Reduced Intrinsic Non‐Radiative Losses Allow Room‐Temperature Triplet Emission from Purely Organic Emitters

Abstract Persistent luminescence from triplet excitons in organic molecules is rare, as fast non‐radiative deactivation typically dominates over radiative transitions. This work demonstrates that the substitution of a hydrogen atom in a derivative of phenanthroimidazole with an N‐phenyl ring can substantially stabilize the excited state. This stabilization converts an organic material without phosphorescence emission into a molecular system exhibiting efficient and ultralong afterglow phosphorescence at room temperature. Results from systematic photophysical investigations, kinetic modeling, excited‐state dynamic modeling, and single‐crystal structure analysis identify that the long‐lived triplets originate from a reduction of intrinsic non‐radiative molecular relaxations. Further modification of the N‐phenyl ring with halogen atoms affects the afterglow lifetime and quantum yield. As a proof‐of‐concept, an anticounterfeiting device is demonstrated with a time‐dependent Morse code feature for data encryption based on these emitters. A fundamental design principle is outlined to achieve long‐lived and emissive triplet states by suppressing intrinsic non‐radiative relaxations in the form of molecular vibrations or rotations.