LAUSR

Vibrational strong coupling (VSC) between molecular vibrations and microcavity photons yields a few polaritons (light-matter modes) and many dark modes (with negligible photonic character). Although VSC is reported to alter thermally activated chemical reactions, its mechanisms remain opaque. To elucidate this problem, we followed ultrafast dynamics of a simple unimolecular vibrational energy exchange in iron pentacarbonyl [Fe(CO)5] under VSC, which showed two competing channels: pseudorotation and intramolecular vibrational-energy redistribution (IVR). We found that under polariton excitation, energy exchange was overall accelerated, with IVR becoming faster and pseudorotation being slowed down. However, dark-mode excitation revealed unchanged dynamics compared with those outside of the cavity, with pseudorotation dominating. Thus, despite controversies around thermally activated VSC modified chemistry, our work shows that VSC can indeed alter chemistry through a nonequilibrium preparation of polaritons. Description Spectroscopy for polaritonic chemistry There is currently considerable interest in understanding the effect of vibrational strong coupling (VSC), a promising route to manipulating chemical dynamics in condensed phases. Previous studies of VSC-modified chemistry used traditional chemical kinetics tools that cannot properly address ultrafast vibrational processes in their natural time scales. Using ultrafast two-dimensional infrared spectroscopy, Chen et al. showed that polaritons (delocalized superpositions of vibrations and electromagnetic cavity modes) can switch the rates of two vibrational energy pathways of a metal carbonyl compound under VSC, making intramolecular vibrational redistribution more favorable over pseudorotation (the opposite is true outside of a cavity) (see the Perspective by Chuntonov). The authors also clarified the role of dark modes in VSC, a longtime question that has been heavily debated but had lacked direct experimental evidence. —YS Two-dimensional infrared spectroscopy resolves ultrafast chemical dynamics in Fe(CO)5 under vibrational strong coupling.