title: Colorless to Neutral Color Electrochromic Devices Based on Asymmetric Viologens.
abstract: Electrochromic materials have extensively been investigated because of their potential fields of application, with a significant growing interest in expanding the provided colorations. However, among all palette of colors, colorless electrochromic devices (ECDs) that provide neutral-grayish colorations with a simple configuration remain a key challenge. The present study reports on the synthesis of asymmetrically 1-alkyl-1'-aryl-substituted viologens and their incorporation in PVA-borax gel polyelectrolytes for ECDs that constitute the simplest device architecture (glass/TCO/EC gel/TCO/glass). We demonstrate herein that these EC gels based on single asymmetric viologens provide more neutral-colored state than their corresponding symmetric viologens (a* and b* ≤ |15|), while maintaining satisfactory colorless bleached state (%Tb> 70% in the whole visible range), transmittance changes (i.e., ∼60%) and cyclability (i.e., ∼15 000 cycles). Additionally, the effect of the solvent on the observed coloration has also been investigated. This easy-to-make neutral-grayish color ECDs may significantly extend the potential of the electrochromic technology, because they adapt better aesthetically to the surrounding environment, as they are easier to implement in different applications.
authors.affiliation: IK4-CIDETEC Research Center , Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain.
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Alesanco
authors.affiliation: IK4-CIDETEC Research Center , Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Viñuales
authors.affiliation: IK4-CIDETEC Research Center , Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain.
authors.forename: Germán
authors.initials: G
authors.lastname: Cabañero
authors.affiliation: IK4-CIDETEC Research Center , Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain.
authors.forename: Javier
authors.initials: J
authors.lastname: Rodriguez
authors.affiliation: IK4-CIDETEC Research Center , Paseo Miramón 196, 20009 Donostia-San Sebastián, Spain.
authors.forename: Ramón
authors.initials: R
authors.lastname: Tena-Zaera
keywords: PVA gels; asymmetric viologens; gray electrochromics; neutral color; smart windows