Electrochromic Devices Based on Disubstituted Oxo- Bipyridinium Ionic Liquids NoØmi Jord¼o, [a] Hugo Cruz, [a] Aida Branco, [b] Fernando Pina,* [a] and Luis C. Branco* [a] Introduction Electrochromism is defined as the reversible change in optical properties that can occur when a material is oxidised or re- duced electrochemically, and it has been explored for academ- ic and industrial applications. [1] Normally, electrochromic mate- rials show a distinct visible colour change between a transpar- ent (bleached) state and a coloured state, or between at least two coloured states. It is known that electrochromism also en- ables the darkening of a window at the flick of a switch. [2] This principle has already been demonstrated in electrochromic car rear-view mirrors. The most important classes of electrochro- mic materials include transition metal oxides, prussian blue systems, viologen derivative salts (disubstituted bipyridinium salts), conducting polymers, transition metal and lanthanide coordination complexes, metal phthalocyanines and metallo- polymers, among others. [3–6] Disubstituted bipyridinium dicat- ions are relevant electroactive species with switchable electro- chromic properties. [7] The three redox states characteristic of vi- ologen salts are well described as the dication (transparent), the radical cation (blue–violet), and the neutral compound, re- spectively. Normally, dication species are more stable, whereas the radical cation is intensely coloured, exhibiting a high molar absorption coefficient related to an intense intramolecular op- tical charge transfer. One interesting feature of these com- pounds is related to the possibility of the colour change of the radical cation according to the substituents selected (type and length) at the N-position of the bipyridinium cation and the type of anion. [8, 9] Different disubstituted bipyridinium salts have been tested in electrochromic devices as well as liquid crystal systems. [10–12] In several publications, it has been report- ed that the write/erase efficiency of electrochromic devices using short-alkyl-chain viologens such as methyl viologen in aqueous electrolytes is low owing to the high solubility of both the dicationic and radical cation states. [10, 13] In electro- chromic devices, methyl viologen should be improved by re- tarding the rate at which the radical cation, produced by elec- tron transfer, diffuses away from the electrode and into the bulk solution. This problem can be avoided by tethering the dication to the surface of an electrode, or by immobilising the viologen species within a semi-solid electrolyte. [14] Another possibility involves the variation of the alkyl-chain substituents from bipyridinium salts and the type of anion, which can also improve the electrochromic devices. [15] Recent applications such as in electrochromic car rear-view mirrors and smart win- dows do not necessarily require short response times. [16] One example of a commercial application of viologen derivative salts is related to Gentex’s commercialised automatic dimming interior “Night Vision Safety” (NVS) mirror functions. [17] This system comprises an ITO (tin-doped indium oxide) glass sur- face (conductive side inwards) and a reflective metallic surface, spaced a fraction of a millimetre apart, to form the two elec- trodes of the cell, and a solvent containing two electroactive chemical species that function as both electrochromic materi- als and supporting electrolyte. The two electroactive chemical species comprise a substituted (cationic) viologen, which serves as the cathodic colouring electrochromic material and a negatively charged (possibly) phenylene diamine as the anodic colouring electrochromic material. After the mirror is switched on, the species migrate to their respective electrodes. This type of electrochromic device therefore requires applica- tion of a continuous small current for replenishment of the col- oured electroactive species lost by their mutual redox reaction Electrochromic symmetric and non-symmetric disubstituted oxo-bipyridinium ionic liquids are developed in high purities and yields. Detailed spectroscopic (NMR, FTIR and elemental analysis), thermal (DSC for melting point and glass transition temperatures, T g determination) and electrochemical (cyclic and square wave voltammetry for determination of redox po- tential and evaluation of reversibility behaviour) analyses are performed. The most promising electrochromic oxo-bipyridini- um ionic liquids in combination with transparent electrolytes are tested as reversible electrochromic devices. More efficient and sustainable electrochromic devices inspired by bipyridini- um ionic liquids can be conceived for industrial applications such as electrochromic car rear-view mirrors and smart win- dows. [a] N. Jord¼o, Dr. H. Cruz, Prof. F. Pina, Dr. L. C. Branco REQUIMTE, Departamento de Química Faculdade de CiÞncias e Tecnologia, Universidade Nova de Lisboa Campus da Caparica, 2829-516 Caparica (Portugal) Fax: (+ 351) 212948550 E-mail : fp@fct.unl.pt l.branco@fct.unl.pt [b] A. Branco Ynivisible, Rua Mouzinho de Albuquerque 7 2070-104 Cartaxo (Portugal) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402232. ChemPlusChem 2015, 80, 202 – 208  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 202 Full Papers DOI: 10.1002/cplu.201402232