Organic Guest-Host System Produces Room-Temperature Phosphorescence at Part-Per-Billion Level.

Manipulation of long-lived triplet excitons in organic molecules are key to applications including next-generation optoelectronics, background-free bioimaging, information encryption and photodynamic therapy. However, organic room-temperature phosphorescence (RTP), which stems from triplet excitons, 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. Here we report a series of fluorescent naphthalimides, which did not originally show observable phosphorescence in solutions, as aggregates, in polymer films, or in any other tested host materials 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.