The Effect of Bromo Substituents on the Multifaceted Emissive and Crystal-Packing Features of Cyclic Triimidazole Derivatives

The understanding of the mechanisms involved in crystal packing effects on the photophysical properties of a molecular entity is fundamental to engineer solid materials for specific applications. In fact, besides rigidification and protection from oxygen quenching, crystal packing influences the emissive properties through specific interactions. In the present work, by a critical comparison among the mono- (1Br), di- (2Br) and tri-bromo (3Br) derivatives of triimidazo[1,2-a:1',2'-c:1",2''-e][1,3,5]triazine, the role of their structural features on the multifaceted emission, spanning from dual fluorescence to ultralong phosphorescence, is elucidated. In particular, conformational distortions are found to be responsible for dual fluorescence, halogen-bonding interactions are at the origin of the long-lived phosphorescence and pi-pi interactions resulting in the formation of columnar or dimeric H-aggregates induce room- temperature ultralong phosphorescence.