Novel dye containing transparent hydrogels responding to changes in tempera
ture with reversible color changes (thermochromism) are presented. The beta
ine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenolate (DTPP), and the
well known indicator dye Cresol Red (Fig. 1) exhibit an outstanding thermoc
hromism, if they are embedded in an aqueous polyvinyl alcohol-borax-surfact
ant gel network. In case of DTPP at pH = 8.5, the color changes gradually f
rom 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 explanatio
n 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 use
ful in a wide range of applications, such as temperature sensors, large-are
a displays, so-called smart or intelligent windows.