Dynamic Transition between Monomer and Excimer Phosphorescence in Organic Near‐Infrared Phosphorescent Crystals

Abstract Achieving efficient near‐infrared room‐temperature phosphorescence of purely organic phosphors remains scarce and challenging due to strong nonradiative decay. Additionally, the investigation of triplet excimer phosphorescence is rarely reported, despite the fact that excimer, a special emitter commonly formed in crystals with strong π–π interactions, can efficiently change the fluorescent properties of compounds. Herein, a series of dithienopyrrole derivatives with low triplet energy levels and stable triplet states, exhibiting persistent near‐infrared room‐temperature phosphorescence, is developed. Via the modification of halogen atoms, the crystals display tunable emissions of monomers from 645 to 702 nm, with a maximum lifetime of 3.68 ms under ambient conditions. Notably, excimer phosphorescence can be switched on at low temperatures, enabled by noncovalent interactions rigidifying the matrix and stabilizing triplet excimer. Unprecedentedly, the dynamic transition process is captured between the monomer and excimer phosphorescence with temperature variations, revealing that the unstable triplet excimers in crystals with a tendency to dissociate can result in the effective quench of room‐temperature phosphorescence. Excited state transitions across varying environments are elucidated, interpreting the structural dynamics of the triplet excimer and demonstrating strategies for devising novel near‐infrared phosphors.