New insights concerning the mechanism of reversible thermochromic mixtures

Abstract

Three-component organic thermochromic materials, consisting of a leuco dye, a weak acid acting as the colour developer, and a low-melting organic solvent, can change their colour in response to temperature changes. Although widely used in applications, their detailed thermochromic mechanism is not fully understood. The present study delineates the role of subtle changes in the solvent's molecular structure and concentration in the crystal violet lactone (CVL, dye)/lauryl gallate (LG, developer)/1-alcohol (dodecanol (DD), tetradecanol (TD), hexadecanol (HD), or octadecanol (OD); solvent) system. Through inkjet printing of the components directly onto a substrate, combinatorial approaches reveal differences when the alkyl chain length of the alcohol solvent is changed slightly. During the process of heating to the melt, followed by cooling to room temperature, CVL/LG/DD showed no colour change. On the other hand, CVL/LG/TD exhibited reversible thermochromism with colour forming in the molten state and colour loss in the solid state. In the composition range investigated, the CVL/LG/HD system showed no colour change during heating, but on cooling from the molten state, at first a blue colour appeared just below the freezing point, and this was followed by a slow colour fading on further cooling. A significant new finding is that the orientationally disordered -phase of the solvent is required to support the dye-developer complex that provides colour. Furthermore, there is an optimal solvent chain length for thermochromism: if too short, there is no disordered phase and no colour; if too long, the formation of the coloured developer-dye complex is prevented in the melt.