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.