Exploring the Thermochromic Properties of Fluoran-Based Complexes: Synthesis, Structural Influence, and Application in Temperature Sensing

Fluoran dyes are extensively used as key components of thermochromic materials. To expand the scope of fluoran-based thermochromic systems, we conducted a comprehensive study to explore the relationships between the thermochromic properties of three-component complexes and the types and ratios of their constituents: fluoran dyes, developers, and solvents. We synthesized twenty-one fluoran dyes with varied chemical structures, including differences in the carbon chain length of the dialkylamino group, substitutions with electron-donating and electron-withdrawing groups, and the enlargement of conjugation systems. By combining these dyes with several phenolic developers and three long-chain alcohols, we prepared numerous thermochromic complexes and evaluated their color-changing behaviors under temperature variations. Complexes utilizing developers, bisphenol (BPA), bisphenol S (BPS), 4-hydroxy-4'-isopropoxydiphenylsulfone (D-8), 2,4'-dihydroxydiphenyl sulfone (2, 4'-BPS), and octyl gallate (OG) exhibited a red-to-colorless transition with increasing temperature, whereas those with developer lauryl gallate (LG) displayed a colorless-to-red transition upon heating. The chain length and conjugation extension of fluoran dyes influenced the optimal ratio of the three components. Additionally, substituents on the xanthene moiety of fluoran dyes didn’t impact their complex performance. Finally, we demonstrated the fabrication of these thermochromic complexes on filter papers, showcasing their sensitivity to temperature fluctuations and highlighting their potential application as temperature sensors.