Effect of varying substituent on the colour change transitions of diacetylene pigments

A library of closely related diacetylenes has been synthesised, comprising docosa-10,12-diynediamides with small n-alkyl or cycloalkyl substituents on the termini. Colour changes of these solids in response to ultraviolet (UV) radiation and heating were analysed using a range of analytical techniques, including UV-visible/near-infrared reflectance, hand-held Raman and solid-state nuclear magnetic resonance (ssNMR) spectroscopies. Colorimetric studies showed that this class of diacetylene undergoes clear colour changes that are very sensitive to even small alterations in molecular structure. The colour change transitions of diacetylenes bearing either cyclopentyl or cyclohexyl substituents were more similar to each other than to those of analogous diacetylenes bearing either n-pentyl or n-hexyl substituents. Differential scanning calorimetry (DSC) also reflected these differences, with the cycloalkyl substituted diacetylenes showing endothermic transitions at significantly higher temperatures than both acyclic analogues. From this library, an n-hexyl substituted diacetylene shows the greatest potential as a colour change pigment, with three distinct colours that are stable at room temperature (white, blue, red), as well as an unstable yellow colour at elevated temperatures.