Highly Efficient Purely Organic Room‐Temperature Phosphorescence of Phenoselenazine Derivatives via a Synergistic Strategy of Intramolecular Locking and Steric Donor Substituent

Abstract Pure organic room‐temperature phosphorescent (RTP) materials have received significant attention recently because of their wide potential applications. However, RTP materials that lack host–guest interactions or hydrogen and ionic bonds typically exhibit limited phosphorescence quantum yields in amorphous films, owing to severe nonradiative decay of transition‐forbidden triplet excitons caused by molecular rotations and vibrations. Herein, a design strategy of an intramolecular‐locking core (FPSeZPh) integrated with steric donor substituents (FPSeZPh‐TMCz) is proposed to modify the parent phenoselenazine molecule (PSeZPh), which can not only restrict the intramolecular motions and suppress the nonradiative decay path of triplet excited states, but also effectively accelerate the phosphorescence radiative decay process simultaneously. Benefiting from these synergistic effects, bright RTP emission is observed for both FPSeZPh and FPSeZPh‐TMCz with phosphorescence efficiencies up to 83.3% in doped polystyrene films, which is among the best results of organic RTP emitters reported so far in amorphous films. Moreover, FPSeZPh and FPSeZPh‐TMCz are applied to construct phosphorescent film sensors for trace oxygen detection with high quenching constants up to 5.5 × 10⁻⁴ ppm⁻¹. These findings not only provide valuable insights into the design of highly efficient metal‐free RTP materials in amorphous films, but also offer broad opportunities for optical and optoelectronic applications.