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

在这项由美国国家科学基金会化学部化学结构动力学与机理A(CSDM-A)和化学测量与成像(CMI)计划资助的项目中，芝加哥大学的格雷戈里·S·恩格尔教授正在开发新型激光光谱仪，以测量分子如何与不同的手性光反应。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，它们与光的相互作用因光场的偏振(方向)扭曲而不同。这个项目正在扩大化学家对惯用手的理解，从静态结构到动态运动。它正在研究化学转化过程中电子的螺旋状运动导致的反应速度的不同。所涉及的运动的时间尺度是飞秒(百万分之一秒)。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。该项目正在创建仪器，以必要的灵敏度测量这些信号。通过扩大化学家对手性(手性)的理解，这个项目正在使化学反应的新控制成为可能，测试现有的分子电子结构理论，并为工程量子材料开辟新的途径。恩格尔实验室的研究生和本科生不仅从事手性分子系统的科学研究，还从事手性2D光谱的开发。因此，他们的教育和培训经验是相当独特的。此外，Engel团队还与总部设在芝加哥的Girls4Science计划和科学与工业博物馆进行了积极的外联合作。将超快非线性光谱和圆二色谱相结合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的尝试阻碍了创建关联手性响应和动力学的完整映射的尝试。该项目利用恩格尔集团开发并获得专利的梯度辅助光子回声光谱(GRPES)实现手性2D光谱。恩格尔小组使用具有偏振序列LXXX和RXXX的葡萄，研究了二维光谱中的交叉峰动态如何严重依赖于光的利手性。可以观察到手性速率常数，但只有当脉冲1有足够的带宽来激发相邻的生色团时，这种动力学才会出现。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)导致了这些手性动力学。更广泛地说，这种光谱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、对不对称环境的耦合强度以及对映体之间的差分系浴耦合。这项工作的更广泛影响包括使这些实验更可靠、更容易被其他实验室和工业使用的仪器设计。该项目正在创造工具，以验证依赖时间的电子结构理论，观察2D材料中的山谷间动力学，建立涉及B场的光-物质相互作用的新范式，演示手性反应动力学，并探索光合作用光收集的设计原则。除了沉浸在手性2D光谱的科学和仪器中，恩格尔小组的学生还在接受科学业务方面的指导，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NSF的法定使命，并通过使用基金会的智力优势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.