An Organic Host–Guest System Producing Room‐Temperature Phosphorescence at the Parts‐Per‐Billion Level

Abstract Manipulation of long‐lived triplet excitons in organic molecules is key to applications including next‐generation optoelectronics, background‐free bioimaging, information encryption, and photodynamic therapy. However, for organic room‐temperature phosphorescence (RTP), which stems from triplet excitons, it is still difficult to simultaneously achieve efficiency and lifetime enhancement on account of weak spin–orbit coupling and rapid nonradiative transitions, especially in the red and near‐infrared region. Herein, we report that a series of fluorescent naphthalimides—which did not originally show observable phosphorescence in solution, as aggregates, in polymer films, or in any other tested host material, including heavy‐atom matrices at cryogenic temperatures—can now efficiently produce ultralong RTP ( ϕ =0.17, τ=243 ms) in phthalimide hosts. Notably, red RTP (λ RTP =628 nm) is realized at a molar ratio of less than 10 parts per billion, demonstrating an unprecedentedly low guest‐to‐host ratio where efficient RTP can take place in molecular solids.