Post-Synthetic Color Tuning of the Ultra-Effective and Highly Stable Surface-Confined Electrochromic Monolayer: Shades of Green for Camouflage Materials

Abstract

We report here on the strategy for the preparation of a series of electrochromic (EC) materials in green shades designed for camouflage purposes. This top-down post-synthetic modification provides access to new EC materials by fine modulation of the color of the surface-confined metalorganic monolayer pre-deposited on indium tin oxide screen-printed supports. Selective on-surface N-quaternization of the outer pyridine unit of the EC metal complex covalently embedded onto an enhanced surface area electrode results in a bathochromic shift of the absorbance signal as well as visual color change from blue to different shades of green. When assembled into solid-state EC devices (ECDs), the materials demonstrate high color differences between colored and bleached states and significant differences in optical density. Upon electrochemical switching, the ECDs initially featuring different shades of green become yellowish or clay. The accessible gamut of colors, fulfilling the requirements for chameleon-like camouflage materials, is able to mimic conditions of various natural environments including forests and sands. Notably, ECDs demonstrate high long-term durability (95% retention of the performance after 3300 cycles), fast coloration (0.6−1.1 s), and bleaching (1.2−3.3 s) times and outstanding coloration efficiencies of 1018−1513 cm 2 /C. Importantly, post-synthetic N-quaternization/color tuning does not deteriorate the performance of the resulting EC materials and devices as judged by cyclic voltammetry, spectroelectrochemistry, and electrochemical impedance spectroscopy. This work adds to the limited number of reports that explore color tuning of EC molecular layers via on-surface modification with the aim to access new non-symmetric materials. Notably, the facile and straightforward technology presented here allows the creation of green-colored EC materials that are difficult to prepare in other ways.

Surface-Enhanced Counter Electrode Materials for the Fabrication of Ultradurable Electrochromic Devices

“…Ideally, a greater value for coloration efficiency is desired, as it indicates that a lower amount of charge is required to create greater optical contrast. In contrast to metal-oxide-based ECDs, as well as ECDs based on triphenylamine polymers and viologen derivative gels with coloration efficiency values that rarely exceed 200 cm 2 /C, ECDs based on metalorganic molecules − and polymers often demonstrate higher values. The coloration efficiency of the device was determined from the charge–optical density plot (Figure B) to be 210 cm 2 /C.…”

“…In modern literature, we see a dramatic growth of interest in the development of effective energy-efficient electrochromic devices (ECDs). This fundamental research has immediate applications in a wide variety of fields such as “smart” windows, anti-glare glass, signage, and wearable technology, as well as for energy storage applications. − Consequently, this has resulted in significant improvements in the performance and stability of the electrochromic materials (ECMs) based on organic molecules, conjugated conductive polymers, − metal oxides, and metal-coordination complexes. − Novel hybrid ECMs are developed and systematically optimized to obtain desired optical properties such as high optical density and coloration efficiency, fast bleaching and coloration speeds, and optical durability. In addition, there has been stemmed interest into the improvement of electrolytes used for solid-state ECDs in order to maximize the ionic conductivity and long-term stability. − …”

Microalgae Chlorella vulgaris and kraft lignin stabilized cellulosic wet foams for camouflage

“…Similar to wildlife, the most basic human camouflage is patterned clothing designed to match natural surroundings, mostly green and brown shades in boreal forests and yellow and brown shades for sand lands. New coloring techniques are being developed all the time, including electrochromics 2,3 and adaptive materials. 4 Other camouflage techniques include structured textiles of metal incorporated yarn 5 and synthetic biology for advanced camouflage and protection materials.…”

Highly Transparent Conjugated Polymer as the Counter Electrode in Electrochromic Smart Windows

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mirrors, [2] electronic paper, [3] spacecraft thermal control, [4] and military camouflage, [5] etc.Generally, there are five functional layers in one electrochromic device (ECD): two conductive layers, one electrochromic layer, one electrolyte layer, and one counter electrode layer (i.e., the ion storage layer). Among those, the electrochromic layer is responsible for the color transformation.

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“…mirrors, [2] electronic paper, [3] spacecraft thermal control, [4] and military camouflage, [5] etc.…”