Polariton Chemistry: Action in the Dark

In a pioneering experiment, Ebbesen, Moran, and coworkers have recently demonstrated control of a thermally activated ground-state chemical reaction using vibrational strong coupling (VSC). Within the Born–Oppenheimer approximation, (electronic) groundstate chemical reactions can often be viewed as occurring on a potential energy surface (PES). Reactions are described by pathways on the PES, starting from a minimum associated with the stable reactants, to another minimum, corresponding to the stable products. In many cases, at least one energy barrier along the reaction pathway will need to be thermally overcome, which is known as the transition state. In the gas phase, it has been shown that for “late barrier” reactions the best way to overcome this transition state is by imparting vibrational excitation energy to the reactants. This begs the question of whether reactions can be triggered by directly exciting the vibrational mode of interest. Inspired by these ideas, significant efforts in the 1970s focused on “modeselective chemistry”, the use of infrared (IR) multiphoton excitation of specific molecular vibrations to induce a reaction in the condensed phase. Only a handful of these efforts were successful, however, with most of them hindered by intramolecular vibrational energy redistribution (IVR)the rapid thermalization of energy from the highly excited vibrational mode into other modes. In fact, most contemporary efforts using light to control the outcome of chemical reactions bypass the exclusive use of IR-activated processes, instead invoking the richer photochemistry generated by UV–visible radiation. However, the findings of Ebbesen, Moran, and co-workers could revive the mode-selective chemistry program within the contemporary context of molecules in optical cavities or “polariton chemistry”. The experiment carried out by Ebbesen, Moran, and co-workers involved enclosing a dense solution of reactant molecules between two metallic mirrors that form an optical cavity in the IR, as shown in Figure 1a. Under VSC, the energy exchange between photons and vibrations is faster than their respective decay rates, leading to the hybridization of light and matter to form quasiparticles known as vibrational polaritons (VPs). This hybridization is mediated by the electric dipole interaction between the vacuum electromagnetic field of the resonant optical mode and a collective totally symmetric vibrational excitation which is delocalized across a large number of molecules, N, distributed with a density ∼10 cm−2. For identical photon and vibrational transition energies, the resulting VPs will be located symmetrically above (P+) and below (P−) the bare vibrational transition and separated by energies of ħΩ ≈ 50−100 cm−1 (see Figure 1a). These VPs can be understood to be symmetric (P+) and antisymmetric (P−) superpositions of the photon and the delocalized vibrational excitation, in analogy with bonding and antibonding orbitals in molecular orbital theory. However, just as in chemical bonding, the number of states before and after hybridization is equal, so there is a macroscopic reservoir of N − 1 subradiant (“nonbonding”) states which do not have the right symmetry to mix with the electric field of the cavity mode and remain parked at the bare vibrational transition energy. Ebbesen, Moran, and co-workers studied the cleavage of the Si−C and Si−O bonds in a model oxysilane compound