Solvatochromic Absorbance and Fluorescence Probe Behavior within Ionic Liquid + γ‑Butyrolactone Mixture

The co-solvent modified ionic liquid (IL) systems offer favorable physicochemical properties, which render them as suitable solvents for several technological applications. Therefore, it is an utmost need to achieve a full understanding of such systems before their further applications. Here, solvation properties of 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim]­[BF₄], and γ-butyrolactone (GBL) mixture, which is an important binary mixture for several electrochemical applications, are explored by multiprobe spectroscopic and theoretical studies. The Reichardt polarity parameter (E_T^N) and Kamlet–Taft parameter (α, β, and π*) are determined with several absorbance probes. The unusual synergism in the probe’s response is observed within the [bmim]­[BF₄] and γ-butyrolactone (GBL) mixture. Such type of anomalous behavior is also observed in the response of the following fluorescence probes: 8-anilino-1-naphthalene sulfonate (ANS), 6-propionyl-2-(dimethylamino)­naphthalene (PRODAN), and pyrene (Py). This unusual behavior is explained by considering the role of solvent–solvent interactions between IL and GBL molecule in the cybotactic region of probes. The occurrence of solvent–solvent interaction and other possible interactions within the binary system are also explored by Fourier transform infrared, dynamic viscosity, and density studies. Further, the preferential solvation model and density functional theory also provide the useful evidence of solvent–solvent interactions within the binary mixture.