Liquid Luminescent Materials Based on 10 H ‐Pyrido[1,2‐ a ]‐5‐indolium Salts

Next‐generation flexible optoelectronics and soft robotics require adaptive luminescent materials that withstand extreme environments while remaining deformable. However, the photoluminescence quantum yield (PLQY) of current solvent‐free ionic liquids suffers from fundamental limitations arising from aggregation‐caused quenching and rapid nonradiative decay, and these liquids degrade under thermal and chemical stress. This study presents a supramolecular locking strategy that overcomes these limitations through kinetically stabilized anion–π + interactions. Specifically, a rigid N‐doped fluorene (10 H ‐pyrido[1,2‐ a ]‐5‐indolium, PIn + ) was paired with selected anions to maintain fluidity and suppress nonradiative molecular motion by locking radiative pathways in the liquid state. This ionic tethering approach yielded an unprecedented PLQY of 80% in the liquid state while preserving the solvent‐free liquid nature of the material. These highly emissive ionic liquids exhibited extreme physicochemical robustness, withstanding thermal decomposition up to 400°C, retaining intense emission without volatilization even under reduced pressure at 250°C, and resisting strongly acidic and basic environments. Derived from a versatile and readily synthesizable fluorene platform, this system also provides broad opportunities for structural customization. These findings establish a general molecular design principle for highly robust and emissive liquid fluorophores, providing a foundation for indestructible and flexible photonic technologies operating under extreme conditions.