Fluorescence emission enhancement in substituted 3-styrylindoles in the solid state

Fluorescence emission properties of substituted 3-styrylindoles viz 3-(2-phenylethenyl-E)-NH-indole (1), 3-[2-(4-cyanophenyl)ethenyl-E]-NH-indole (2), and 3-(2-cyano-2-phenylethenyl-Z]-NH-indole (3) in n-hexane, THF-H2O binary mixtures and solid state are reported. In general, fluorescence efficiency in solid state is found to be much higher than in n-hexane solution. The fluorescence enhancement in solid state is attributed to restricted intramolecular motions. Interestingly, the molecules of 3 in solid state are also bound together through intermolecular hydrogen bonds yielding polymer like structure, leading to further enhancement of fluorescence emission. In THF-H2O binary mixtures, drastic enhancement of fluorescence efficiency is observed due to aggregate formation.