Chiral Rates and Dynamics: Time- and Frequency-Resolved Chiral Electronic Spectroscopy for Electronic Structure, Dynamics, and Quantum Optics

在这个由美国国家科学基金会化学部化学结构动力学与机制a （CSDM-A）和化学测量与成像（CMI）项目资助的项目中，芝加哥大学的Gregory S. Engel教授正在开发新型激光光谱，以测量分子在不同的光旋向下的反应。有些分子，就像我们的手一样，是彼此不可重叠的镜像。这些分子被称为手性分子，它们和光的相互作用取决于光场偏振（方向）的扭曲。这个项目扩展了化学家对手性的理解，从静态结构到动态运动。它研究的是在化学转化过程中，由于电子的螺旋状运动，反应速率是如何变化的。所涉及的运动的时间尺度是飞秒（十亿分之一秒的百万分之一）。含有手性信息的信号比传统光谱学测量的典型信号弱许多倍。这个项目正在创造仪器，以必要的灵敏度测量这些信号。通过扩展化学家对手性（手性）的理解，该项目实现了对化学反应的新控制，测试了分子电子结构的现有理论，并为工程量子材料开辟了新的途径。恩格尔实验室的研究生和本科生不仅从事手性分子体系的研究，而且从事手性二维光谱学的发展。因此，他们的教育和培训经历非常独特。此外，恩格尔团队还与总部位于芝加哥的Girls4Science项目和科学与工业博物馆开展了积极的外展合作。将超快非线性光谱与圆二色性相结合一直是光谱学家的梦想。但是，信号强度比无手性背景弱几个数量级，使得试图建立一个完整的手性反应和动力学相关图谱的努力受挫。该项目利用Engel集团开发并获得专利的梯度辅助光子回波光谱（GRAPES）来实现手性二维光谱。Engel小组正在使用带有LXXX和RXXX偏振序列的GRAPES，研究二维光谱中的交叉峰动力学如何严重依赖于光的旋向性。手性速率常数可以观察到，但这种动态只有在脉冲1有足够的带宽来激发相邻的发色团时才会出现。也就是说，与邻近分子的离域激发态的手性相互作用（可能是磁性）导致了这些手性动力学。更一般地说，这种光谱学直接监测离域电子态的崩溃，微分手性动力学，绝对构型的变化，与不对称环境的耦合强度，以及对映体之间的微分体系池耦合。这项工作的广泛影响包括仪器设计，使这些实验更可靠，更容易为其他实验室和工业使用。该项目正在创建工具来验证随时间变化的电子结构理论，观察二维材料的谷间动力学，建立涉及b场的光-物质相互作用的新范例，展示手性反应动力学，并探索光合作用光收集的设计原则。除了沉浸于手性二维光谱学的科学和仪器之外，Engel小组的学生还接受了？“科学的商业”，从实验室基础设施的实际维护，到确定和确保研究资金来源。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。

In this project funded by the Chemical Structure Dynamics and Mechanisms-A (CSDM-A) and Chemical Measurement and Imaging (CMI) programs of the Chemistry Division of NSF, Professor Gregory S. Engel of the University of Chicago is developing novel laser spectroscopies to measure how molecules react with different handedness of light. Some molecules, much like our hands, are non-superimposable mirror images of one another. These molecules are called chiral molecules and they interact with light differently depending on the twist of polarization (direction) of the light field. This project is expanding chemists' understanding of handedness, from static structure to dynamic motions. It is looking at how rates of reaction differ due to corkscrew-like motions of electrons during chemical transformations. The timescales of the motions involved are femtoseconds (millionths of a billionth of a second). The signals containing the chiral information are many times weaker than typical signals measured with traditional spectroscopy. This project is creating instrumentation to measure these signals with the necessary sensitivity. By extending chemists' understanding of chirality (handedness), this project is enabling novel control of chemical reactions, testing existing theories for the electronic structure of molecules, and opening new avenues for engineering quantum materials. The graduate and undergraduate students in the Engel laboratory are engaged not only in the science of chiral molecular systems, but in the development of chiral 2D spectroscopy. Their education and training experience is therefore quite unique. In addition, the Engel team has an active outreach collaboration with the Chicago-based Girls4Science program and with the Museum of Science and Industry. Combining ultrafast nonlinear spectroscopy and circular dichroism has long been a dream of spectroscopists. But signal strengths orders of magnitude weaker than the achiral background frustrate attempts to create a complete map correlating chiral response and dynamics. The project exploits GRadient Assisted Photon Echo Spectroscopy (GRAPES) developed and patented in the Engel Group to realize chiral 2D spectroscopy. Using GRAPES with the polarization sequences LXXX and RXXX, the Engel Group is investigating how the cross peak dynamics in the two dimensional spectra depend critically on the handedness of light. Chiral rate constants can be observed, but this dynamics only appears when sufficient bandwidth in pulse 1 is present to excite the adjacent chromophores. That is, a chiral interaction (likely magnetic) with the delocalized excited state of adjacent molecules is causing these chiral dynamics. More generally, this spectroscopy directly monitors collapse of delocalized electronics states, differential chiral dynamics, changes in absolute configuration, strength of coupling to asymmetric environments, and differential system-bath coupling amongst enantiomers. The broader impacts of this work include instrument designs that make these experiments more reliable and more accessible to other laboratories and to industry. The project is creating tools that validate time-dependent electronic structure theories, observe intervalley dynamics in 2D materials, establish new paradigms for light-matter interactions involving B-fields, demonstrate chiral reaction dynamics, and probe design principles of photosynthetic light harvesting. In addition to their immersion into the science and instrumentation of chiral 2D spectroscopy, students in the Engel group are being mentored in the ?business of science," from the practical maintenance of laboratory infrastructure, to identifying and securing sources of research funding.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.