Understanding Excimers in Molecular J- and H-aggregates: A Holstein-Peierls Approach

该奖项支持光与由有机分子组成的半导体材料相互作用的理论/计算研究和教育。我们更熟悉的半导体，如广泛用于微电子和计算机芯片的硅，都是无机的。然而，基于有机分子的半导体具有诸如更便宜的材料加工和更有利的机械性能等优点。有机材料继续进入商业设备，例如用于显示器（“OLED”电视）的有机发光二极管或OLED，将阳光转化为电能的太阳能电池，甚至是“可穿戴”电子设备和传感器。PI和他的研究小组将研究有机晶体和聚集体中的光吸收和光发射等基本过程，以及光吸收的能量如何在分子之间传输——类似于植物在光合作用过程中发生的过程。研究小组将通过解决基于量子力学的方程来进行理论研究，量子力学描述了有机分子如何对光的反应。这些方程将用复杂的计算机算法求解。特别令人感兴趣的是某些被称为准分子的电子激发态，它们发射波长较长的光，并且将电子能量转换为光能的效率较低。PI和他的团队将研究这些准分子是如何形成的，以及最终如何设计避免准分子形成的分子聚集体。拟议的活动还将通过与普渡大学的黄利白教授等实验家的合作，加强研究基础设施，他将采用最先进的实验技术来探测有机薄膜中的能量传输。总的来说，这项研究工作应该有助于为下一代基于有机材料的电子设备绘制蓝图。该奖项支持关于光如何从有机分子制成的半导体材料中吸收或发射的理论和计算研究和教育。作为场效应晶体管、发光二极管和太阳能电池中的半导体材料，π共轭分子和聚合物的固相继续受到广泛关注。然而，尽管在Kasha对分子H-和j -聚集体的开创性工作之后进行了超过50年的密集实验和理论研究，关于光激发的命运以及它们的光谱特征如何依赖于晶体包装和形态的重要问题仍然存在。PI的团队已经扩展了Kasha的模型，该模型完全基于远程库仑耦合，包括由分子间电荷转移引起的短程（超交换）耦合，以及乙烯基拉伸模式的局部耦合，这导致了许多共轭分子的紫外-可见光谱中明显的振动进展。虽然该模型可以定量准确地预测吸收谱线形状的细节，并将光谱特征与潜在激子的性质相关联，但它在描述光致发光和能量传输方面的能力有限，这些过程通常需要包含准分子。准分子常见于π共轭分子，当电子激发态沿着“慢”分子间（声子）坐标松弛导致无特征的红移发射时，就会产生准分子。在这个项目中，下一代分子H-和j -聚集模型将被开发出来，用于解释准分子的形成和发射。该方法基于荷尔斯坦-佩尔斯哈密顿量，其中包括沿慢坐标的电子耦合，在π堆叠等紧密堆积的系统中特别强，其中分子间电子和空穴转移积分对相邻发色团相对方向的小，亚埃的变化非常敏感。Holstein-Peierls方法使所有重要的物理过程都能以平等的基础和完全的量子力学来处理。该模型将用于解释准分子形成的苝二亚胺二聚物和较大π堆积的吸收和光致发光谱线形状。准分子作为能量陷阱的能力将通过分析密度矩阵运动方程来研究。普渡大学的黄立白将合作，并通过对几种显示不同程度准分子发射的苝二亚胺衍生物进行光谱测量和飞秒分辨输运测量，提供实验验证。PI的方法可能会增加发现新的和潜在有用的物理现象的可能性，以及控制器件应用中准分子形成的设计策略。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。

NONTECHNICAL SUMMARY This award supports theoretical/computational research and education on how light interacts with semiconductor materials made of organic molecules. The more familiar semiconductors like silicon, which is widely used in microelectronics and computer chips, are inorganic. However, semiconductors based on organic molecules offer advantages such as less expensive materials processing and more favorable mechanical properties. Organic materials continue to make inroads into commercial devices, such as organic light-emitting diodes or OLEDs used in displays (the “OLED” TV), solar cells, which convert sunlight into electrical energy, and even “wearable” electronic devices and sensors. The PI and his research team will investigate fundamental processes such as light absorption and light emission in organic crystals and aggregates, as well as how the energy from light absorption is transported between molecules - similar to what occurs in plants during the process of photosynthesis. The research team will conduct a theoretical investigation by solving equations based on quantum mechanics which describe how organic molecules respond to light. The equations will be solved using sophisticated computer algorithms. Of particular interest are certain electronic excited states known as excimers, which emit light of longer wavelengths and convert electronic energy into light energy less efficiently. The PI and his team will study how such excimers form and ultimately how to design molecular aggregates which avoid excimer formation. The proposed activities will also enhance research infrastructure through collaborations with experimentalists such as Professor Libai Huang at Purdue University, who will employ state-or-the-art experimental techniques to probe energy transport in organic films. Overall, this research effort should contribute to a blueprint for the next generation of electronic devices based on organic materials.TECHNICAL SUMMARY This award supports theoretical and computational research and education on how light is absorbed or emitted from semiconductor materials made of organic molecules. Solid phases of pi-conjugated molecules and polymers continue to receive widespread attention as semiconducting materials in field effect transistors, light emitting diodes, and solar cells. However, despite the more than five decades of intensive experimental and theoretical research following Kasha's pioneering work on molecular H- and J-aggregates, important questions remain regarding the fate of photo-excitations and how their spectral signatures depend on crystal packing and morphology. The PI’s group has extended Kasha’s model, which is predicated entirely on long-range Coulombic coupling, to include short-range (super-exchange) coupling arising from intermolecular charge-transfer, as well as local coupling to the vinyl-stretching mode responsible for pronounced vibronic progressions in the UV-Vis spectra of a great many conjugated molecules. Although the model can predict with quantitative accuracy details of the absorption spectral line shape and correlate spectral features to the nature of the underlying excitons, it is limited in its ability to describe photoluminescence and energy transport, processes which often require the inclusion of excimers. Excimers, common in pi-conjugated molecules, arise when an electronic excited state relaxes along a “slow” intermolecular (phonon) coordinate resulting in featureless, red-shifted emission. In this project, the next generation of molecular H- and J-aggregate models will be developed which account for excimer formation and emission. The approach is based on a Holstein-Peierls Hamiltonian which includes electronic coupling along the slow-coordinate, is particularly strong in closely packed systems like π-stacks, where the intermolecular electron and hole transfer integrals are hypersensitive to small, sub-Angstrom, changes in the relative orientation of neighboring chromophores. The Holstein-Peierls approach enables all of the important physical processes to be treated on equal footing and fully quantum-mechanically. The model will be employed to account for the absorption and photoluminescence spectral line shapes in excimer-forming perylene diimide dimer complexes and larger π-stacks. The ability of excimers to function as energy traps will be investigated through analysis of the density matrix equations of motion. Libai Huang at Purdue University will collaborate and provide experimental validation by conducting spectroscopic measurements and femtosecond-resolved transport measurements of several perylene diimide derivatives which display varying degrees of excimer emission. The PI’s approach may enhance the likelihood for discovering new and potentially useful physical phenomena, as well as design strategies for controlling excimer formation for device applications.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.