Self-aggregation behavior of fluorescent carbazole-tailed imidazolium ionic liquids in aqueous solutions

Three amphiphilic imidazolium ionic liquids (ILs), 1-[n-(N-carbazole)alkyl]-3-methylimidazolium bromide [carbazoleC(n)mim]Br (n = 6, 10, and 12), that incorporate a fluorescent carbazole moiety at the terminal of the hydrocarbon chain, were designed and synthesized. Their surface activity, aggregation behavior, and thermodynamics of micelle formation in aqueous solutions were systematically investigated by surface tension, electrical conductivity, and (1)H NMR spectroscopy. The incorporation of bulky carbazole groups led to the loose arrangement of [carbazoleC(n)mim]Br molecules at the air-water interface. Both the enhanced hydrophobicity and the pi-pi interaction due to the incorporation of carbazole groups contributed to the lower critical micelle concentration (cmc) and higher micellar aggregation number, reflecting a strong capability of the designed carbazole-tailed imidazolium ILs for the formation of micelle. The analysis of (1)H NMR spectra revealed that the carbazole moieties may overlap with the middle methylenes of the alkyl chains. The amphiphilic carbazole-tailed imidazolium ILs demonstrated strong and stable fluorescence properties, indicating their potential application in the combined field of surface chemistry and photochemistry.