Thermochromic cobalt(II) chloro-complexes in different media: Possible application for auto-regulated solar protection

The well known fact that cobalt(II) halide complexes in various media undergo changes of co-ordination, absorbance and colour with the increase of temperature was combined with the low melting point of new materials suitable for simultaneous control of temperature and light intensity. Hence, the increase of absorbance with temperature acts as an auto-regulated shading protection from overheating. In this work, the thermochromic behaviour of cobalt(II) chloro-complexes was investigated by VIS–spectroscopy in four different ammonium nitrate+organic component based solvents and in ammonium nitrate+calcium nitrate tetrahydrate melt. The selected organic components were formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF) and dimethylsulphoxide (DMSO). In all binary mixtures the mole ratio of ammonium nitrate to the second component of the system was 1:3 and they were studied as suitable solvents for the formation of cobalt(II) chloro-complexes. Absorption spectra have been investigated in the visible spectral range 400–800 nm at two different temperatures (298.15 and 323.15 K). The overall stability constants of all formed complexes were calculated using a non-linear regression program at different temperatures: 308.15, 318.15, 328.15, 338.15 and 348.15 K. Based on these stability constants, the thermodynamic data, i.e. standard enthalpy, entropy and Gibbs energy, were calculated for all complexation reactions. Influence of the solvent composition on the complex stability and thermochromism was also discussed.

► Thermochromic behaviour of cobalt(II) chloro-complexes in five binary mixtures was investigated.
► Size of the organic component and its relative permittivity have the biggest influence on cobalt(II) thermochromism.
► Additional chemical energy can be stored in studied systems.
► Investigated mixtures are suitable for auto-regulated protection and energy storage.