Maroof Ali

Effect of Ionic Liquid on Prototropic and Solvatochromic Behavior of Fluorescein

The effect of a water-miscible room-temperature ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF(4)]), on the behavior of a common and popular probe fluorescein is assessed. Depending on solubilizing milieu and conditions, fluorescein may exist in one or more of its prototropic forms (cationic, neutral (zwitterionic, quinoid, and lactone), monoanionic (phenolate and carboxylate), and dianionic) in the solution. Information regarding the various prototropic forms of fluorescein in buffer-rich aqueous IL mixtures ([bmim][BF(4)] ≤ 50 wt %, mole fraction <0.1) at different pH is obtained via UV-vis absorbance and fluorescence. The addition of [bmim][BF(4)] is found to cause lactonization of fluorescein in the solution. Hydrolytic instability of IL [bmim][BF(4)] is observed to be much lower than that of NaBF(4). Absorbance and fluorescence from fluorescein are observed to be insignificant when dissolved in neat [bmim][BF(4)], suggesting the presence of predominantly the lactone form. The IL [bmim][BF(4)] behaves similar to apolar media in this context. It is demonstrated that various prototropic forms of fluorescein may be generated within [bmim][BF(4)] by the addition of buffer of appropriate pH. Significant bathochromic shift in absorbance and fluorescence band maxima of dianionic fluorescein as concentration of [bmim][BF(4)] is increased correlates well with the decrease in the hydrogen-bond-donating acidity of the medium. The lack of good linear correlation between the Stokes shift and the orientational polarizability suggests the possible presence of specific Coulombic interaction between the IL cation and the fluorescein dianion within the mixture. It is established that IL [bmim][BF(4)] has an interesting and unique effect on the prototropic behavior of fluorescein.

Dilute aqueous 1-butyl-3-methylimidazolium hexafluorophosphate: properties and solvatochromic probe behavior

Limitations of the properties of water, the most common ‘green’ solvent, may be overcome by adding small amounts of ionic liquids (ILs). Important physical properties, such as density, viscosity and refractive index, of dilute aqueous IL 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) are shown to be significantly different than those of pure water at different temperatures. Up to 2.00 wt% bmimPF6 behaves like a strong electrolyte in water. Solvatochromic absorbance probe investigation indicates sensitive spectral behavior of a water-soluble betaine dye upon bmimPF6 addition to water. With considerably modified physicochemical properties, dilute aqueous IL solutions may find use in a variety of chemical applications.

Binding of the ionic liquid cation 1-alkyl-3-methylimidazolium to p-tetranitrocalix[4]arene probed by fluorescent indicator displacement

AbstractAcridine orange (AO) was used as a fluorescent probe molecule to study the encapsulation of an alkylimidazolium cation from a water-soluble ionic liquid (IL) within two cavitand species, p-tetranitrocalix[4]arene (1) and calix[4]resorcinarene (2), both in alkaline aqueous media. The addition of IL to the preformed [1⋅AO] adduct resulted in significantly increased fluorescence due to the expulsion of AO from the inclusion complex to the aqueous phase by competitive recognition of the 1-alkyl-3-methylimidazolium cation ([Cnmim]+, n = 4 and 6) by 1. Conversely, the fluorescence signal dropped upon the addition of IL to the [2⋅AO] host–guest complex due to unfavorable binding between [Cnmim]+ and 2. The formation of these postulated adducts is corroborated using ab initio calculations, which also provide evidence for the location of [bmim]+ at the lower external rim of [2⋅AO], providing an explanation for the observed luminescence quenching in the latter case. These results point to a number of different paths for exploration, ranging from the fluorescence monitoring of IL contamination in groundwater to the “daisy chaining” of macrocyles toward supramolecular ionic networks. They also broadly encourage the exploration of ILs in host–guest-based optical and mass spectrometric sensory systems. FigureThe acridine orange (AO) adduct with the p-tetranitrocalix[4]arene cavitand (1) displays fluorescence turn-on when challenged with the 1-butyl-3-methylimidazolium ([bmim]+) cation in aqueous media due to expulsion of AO from the inclusion complex to the aqueous phase by competitive recognition of [bmim]+.