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

本项目由美国国家科学基金会化学部化学结构动力学和机理-A（CSDM-A）和化学测量与成像（CMI）项目资助。芝加哥大学的恩格尔正在开发新型激光光谱学，以测量分子如何与不同旋向的光反应。有些分子，就像我们的手一样，彼此是不可重叠的镜像。这些分子被称为手性分子，它们与光的相互作用取决于光场偏振（方向）的扭曲。这个项目正在扩展化学家对手性的理解，从静态结构到动态运动。它正在研究在化学转化过程中，由于电子的螺旋状运动，反应速率如何不同。所涉及的运动的时间尺度是飞秒（百万分之一秒）。包含手性信息的信号比用传统光谱测量的典型信号弱许多倍。该项目正在创建仪器，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，该项目能够实现化学反应的新控制，测试分子电子结构的现有理论，并为工程量子材料开辟新途径。 恩格尔实验室的研究生和本科生不仅从事手性分子系统的科学，而且从事手性2D光谱学的发展。 因此，他们的教育和培训经验非常独特。 此外，恩格尔团队还与芝加哥的Girls 4Science项目和科学与工业博物馆进行了积极的外展合作。结合超快非线性光谱和圆二色性一直是光谱学家的梦想。但是信号强度比非手性背景弱几个数量级，使创建一个完整的手性响应和动力学相关图谱的尝试受挫。该项目利用Engel集团开发并获得专利的GRadient辅助光子回波光谱（GRAPES）实现手性2D光谱。Engel小组使用GRAPES和偏振序列LXXX和RXXX，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。可以观察到手性速率常数，但只有当脉冲1中存在足够的带宽以激发相邻的发色团时，才会出现这种动力学。 也就是说，与相邻分子的离域激发态的手性相互作用（可能是磁性的）引起了这些手性动力学。更一般地，该光谱直接监测离域电子状态的崩溃、差分手性动力学、绝对构型的变化、与不对称环境的耦合强度以及对映异构体之间的差分系统-浴耦合。这项工作的更广泛的影响包括仪器设计，使这些实验更可靠，更容易进入其他实验室和行业。该项目正在创建工具，以验证时间相关的电子结构理论，观察2D材料中的谷间动力学，建立涉及B场的光-物质相互作用的新范例，演示手性反应动力学，并探索光合光捕获的设计原理。除了他们沉浸到科学和仪器的手性2D光谱，学生在恩格尔集团正在指导？该奖项反映了NSF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.