A bifunctional porous organic polymer for NIR-selective electrochromism and bright-to-quenched electrofluorochromic smart windows

Abstract

As demand for selective near-infrared (NIR) modulation grows in applications such as wearable optical encryption, adaptive thermal regulation, infrared camouflage, and multi-band communication, organic π-conjugated polymers have attracted significant interest. Their tunable bandgap, solution processability, and mechanical flexibility make them ideal candidates for nextgeneration NIR-selective electrochromic devices. A conjugated low bandgap covalent porous polymer (CPF), TPA-TZTZ, consisting of a triphenylamine (TPA) based donor and thiazolothiazole (TZTZ) containing acceptor for dual performance vis/NIR selective electrochromic (EC) and bright-to-quenched electrofluorochromic (EFC) properties, has been designed and thoroughly characterized. The TZTZ core provides efficient charge transport and inherent n-type behavior, while the TPA unit enables reversible multistep oxidation and stable radical-cation formation. TPA-TZTZ-based solid-state EC device exhibits anodic visible-toselective NIR electrochromism and yellow-to-quenched electrofluorochromism, with sufficient durability, high colour efficiency, and fastness. The stepwise electrochemical oxidation of the CPF produces redox-induced polaron and bipolaron states, enabling voltage-induced selective Vis/NIR absorption with notable spectral contrast. Moreover, the porous structure promotes ion diffusion, enhancing device stability and increasing EC/EFC durability. The low energy consumption and good EC performance highlight the potential of TPA-TZTZ for practical uses in solar irradiance modulation for smart windows and EC-based display technologies. The yellow to dark EC and greenish-yellow to quenched EFC behaviors are collectively used to mimic an S-R "Flip-Flop" molecular logic gate operation. Overall, TPA-TZTZ represents a promising bifunctional CPF with a high contrast ratio, low energy consumption, and a high coloration efficiency, making it suitable for multifunctional smart windows and NIR-responsive optoelectronic technologies.