Enhanced S sub(2) emission in carbazole-based ionic liquids

Ionic liquids composed of a carbazoleimidizolium-based cation and various hydrophobic anions have been synthesized and characterized. Analyses of the absorption spectra of these compounds indicate significant increases in energy gaps between the first two excited singlet states, which results in inhibition of internal conversion from the S sub(2) to S sub(1) states. Detailed studies of the spectral properties of these compounds support emission from multiple excited states including possible emission from the second excited singlet state (S sub(2) emission) in combination with an intramolecular charge transfer state. This conclusion is also consistent with fluorescence lifetime data, which suggest fluorescence emission from multiple electronic excited states. In addition, theoretical calculations of the excited states support these conclusions.