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.