Modeling Molecular Aggregate Photophysics in Free Space and in Optical Microcavities

非技术摘要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。最熟悉的半导体可能是硅，它用于微电子学和现代计算机内部的芯片。然而，基于有机分子的半导体继续进入商业设备，例如有机发光二极管或OLED。有机薄膜可以通过电力驱动发光，或者可以用于将太阳能转化为电能。首席研究员和他的研究团队将研究有机晶体或聚集体吸收和发光时的基本过程。 PI还将研究吸收的能量如何在分子之间传输，这类似于植物在光合作用过程中如何传输能量。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如何响应光。这些方程将使用复杂的计算机算法来求​​解。 PI还将研究将有机薄膜封装在一个非常小的“微型”空腔中的效果，该空腔由两个反射镜形成，两个反射镜之间的距离约为光的波长。微腔增强了光与封闭分子之间的相互作用，并且可以显着改变封闭有机薄膜的行为，从而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，普渡大学的 Libai Huang 教授将采用最先进的实验技术来探测有机薄膜中的能量传输，智利圣地亚哥大学的 Felipe Herrera 博士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基于有机材料的下一代电子设备制定蓝图。技术摘要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。 π共轭分子和聚合物的固相作为场效应晶体管、发光二极管和太阳能电池中的半导体材料继续受到广泛关注。然而，尽管 Kasha 对 H 和 J 聚集体的开创性工作进行了超过 50 年的深入实验和理论研究，但关于分子聚集体中光激发的性质以及光学响应如何与晶体堆积和形态相关，仍然存在重要问题。 PI 和他的研究团队最近扩展了 Kasha 的模型，该模型完全基于长程库仑耦合，包括由于分子间紧密接触的堆积排列中分子间电荷转移而产生的短程（超交换）耦合。尽管该模型可以定量准确地预测吸收谱线形状的细节，但其描述能量传输的能力受到限制，因为它没有考虑准分子，而准分子在许多染料聚集体和晶体中常见。当光激发态与分子间坐标强烈耦合时，准分子会捕获能量并限制传输。因此，PI 研究活动的主要目标是扩展后 Kasha 模型以包括准分子。该方法基于荷斯坦型哈密顿量的多粒子表示，该方法优于大多数其他方法，因为它平等地处理所有重要的物理过程，包括激子耦合、弗兰克尔和电荷转移激子之间的混合、激子-振动耦合以及激子-光子耦合。在大相空间内对物理可观察量进行本质上精确的处理增强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，就可以预测激子传输的效率。普渡大学的黄氏研究小组将通过对几种具有不同准分子发射程度的苝二酰亚胺 (PDI) 衍生物进行飞秒分辨输运测量来提供实验验证。在另一个推动力中，PI 将研究光学微腔内有机材料的行为，其中强腔场可用于控制材料本征态之间的混合。 特别令人感兴趣的是调节弗兰克尔和电荷转移激子之间的混合的可能性，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而成的复合准粒子）的基本光物理特性将与智利圣地亚哥大学的费利佩·埃雷拉（Felipe Herrera）合作进行探索。分析将基于荷斯坦式哈密顿量，用于自由空间和空腔限制分子聚集体，以多粒子基础组表示，足以获得高度准确的光谱和传输可观测值。 总体而言，该项目有可能显着增进我们对以下方面的理解：i) 分子聚集体特性之间的关系，特别是光物理和传输以及堆积形态； ii) 利用微腔耦合作为控制 pi 堆叠中超交换耦合和准分子形成的方法，以及 iii) 涉及所有三个自由度（电子、振动和光子）的新型极化激元。该奖项由材料研究部的凝聚态和材料理论项目以及化学部的化学理论、模型和计算方法项目共同支持。该奖项反映了 NSF 的法定使命，并已被通过使用基金会的智力优点和更广泛的影响审查标准进行评估，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.