The first example of thermochromism of dyes embedded in transparent polymer gel networks

Novel dye containing transparent hydrogels responding to changes in temperature with reversible color changes (thermochromism) are presented. The betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTPP), and the well known indicator dye Cresol Red (Fig. 1) exhibit an outstanding thermochromism, if they are embedded in an aqueous polyvinyl alcohol-borax-surfactant gel network. In case of DTPP at pH = 8.5, the color changes gradually from colorless at 10 degrees C to a deep violet at 80 degrees C. Similarly, for Cresol Red a change from yellow to wine-red was observed. An explanation for the observed reversible color changes in the closed hydrogel systems is given by a temperature-induced shift of the proton-transfer equilibrium between the phenolate and the phenol form of the dye molecules in the micro-environment of the gel network. The new thermochromic hydrogels can be useful in a wide range of applications, such as temperature sensors, large-area displays, so-called smart or intelligent windows.