Large Stokes Shift Ionic-Liquid Dye

Abstract

We have incorporated the dye N-methyl-6-oxyquinolone [6MQz] in its protonated form as a cation into an ionic liquid (IL) and thus to synthesize an IL dye. The IL dye N-methyl-6-hydroxyquinolinium bis(trifluoromethylsulfonyl) imide [6MQc][NTf2] was characterized by NMR, ATR IR spectroscopy and X-ray crystallography. The fluorescence of the IL dye has a large Stokes shift of Δλ=116 nm and a quantum yield of φF=0.56 in acetonitrile. Characteristic solvent dependent shifts can be detected in the emission spectra. In other ILs, acetonitrile and THF we observe a bathochromic shift of up to 28 nm compared to the pure IL dye at 467 nm. For stronger polar solvents the fluorescence signals are strongly red-shifted to 650 nm indicating proton transfer to the solvent molecules in the excited state. This underlines the importance of the IL building block [MQc]+ as photo acid.