Photoluminescence Characteristics of Merocyanine Dyes in Ionic Liquids

The photoluminescence (PL) characteristics of merocyanine dye molecules in ionic liquids of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF6) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4) with different purity grades were investigated. The absorption spec-tra for this dye in all the ionic liquid solvents investigated looked very similar while the PL emission spectra differed markedly depending on the type of ionic liquid and the impurity content. In all purity grades of [BMIM]BF4 and the ultrahigh grade of [BMIM]PF6, the PL spectra of these mero-cyanine solutions showed a single peak around 480 nm. However, when a lower grade of [BMIM]PF6 was used as the solvent, an additional PL peak appeared around 575 nm. The deprotonation from the hydroxyl group of the merocyanine molecule after photoexcitation is proposed to explain the phenomenon in this unique solvent system