Fast Microsecond Room‐Temperature Phosphorescence in Purely Organic Phosphors by Conformationally Directed Carbonyl Orientation

ABSTRACT Metal‐free purely organic phosphors (POPs) are attractive triplet emitters, but their room‐temperature phosphorescence (RTP) is often slow at milliseconds or longer because the radiative T 1 →S 0 transition is spin‐forbidden. Here, we introduce directional carbonyl orientation (DCO) as a packing‐aware conformational descriptor for understanding and designing fast metal‐free RTP. In brominated terephthalate derivatives, co‐directional carbonyl torsions are associated with fast RTP, whereas counter‐directional torsions are associated with slower RTP. Using two polymorphs of dimethyl dibromoterephthalate (MeBrTPA), we show that the co‐directional Form I emits blue RTP with Φ phos = 24%, τ phos = 15 µs, and an estimated k r = 1.6 × 10 4 s −1 , while the counter‐directional Form II is slower despite its higher quantum yield. State‐resolved TD‐DFT/SOC calculations further reveal that the difference cannot be described by direct S 0 –T 1 coupling alone, but instead reflects distinct state‐specific combinations of T 1 –S n SOC and S n –S 0 transition strength. Finally, translating the DCO‐favored geometry from crystal packing into molecular design, conformational locking in CyclicBrTPA produces fast blue RTP in amorphous PMMA. Together, these results establish DCO as a practical molecular design strategy for organizing favorable spin‐coupling geometries beyond a single crystal‐packing environment, while highlighting the continued importance of a sufficiently rigid environment for room‐temperature emission.