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

University Of Chicago, Chicago IL

Investigators

Abstract

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.