ELECTROCHROMIC, ELECTROFLUOROCHROMIC, AND PHOTOACTIVE PROPERTIES OF THIAZOLOTHIAZOLE-BASED, MULTIFUNCTIONAL MATERIALS

There have been an increasing number of materials developed that show multifunctional chromogenic properties (such as electrochromism, electrofluorochromism, or photochromism), but to date, few materials have shown all three properties. Materials that are electrochemically and optically active are attractive for a diverse set of applications that include smart-windows, lighting, sensing, energy production, and conservation. Achieving systems made from organic, cost-effective, readily synthesized materials would make them easy to utilize in a variety of fields. Multifunctional chromogenic dipyridinium thiazolo(5,4-d)thiazole (TTz) show promise in achieving these needs as they offer high contrast color change, high fluorescent quantum yields above 90%, and water processability while made from inexpensive starting materials. The planar, rigid, heterocyclic TTz core improves stability and reversibility as the TTz reduces from yellow TTz2+ to purple TTz•+ to blue TTz0 compared to other viologen systems.
When implemented in a low-cost poly(vinyl alcohol) (PVA)/borax hydrogel device using conductive glass electrodes, the TTz can change color and fluorescence intensity with applied voltage or light exposure. The electrochromism offers 75% transmittance contrast that is stable for 250 on/off cycles and electrofluorochromism with >90% contrast. By adjusting gel components and coating/drying the hydrogel, a variety of photochromic thin films were produced. Remarkably, the TTz-embedded films retain their high contrast chromogenic properties showing photochromism (yellow TTz2+ to blue TTz0 color change) and photofluorochromism after only one minute of light exposure. After turning blue, the oxidation back to yellow occurs through interactions with oxygen. This is potentially an effective way to optically gauge the presence of oxygen which is useful for a variety smart packaging applications for food, pharmaceuticals, and electronics. The color change speed and contrast can be tuned by adjusting borax and TTz concentrations. Because the TTz’s show multifunctional capabilities, photo-charging battery devices are tested to indicate the creation and storage of electrical charge when illuminated. When paired with the appropriate catholyte and membrane, TTz shows evidence of photocharging in hydrogel and film devices. Comparing charge discharge curves of the battery devices, illumination can increase voltages by 0.2 V and improve charging capacity. This work shows the remarkable multifunctional electroactive and photoactive properties of dipyridinium thiazolothiazole materials as well as their implemented to yield reversible, high contrast electrochromism, electrofluorochromism, photochromism, photofluorochromism, and light responsive charging.