Modeling Molecular Aggregate Photophysics in Free Space and in Optical Microcavities

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。最熟悉的半导体可能是用于微电子和现代计算机内部芯片的硅。然而，基于有机分子的半导体继续进入商业设备，如有机发光二极管或oled。薄的有机薄膜可以用电驱动来发光，或者可以用来将太阳能转化为电能。PI和他的研究小组将研究有机晶体或聚集体吸收和发射光的基本过程。PI还将研究吸收的能量如何在分子之间运输，这与植物在光合作用过程中如何运输能量类似。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。PI还将研究将一层薄薄的有机薄膜包裹在一个非常小的“微”腔内的效果，该腔由两个反射镜组成，反射镜之间的距离大约等于一个波长的光。微腔增强了光与封闭分子之间的相互作用，可以显著改变封闭有机膜的行为，使人们能够更好地控制其光学特性。拟议的活动还将通过国内和国际合作加强研究基础设施，其中包括普渡大学的黄利白教授，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，以及智利圣地亚哥大学的Felipe Herrera博士，他将协助有机微腔的理论研究。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。该奖项支持关于如何从有机分子制成的半导体材料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，但关于分子聚集体光激发的性质以及光学响应与晶体堆积和形态的关系仍然存在重要问题。PI和他的研究小组最近扩展了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，这些电荷转移是在分子间紧密接触的填料安排中产生的。虽然该模型可以定量准确地预测吸收谱线形状的细节，但它在描述能量传输方面的能力有限，因为它没有考虑在许多染料聚集体和晶体中常见的准分子。当光激发态与分子间坐标强烈耦合时，会产生能捕获能量并限制输运的准分子。因此，PI的研究活动的主要目标是扩大后卡沙模型，以包括准分子。该方法基于荷尔斯坦式哈密顿量的多粒子表示，优于大多数其他方法，因为它平等地对待所有重要的物理过程，包括激子耦合、Frenkel和电荷转移激子之间的混合、激子-振动耦合和激子-光子耦合。对大相空间内的物理观测进行本质上的精确处理，增加了发现新的和潜在有用的物理现象的可能性。有了吸收光谱和光致发光光谱的定量再现，就可以预测激子输运的效率。普渡大学的Huang小组将通过对几种具有不同程度准分子发射的苝二酰亚胺（PDI）衍生物进行飞秒分辨输运测量来提供实验验证。在另一个推力中，PI将研究光学微腔内有机材料的行为，在光学微腔中，强腔场可以用来控制材料本征态之间的混合。特别令人感兴趣的是调节弗伦克尔激子和电荷转移激子之间混合的可能性，从而控制准分子的形成。此外，最近发现的“暗”极化子（由电子、光子和振动自由度混合组成的复合准粒子）的基本光物理性质将与智利圣地亚哥大学的Felipe Herrera合作进行探索。分析将以荷尔斯坦式哈密顿量为基础，以多粒子基集表示自由空间和腔限制分子聚集体，足以获得高精度的光谱和输运观测值。总的来说，该项目有可能显著提高我们对以下方面的理解：1)分子聚集体性质（特别是光物理和运输）与包装形态之间的关系；Ii)微腔耦合可以作为控制超交换耦合和pi-stack中准分子形成的手段；iii)涉及所有三个自由度（电子、振动和光子）的新型极化子。该奖项由材料研究部门的凝聚态物质和材料理论项目以及化学部门的化学理论、模型和计算方法项目共同支持。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 SUMMARYThis award supports theoretical and computational research and education on how light is absorbed or emitted from semiconductor materials made of organic molecules. The most familiar semiconductor is probably silicon which is used in microelectronics and the chips inside modern computers. However,semiconductors based on organic molecules continue to make inroads into commercial devices, such as organic light-emitting diodes or OLEDs. Thin organic films can be driven electrically to emit light or can be used to convert solar energy into electrical energy. The PI and his research team will investigate the fundamental processes in organic crystals or aggregates when they absorb and emit light. The PI will also investigate how absorbed energy is transported between molecules which is similar to how plants transport energy 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. The PI will also investigate the effect of enclosing a thin organic film in a very small "micro" cavity formed by two reflecting mirrors separated by a tiny distance equal to about a wave length of light. A microcavity enhances the interaction between light and the enclosed molecules and can dramatically alter the behavior of the enclosed organic film, allowing one to better control its optical properties. The proposed activities will also enhance research infrastructure through domestic and international collaborations involving Professor Libai Huang at Purdue University, who will employ state-or-the-art experimental techniques to probe energy transport in organic films, and Dr. Felipe Herrera at the University of Santiago, Chile, who will assist in the theoretical investigations of organic microcavities. Overall, this research effort should contribute to a blueprint for the next generation of electronic devices based on organic materials.TECHNICAL SUMMARYThis 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 H- and J-aggregates, there remain important questions regarding the nature of the photo-excitations in molecular aggregates and how the optical response is related to crystal packing and morphology. The PI and his research team have recently 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 in packing arrangements hosting close intermolecular contacts. Although the model can predict with quantitative accuracy details of the absorption spectral line shape, it is limited in its ability to describe energy transport, as it does not account for excimers, which are commonly encountered in many dye aggregates and crystals. Excimers, which can trap energy and limit transport, arise when an optically-excited state couples strongly to an intermolecular coordinate. Hence, a primary goal of the PI's research activity is to expand the post-Kasha model to include excimers. The approach is based on a multi-particle representation of a Holstein-style Hamiltonian which is superior to most others in that it treats all the important physical processes including exciton coupling, the mixing between Frenkel and charge-transfer excitons, exciton-vibrational coupling, and exciton-photon coupling, on equal footing. The essentially exact treatment of physical observables within a large phase space enhances the likelihood for discovering new and potentially useful physical phenomena. With quantitative reproductions of both the absorption and photoluminescence spectra in hand, predictions of the efficiency of exciton transport will be made. The Huang Group at Purdue will provide the experimental validation by conducting femtosecond-resolved transport measurements of several perylene diimide (PDI) derivatives with varying degrees of excimer emission. In another thrust, the PI will investigate the behavior of organic materials inside optical microcavities, where a strong cavity field can be used to control the mixing between material eigenstates. Of particular interest is the possibility of modulating the mixing between Frenkel and charge-transfer excitons, thereby controlling the formation of excimers. In addition, the fundamental photophysical properties of the recently discovered "dark" polaritons - composite quasiparticles consisting of a mixture of electronic, photonic and vibrational degrees of freedom - will be explored in collaboration with Felipe Herrera at the University of Santiago, Chile. The analyses will be based on Holstein-style Hamiltonians for free-space and cavity-confined molecular aggregates represented in a multi-particle basis set sufficient for obtaining highly accurate spectral and transport observables. Overall, the project has the potential to significantly advance our understanding of i) the relationship between molecular aggregate properties, particularly photophysics and transport, and packing morphology; ii) the way microcavity coupling can be exploited as a means for controlling super-exchange coupling and excimer formation in pi-stacks and iii) novel types of polaritons involving all three degrees of freedom, electronic, vibrational, and photonic.This award is jointly supported through the Condensed Matter and Materials Theory Program in the Division of Materials Research and the Chemical Theory, Models and Computational Methods Program in the Chemistry Division.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.