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

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 (λRTP =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.