Ketocyanine-Based Materials for Near Infrared-to-Visible Thermochromism

Thermochromic materials with tunable optical absorption over the NIR and visible spectral regions, and exhibiting colorless-to-colored transitions, are of interest in relevant applications such as invisible security inks or solar light-energy management. Despite so, the most common commercial thermochromic materials, based on phase-change-material mixtures of spirocompounds (leuco-dyes), allow narrow spectral modulation within the UV/vis regions and mostly produce colored-to-colorless transition, hampering their applicability beyond novelty products. In contrast, materials intrinsically thermochromic in the NIR, require time-consuming syntheses for their spectral and transition temperature tunability. Herein these challenges are overcome by replacing conventional leuco-dyes with Ketocyanines that exhibit strong color sensitivity on external multifactor, i.e. polarity, aggregation state, and pH, through different intermolecular interaction mechanisms, allowing fine and straightforward spectral tunability. Their absorption can be further shifted to the NIR region (λ_max = 870 nm) by increasing the number of methine units, giving access to the desired colorless-to-colored transition (Δλ = 280 nm), via negative thermochromism. The preserved transitions of the microstructured mixtures are of interest for future thermochromic inks. Finally, their acid/base vapors response makes these multistimuli responsive materials highly attractive for applications such as anti-counterfeiting, bioimaging, and optical information storage for food packaging.