Exploring the Synergy between π-π Interactions and Hydrogen-Bonding in the Formation of Type V Deep Eutectic Solvents

We present the design and characterization of two novel hydrophobic eutectic mixtures, where π-π attractive forces may drive deep eutectic solvent (DES) formation and provide potential capabilities for sustainable extractions. As model systems, we selected precursor molecules where ad hoc functional groups modulate the electron density of the aromatic rings in opposite directions, namely, 1,3-diacetylbenzene (DAB), 3,5-dimethoxyphenol (DMP), and 1,3,5-trimethoxybenzene (TMB). Solid-liquid equilibrium analysis, conducted through differential scanning calorimetry measurements, and the conductor-like screen model for realistic solvents predictive tool reveal a different thermal behavior between the DMP/DAB and TMB/DAB mixtures. A remarkable depression of the melting point (MP) as compared to thermal ideality is found for the DMP/DAB system, that can be classified as a type V DES. This deviation originates both from the strong hydrogen-bonds (H-bonds) between the DAB and DMP molecules in the mixture and the presence of noncovalent π-π interactions between the aromatic rings of the two components. Conversely, only a slight but detectable MP depression is observed upon DAB addition to TMB and this thermal behavior is explained by the absence of H-bonds and the existence of π-π interactions between the electron-rich TMB and the electron-deficient DAB aromatic systems. The π-π interaction between the TMB and DAB molecules is more favorable than the self-aggregation among TMB molecules in the pure state due to the high electron density of the aromatic ring and the steric hindrance among the bulky methoxy groups.