Exciton‐Polaritons in Nanoscale Metal‐Organic Frameworks: A Platform for the Reversible Modulation of Strong Light‐Matter Coupling via the Chemical Environment

ABSTRACT We demonstrate robust exciton‐polariton formation in planar metallic optical cavities incorporating porphyrin‐based Metal‐Organic Framework (MOF) nanoparticles. Our fabrication strategy employs highly monodisperse fluorinated MOF nanoparticle monolayers, sequentially coated with transparent silicon dioxide nanoparticles, enabling precise control over film thickness while preserving MOF pore accessibility. This architecture ensures well‐defined excitonic transitions and facilitates alignment of the MOF exciton transition dipole moments with the cavity electric field, thereby maximizing coupling strength. Experimental polariton absorption energy dispersion relations unequivocally reveal light‐matter hybridization through clear anticrossing of the upper and lower polaritonic branches, yielding Rabi splittings as large as 440 meV, placing the system at the threshold of the ultra‐strong light‐matter coupling regime. Critically, these MOF nanoparticle optical cavities exhibit unprecedented, precise, and reversible tuning of the polariton absorption splitting upon exposure to gradual vapor pressure changes. This tunability stems both from the adsorption/desorption properties of the MOF nanoparticle mesostructured pore network and the porphyrin ligand solvatochromic character. Our work constitutes an unparalleled demonstration of chemical environment‐controlled exciton‐polariton fine tuning, opening new avenues for responsive polaritonic materials in sensing, catalysis, and photochemistry.