CAREER: Ultrafast Nanoscopy of Energy Transport in Molecular Assemblies

美国国家科学基金会化学分部的化学结构、动力学和机理A部分（CSDMA）项目支持普渡大学的黄利白教授开发创新的显微镜技术，为分子吸收的光能如何在空间和时间上运动提供“电影”。这项研究中的分子强烈吸收光，对太阳能电池等设备有潜在的用处。为了在太阳能转换装置中使用它们，分子必须紧密地聚集在一起，而这种聚集极大地影响了能量如何从一个分子转移到另一个分子。这些因素决定了太阳能电池的效率。通过跟踪能量如何在不同包装的分子组装中移动，这项工作正在解决控制能量迁移速度和距离的机制，并为设计高效太阳能收集的结构提供指导。为了实现这些目标，人们正在开发显微镜技术，以100飞秒（1飞秒是1千万亿分之一秒）的分辨率记录快速能量转移事件，并以20纳米（1纳米是10亿分之一米）的分辨率成像能量迁移距离。黄博士正在开发一种新的教学模式，将主动学习与科学写作相结合，旨在更好地教授物理化学中的抽象概念。特别是，太阳能在该奖项中的应用正在发展成演示，以说明量子力学中的关键概念。这些演示的写作作业被用来促进对抽象概念的高层次认知理解。研究人员还在制定一项科学推广计划，将最先进的太阳能研究带到印第安纳州西北部的高中。在这个项目中开发的超快纳米显微镜方法直接成像激子在多个长度和时间尺度上的传输，以阐明分子组装中的相干和非相干能量传递途径。同时以100fs的时间分辨率和~ 20nm的空间精度绘制了光激发后的激子种群和动力学。这些测量提供了相干输运空间范围的首次可视化。采用两个模型分子聚集体系统地探测了中间耦合状态下的能量传递。线性H聚集体作为模型1D激子量子线，其中远程波状相干输运有望实现。管状聚集体被用来阐明作为维数函数的激子输运。基于模型系统的结果，控制超分子组装中输运的长期目标是通过调节短期和长期分子间耦合来实现的。研究和教育活动被整合在一起，以教育K-12、本科和研究生水平的下一代太阳能研究人员。

The Chemical Structure, Dynamics and Mechanisms Part A (CSDMA) Program in the Chemistry Division of the NSF supports Professor Libai Huang at Purdue University to develop innovative microscopy techniques that provide 'movies' of how light energy absorbed by molecules moves in space and in time. The molecules in this research absorb light strongly and are potentially useful for devices such as solar cells. In order to use them in solar energy conversion devices, the molecules have to be closely packed together and the packing greatly influences how energy is transferred from one molecule to the other. These factors determine the efficiency of the solar cell. By tracking how energy moves in molecular assemblies with different packing, this work is resolving mechanisms that control the speed and distance of energy migration and providing guidelines for designing structures for efficient solar energy harvesting. To achieve these goals, microscopy techniques are being developed to record fast energy transfer events with a resolution of 100 femtoseconds (a femtosecond is one quadrillionth of a second) and to image energy migration distance with a resolution of 20 nanometers (a nanometer is one billionth of a meter). Dr. Huang is developing a new instructional model that integrates active learning with scientific writing aimed at better teaching abstract concepts in physical chemistry. In particular, solar energy applications in this award are being developed into demonstrations to illustrate key concepts in quantum mechanics. Writing assignments of these demonstrations are being utilized to promote high-level cognitive understanding of abstract concepts. The researchers are also developing a scientific outreach program to bring state-of-the-art solar energy research to high schools in Northwest Indiana. The ultrafast nanoscopy methods being developed in this project directly image exciton transport across multiple length and time scales to elucidate coherent and incoherent energy transfer pathways in molecular assemblies. Exciton populations and dynamics following photoexcitation are being mapped with simultaneous 100 fs temporal resolution and ~20 nm spatial precision. These measurements provide first-of-a-kind visualization of the spatial extent of coherent transport. Two model molecular aggregates are employed to systematically probe energy transfer in the intermediate coupling regimes. Linear H aggregates serve as model 1D excitonic quantum wires in which long-range wavelike coherent transport is expected. Tubular aggregates are employed to elucidate exciton transport as a function of dimensionality. Building on results from the model systems, the long-term goal of controlling transport in supramolecular assemblies is achieved by modulating both short- and long-range intermolecular coupling. The research and educational activities are integrated to educate the next generation of solar energy researchers at the K-12, undergraduate, and graduate levels.