Thermal chemical reactivity in Frenkel exciton-polariton cavities.

Hybrid light-matter states formed under strong coupling between molecular excitations and confined electromagnetic modes provide a potential route to modify chemical properties. Here, we analyze the equilibrium statistical mechanics of Frenkel exciton-polaritons in a planar microcavity while explicitly retaining the in-plane photon dispersion and the resulting mode counting. We then apply this framework to a thermally averaged, population-based proxy for excitation-enabled molecular chemical activity. Within a generalized Tavis-Cummings description, we find that the cavity-induced change in thermal chemical activity is most pronounced for small molecular ensembles (low areal density within a given cavity mode volume) and increases with the collective coupling strength (Rabi splitting), particularly at low temperatures. These results highlight the importance of polariton dispersion and molecular-mode counting in assessing cavity modifications of thermally driven molecular reactivity.