Understanding Carrier Delocalization and Transport in Micelle Forming Amphiphilic Conjugated Polymers

在化学系大分子、超分子和纳米化学项目的支持下，加州大学洛杉矶分校的Sarah H Tolbert、Benjamin J Schwartz和Yves Rubin教授正在系统地研究水溶液中半导体聚合物的组装，以寻求更有利的导电几何形状。半导体聚合物因其溶液可加工性、低成本和结构可调性而成为一类令人兴奋的光电材料。这些特性使它们在一系列有机电子器件中非常有用，包括光伏、热电、发光二极管和晶体管。然而，共轭聚合物的构象自由导致固有的无序，从而导致导电性差，因此限制了商业适用性。这项研究将解决这些问题，并利用有机合成、结构研究和现代光谱学来探索水溶性两亲性半导体聚合物，这些聚合物可以自组装成圆柱形胶束，作为一种不需要晶体网络就能拉直聚合物链和减少缺陷的方法。在控制聚合物自组装的同时，研究载流子迁移率方面的链构象，在有机电子领域具有广泛影响的潜力，并可能导致开发新的低成本聚合物系统，用于各种将温度和/或光转换为电能的应用，反之亦然。该项目将为本科生和研究生提供参与前沿跨学科研究的机会。为了将纳米结构材料、有机电子学和自组装的想法带给更广泛的受众，与这项工作相关的实验将通过一系列研究生为教师举办的研讨会带到整个洛杉矶地区的中学教室。本研究将重点研究基于聚（环戊二噻吩）-邻噻吩（PCT）骨架的胶束形成两亲性共轭聚合物的合成，并将研究其组装结构如何控制掺杂后的电荷迁移率。在第一个目标中，有机合成和两亲性组装将用于精确控制化学掺杂PCT聚合物中电荷平衡反离子的位置。将制备具有阳离子、阴离子、非离子和两性离子尺寸链的基于pct的聚合物，并使用液相小角x射线散射（SAXS）来表征胶束形成。将用铁（III）盐在水中掺杂，并使用稳态和瞬态红外/可见吸收光谱探测载流子的数量和性质。基于理论计算和模型，双电荷侧链和/或二价溶液相反离子将被用于控制极化子在高掺杂密度下成对成双极化子。为了创造阴离子侧链作为极化子的反离子的体系，阴离子和两性离子或非离子聚合物的共聚物将被应用。这种方法将消除溶液中额外离子的需要。第二个目标将针对更容易化学掺杂的新型聚合物骨架。最终目标将寻求开发方法，将优化的组件从水溶液过渡到固态，以创造具有更高导电性的新材料。与本研究相关的控制聚合物构象和电荷定位的综合方法有可能为进一步了解共轭聚合物中的基本载流子动力学提供重要的策略。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

With the support of the Macromolecular, Supramolecular and Nanochemistry program in the Division of Chemistry, Professors Sarah H Tolbert, Benjamin J Schwartz and Yves Rubin of the University of California at Los Angeles are systematically studying assemblies of semiconducting polymers in aqueous solutions in order to seek more favorable geometries for electrical conductivity. Semiconducting polymers are an exciting class of optoelectronic materials because of their solution processability, low cost, and structural tunability. These characteristics make them useful in a range of organic electronic devices, including photovoltaics, thermoelectrics, light-emitting diodes, and transistors. However, the conformational freedom of conjugated polymers leads to intrinsic disorder that can result in poor electrical conductivity and hence limited commercial applicability. This research will address these issues and use organic synthesis, structural studies, and modern spectroscopy to explore water-soluble amphiphilic semiconducting polymers that self-assemble into cylindrical micelles as a way to straighten polymer chains and reduce defects without the need for a crystalline network. The efforts toward controlling polymer self-assembly while interrogating chain conformation with respect to carrier mobility have the potential for broad impact in the field of organic electronics and could lead to a development of new and low-cost polymeric systems for a variety of applications in which temperature and/or light are converted to electricity and vice versa. The project will provide opportunities for undergraduate and graduate students to be involved in cutting-edge interdisciplinary research. In an effort to bring the ideas of nanostructured materials, organic electronics, and self-assembly to a broader audience, experiments related to this work will be brought to secondary school classrooms throughout the greater Los Angeles area via a series of graduate-student run workshops for teachers. This research will focus on the synthesis of micelle-forming amphiphilic conjugated polymers based on poly(cyclopentadithiophene)-alt-thiophene (PCT) backbones, and will investigate of how their assembled structure controls charge mobility upon doping. In the first objective, organic synthesis and amphiphilic assembly will be used to precisely control the position of charge-balancing counterions in chemically-doped PCT polymers. PCT-based polymers with cationic, anionic, non-ionic, and zwitterionic size chains will be prepared and solution-phase small-angle X-ray scattering (SAXS) will be used to characterize micelle formation. Doping will be achieved with iron(III) salts in water and the number and nature of charge carriers will be probed using steady-state and transient IR/visible absorption spectroscopy. Based on theoretical calculations and modeling, doubly charged side chains and/or divalent solution-phase counterions will be employed to control polaron pairing into bipolarons at high doping densities. In order to create systems where the anionic side chains serve as counterions for the polarons, copolymers of anionic and either zwitterionic or non-ionic polymers will be applied. Such an approach will eliminate the need for additional ions in solution. The second objective will target new polymer backbones that are easier to chemically dope. The final goal will seek to develop methods to transition optimized assemblies from aqueous solutions into the solid state to create new materials with improved conductivity. The comprehensive approach for controlling polymer conformation and charge localization associated with this research has the potential to provide important strategies to further understand fundamental charge carrier dynamics in conjugated polymers.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.