Understanding and Controlling Reaction Mechanisms Under Vibrational Strong Coupling

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With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Wei Xiong and his group at the University of California-San Diego aim to understand the mechanisms of a specific set of chemical reactions in optical cavities. When reactants and/or products are in a cavity composed of two partial-reflective mirrors, and are highly concentrated to reach the so-called strong coupling regime, the activity and selectivity of reactions in the cavity can differ from the same reaction outside cavities. Such a phenomenon opens a new way to control chemical reactions and can profoundly impact reaction engineering through a simple optical method. However, there has been debate about the experimental observations that support the claim of cavity-modified reactions, and the molecular-level mechanisms of how the cavities modify reactions remain unclear. Dr. Xiong and his group aim to quantify the reaction in cavities and understand the underlying mechanisms through optical spectroscopy. A proper characterization of the cavity reactions and understanding of the mechanisms will lay a solid foundation for rationally designing cavities to modify chemistry. To enhance the general public's interest in chemistry, Dr. Xiong is setting up a food and chemistry YouTube channel to discuss the basic chemistry of food and cooking. Videos will feature scientists cooking food based on their own backgrounds to embrace the diverse culture in our society. While one of the purposes of this activity is to broadcast chemistry through food, something everyone can enjoy, another purpose is to show scientists in real-life settings. Observing scientists in more familiar settings can help motivate young learners to pursue STEM (science, technology, engineering and mathematics) careers and encourage their families to support such aspirations.The goal of this project is to study chemical reactions modified by molecular polaritons to gain new insight into the roles that dark modes and cooperativity play in the reactions. Vibrational strong coupling (VSC) of light and matter occurs when molecular vibrational and photon cavity modes exchange energy faster than the lifetime of both modes. Under VSC, cavity photons and molecular vibrational excitations hybridize to form molecular vibrational polaritons. Recent reports have shown several fascinating examples of how molecular potential energy landscapes and concomitant reaction pathways can be modified under VSC conditions, including modifying reaction branching ratios and enhancing or suppressing chemical reaction rates, making VSC a promising new tool to manipulate chemical reactions. However, there are several challenges in this emerging field. This project focuses on addressing three outstanding challenges by developing an alternative and more direct analytical method to quantify chemistry under VSC; quantifying the reaction performance and polariton/dark mode dynamics to understand the interplay between dark modes and polaritons and how it influences chemistry under VSC; and aiming to understand the role of energy transfer in chemistry under VSC. The main research tools include ultrafast spectroscopy and analytical instruments such as GC-MS (gas chromatography/mass spectrometry). The outcomes of this project include the development of an alternative way to quantify chemistry under VSC and better understanding of the mechanisms of chemistry under VSC. The broader impacts include new design principles for the rational design of VSC conditions to control reactions and influence the field of catalysis, pharmaceutical molecule synthesis, green chemistry, and photochemistry. The team also hosts undergraduate research students from groups that are underrepresented in science, in order to help them become familiar with graduate life and introduce them to cutting-edge research programs in an effort to broaden participation in graduate research.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.