Electrochromic Devices and Smart Window Applications of Near-Infrared Electrochromic Thienoviologens Polymer Properties

In recent years, with the increasing demand for building energy efficiency and comfort in daily life, electrochromic smart windows have attracted widespread attention. Electrochromic smart windows utilize electrochromic material (ECM) technology to change the color or transparency of the window under the action of an electric field, thereby regulating the light and heat entering the room and achieving energy-saving effects. A bistable electrochromic material is a material that can switch between two color states and maintain one of these states without an external electric field. Although ECMs with bistable properties can significantly reduce energy consumption, current research on their application in smart windows is limited. Therefore, three conjugated extended viologen polymers with thiophene derivative bridges, <b>PDV-MMA</b>, <b>PBV-MMA</b>, and <b>PTV-MMA</b>, were designed and synthesized, and electrochromic devices (ECDs) were prepared. The introduction of thiophene derivatives not only prolongs the effective conjugated chain length of viologen, but also extends the electrochromic (EC) response to the near-infrared (NIR) region. In addition, by inserting different thiophene derivatives to separate the two pyridinium moieties, viologen polymers emit strong fluorescence that can be quenched by applying a negative voltage. The electrochromic device based on <b>PDV-MMA</b> material has fast response time (1.8 s), low turn-on voltage (-0.9 V), high color contrast (73.7%), high coloring efficiency (550 cm²/C), and cycling stability (3300 cycles). In addition, ECD based on <b>PDV-MMA</b> exhibits bistable characteristics, with only a 10% transmittance decay within 221 min, displaying clear patterns even after 20 h of power outage, and significantly reducing indoor temperature (7.0 °C) in simulated indoor environments. These characteristics make the application of electrochromic smart windows highly attractive in energy-efficient buildings. The study also demonstrated an intelligent window system driven by a solar cell (SC), further validating its feasibility in practical applications.