Position and Number Do Matter: Tuning Room Temperature Phosphorescence in Bromo‐1,8‐Naphthalimides through H‐Aggregation and Halogen Bonding

Abstract Development of purely organic room temperature phosphors can often be obtained via introduction of heavy atoms into the organic scaffold. However, the task of controlling emission properties in room temperature phosphorescent (RTP) crystals via heavy atom substitution has met limited success. Herein, two classes of molecules ( NI p Br 1‐2 and NI m Br 1‐2 ) are designed by varying position ( para and meta ) as well as number of bromo substituents in 1,8‐naphthalimide to modulate intermolecular interactions and their phosphorescence properties. NI p Br 2 features only halogen bonding. NI p Br , NI m Br , and NImBr 2 feature H‐aggregation in their crystalline packing wherein the efficiency of H‐aggregation increases as NI p Br < NI m Br < NImBr 2 . NI m Br 2 also features halogen bonding in addition to H‐aggregation. Relative phosphorescence intensity, phosphorescence lifetime and quantum yield increase in the order NI p Br 2 < NI p Br < NI m B r < NI m Br 2 , endorsing cruciality of bromine atom positioning and numbering in tuning the RTP performance. Amplification of phosphorescence in meta derivatives over the para congeners leads to tunable emission color of the crystals from blue to orange. These results outline a predictive structure‐property relationship wherein judicious choice of both position and number of bromine substitution can help in regulating emission behavior, providing a major step forward in expanding the fundamentals of organic RTP.