Red Room-Temperature Phosphorescent Diimides: Molecular Design and Application

N-Substituted naphthalene diimides (NDIs) were explored as purely organic room temperature phosphorescence (RTP) materials based on the strategy of intramolecular charge-transfer (ICT) mediation. A series of NDIs were designed and investigated for their luminescence properties. All emissive NDIs exhibited similar red RTP (emission range from ~600 to ~800 nm) in solid state media; the quantum yield (QY) of the obtained RTP relies heavily on the N-substitution. From poor to moderate to strong donor moieties, the QY increases but suddenly disappears. First-principle calculations reveal that a “sweet spot” for strong NDI RTP exists: while a suitable charge-transfer state can enhance RTP, a strong donor may cause total triplet quenching through the photo-induced electron transfer (PET) mechanism. Furthermore, combining NDI aggregation effects, we realized the reddest RTP ever reported for purely organic materials (Max emission = 675 nm, QY = 21.7%). Given the red emission, respectable QY and oxygen sensitive properties for some of the NDIs, they were tested as imaging agents for in vivo imaging studies.