In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

Water can accelerate a variety of organic reactions far beyond the rates observed in classical organic solvents. However, using pure water as a solvent introduces solubility constraints that have limited the applicability of efficient photochemistry in particular. We report here the formation of aggregates between pairs of arenes, heteroarenes, enamines, or esters with different electron affinities in an aqueous medium, leading to an oil-water phase boundary through substrate melting point depression. The active hydrogen atoms in the reactants engage in hydrogen bonds with water, thereby accelerating photochemical reactions. This methodology realizes appealingly simple conditions for aqueous coupling reactions of complex solid molecules, including complex drug molecules that are poorly soluble in water.

The amino alcohol meglumine solubilizes organic compounds in water and enforces the formation of electron donor acceptor (EDA) complexes of haloarenes with indoles, anilines, anisoles or thiols, which are not observed in organic solvents. UV‐A photoinduced electron transfer within the EDA complexes induces the mesolytic cleavage of the halide ion and radical recombination of the arenes leading, after rearomatization and proton loss to C−C or C−S coupling products. Depending on the substitution pattern selective and unique cross‐couplings are observed. UV and NMR measurements reveal the importance of the assembly for the photoinduced reaction. Enforced EDA aggregate formation in water allows new activation modes for organic photochemical synthesis.