On the Role of Symmetry in Vibrational Strong Coupling: The Case of Charge‐Transfer Complexation

Abstract It is well known that symmetry plays a key role in chemical reactivity. Here we explore its role in vibrational strong coupling (VSC) for a charge‐transfer (CT) complexation reaction. By studying the trimethylated‐benzene–I 2 CT complex, we find that VSC induces large changes in the equilibrium constant K DA of the CT complex, reflecting modifications in the Δ G ° value of the reaction. Furthermore, by tuning the microfluidic cavity modes to the different IR vibrations of the trimethylated benzene, Δ G ° either increases or decreases depending only on the symmetry of the normal mode that is coupled. This result reveals the critical role of symmetry in VSC and, in turn, provides an explanation for why the magnitude of chemical changes induced by VSC are much greater than the Rabi splitting, that is, the energy perturbation caused by VSC. These findings further confirm that VSC is powerful and versatile tool for the molecular sciences.