(Invited) Organic Based Electrochromic and Thermochromic Materials: Design, Characterization and Integration in Flexible Devices
Authors: Beverina Luca , Sassi Mauro , Brazzo Paolo , Galliani Daniela 
Published: February 2020

Electrochromism and thermocromism are amongst the most promising chromogenic phenomena having real technological appeal. In particular, Organic electrochromic materials (OEMs) - the focus of the first part of the talk -offer a low power input and low cost solution for applications like smart windows, electrochromic sunglasses, electrochromic paper and displays.

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Depending on the specific application, suitable OEMs should offer a wide color palette, high electrochromic contrast, fast switching, low cost and high stability. Relevant applications like sunglasses and automotive additionally require that the transmitted light experiences negligible color distortion in both device’s limiting states. The Donor-Acceptor (D-A) polymers concept so far gave best results on this very challenging topic. Indeed, the preparation of copolymers constituted by alternating electron rich and electron poor groups leads to devices having brown and even grey low transmittance states. Such result comes at the expenses of a somewhat limited transmittance in the bleached state (T ≈ 60 %).

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Multichromophoric polymers (MCPs) represent a viable alternative to the D-A concept. Typical MCPs feature a PEDOT conjugated backbone, side chain functionalized with small molecule electrochromic derivatives acting as contrast enhancers and color tuning additives.

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The most performing MCP so far reported, a naphthalenediimide containing cross-linked PEDOT, features a transmittance level above 80 % all over the visible region in the bleached state, along with a grey (T < 20 %) colored state. Cycling stability in the excess of 1000 cycles was also demonstrated.

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We will discuss the synthesis and characterization of two new MCPs based upon specifically designed small molecule contrast enhancers pertaining to the class of Weitz electrochromes. The resulting side chain PEDOT polymers are able to reversibly switch between a brown and a colorless state, both of them complying with the performance of photochromic lenses. Figure 1a shows a representative example of an electrochromic devices based on said materials.
In the second part of the talk we will focus on a particular use of organic thermochromic materials: Time Temperature Indicators for food safety. The spontaneous spoilage of fresh goods – milk might probably be the best everyday life example – due to inappropriate handling and storage, particularly during long distance delivery, is an acknowledged treat for public health, even more serious than adulteration. As such, there is a relevant technological interest towards the development of simple and low-cost smart labels that can be integrated into packages giving information on their whole thermal history. The current Time Temperature Indicator (TTI)

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devices include two classes: a) Integrated circuits featuring data loggers and radio frequency identification chips, and b) simpler color indicators relying on chemical reactions such as enzymatic reactions. The second class is inexpensive and attractive for the packaging industry. We will show a new class of irreversible thermochromic molecular materials whose chemical transformation from a colorless state to a strongly colored one depends upon temperature, chemical substitution and substrate. The selection of the appropriate active molecule/substrate combination provides sensitivity to different targeted time/temperature regimes.

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The key process providing the required irreversible color change is connected with the “latent pigment” approach.

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Figure 1b shows one example of the two limiting states of demonstrator smart labels based upon latent squaraine pigments.
Figure 1. a) Bleached (left) and colored (right) limiting states of an electrochromic device having the structure PET-ITO/organic electrochrome/electrolyte polymer membrane /Prussian blue/PET-ITO. b) bottom: comparison between a reference and a smart label based on a thermochromic material after 3 h at 4°C and at 25 °C; top: pictures taken at regular intervals of a TTI based on the molecule shown at 25°C.
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