Switching of Exciton-Polaritons in Covalent Organic Framework-Based Optical Resonators through Interaction with the Molecular Environment

We report a demonstration of a covalent organic framework (COF) integrated into an optical cavity for the realization of exciton–polaritons and their reversible switching through changes in the surrounding molecular environment. Films of COF nanoplates were integrated into Fabry–Pérot resonators engineered to achieve spectral matching between cavity modes and COF excitonic transitions, yielding strong light–matter coupling with a Rabi splitting above 300 meV. The cavity architecture preserves accessibility of the porous framework, enabling interaction with guest molecules. The potential of this interplay is shown by exposing the resonators to water vapor, which induces both a redshift and a pronounced reduction of the COF optical absorption. The observed hypochromic effect drives a reversible shift from strong to weak coupling, effectively switching exciton–polaritons on and off, as guest molecules are desorbed from or adsorbed in the reticular material host. The process is reversible upon desorption, with preservation of structural and optical integrity. These results establish COFs as a versatile platform for chemically tunable polaritonic systems, opening avenues for responsive photonic devices, sensing, and control of light–matter interactions in nanostructured materials.