Molecular insight into the polymorphism-dependent organic phosphorescence

Room temperature phosphorescence (RTP) in organic aggregates generally depends on the molecular conformation and packing, but the coupling between them presents a formidable challenge to uncovering the inherent mechanism in polymorphism-dependent RTP. Herein, organic polymorphs of DOS compounds (DOS-O and DOS-S), exhibiting different RTP lifetimes, were chosen as prototypes. By exploring the molecular and electronic structures, and the decay dynamics of T₁ states for DOS polymorphs, we proposed that the decreased energy gap of T₁ states caused by conformational variation governs the shortening of RTP lifetime from DOS-O to DOS-S. This is because it hugely accelerates the nonradiative decay rate of T₁ → S₀, in contrast to the negative contribution from the spin-orbit coupling and reorganization energy. This point was further demonstrated by the good consistency between theory and experiment in RTP lifetime.