Confinement‐Induced Enhancement of Solid‐State Emission Quantum Yield in Rubrene@MOF Hybrid Materials

Aggregation‐caused quenching (ACQ) severely limits the solid‐state luminescence of rubrene despite its outstanding optical properties. Here, we demonstrate that pore‐size‐controlled confinement in the zirconium‐based metal–organic frameworks (MOFs) UiO‐66 and UiO‐67 effectively suppresses parallel stacking and restores bright emission. Solvent‐free gas‐phase incorporation of rubrene yields hybrid materials that exhibit solution‐like UV/Vis absorption bands and fluorescence quantum yields of up to 25%, in stark contrast to non‐emissive bulk rubrene. The smaller pores of UiO‐66 enforce spatial isolation of single guest molecules, while the larger cavities of UiO‐67 allow higher loadings without inducing aggregation. DFTB3‐based molecular dynamics simulations reveal two complementary mechanisms of aggregation suppression: geometric exclusion in UiO‐66 and energetic destabilization of co‐occupation in UiO‐67. Confinement thus reverses the intrinsic stacking preference of rubrene and establishes MOF nanoconfinement as a general strategy to activate solid‐state emission in π ‐conjugated fluorophors.