Electronic Coupling and Polymorphic Heterogeneity in Singlet Fission Microcrystals Studied with 2D White-Light Microscopy

在这个由化学系化学结构、动力学和机制A(CSDM-A)计划资助的项目中，威斯康星大学马丁·赞尼教授的实验室探索了一些材料的一种新特性，可以将光转化为电。在这些材料中，当一个光子或光单位击中材料时，多个电子可以带走它的电能，这一过程被称为单态裂变，而在大多数材料中，只有一个电子被一个光子激发。进行单线态裂变的系统有助于太阳能电池和光电探测器等光伏设备的设计。虽然已经在晶体中观察到单线态裂变，但它发生的详细机制还没有建立起来。理论和实验研究表明，分子在晶体中的排列对决定驱动单线态裂变的电子和电荷转移耦合起着重要作用。因此，不同的晶型(即同一分子的不同晶体堆积/排列)往往表现出不同的单线态裂变效率。通过研究两个模型系统，这个项目提供了更好的理解分子结构，它的堆积成晶型，以及它的电荷产生是如何关联的。这些实验有助于更好地理解如何利用单线态裂变来更有效地产生电荷。Zanni教授和他的学生还继续参与教育活动，包括指导本科生研究学生，参与为少数族裔、低收入和第一代研究生提供同行指导和其他支持的计划，以及向当地小学展示科学表演和化学练习册活动。Zanni教授和他的学生正在研究三异丙基硅乙炔(TIPS)-并五苯和Rubrene两个分子系统中的单线态裂变，具体目标是识别和了解非平衡结构对单线态分裂的影响。TIPS-并五苯是一种溶液可加工的有机半导体，能够进行快速有效的单线态裂变。在之前由美国国家科学基金会资助的工作中，Zanni教授的团队揭示了TIPS-五苯根据成膜过程采用不同的堆积结构。即使晶体结构的微小变化也会改变两个相邻分子之间的相互作用，改变单态、三态和电荷转移态的电子耦合。研究发现，TIPS-并五苯微晶包含少量滑移堆积几何结构，改善了三重态的产生，并实现了单态和三重态之间的平衡。在这个项目中，Zanni教授和他的团队旨在扩大对电子耦合中的空间变化如何通过缺陷和多态影响单线态裂变和其他能量转移过程的理解。通过改变生长条件、退火、成像晶体生长等实验对滑移堆积结构进行了研究。Rubrene是另一种经历单态分裂并结晶成几种不同晶型的材料。为了了解分子几何和电荷转移动力学之间的关系，对每一种晶型的单态裂变动力学都进行了详细的研究。研究中使用了二维白光(2D WL)光谱和宽带瞬变吸收显微镜()。第二代2D WL显微镜正在建造中，脉冲位于光谱的蓝色部分。这台新仪器的光谱范围允许研究Rubrene和许多其他重要物质，它们比TIPS--并五苯吸收波长更短。这种显微镜有望通过独立扫描泵浦和探测脉冲的位置来实现激子扩散的成像。晶体堆积和电子耦合的空间异质性对激子和电荷输运过程有很大的影响。此外，Zanni教授和他的团队帮助科学界和业界成员收集和模拟2D IR和2D WL数据，并参与上述教育和推广活动。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In this project, funded by the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program of the Division of Chemistry, Professor Martin Zanni’s laboratory at the University of Wisconsin explores a novel property of some materials that can convert light into electricity. In these materials, when one photon, or unit of light, hits the material, multiple electrons can carry away its electrical energy, a process known as singlet fission, whereas in most materials only one electron is excited by one photon. Systems undergoing singlet fission can benefit the design of photovoltaic devices like solar cells and photodetectors. Although singlet fission has been observed in crystals, the detailed mechanism by which it occurs is not established yet. Theoretical and experimental studies show that the arrangement of molecules in a crystal plays an important role in determining electronic and charge transfer couplings that drive singlet fission. Hence, different crystal polymorphs (i.e. different crystal packing/arrangement of the same molecule), often exhibit different singlet fission efficiencies. By studying two model systems, this project is providing better understanding of how molecular structure, its packing into polymorphs, and its charge generation are related. These experiments are contributing to a better understanding of how singlet fission might be utilized for more efficient charge generation. Professor Zanni and his students also continue their involvement in educational activities, including mentoring undergraduate research students, participating in a program to provide peer-mentoring and other support for minority, low-income and first-generation graduate students, and presenting a science show and chemistry worksheet activities to local elementary schools.Professor Zanni and his students are studying singlet fission in two molecular systems, triisopropylsilylethynyl (TIPS)-pentacene and rubrene, with the specific goal of identifying and understanding the impact of non-equilibrium structures on singlet fission. TIPS-pentacene is a solution processable organic semiconductor capable of undergoing rapid and efficient singlet fission. In previous NSF-funded work, Professor Zanni’s group revealed that TIPS-pentacene adopts different packing structure depending on the film formation procedure. Even small changes to the crystal structure alter the interactions between two neighboring molecules, changing the electronic couplings of the singlet, triplet, and charge transfer states. It was found that TIPS-pentacene microcrystals contain small populations of slip-stacked geometries that improve triplet state generation and also enable equilibration between the singlet and triplet states. In this project, Professor Zanni and his group aim to extend the understanding of how spatial variation in electronic couplings through defects and polymorphs impact singlet fission and other energy transfer processes. The slip-stacked structures are being studied by varying growth conditions, annealing, imaging crystal growth, and other experiments. Rubrene is another material that undergoes singlet fission and crystallizes into several different polymorphs. Singlet fission kinetics are being studied in each polymorph, in detail, to understand the relation between molecular geometry and charge transfer dynamics. State of the art two dimensional white light (2D WL) spectroscopy and broadband transient absorption microscopy (TAM) are used for the studies. A second-generation 2D WL microscope is being built with pulses centered in the blue part of the spectrum. The spectral range of this new instrument allows for study of rubrene and many other important materials which absorb at shorter wavelengths than TIPS-pentacene. This microscope is expected to enable the imaging of exciton diffusion by independently scanning the position of the pump and probe pulses. Crystal packing and spatial heterogeneity of electronic couplings can strongly impact exciton and charge transport processes. In addition, Professor Zanni and his group assist members of the scientific community and industry in collecting and simulating 2D IR and 2D WL data, as well as participate in the educational and outreach activities discussed above.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.