Direct Transition from Triplet Excitons to Hybrid Light-Matter States via Triplet-Triplet Annihilation.

Strong light-matter coupling generates hybrid states that inherit properties of both light and matter, effectively allowing the modification of the molecular potential energy landscape. This phenomenon opens up a plethora of options for manipulating the properties of molecules, with a broad range of applications in photochemistry and photophysics. In this article, we use strong light-matter coupling to transform an endothermic triplet-triplet annihilation process into an exothermic one. The resulting gradual on-off photon upconversion experiment demonstrates a direct conversion between molecular states and hybrid light-matter states. Our study provides a direct evidence that energy can relax from nonresonant low energy molecular states directly into hybrid light-matter states and lays the groundwork for tunable photon upconversion systems that modify molecular properties in situ by optical cavities rather than with chemical modifications.