Heavy Atom Effect on T _n (n ≥ 2) Afterglow

Abstract The properties of higher triplet excited states (T n , n ≥ 2) are essential for deep understanding of excited state dynamics, but direct observation of T n phosphorescence remains rare, let alone systematic studies of T n behaviors based on well‐designed luminescence molecules. Recently, a serendipitous finding of T n (n ≥ 2) afterglow (phosphorescence lifetime > 0.1 s) in benzophenone‐containing difluoroboron β‐diketonate (BPBF 2 ) system is communicated. Here Cl/Br/I atoms are covalently incorporated into BPBF 2 molecules and report the first systematic study of heavy atom effect (HAE) on T n (n ≥ 2) afterglow. It is known that Cl/Br/I HAE has been widely reported and thoroughly studied in T 1 afterglow systems, which shows that Br atom is usually the optimized selection for balancing T 1 afterglow efficiency and lifetime. In contrast, the covalent linkage of Br atom to BPBF 2 has been found to dramatically suppress T n (n ≥ 2) afterglow. The experimental emission intensity(T 2 )/intensity(T 1 ) ratios are positively correlated with k IC (T 1 −T 2 )/ k IC (T 2 −T 1 ) × k P (T 2 )/ k P (T 1 ), where k IC and k P refer to the rates of calculated internal conversion and phosphorescence emission. These fundamental studies would be helpful for elucidating the panorama of Perrin‐Jablonski diagram of organic systems and provide intriguing T n (n ≥ 2) afterglow materials for future applications.