Thermochromism and Solvatochromism by Reversible Dye Aggregation in Sugar-Gel Matrices

Upon changes in temperature or solvent, extensive and reversible color changes (e.g., blue to pink) were observed with a cationic flavylium dye in sugar-gel matrices, such as agar, agarose, pectin, sodium alginate, and carrageenan gels, whereas only small spectral changes were found in methylcellulose gel or in the absence of any sugar gels. During many repeated cycles of warming and cooling no indications of significant fatigue were observed, demonstrating a high thermochromic reversibility. Comparable thermochromism and solvatochromism were observed with the well-known cationic dye Methylene Blue in the sugar-gel matrices. In the presence of an anionic surfactant in place of sugar gel, extensive color changes were also observed while no changes were observed at all in the presence of a cationic surfactant, thus implying an important role of the electrostatic interactions between the cationic dye and anionic surfaces or species. It has been assumed that the cationic dyes are reversibly coagulated or condensed on the negatively charged colloid-gel surface to enhance dye aggregation.