Colorless‐to‐Black/Gray Electrochromic Devices Based on Single 1‐Alkyl‐1′‐Aryl Asymmetric Viologen‐Modified Monolayered Electrodes

Abstract

In the rising tendency of electrochromics and incipient development of multicolor systems, big attention is recently being paid to the colorless‐to‐black or gray electrochromic (EC) materials, since they can extend the applicability of the EC technology. However, the achievement of high performance EC materials which intrinsically provide colorless‐to‐gray/black electrochromism remains a strategic challenge. The present study reports on colorless‐to‐black/gray EC devices (ECDs) based on new 1‐alkyl‐1′‐aryl asymmetric viologens assembled in a nanostructured device configuration with poly(vinyl alcohol) (PVA)–borax gel electrolyte (conducting glass/viologen‐modified‐TiO2/PVA–borax gel electrolyte/conducting glass). The asymmetric viologens reported herein have been conveniently functionalized with anchoring groups in both alkyl and aryl substituents, ensuring rapid anchoring process (≈1 min) and strong binding affinity toward TiO2 monolayered working electrode. It is demonstrated herein that these ECDs based on a single asymmetric viologen provide not only gray but also real black electrochromism (a* and b* ≤ |5|) with great variation of the level of coloration (ΔL* = 53) and transmittance change (Δ%T ≥ 60), along with satisfactory switching times (5–10 s), transparent colorless‐off state, and good cyclability (≈10 000 cycles). Furthermore, these asymmetric viologens have also been tested in light‐scattering electrodes to prove their compatibility with them, extending their field of application.

High‐Performance Electrochromic Optical Shutter Based on Fluoran Dye for Visibility Enhancement of Augmented Reality Display

“…The LCD utilizing dye‐doped cholesteric LC (chLC) and electrowetting devices have excellent optical properties and fast response time but too high driving voltage limits the application to portable devices like OST‐HMD . On the other hand, reported ECDs using various electrochromic materials have very low driving voltage suitable for the portable devices . Among reported electrochromic materials, fluoran dye exhibits superior characteristics than viologen and polymer materials in all evaluation items.…”

Transparent to Black Electrochromism—The “Holy Grail” of Organic Optoelectronics

“…Those include: transition metal oxides, prussian blue systems, viologens, conjugated polymers, transition metal complexes and metal phthalocyanines [ 9 , 10 ]. Of those, transition metal oxides, viologens and conjugated polymers have been reported to exhibit bleached and dark coloured states [ 6 , 11 , 12 ]. Each of those systems has its strong and weak points, therefore, each may excel when presented with different requirements.…”

“…Alesanco et al [ 11 ] report another approach to the use of viologen derivatives as electrochromic systems ( Scheme 1 ). In this case, the Authors focused on asymmetric derivatives, equipped with terminal phosphate groups to promote adherence to the TiO 2 monolayer, which the authors deposited on the working electrode of their devices.…”

All-in-One Gel-Based Electrochromic Devices: Strengths and Recent Developments

“…Another clear example of using gel electrolytes to modulate the equilibrium of the radical cation monomer and dimer is from recent work based on 1-alkyl-1′-aryl substituted asymmetric viologens ( Figure 9 aI and bI) [ 155 ]. The latter, with electrochromic and electrochemical behavior about midway between the corresponding symmetric viologens, have been demonstrated to provide neutral tones (i.e., gray, blackish) [ 156 ] when conveniently tested in a water-based gel-type electrolyte ( Figure 9 aII and bII). This report confirmed that using these asymmetric viologens (i.e., 1-ethyl-1′-( p -cyanophenyl)-4,4′-bipyridinium dibromide (et-pCNVio, Figure 9 aI) and 1-benzyl-1′-( p -cyanophenyl)-4,4′-bipyridinium dibromide (Bn-pCNVio, Figure 9 bI) in a PVA-borax gel-type electrolyte led to different colored states, and therefore different absorption profiles, from the ones registered for anhydrous liquid systems.…”