Halogen atom substitution effect on the carbazole fluorescence properties and supramolecular interactions in the solid-state†

The halogen substitution in an organic fluorophore exerted significant influence on the molecular organization via halogen intermolecular interactions and solid-state fluorescence. The carbazole core unit has been extensively utilized for developing optoelectronic materials because of its excellent fluorescence and phosphorescence properties. Herein, we have synthesized dihalogen substituted carbazoles (Cz-Cl, Cz-Br, Cz-I) and investigated their role in the supramolecular organization and solid-state fluorescence. Solid-state structural analysis revealed that Cz and halogen derivatives produced a herringbone structure but with varied intermolecular interactions and molecular orientation. Cz showed N–H⋯π intermolecular interactions between the amine hydrogen and middle ring of carbazole. Cz-Cl and Cz-Br also showed N–H⋯π intermolecular interactions but between the amine hydrogen and halogen decorated phenyl ring of carbazole. Cz-I showed only weak halogen–halogen and halogen–hydrogen intermolecular interactions. Cz showed strong fluorescence in solution as well as the solid-state (Φ_f = 55.9%). However, halogen substituted carbazole derivatives showed only solid-state fluorescence with strong reduction of fluorescence efficiency (Φ_f = 6.5% (Cz-Cl), 1.2% (Cz-Br)). Further, Cz-I did not show any fluorescence even in the solid-state. The significant reduction of fluorescence might be attributed to the heavy atom effect. Halogen substitution also caused varied electron density distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Thus, halogen substitution influenced the molecular organization via halogen intermolecular interactions and solid-state fluorescence due to the heavy atom effect.