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The Coblentz Society annually presents an award to an outstanding young molecular spectroscopist under the age of 40. This year’s recipient, Wei Xiong of the University of California, San Diego, is being honored for his work, as noted on the Coblentz Society webpage, “using and developing ultrafast nonlinear spectroscopic and imaging tools to reveal molecular structures and dynamics of materials, including ultrafast dynamics of polaritonic systems, guest molecule adsorptions in self-assembled materials, and femtosecond charge transfer dynamics on organic material interfaces.” Spectroscopy spoke to Xiong about his work and his feelings about receiving this award.
In a recent paper (1), you discuss polaritons and how they can modify chemical events. Briefly explain what a polariton is.
Polaritons are hybrid quasiparticles between photons and molecules through strong coupling. when a carbon atom and an oxygen atom hybridize with each other. CO molecules have much different properties than the carbon and oxygen atoms, so the newly formed molecules are distinct to the atomic ones—just like when photon and molecules can hybridize with each other and form polaritons, the newly formed polaritons have new properties different from photons or the molecules forming them. Two most distinct features are the polariton wavefunctions are delocalized comparing to the localized molecular orbitals, and polaritons have different energy levels from its molecular counterparts. The concept of polaritons is well-established in solid-state physics, but only recently, it came to attention to chemists, because people proposed that the distinct wavefunction and energy of polaritons may lead to new chemistry pathways and can act as a new catalyst.
Your paper goes on to examine the modification of chemical events by vibrational strong coupling (VSC) and how this modification was supported by several research results using multiple techniques, including infrared (IR), ultraviolet–visible (UV–vis), and mass spectrometry (MS), leading to an exciting emerging new field termed polariton chemistry. Why did you use those specific analytical techniques over others that are available?
The most critical technique we used is a newer technique, called two-dimensional infrared spectroscopy (2D-IR). This is a technique like the well-known 2D-NMR, except that it probes the vibrational resonance, instead of magnetic resonances. Furthermore, 2D-IR can track molecular dynamics at a femtosecond timescale. This turns out to be important to understand polariton chemistry, because it allows us to follow the energy flow and barrier crossing events that eventually lead to chemical reactions at its natural timescales. Another feature of 2D-IR that is useful is that it can resolve different quantum states in the systems under VSC. This is important, because, in the systems, not only are there polaritions, but there are also so-called dark modes, which resembles normal molecules without VSC. However, the population of dark modes is much more than the one of polariton states. Thus, it is critical to resolve the dynamic of these various quantum states and pin down which exact states oversee the VSC modified chemistry.
Please summarize your findings for your polariton research and describe what is most surprising or exciting about it.
In our research, we showed that polaritons can enable vibrational energy transfer among molecules through spaces, which was often unfavorable in regular liquid phase systems. Thus, we showed that, by taking advantage of the delocalized nature of polaritons, localized vibrational energy can be transported over a longer distance. This is exciting because it opens to us a new way to transfer vibrational energy between molecules and may enable remote catalysis in the future.
In other research, we showed explicitly how VSC modifies chemistry through an isomerization dynamic. The dynamic, called Berry’s peudorotation, involves CO ligands rearrangement around the metal center. Using 2D-IR spectroscopy, we unambiguously showed that only polariton states can modify the isomerization dynamics, while the dark modes cannot, which is an exciting result. Thus, this research confirmed the central ideal that polaritons’ unique features can modify chemical reactions. However, as mentioned above, since the dark modes have a much larger population compared to the dark modes, this work pointed to the discovery that reducing the population of dark modes should promote the polariton chemistry effects. I am excited about this result because it truly required sophisticated spectroscopic skills and knowledge to finally solve the problem, showing the value of ultrafast nonlinear spectroscopy to modern science.
Were there any limitations or challenges you encountered in your work?
As any fields, there are always exciting challenges; so is it in my work. Among them was that there is still a gap between the theoretical description and experimental results of polariton chemistry. It would be great for experimentalists and theorists to work together to reduce this gap. It also remains to be seen how universal these polariton-modified chemistry and chemical dynamic systems are, and whether we can discovery mechanism and use them to establish rational design rules for polariton chemistry.
Can you please summarize the feedback that you have received from others regarding your polariton studies?
Many people have found this work to be fascinating and innovative, but, at the same time, it is not the easiest subject to understand. The most challenging part to rationalize it is how the collective strong coupling effect, which is an effect shared by molecules, influences a chemical reaction event, that is essentially a single molecule event. I think our work played an important part on elucidating these questions and additional future work will be pursued to fully understand it.
What are the next steps in this research?
There are many steps, including moving to nanocavity structures to reduce the number of dark modes, and extending our studies to more complex chemical reaction systems, now that we understand the simplest elementary reactions.
What does your being named the recipient of the Coblentz Award mean to you professionally? Personally?
I am deeply humbled to receive this tremendous honor bestowed upon me by the Coblentz society. As a spectroscopist, I have had the privilege of working with numerous brilliant scientists who have been recognized with this prestigious award. Joining the ranks of these revered individuals whom I have long admired is truly awe-inspiring. The recognition of our fundamental spectroscopy work is truly gratifying, as it reinforces my passion for the research fields that I am deeply interested in. Knowing that the significance of spectroscopy is acknowledged and celebrated motivates me to explore new frontiers and contribute to the advancement of knowledge in these fields.
What advice can you offer those starting out in their post-doctorate career?
To me, the post-doctorate career represents a unique opportunity to nurture and actualize one's own ideas. It is a time to embrace courage, foster innovation, and dare to think big. Only the sky is the limit!
(1) Xiong, W. Molecular Vibrational Polariton Dynamics: What Can Polaritons Do? Acc. Chem. Res. 2023, 56, 7, 776–786. DOI: 10.1021/acs.accounts.2c00796