Visualizing the multi-level assembly structures of conjugated molecular systems with chain-length dependent behavior

It remains challenging to understand the structural evolution of conjugated polymers from single chains to solvated aggregates and film microstructures, although it underpins the performance of optoelectrical devices fabricated via the mainstream solution processing method. With correlative imaging of cryogenic electron microscopy and liquid-phase electron microscopy, we observed the morphological evolution of multi-level assembly of a model system of isoindigo-based conjugated polymer. In confirming the ensemble measurements in solution (UV-vis) and solid state (grazing-incidence wide-angle X-ray scattering), we further unraveled the hidden molecular assembly pathways, the mesoscale network formation, and their unorthodox chain dependence. Oligomers and short polymer chains showed rigid conformations forming discrete aggregates in solution, which further grew to form a highly ordered film that exhibits poor charge transport performance. In contrast, long polymer chains exhibited flexible chain conformations, creating interlinked aggregates networks in solution, which directly imprinted into films, forming interconnective solid-state microstructure with excellent electrical performance. Our work reports direct visualization methods of the structural evolution of polymer aggregates in solution and systemically studies the chain-length dependent aggregation and charge transport of conjugated polymers. Visualizing multi-level assembly structures of conjugated polymers provides a deep understanding of the inheritance of assemblies from solution-state aggregation to solid-state microstructures, accelerating the optimization of solution processing and device fabrication.共轭聚合物从单链到溶剂化聚集体及薄膜微结构的结构演化，是主流溶液加工法制备光电器件性能的核心支撑，但阐明该演化过程仍颇具挑战。依托冷冻电子显微镜（cryogenic electron microscopy）与液相电子显微镜（liquid-phase electron microscopy）的关联成像技术，我们针对基于异靛的共轭聚合物模型体系，观测到其多级组装的形貌演化过程。在验证溶液相中（紫外-可见光谱，UV-vis）与固态下（掠入射广角X射线散射，grazing-incidence wide-angle X-ray scattering）的系综测量结果后，我们进一步揭示了隐藏的分子组装路径、介观尺度网络的形成过程，及其反常的链长依赖性。低聚物与短链聚合物呈现刚性构象，在溶液中形成离散的聚集体，随后进一步生长为高度有序的薄膜，但该薄膜的电荷传输性能较差。与之相反，长链聚合物则呈现柔性链构象，在溶液中形成相互连接的聚集体网络，该网络直接复制至薄膜中，形成具有优异电学性能的互连固态微结构。本工作报道了溶液中聚合物聚集体结构演化的直接可视化方法，并系统研究了共轭聚合物的链长依赖型聚集行为与电荷传输特性。可视化共轭聚合物的多级组装结构，可深入理解组装体从溶液态聚集到固态微结构的继承关系，从而加速溶液加工工艺与器件制备的优化进程。