Organic materials have recently gained considerable attention for electronic applications, improving performance and sustainability to current technologies. Commercialized metal-based systems are generally expensive, toxic and difficult to recycle, however organic materials offer promising solutions. Viologens, N,N' di-quaternized bipyridyl salts, are a well-studied species exhibiting three reversible redox states, possessing valuable electrochromic and electron-accepting properties. These properties can be fine-tuned through synthesis by altering the nitrogen substituents and various counteranions. Currently, viologens have become of great interest as functional materials in a wide array of applications; a few to name include electrochromic devices, molecular machines, and organic batteries. This review highlights representative recent work and advances towards utilizing viologens in practical applications that currently compete with metal-based technologies. Additionally, modified viologens that can be further fine-tuned will be discussed.

Viologens and Their Application as Functional Materials

Antibacterial activity of polymeric substrate with surface grafted viologen moieties

The magnetic properties of two series of dinuclear complexes, and one trinuclear complex, have been examined as a function of the bridging pathway between the metal centers. The first series of dinuclear complexes is [{MoV(O)(Tp*)Cl}2(μ−OO)], where “OO” is [1,4-O(C6H4)nO]2- (n = 1, 1; n = 2, 3), [4,4‘-O(C6H3-2-Me)2O]2- (4), or [1,3-OC6H4O]2- (2) [Tp* = tris(3,5-dimethylpyrazolyl)hydroborate]. The second series of dinuclear complexes is [{MoI(NO)(Tp*)Cl}2(μ-NN)], where “NN” is 4,4‘-bipyridyl (5), 3,3‘-dimethyl-4,4‘-bipyridine (6), 3,8-phenanthroline (7), or 2,7-diazapyrene (8). The trinuclear complex is [{MoV(O)(Tp*)Cl}3(1,3,5-C6H3O3)] (9), whose crystal structure was determined [9·5CH2Cl2:  C56H81B3Cl13Mo3N18O6; monoclinic, P21/n; a = 13.443, b = 41.46(2), c = 14.314(6) A; β = 93.21(3)°; V = 7995(5) A3; Z = 4; R1 = 0.106]. In these complexes, the sign and magnitude of the exchange coupling constant J is clearly related to both the topology and the conformation of the bridging ligand [where J is derived fr...

Roles of Bridging Ligand Topology and Conformation in Controlling Exchange Interactions between Paramagnetic Molybdenum Fragments in Dinuclear and Trinuclear Complexes.

Water-soluble silsesquioxane nanoparticles (NPs) incorporating viologen groups (PXV; 1,1′-bis[3-(trimethoxysilyl)propyl]-4,4′-bipyridinium iodide) have been synthesized by sol−gel polymerization. The electrochromic properties of the bulk film fabricated by layer-by-layer (LbL) assembly have been examined, along with their incorporation into solid-state devices. The orange LbL films show high thermal stability and exhibit a maximum UV−vis absorption at 550 nm. Electrochromic switching of the NPs in liquid electrolyte as well as in the solid state was evaluated by a kinetic study via measurement of the change in transmission (% T) at the maximum contrast. Cyclic voltammograms of the PXV NP LbL films exhibit a reversible reduction at −0.6 V vs Ag/AgCl in a 0.1 M NaClO4(aq) solution, revealing good electrochromic stability, with a color change from orange to dark purple-blue at applied potentials ranging from −0.7 to −1.3 V. Cathodically coloring PXV NP solid-state devices exhibit a switching time of a few se...

High-Contrast Solid-State Electrochromic Devices of Viologen-Bridged Polysilsesquioxane Nanoparticles Fabricated by Layer-by-Layer Assembly

This work describes the synthesis and characteristics of a novel electrochromic ionic liquid (IL) based on a phosphonium core tethered to a viologen moiety. When integrated into a solid-state electrochromic platform, the viologen modified IL behaved as both the electrolyte and the electrochromic material. Platform fabrication was achieved through in situ photo-polymerization and encapsulation of this novel IL within a hybrid sol–gel. Important parameters of the platform performance, including its coloration efficiency, switching kinetics, and optical properties were characterised using UV–vis spectroscopy and cyclic voltammetry in tandem. The electrochromic platform exhibits a coloration efficiency of 10.72 cm2 C–1 and a varied optical output as a function of the incident current. Despite the rather viscous nature of the material, the platform exhibited approximately 2 orders of magnitude faster switching kinetics (221 s to reach 95 % absorbance) when compared to previously reported electrochromic ILs (18...

An electrochromic ionic liquid: design, characterization, and performance in a solid-state platform.

A range of diquaternary salts of 4,4′-bipyridine and 3,8-phenanthroline were prepared and subsequently screened using electrochemical and spectroscopic techniques. It was determined that the stability of the radical film deposited on cathodic reduction of aqueous solutions of these salts may be strongly influenced by symmetry, steric, and electronic factors. From this basis, it was possible to design a viologen, 1,1′-bis-(2-methylbenzyl)-2-methyl-4,4′-bipyridinium dibromide (9), having electrochromic properties for electronic display applications superior to those already in existence. A radical film of mixed composition was found to be less susceptible to ageing effects than films generated from the individual dicationic salts.

John A. Barltrop

Papers

The synthesis and electrochemical study of new electrochromic viologen-based materials