High-Contrast Colorless-to-Colored Thermochromic Materials

Herein, high-contrast colorless-to-colored thermochromic materials are accomplished through novel 3-component mixtures based on catechol/pyrogallol derivatives (color developer - CD), fatty acid phase change materials (PCM), and leuco dyes. Such components ensure: I) always highly colored hot liquid states, promoted by strong stabilization of the dye open form through hydrogen-bonding interactions with the CDs' vicinal -hydroxyl groups, as also demonstrated by density functional theory calculations; II) solid mixtures with fully controllable coloration state, which directly relies on the relative length of the alkyl chain of CD (C_CD) and PCM (C_PCM). As also rationalized by thermal and spectoscopic measurements, colorless solids are obtained when C_CD is larger than C_PCM (ΔC = C_PCM-C_CD<0), due to self-assembly and phase segregation of CD molecules from PCMs, preventing their interaction with the dye. Instead, when ΔC≥0, the CD solubilizes in PCMs, developing the color of the co-dissolved dye. The rule is extendable to several leuco dye families (lactams, lactons, fluorans) and acid PCMs, fostering a tunable hot state color palette and transition temperature. The target transition is also preserved in structured mixtures (solid lipid particles), granting for the preparation of polymer composites and screen-printed thermochromic patterns, of interest for sensors and invisible security inks.