Ionic Liquids-Cobalt (II) Thermochromic Complexes: How the Structure Tunability Affects "Self-contained" Systems

With the aim of obtaining thermochromic systems with potential applications in solar energy storage, we evaluated the behavior of some sugar-based ionic liquids (ILs)–Co(NTf2)2 complexes, in IL solution, as a function of temperature. Different structural changes on the cation, the nature of the anion, and the nature of the IL used as the solvent were considered. The analysis of the above factors was carried out through a combined approach of different techniques, that is, variable temperature UV–vis and NMR spectroscopies, conductivity, and thermal gravimetric analysis. The thermochromic systems were analyzed both as solutions and as thin films, and the data collected highlight the defining role played by both the cation structure and the solvent nature in determining their performance. Most of the investigated systems show a chromogenic transition from pink to blue, occurring in a temperature range suitable for practical applications (40–60 °C). Interestingly, when embedded in a polymeric matrix, thin films with high recyclability and long life are also described.