Strong coupling to the electronic or vibronic transitions of an organic semiconductor has been extensively studied in microcavity structures in which a molecular film is placed between two closely spaced mirrors. Recent experiments suggest that such strong coupling can be used to modify chemical reactions; however, the geometry of conventional microcavity structures makes such studies difficult as they limit the ability of molecules to interact with their local environment. Here, we show that optical strong coupling to a molecular film can be achieved even when such molecules are located on the surface of a dielectric slab. We then show that such molecules on the surface of the slab can undergo facile interactions with molecules in their surrounding environment, and evidence a reversible protonation/deprotonation reaction by exposing a surface-bound porphyrin to an acidic or basic vapor. Although our proof-of-principle measurements do not evidence any change in reaction rates, we believe our structures represent a promising system in which to explore polariton-driven chemical phenomena.