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.