Reversible thermochromic systems based on a new library of flavylium spirolactone leuco dyes

Abstract

The aim of this work was the design, synthesis, and characterization of new chromogenic spirolactone dyes based on a 2-phenyl-1-benzopyrylium (flavylium) backbone for further development of thermochromic smart labels. The influence of electron-donating and electron-withdrawing groups on the five synthesized dyes was studied in terms of color modification, acid-base behavior and thermochromic properties. By using UV–Vis absorption and emission spectroscopies, molar absorptivity, oscillator strength, Stokes shift, and fluorescence quantum yield were determined for each dye to evaluate the impact on color and excited state properties of the designed structures. The acid-base behavior and the ability to evolve towards leuco species was assessed by titrations with triethylamine and N,N-diisopropylethylamine in non-polar and polar aprotic solvents such as acetonitrile and octan-1-ol. In addition, the reversibility of the color was also evaluated by titrations with polar protic solvents (water and ethanol) and Lewis acids (gallic acid and iron(III) chloride). The number of equivalents of base/acid required to reach the leuco product and then to reverse the color were determined by representing the absorption maximum as a function of added base/acid in acetonitrile (water content less than 0.01-v/v). Finally, the thermochromic properties were evaluated in the presence of long-chain organic amines and water-insoluble solvents with melting points close to room temperature. The synthesized compounds can switch between two species if the appropriate conditions are used. Moreover, the tests carried out to assess their thermochromic behavior concluded that all dyes display thermochromism to some extent. The dyes that have electron-withdrawing groups attached, such as cyano, display stronger chromogenic effects useful for future applications.