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Abstract Vibrational polaritons have shown potential in influencing chemical reactions, but the exact mechanism by which they impact vibrational energy redistribution, crucial for rational polariton chemistry design, remains unclear. In this work, we shed light on this aspect by revealing the role of solvent phonon modes in facilitating the energy relaxation process from the polaritons formed of aT_1umode of W(CO)₆to an IR inactiveE_gmode. Ultrafast dynamic measurements indicate that along with the direct relaxation to the darkT_1umodes, lower polaritons also transition to an intermediate state, which then subsequently relaxes to theT_1umode. We reason that the intermediate state could correspond to the near-in-energy Raman activeE_gmode, which is populated through a phonon scattering process. This proposed mechanism finds support in the observed dependence of the IR-inactive state’s population on the factors influencing phonon density of states, e.g., solvents. The significance of the Raman mode’s involvement emphasizes the importance of non-IR active modes in modifying chemical reactions and ultrafast molecular dynamics. 

Polaritons lose delocalization in energetically disordered systems. A large Rabi splitting about 3–4 times of the inhomogeneous linewidths is required to restore delocalization. This study can guide future rational experiment designs. 

Two-dimensional infrared spectroscopy resolves ultrafast chemical dynamics in Fe(CO) 5 under vibrational strong coupling.