Real-time spectroscopic tracking of efficient intersystem crossing triggered by the heavy-atom effect in di-heteroatomic organic phosphorescent molecules

The development of efficient and long-lived halogen-free organic phosphorescent molecules remains a challenge. For the single-heteroatomic 9,10-dihydroacridine (AcH₂), the evolution of singlet and triplet excited state absorption signals reveals an intersystem crossing (ISC) lifetime of 8.2 ns and a triplet state lifetime of 0.52 µs. In contrast, the ISC lifetimes of di-heteroatomic phenoxazine (PXZ) and phenothiazine (PTZ) are significantly accelerated to 1.7 ns and 1.1 ns, respectively, while the triplet state lifetimes are extended to 0.72 µs and 4 µs. These results confirm that the introduction of di-heteroatomic synergistic effects enhances ISC efficiency while simultaneously prolonging the triplet state lifetimes. Notably, these two critical factors are further improved in PTZ due to the heavy-atom effect of sulfur atom. The work emphasizes the di-heteroatomic synergistic effect, particularly the role of heteroatoms with large atomic numbers, which is crucial for the design of halogen-free organic phosphorescent materials.