Temperature-Dependent Solvatochromic Probe Behavior within Ionic Liquids and (Ionic Liquid + Water) Mixtures

Spectroscopic responses of absorbance probes, betaine dye 33, N,N-diethyl-4-nitroaniline, and 4-nitroaniline, and fluorescence dipolarity probes, pyrene (Py) and pyrene-1-carboxaldehyde (PyCHO) within ionic liquids (ILs) 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), and aqueous mixtures of [bmim][BF4] are used to assess the changes in important physicochemical properties with temperature in the range 10−90 °C. E T N obtained from betaine dye 33, indicating dipolarity/polarizability and/or hydrogen bond donating (HBD) acidity, decreases linearly with increasing temperature within the two ILs. Changes in Kamlet−Taft parameters dipolarity/polarizability (π*), HBD acidity (α), and HB accepting (HBA) basicity (β) with temperature show interesting trends. While π* and α decrease linearly with increasing temperature within the two ILs, β appears to be independent of the temperature. Similar to E T Nand π*, the first-to-third band intensity ratio of probe Py also decreases linearly with increasing temperature within the ILs. The lowest energy fluorescence maxima of PyCHO, though it decreases significantly within water as the temperature is increased from 10 to 90 °C, it does not change within the two ILs investigated. The temperature dependence of the dipolarity/polarizability as manifested via betaine dye 33 behavior is found to be more within the aqueous mixtures of [bmim][BF4] as compared to that within neat [bmim][BF4] or neat water. The sensitivity of π* toward temperature increases as IL is added to water and that of α decreases. The temperature dependent Py behavior shows no clear-cut trend within aqueous mixtures of [bmim][BF4]; insensitivity of the PyCHO response toward temperature change is reasserted within aqueous IL mixtures. All-in-all, the temperature-dependent behavior of solvatochromic probes within [bmim][PF6], [bmim][BF4], and aqueous mixtures of [bmim][BF4] is found to depend on the identity of the probe.