface nano-fabrication.In chapter five, we synthesized star-shaped {C}₃-oligofluorenes end-capped with one, two, three carboxylic groups, respectively. Hydrogen bondings induced from carboxylic groups were intentionally designed at the end of the arm（s） of these star-shaped molecules. The hydrogen bonding positions of the so-perpared star-shaped oligofluorenes were similar to those of trimestic acids （TMA） which has been intensively studied, and this enlightened us to design these moleucles. Based on the simplest hydrogen bonding interactions from carboxylic acid groups, we initialize our research on hydrogen bonding induced well-ordered self-assembly of opto-electronic molecules at the interface. The assembly of these star-shaped oligomers at the interface between highly oriented pyrolytic graphite and octylbenzene or octanoic acid under ambient conditions was revealed by STM. The results demonstrated that the length of the side chains and the number of carboxylic acid groups considerably affected the final structure of self-assembly monolayers. Under the assistance of hydrogen bonding interactions,π-πinteractions between the molecules and the substrate and the stablization effects from long side chains, every star-shaped oligomers with dedocane as side chains could form a stable assembly structure, on the contrary, those with butane as side chains could not. Moreover, the number of hydrogen bonds also led to completely different assembly patterns, especially as to star-shape oligofluorene with three -COOH end-groups which fabricated honeycomb structures like TMA only when they were dissolved in octanoic acid solution, and could not produce such an ordered network at the octylbenzene-HOPG interface unless the coadsorption with guest molecules. We also investigated these two conditions respectively.The research in the chapter 6 is based on my works on the operation of Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry as a research assistant in nearly two years. When employing this equipment to analyze some conjugated polymers, I found some novel and interesting results which was also related to the intrinsic properties of these opto-electronic materials. And I concluded these features and gave explanations in this chapter.更多还原