Major advance in the development of next-generation thermochromic materials

a) Molecular structures of ketocyanine dyes used in this work (Cyn1, Cyn2, Cyn3); b) scheme of the designed three-component thermochromic material, made of PCM, ketocyanine dye (Cyn3 was drawn as an example), and color developer, and the expected thermochromic transition through keto-enol equilibrium. @ Advanced Optical Materials / ICN2

Scientists at ICN2 have developed an innovative material based on a solution of fluorescent dyes (ketocyanines) and phase-change materials (PCM). This combination enables a reversible and controllable colour change in response to temperature-induced solid-liquid transitions. These findings pave the way for the development of a wide range of new technologies. Thermochromic materials are substances that can change colour, either reversibly or irreversibly, in response to variations in temperature. These modifications can be due to changes in their chemical structure or in the way they interact with light. This property makes them extremely useful in a variety of applications, such as smart windows that regulate the amount of light entering the room, or smart inks that help preventing counterfeiting of banknotes and other products.

Researchers from the ICN2 Nanostructured Functional Materials Group have recently developed a new type of thermochromic material based on a group of fluorescent dyes known as ketocyanines, synthesised by the group of Prof. Mar Puyol (UAB) and collaborators. It also incorporates phase-change materials (PCM), which are capable of absorbing and emitting heat when switching between solid/liquid states. This combination allows for more varied and adjustable colour changes in response to temperature. The study, led by Dr Claudio Roscini and CSIC Researcher Prof. Daniel Ruiz-Molina, has been published in the journal Advanced Optical Materials.

“Unlike conventional thermochromic materials, which only allow colour changes in certain regions of the light spectrum (such as ultraviolet and visible light) and usually become colourless when heated, ketocyanine-based thermochromic mixtures allow a much wider range of colour changes and become coloured when the mixture is heated,” say Noel Muñoz and Dr Alex Julià, who contributed to the experimental development. By modifying certain factors, such as the proportions and types of their components (e.g., colour developer, PCM), the researchers were able to make these dyes absorb light in the near-infrared range. This means that, unlike conventional materials, inks made from these new materials can change from colourless to a specific colour when heated.

Another major advantage is that they can be incorporated into cellulose paper or capsules to form leakage-free solid materials, more relevant for practical applications. In addition, they have shown the ability to change colour when exposed to substances such as ammonia or certain acid vapours, further increasing their versatility for various applications (gas/pH sensors).

The list of potential applications is long. In addition to the already mentioned uses, such as smart windows and invisible inks, areas in which the Nanostructured Functional Materials Group is well known, thanks to the successful spin-offs Futurechromes and Distinkt, other potential applications could involve food packaging. In this context, thermochromic materials could be used to monitor real-time temperature or pH to improve preservation. Reference Ketocyanine-Based Materials for Near Infrared-to-Visible Thermochromism

Alex Julià López, Noel Muñoz Pérez, Ruslan Magerramov, Mar Puyol, Ferran Crugeira, Sergey Miltsov, Daniel Ruiz-Molina, Claudio Roscini https://advanced.onlinelibrary.wiley.com/doi/10.1002/adom.202401411 Catalan Institute of Nanoscience and Nanotechnology (ICN2)