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

该奖项支持有关光如何与有机分子构成的半导体材料相互作用的理论/计算研究和教育。人们更熟悉的半导体，如广泛用于微电子和计算机芯片的硅，是无机的。然而，基于有机分子的半导体具有材料加工成本较低、机械性能较好等优点。有机材料继续进军商业设备，如用于显示器(OLED电视)的有机发光二极管或OLED，将太阳光转换为电能的太阳能电池，甚至“可穿戴”的电子设备和传感器。PI和他的研究团队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情况。研究小组将通过求解基于量子力学的方程来进行理论研究，这些方程描述了有机分子对光的反应。这些方程将使用复杂的计算机算法来求解。特别令人感兴趣的是某些被称为准分子的电子激发态，它们发射波长更长的光，将电子能量转化为光能的效率较低。PI和他的团队将研究这种准分子是如何形成的，并最终研究如何设计避免形成准分子的分子聚集体。拟议的活动还将通过与普渡大学的黄立白教授等实验者合作来加强研究基础设施，黄立白教授将使用国家或最先进的实验技术来探索有机薄膜中的能量传输。总体而言，这项研究工作应该有助于绘制基于有机材料的下一代电子设备的蓝图。该奖项支持有关有机分子半导体材料如何吸收或发射光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的半导体材料，pi共轭分子和聚合物的固相材料继续受到广泛的关注。然而，尽管Kasha在分子H-和J-聚集体方面的开创性工作进行了50多年的密集的实验和理论研究，但关于光激发的命运以及它们的光谱特征如何依赖于晶体堆积和形态仍然存在重要的问题。Pi的小组扩展了Kasha的模型，该模型完全建立在长程库仑耦合的基础上，包括了分子间电荷转移引起的短程(超交换)耦合，以及与乙烯基伸缩模式的局部耦合，这些耦合导致了大量共轭分子的UV-Vis光谱中显著的振动级进。尽管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，但它描述光致发光和能量传输的能力有限，这一过程往往需要包括激子。激发子，常见于圆周率共轭分子，当电子激发态沿“慢”分子(声子)坐标松弛时，产生无特征的红移发射。在这个项目中，将开发下一代分子H-和J-聚集体模型，用于解释准分子的形成和发射。该方法基于包含沿着慢坐标的电子耦合的Holstein-Peierls哈密顿量，在像π堆栈这样的紧密堆积系统中尤其强烈，其中分子间电子和空穴转移积分对相邻发色团的相对取向的小的亚埃变化非常敏感。Holstein-Peierls方法使所有重要的物理过程都能在平等的基础上完全量子力学地处理。该模型将被用来解释准分子形成的二聚体二聚体络合物和较大的π堆栈的吸收和光致发光谱线形状。我们将通过对密度矩阵运动方程的分析来研究准分子作为能量陷阱的能力。普渡大学的黄立白将合作进行光谱测量和飞秒分辨输运测量，以提供实验验证，这些测量显示出不同程度的准分子发射。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，以及为设备应用控制准分子形成的设计策略。这一奖项反映了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。

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.