Electrochromism is the reversible and visible change in transmittance and/or reflectance that is associated with an electrochemically induced oxidation-reduction reaction. It has been well studied due to its promising applications in displays, sensors, smart windows, data storage, and information displays, etc. Materials exhibiting reversible color changes under an electric stimulus, called electrochromic materials. Electrochromic materials have the property of a change, evocation, or bleaching of color as effected either by an electron-transfer (redox) process or by a sufficient electrochemical potential. An ideal electrochromic material is expected to have high optic contrast between its extreme states, a short response time and high stability. In 1993, Lehn and co-workers  developed the first electrochromic Ru (II) complex with an electroactive quinone moiety in bipyridine ligands, which showed a reversible electrochemical “on-off” luminescent switch. Since then, many electrochromic materials have been rapidly exploited, including luminescent organic small molecules, coordination complexes, conducting polymers, polyoxometalates and quantum dots. Among them, coordination complexes have attracted increasing attention as electrochromic materials because of their intense coloration and redox reactivity.
Figure 1. Schematic diagram of electrochromic smart windows
Coordination complexes have been widely used as electrochromic materials, and a few simple examples are given below.
- Electrochromism based on Ru complexes
Forster and co-workers  reported on a metallopolymer, [Ru(terpy)-(box) PVP20]PF6 [terpy=2,2’:6’,2’’-terpyridine; box=2-(2-hydroxyphenyl) benzoxazole; PVP is poly(4-vinylpyridine), with 1 in 20 of the monomer units labeled with the Ru(terpy)(box)]. The metallopolymer exhibits rich electrochromic behaviour both in solution and within thin films. Oxidation at +0.5 V switches the colour from wine red to green while potentials above 1.0 V produce a change in the NIR region of the spectrum. The film can be oxidised at a rate of 10-13 cm2 s-1 and is consistent with the colour switching rate being controlled by electron hopping or segmental polymer chain motion.
Figure 2. Chemical structures of [Ru(terpy)-(box) PVP20]PF6
- Electrochromism based on Ir complexes
Huang and co-workers  have reported a series of ionic Ir (III) complexes sharing the same phosphorescent Ir (III) cation with a N-H moiety in the N^N ligand and containing different counterions. The anionic counterions cause a variation in the emission colours of the complexes by forming hydrogen bonds with the N-H proton. Interestingly, the emission colours of these complexes are sensitive to electrical stimuli. On the basis of these findings, a quasi-solid data recording and storage device is fabricated to show the evident luminescence colour change from yellow to green at a low voltage.
Figure 3. The constructed rewritable data recording device based on vapochromic phosphorescence
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- Lehn, J. M., et al. An electro-photoswitch: redox switching of the luminescence of a bipyridine metal complex. Chem. Commun. 1993: 1034-1036.
- Forster, R. J.; et al. Three colour electrochromic metallopolymer based on a ruthenium phenolate complex bound to poly(4-vinyl) pyridine. Electrochem. Commun. 2008, 10: 466-470.
- Huang, W.; et al. Smart responsive phosphorescent materials for data recording and security protection. Nat. Commum. 2014, 5: 3601.