Covalent Attachment Strategies of Molecular Electrochromes for Enhancing Electrochromic Performance

Abstract

Electrochromes are emerging materials for enabling sustainable energy devices such as smart windows and low power‐consuming displays along with automotive mirrors. This is owing to their electrochemical activity that results in unique optical transmission, modulating with applied potential. The molecular design rules of electrochromes are well established, consisting of electroactive components such as viologens, rhodamines, and transition metal complexes. While molecular electrochromes offer the advantage of establishing accurate structure/property relationships for tuning the optical transmission contingent on molecular structure, their physisorption on the electrodes limits the performance of electrochromic devices. Indeed, molecular electrochromes suffer from poor performance compared to their polymer counterparts in operating electrochromic devices. This perspective presents approaches to overcome these challenges. Focus is given to various strategies of covalently attaching molecular electrochromes to the device electrode for improving key electrochromic properties of contrast ratio and coloration efficiency. These operating device metrics are improved compared with the physisorption of molecular electrochromes via noncovalent interactions. The overarching goal is to provide useful insight that can be leveraged for the rational design of molecular electrochromes for their covalent attachment to electrodes toward matching device metrics of their polymer counterparts.

The covalent attachment of molecular electrochromes to the conductive surface is an emerging approach for improving the performance of electrochromic devices. The perspective outlook surveys different functionalizations for covalently bonding molecular electrochromes toward enhancing the device metrics consisting of color switching with applied potential and coloration efficiency, among others, by framing improvements against their physisorbed counterparts.

1. Introduction

Electrochromes can be defined as electroactive organic, inorganic, or hybrid materials alike that change their optical transmission color with an applied potential. The electrochemically mediated color change dates back to the 1960s with Prussian Blue inorganic salts of Fe₄[Fe(CN)₆]₃ ^{[ 1 ]} and oxides of tungsten (WoO₃) and molybdenum (MoO₃) among the first examples of color switching compounds with applied potential.^{[ 2} , ^{3 ]} The electrochemically promoted electron transfer that converts the electrochrome in its resting state to a redox state having a different optical transmission was further spurred on by extensive studies by Mortimer et al.^{[ 4 ]} Organic chromophores with high degrees of conjugations were also found to be suitable electrochromes. This was courtesy of their intrinsic electroactivity and visible color changes with an applied potential owing to their extended degree of conjugation. Indeed, various electroactive chromophores (Figure  1 ) have been demonstrated to be ideal for preparing electrochromes that exhibit visible color changes with applied potential including phenothiazine (1), carbazole (2), 3,4‐ethylenedioxythiophene (EDOT) (3), 2,3‐propylenedioxythiophene (ProDOT) (4), triphenylamine (TPA) (5), 1,1′‐disubstituted‐4,4′‐bipyridinium salts (viologens) (6), and terpyridines (7).^{[ 4} , ⁵ , ⁶ , ^{7 ]}

Figure 1.

Representative examples of electroactive organic chromophores used for preparing molecular electrochromes: phenothiazine (1), carbazole (2), EDOT (3), ProDOT (4) triphenylamine (5), viologen (6), and metallo‐terpyridines (7).

Since the original understanding of redox mediated optical transmission change, the field of organic electrochromes has matured. This has resulted in well‐established design rules for developing molecular electrochromes. These were courtesy of extensive molecular structure/property relationships that have been reviewed by Stolar,^{[ 7 ]} Reynolds,^{[ 8 ]} Li,^{[ 9 ]} Baumgartner,^{[ 10 ]} Wang,^{[ 11 ]} and Skene.^{[ 12 ]} Hybrids of organic/inorganic materials have also emerged as viable electrochromes.^{[ 13} , ^{14 ]} Since the original understanding of electrochemical mediated optical transmission change, the field of organic electrochromes has matured. This has resulted in well‐established design rules for developing molecular electrochromes. The reversible color change with applied potential has been exploited for use in various devices such as sensors,^{[ 15 ]} smart windows,^{[ 9} , ^{16 ]} displays,^{[ 17} , ^{18 ]} energy storage devices,^{[ 9} , ^{19 ]} and artificial skin.^{[ 20} , ^{21 ]} While electrochromes underpin the color changing behavior, the substrate used for operating electrochromic devices and the interaction of the electrochrome on the substrate are also of importance.^{[ 7} , ¹⁰ , ¹¹ , ¹² , ^{13 ]} Electrochromic devices are typically solid‐state stacked layers, with the simplest architecture consisting of the discrete electrochrome and electrolyte layers sandwiched between two transparent electrodes (Figure  2A).^{[ 3} , ^{22 ]} The architecture can be augmented to include an ion storage layer for the device to retain its color state after the potential is no longer applied (Figure 2B).^{[ 23} , ²⁴ , ²⁵ , ^{26 ]}

Figure 2.

Schematic representation of the A) simplistic and B) augmented stacked electrochromic device architectures.

While molecular electrochromes offer the advantage of accurately establishing structure/property relationship owing to their well‐defined structure and characterization using conventional methods, the mid‐ to long‐term performance of electrochromic devices they enable lags behind that of their polymer counterparts. Indeed, amorphous conjugated organic polymers play an important role in electrochromics. This is in part owing to their electrochemical robustness. As a result, they can sustain multiple redox cycles without color fatigue. Their high molecular weight is also beneficial. This ensures their limited mobility in an operating device, further ensuring consistent color and electrochemical performance.^{[ 8} , ¹² , ^{27 ]} Bearing this in mind, it would be ideal to take advantage of the long‐term electrochromic stability of polymers along with the well‐defined and tunable properties that molecular electrochromic provide. Given the performance limitations of molecular electrochromes are in part responsible for their mobility in operating devices that can result in undesired intermolecular reactions, anchoring them to the surface would improve their electrochemical stability along with consistent optical transmission changes with repeated cycles of applied potential. Toward addressing these shortcomings, this perspective highlights different strategies that have been adopted to immobilize molecular electrochromes on the working device electrode for preventing their unwanted intermolecular redox mediated reaction by diffusion. The improvements afforded from the covalent attachment of molecular electrochromes are framed against their unattached counterparts according to key electrochromic metrics, including contrast ratio, coloration efficiency, color switching kinetics, and electrochemical stability (vide infra). Focus is placed on the functionalization of various electrochromes with phosphonic acids, carboxylic acids, and silanols. These functionalities for covalently attaching the electrochrome to electrodes and the resulting electrochromic enhancement are surveyed to provide key insights that can be leveraged for the rational design of the next generation of high performance molecular electrochromes with predefined electrochromic properties.

2. Electrochromic Metrics

Key metrics that define the true performance of electrochromes are the contrast ratio (CR), coloration efficiency (η), response time (τ), and electrochemical reversibility/stability. CR is defined as the difference between the electrochemically generated bleached (off) and colored (on) states at a given monitoring wavelength. The wavelength of choice is usually the maximum absorption of the colored stated that is electrochemically generated. The desired CR of 100% is possible with electrochromes at the onset of generating the colored state (Figure  3A). However, a CR decay after an extended number of cycles of switching between the colored (R _x ) or bleached states (R ₀) is symptomatic of degradation of either of the colored states. CR is described as a ratio between R ₀ and R _x according to Equation (1)

$$\text{CR}=\frac{R_0}{R_{\mathrm{x}}}$$ (1)

Figure 3.

Representative electrochromic device metrics. A) Ideal variation in absorbance of an electrochrome with applied potential monitored at a given wavelength. B) Transmittance per cent differences showing color fatigue/electrochrome degradation over extended periods of switching between the colored and bleached states. C) Variation of absorbance with charge density to obtain η from the slope of the linear region. D) Coloration and bleaching kinetics of the electrochrome monitored at a certain wavelength at 90% conversion.

The color degradation, associated with a decrease in optical transmission, when switching between the colored and bleached states over multiple cycles of switching the applied potential (Figure 3B) points toward electrochemical instability of the electrochrome. It can also denote diffusion of the electrochrome from the electrode. Monitoring the CR at a given wavelength contingent on switching between the bleached and colored states was proposed by Padilla et al. as a universal means for directly comparing different electrochromes.^{[ 28} , ^{29 ]}

While CR provides relevant information about the optical transmission differences, it does not yield tangible electrochemical data. This is the benefit of η that correlates the optical contrast change (ΔA) with the charge density (Q) applied per area of the working electrode according to both Equation (2) and the linear region of Figure 3C.

$$\eta =\frac{\Delta A}{Q}$$ (2)

Large values of η are desirable because they imply the electrochrome undergoes a maximum change in optical transmission with minimal current density. While values upward of 900 cm² C⁻¹ are possible with dissolved electrochromes,^{[ 30 ]} these are often reduced in operating electrochromic devices. This can be ascribed to the increased resistance of the electrolyte film relating to the conducting solution, in addition to the thickness of the electrochromic layer on the electrode along with unwanted electrochemically induced reactions of the electrochrome among others. An absolute benchmark for η cannot be given because this metric is highly dependent on the molecular structure of the electrochrome, with the η of metal containing electrochromes being larger than their purely organic counterparts. The switching kinetics are defined as τ _b and τ _c , corresponding to the bleaching and coloration times, respectively. Bleaching times are typically taken as the time required for either 90% or 95% of the optical transmission change of the colored state to return to the resting state while coloration time refers to the rate of forming 90% to 95% of a color from the neutral state with an applied potential (Figure 3D).

The kinetics of switching between the two optically different states are underpinned by many factors. For example, the τ is slow (ca. sec.) in solution because the rate determining step is the diffusion of the electrochrome to the electrode. In contrast, fast switching times are possible in the solid state (≤ s) because the rate determining step is electron transfer between the electrochrome and the electrode with electron transferring being at least two orders of magnitudes faster than diffusion. Bearing this in mind, the switching kinetics are contingent on the given electrochrome. While large color contrasts are desired in the end use of the electrochromic device in true applications, the time required to reach these values is longer for larger changes in the optical transmission compared with the time required to reach low optical differences. A universal approach was established by et Padilla et al.^{[ 31 ]} to overcome this challenge by directly comparing the switching kinetics of electrochromes regardless of the optical transmission. Finally, the electrochemical stability is assessed by either cyclic or differential pulse voltammetry. This is to validate that electrochromes can be reversibly oxidized/reduced over the many redox cycles when exposed in an operating device with degradation. These electrochromic performance standards will be given for the various electrochromes contingent on their immobilization.

3. Benchmarking Electrochromic Device Performance

3.1. Electrochrome Physisorption

To appreciate the enhancement in device performance with the covalent attachment of electrochromes to the surface, the performance of devices using conventional deposition methods must first be understood. For this reason, the performance of electrochromic devices with the electrochrome physisorbed on the electrode will be succinctly reviewed contingent on electrochrome structure.

Electroactive donor–acceptor conjugated copolymers have been prepared by both electro‐ and chemical polymerizations. Copolymers with electron‐rich ProDOT coupled with electron acceptors such as 2,1,3‐benzothiadiazole were synthesized by Reynolds et al. (8; Figure  4 A). The spray‐coated copolymer on transparent indium tin oxide (ITO) electrodes showed ≈51.5% change in transmission at 595 nm. The copolymer was also stable, maintaining its color contrast over 10 000 cycles of switching between the bleached and colored states.^{[ 32 ]} The seminal work by Reynolds et al. spurred on Yan et al. who extended the copolymer by incorporating various aromatics including carbazole, naphthalene, and phenylene into the conjugated framework (9). Their copolymers are absorbed in the visible region between 400 and 750 nm wavelength.^{[ 33 ]} Huang et al. improved the low molar absorptivity in the visible range (400–500 nm) optical limitation of the copolymers by introducing spirofluorene into the ProDOT copolymer. The copolymer retained its electroactivity along with a panchromatic absorption. Indeed, the polymer electrode absorbed from 300 to 800 nm. The electrochromic device prepared from 10 had a high coloration efficiency of 1078 cm² C⁻¹ along with fast coloration/bleaching times of 0.82 s/0.86 s. The device maintained its contrast ratio upward of 10⁵ cycles of switching between the bleached and colored states.^{[ 34 ]}

Figure 4.

A) Representative electrochromic conjugated polymers 8, 9, and 10 from refs. [32, 33, 34] used for preparing electrochromic devices. B) Cyclic voltammogram of electrochromic device prepared with 9. Inset: photographs of the operating electrochromic device corresponding to the perceived color changes of the electrochromic device of 9 with applied potential. Adapted with permission.^{[ 34 ]} Copyright 2024, Springer.

The electrochromes 11 and 12 were physisorbed on the working electrode by electropolymerizing the respective electroactive monomers directly.^{[ 35 ]} 11 and 12 had reasonable η of 442 and 481 cm² C⁻¹, respectively. The switching times were slow; τ = 10 s. This aside, their colors spanned the visible spectra from yellow to orange when oxidized (Figure  5B), confirming their electrochromism. The reduced metrics of the PEDOT polymers relative to its ProDOT counterparts (8–10) illustrate the importance of both molecular structure and electronic effects on the electrochromic device performance.

Figure 5.

A) Electrochromic polymers 11 and 12 from Ref. [35] B) Change in absorption spectra and the corresponding change in perceived colors for 11 upon electrochemical oxidation Inset: cyclic voltammogram of polymer films of 11 on ITO working electrode measured at a scan rate of 10 mV s⁻¹ in 0.1 M ACN/TBABF₄. Adapted with permission.^{[ 35 ]} Copyright 2005, Elsevier.

Extending the π conjugation of 3 similar to the copolymers, together with integrating long alkyl chains for improving its processibility was studied by Baumgartner et al. They developed electrochromic gels from dithieno[3,2‐b:2',3'‐d ]phosphole (13; Figure  6 A) and dodecyloxy chains. The gel could be deposited on the ITO electrode by straightforward spin coating to address the challenges of device preparation with gels that cannot either be evenly or readily coated on the electrode (Figure 6b).^{[ 36 ]}

Figure 6.

A) Electrochrome 13 from ref. [36] used to fabricate an electrochromic gel. B) Top: change in absorption spectra of 13 with applied potential. Arrows indicate spectral changes with applied potential. Bottom: photoluminescence of the electrochromic device before (left) and after applying potential (right). Adapted with permission.^{[ 36 ]} Copyright 2011, Wiley. C) Selected phosphaviologen electrochromes (13–17) from ref. [37] D) Reversible electrochromic device based on solution‐processed 17. Adapted with permission.^{[ 37 ]} Copyright 2015, ACS.

Along the same lines of improving the fabrication of the electrochromic device, the surface on which the molecular electrochromes are adsorbed by solution deposition methods can be replaced. Baumgartner et al. replaced ITO with fluorine‐doped tin oxide (FTO) to assess the electrochromism of phosphoryl‐bridged viologen (5) and phosphaviologens (13–17; Figure 6C) with this electrode.^{[ 37 ]} It was found that counter anion (Br^‐ or OT^‐) affected the optoelectronic properties. Indeed, bibenzyl ditriflate 17 (Figure 6C) exhibited the best processibility with sufficient physisorption on the FTO (Figure 6D) to give optimal optoelectronic properties including an optically transparent and a colorless solid‐state electrochromic device.

These examples illustrate the electrochromic device upper metrics that are possible with the physisorption of various conjugated copolymer electrochromes. While device performance is contingent on the given electrochrome, coloration efficiency (≥500 cm² C⁻¹), coloration times (<1 s), and consistent optical difference when cycling between the colored and bleached states (>1000 cycles) are threshold electrochromic device metrics values of conjugated organic polymers. To frame the electrochromism of the various polymers and coatings, the electrochromic device metrics should be compared to their molecular counterparts. Moreover, comparing the performance of similar molecular electrochromes in both solution and operating devices provides sound evidence for their limited electrochromism in devices. Such is the case with molecular electrochromes 18–20 (Figure  7 ) consisting of triphenylamine as donor conjugated with a benzothiadiazole electron acceptor.^{[ 38 ]} While the electron acceptors of varying strength of 18–20 moderately affected the electrochromism, the CE of the best electrochrome (19) was ≈656 cm² C⁻¹ in solution. The value decreased twofold to 266 cm² C⁻¹ when the molecular electrochrome was used in an operating electrochromic device.^{[ 38 ]} The kinetics of switching between the bleached and colored states of the device were also extremely slow, being 34 and 52 s, respectively. These illustrate that although molecular electrochromes have desired electrochromic performance in solutions such as CR upward of 100%, and η > 500 cm² C⁻¹, these metrics are not carried over to operating electrochromic devices. This is further supported by the limited number of molecular electrochrome studies that compare the electrochromic performance in both solutions and fully operating devices.^{[ 39} , ⁴⁰ , ^{41 ]} Electrochromic devices prepared from molecular electrochromes further oft fail to meet a threshold of 10³ cycles of consistent coloration when switching between the colored and bleached states. The limited electrochromic performance of molecular electrochromes in devices highlights the importance of developing strategies that can carry over the desired electrochromic properties of molecular electrochromes to functioning devices from solution.

Figure 7.

A) Representative molecular electrochromes with reduced electrochromism in operating devices compared to solution. B) Change in transmission percent of the colored state of 20 by switching the applied potential in a working electrochromic device. Inset: color change of the bleached (left) and colored (right) states of the electrochromic device of 20. Adapted with permission.^{[ 38 ]} Copyright 2025, Wiley.

4. Electrochrome Functionalization

The covalent attachment of molecular electrochromes is an alternative method for obtaining immobilized films of the electroactive and color switching material on a surface. This contrasts with other approaches such as photopolymerization and electropolymerization that convert an otherwise monomer to either its unconjugated or conjugated polymer, respectively. The requirements of these approaches typically are either a vinylene or an electroactive monomer for the photo‐ and electrochemical polymerization, respectively. Photopolymerization does not alter the intrinsic electrochromic behavior of the electrochrome. In contrast, electropolymerization affords a conjugated polymer similar in composition to its counterpart prepared by synthetic aryl–aryl coupling methods. Electropolymerization is not void of challenges. These include controlling the degree of polymerization, and hence the electrochromism, and the thickness of the polymer film. Whereas photopolymerization affords an immobilized film that resembles its molecular electrochrome counterpart. An advantage of covalently attaching the molecular electrochrome other than photo‐ and electrochemical polymerization is that the thickness of the layer can be accurately controlled. For example, a uniform monolayer is possible by self‐assembly on the surface.

Similar to both photo‐ and electrochemical polymerization, molecular electrochromes must be functionalized for their covalent attachment to the surface toward improving the overall stability electrochromic devices. Both the intrinsic electroactivity and color switching of molecular electrochromes must not be altered by their functionalization. Functional groups that can readily be incorporated for the immobilization electrochromes on electrodes include phosphonic acids (O=P(OH)₂ R), carboxylic acids (RCOOH), silanes (SiR₃) and silanols (SiOR₃). These functional groups are highlighted in the following sections along with their merits when incorporated into molecular electrochromes and the overall impact on the electrochromic performance.

4.1. Phosphonic Acids

Phosphonic acids (PAs) are well known to bind strongly to metal oxides. This has been taken advantage of plastic electronics for improving the performance of devices such as organic light‐emitting diodes (OLEDs),^{[ 42 ]} perovskites,^{[ 43 ]} and field effect transistors. PAs have several advantages over silanes and carboxylic acids (vide infra). Indeed, the covalent bonds between phosphonic acids and the metal oxides that are commonly used as the transparent conductive for electrodes are more robust and less sensitive to hydrolysis than their carboxylic acid and silane counterparts. PAs can covalently be attached to the electrodes such as ITO and FTO by thermal annealing^{[ 44 ]} along with chemical and electrochemical reactions.^{[ 45 ]} Hotchkiss et al. confirmed the mechanism of PA binding to surfaces of ITO‐coated electrodes by X‐ray photoelectron spectroscopy and infrared reflection adsorption spectroscopy.^{[ 46 ]} They showed that PAs bind to ITO in a bidentate fashion, involving two of their three oxygens. These are, in part, the reasons for the robust attachment of PAs to metal oxides. PAs have gained importance as an anchoring group because of their straightforward synthesis. They can be prepared in two different routes from the corresponding phosphonate ester, which in turn, can be prepared from the corresponding alkyl/aryl halide. The phosphonate ester can be hydrolyzed to the subsequent acid either by refluxing with dilute HCl or with bromotrimethylsilane at room temperature (Figure  8 ). The scope of PA synthesis has been previously detailed.^{[ 47 ]} Given most electrochromes either are aromatic or contain an aromatic, PAs can readily be integrated using common reagents according to Figure 8. Different molecular electrochromes that have successfully been converted to PAs and their subsequent immobilization as monolayers on device electrodes are herein outlined along with their electrochromic performance.

Figure 8.

General synthetic routes for the functionalization with phosphonic acids (PAs).

4.1.1. Viologens

Viologens have been well studied as electrochromes owing to their reversible cathodic behavior and corresponding visible color changes.^{[ 48} , ^{49 ]} Viologens can further play two roles in electrochromic devices. On one hand, they can serve as the prinicpal electrochrome in the device. On the other hand, they can be used as an ion‐storage material with anodic electrochromes. An interesting element of viologens is they can be physisorbed on the surface of metal surfaces including ITO nanoparticles,^{[ 50 ]} ZnO nanotubes,^{[ 51 ]} and ZnO nanowires electrodes.^{[ 52 ]} For example, viologen (29; Figure  9 ) and an ITO nanoparticle slurry were physisorbed on an ITO film. The resulting electrochromic layer exhibited fast coloration and bleaching times (500/380 ms). This aside, the coloration efficiency (140 cm² C⁻¹) of the device was below par and it had a poor electrochemical stability.^{[ 50 ]} Polymers incorporating viologens have nonetheless been used as electrochromes.^{[ 53 ]}

Figure 9.

Representative viologens and phosphonic acid (PA) functionalized viologens electrochromes taken from refs. [18, 41]. Representative ion storage materials A) and electrochromic devices based on viologen phosphonic acids B). Insets: photographs of the corresponding electrochromic devices switching between the bleached and colored states. Adapted with permission.^{[ 18} , ^{41 ]} Copyright 2012 and 2022, Elsevier.

PA‐functionalized viologen (31) could be used as the cathode layer in a working electrochromic device. Here, 31 was anchored to TiO₂ nanoparticles, while the anode was prepared by coating Prussian blue on antimony‐doped tin oxide (SnO₂:Sb; ATO) nanoparticles. The functioning device had a sixfold increase in coloration efficiency (916 cm² C⁻¹) and fast color switching kinetics (600–720 ms).^{[ 54 ]}

Vlachopoulos et al. used PA‐functionalized viologens (33 A‐C) to develop a reflective nanosized electrochromic display. This was done by anchoring the viologen to TiO₂ nanoparticles along with using ATO as the counter electrode. TiO₂ was deposited only on a given area of the electrode to create pixels. The reflecting viologen electrochromic device had a contrast ratio of 5 and extremely fast response times that varied between 10 ms and seconds depending on the pixel size.^{[ 55 ]}

PA‐functionalized viologens (34–36) were also covalently attached to mesoporous TiO₂ particles.^{[ 18 ]} A range of colors of the electrochrome was possible by varying the substitution of 6. For example, 34 was cyan, 35 was magenta, and 36 was yellow. The corresponding electrochromic devices had moderate switching kinetics (τ = 2 s) and high coloration efficiencies (η = 330 cm² C⁻¹) for this class of electrochrome. The electrochromes had three states: neutral, radical, and radical cation, whose absorption collectively spanned the visible spectrum. Anchoring the viologens with phosphonic acid to the electrode had the additional advantage of increasing the electrochemical stability. Immobilizing the electrochrome prevented unwanted coupling reactions of the intermediates during device operation that irreversibly altered the electrochromic properties. Khodorkovsky et al. synthesized various viologens with benzyl phosphonic acids (37–39) for their covalent attachment to the electrode. They anchored these electrochromes to TiO₂ nanoporous‐coated FTO glass and fabricated and operated RGB electrochromic device.^{[ 41 ]}

4.1.2. Phenothiazines

Phenothiazines (1, PTZs) have also been used in electrochromic devices. PTZ consists of electron‐rich heteroatoms: nitrogen and sulfur. These electron donors make PTZ hole transporters. Fu et al. synthesized various PTZ by nucleophilic substitution to vary the N‐substitution (40X; Figure  10 ) and they assessed the effect of substitution on the electrochromism. The absorbance increased and the absorption wavelength redshifted contingent on the N‐substitution compared to their unsubstituted counterparts. The PTZ electrochromes that were sandwiched between ITO electrodes indeed were electrochromic with reversible and distinct color changes with an applied potential of 2.4 V. The unsubstituted PTZ turned green while the N‐substituted PTZs turned red color with applied potential. The bleaching kinetics of the electrochemically produced state of the electrochromic device were extremely fast: τ = 250 ms.^{[ 56 ]}

Figure 10.

Phenothiazines and PA functionalized phenothiazines. Adapted with permission.^{[ 57} , ^{101 ]} Copyright 2000, ACS and 2024, Elsevier.

Cummins et al. developed an ultrafast electrochromic device using PA‐functionalized PTZ (41; Figure 10) as the anodic layer and viologen (34) as the cathodic electrochrome. A anodic layer of 41 was anchored to nanostructured TiO₂ while the cathodic layer of 34 was anchored to nanostructured SnO₂:Sb. The resulting films were sandwiched into an electrochromic device and the device was stable with a coloration efficiency of 270 cm² C⁻¹.^{[ 57 ]} This is considered a high value for this class of electrochrome. However, the bleaching time of the transiently colored state of the immobilized film was consistent with its untethered counterpart investigated by Tu et al. being ≈210–260 ms.^{[ 56 ]}

Park et al. introduced an electron‐donating methoxy substituent at the 3,7‐positions of PTZ (42; Figure 10). This substitution increased the electrochemical stability of the electrochrome. Their electrochrome was also functionalized with PA for anchoring the electrochrome to an ATO electrode. The device fabricated had two colored states upon electrochemical oxidation. One transition changed from colorless to greenish blue at 0.8 V while the second transition changed from greenish blue to blue at 1.2 V. The immobilized layer had a CE ≈470 cm² C⁻¹ for the first oxidation. Again, this is a high value for this class of electrochrome. Moreover, the hybrid device had an excellent electrochromic stability and it could be switched between the different states upward of 2000 cycles. The response time was also fast; τ<1 s.^{[ 58 ]}

Zhang et al. introduced aromatic substituents at the 3,7‐positions of PTZ without modifying the N‐substitution (40J–M). They showed these derivatives could be used in electrochromic devices as a proof of concept. They physisorbed the electrochromes on the ITO surface by dissolving them in polycaprolactone.^{[ 59 ]} They showed the electrochromic devices changed from yellow to green and then to dark brown with applied potential. The optical transmission of the electrochromic device prepared from 40M was reduced to 50% after 20 cycles of switching the potential between the colored and bleached states. Anchoring the electrochrome with PA is a viable alternative to improve the device's performance.

4.1.3. Triphenylamines

Triphenylamine (5; TPA) is among the most studied anodic electrochrome.^{[ 60} , ^{61 ]} This is a result of its high electrochemical stability. Its electroactivity can also be exploited for electropolymerizing directly on the electrochromic device electrode. This approach for immobilizing electroactive films has previously been reviewed.^{[ 60} , ⁶¹ , ⁶² , ⁶³ , ^{64 ]} The functionalization of TPA for anchoring to the electrode and the effect of tethering on the electrochromic performances have subsequently been explored to improve the metrics of these molecular electrochromes. Shao et al. studied the electrochromic properties of amino‐substituted TPAs (43‐46; Figure  11 ). They developed a proof‐of‐concept electrochromic device that was not a solid‐state device by using heptyl viologen (HV; 30) as the complementary layer. The device had a high coloration efficiency of 631 cm² C⁻¹ albeit a modest coloration time of 1.9 s.^{[ 40 ]} This contrasts with 20 that was not immobilized and whose corresponding metrics were twofold and thirtyfold worse (Figure 7).

Figure 11.

Triphenylamines along with PA functionalized triphenylamine electrochromes from refs. [40, 67, 68, 69, 70].

Nguyen et al. also demonstrated that mono‐amino substituted triphenylamine (47) could be anchored to mesoporous doped indium tin oxide (mITO) similar to dye sensitized solar cells (DSSCs).^{[ 65 ]} Their electrochromic device switched from colorless to black with moderate coloration and bleaching times of 4.3 and 2.0 s, respectively. However, their electrochromic stability decreased after 50 cycles of switching between the bleached and colored states due to the leaching of tin and indium from the electrode surface. This clearly shows PA is a better anchoring group than amines.

PA‐functionalized TPA (48) and PA‐functionalized MV (34) were chosen as anodic and cathodic electrochromes, respectively, and they were anchored to nanostructured TiO₂ that was coated on an FTO electrode surface. The color change of the MV (34) layer in a half‐device electrochromic device (only one transparent solid electrode) was from transparent to purple upon electrochemical reduction. TPA (48) changed color from transparent to golden yellowish upon electrochemical oxidation. The electrochromic device with both the cathodic and anodic PAs changed from transparent to blackish color with applied potential. The optical and the electrochromic properties of the device could be modulated by altering both the distance between the two electrodes (cell gap; Figure  12A) and the concentration of the electrolyte. The coloration and bleaching times of the operating electrochromic device were ≈2–3, and 1 s, respectively, when the electrolyte concentration was in the 0.2–0.5 M range.^{[ 66 ]}

Figure 12.

A) Electrochromic device architecture using 34 and 48 as electrochromes. B) Photographs showing the colored (bottom) and bleached (states) of the electrochromic device prepared from 34‐48. Adapted with permission.^{[ 66 ]} Copyright 2016, Wiley.

The TPA‐PA (48) could be anchored to a mesoporous ATO surface and it was electrochemically dimerized to the electrochromic (49). The dimer could also be used as an anodic electrochromic material. This hybrid electrochromic device changed from a colorless state to bronze for the neutral and radical cation, respectively. Further oxidation to the dication gave rise to a green color. The film had a fast coloration (τ = 0.3 s) and slower bleaching (τ = 2.6 s) times along with a high optical contrast CR = 82% at 700 nm.^{[ 67 ]} Kortz et al. used the same approach to prepare an anodic material. They additionally incorporated nitriles into the MV‐PAs (36) as a complementary material in the working electrochromic device. This approach enhanced the coloration efficiency to 440 cm² C⁻¹ along with accelerating the bleaching time of the colored state to 0.5 s.^{[ 68 ]}

Building upon the success of immobilized TPAs, Xu et al. prepared electrochromic devices by anchoring both the anodic and cathodic electrochromes to TiO₂ electrodes using PAs. They used 48 as the anodic layer and modified MV PAs (32) as the cathodic layer. The device had a high electrochromic stability, capable of switching between its bleached and colored states for over 10⁵ cycles.^{[ 69 ]} They further modified their device by incorporating electron‐donating groups into the functionalized PA‐TPA (50). This was used as the anodic layer and bidentate MV‐PA (31) was used as the cathodic layer. The resulting device had improved electrochemical stability along with consistent color contrast when switching between the bleached and colored states for >10⁵ cycles.^{[ 70 ]}

4.1.4. EDOT

Owing to its intrinsic electron richness, 3.4‐ethylenedioxydithiophene (EDOT; 3; Figure 1) can be readily electropolymerized at a low oxidation potential. The resulting doped conjugated polymer (PEDOT) has good adhesion to the working electrode. It therefore does not delaminate from the electrode and it can be used in many devices as deposited. The optical and electrochemical properties of PEDOT have made it ideal for use as an electrochrome. Notably, Its reversible electrochemical redox and visible color change with applied potential. These properties can be tailored by coupling the intrinsic electron donating EDOT (D) with electron acceptors (A). Indeed, D–A–D conjugated structures have enhanced optical properties owing in part to an intramolecular charge transfer. This was taken advantage of preparing electrochromic polymers.^{[ 71 ]} Molecular electrochromes with D–A–D structures of varying degrees of conjugation and incorporating EDOT have also been developed and evaluated.^{[ 63 ]} Reynolds et al. theoretically calculated that molecular EDOT chromophores had ideal properties to be used as anodically colored electrochromes.^{[ 72 ]} The absorption wavelength of the charge transfer could be tailored contingent on the electron donor/acceptor that was incorporated into the conjugated framework. Indeed, the electrochromes varied in color from yellow, to green, and red.^{[ 72 ]} The electrochromes could be evenly distributed on the metal oxide surface by the covalent attachment with PA. DFT calculations of a series of nine PA‐functionalized EDOTs confirmed their desired electrochromism.^{[ 73} , ^{74 ]} The immobilization of the EDOT electrochrome with PA (51; Figure  13 ) enhanced its color density. More importantly, the color switching times of the operating electrochromic device could be accelerated to 3 s along with a contrast ratio of 65% over 100 cycles of switching between the bleached and colored states, demonstrating the usefulness of covalent attachment of the electrochromes to the surface.^{[ 73 ]}

Figure 13.

Phosphonic acid functionalized EDOT and its color change (right). Adapted with permission.^{[ 74 ]} Copyright 2022, RSC.

4.1.5. Metal Complexes

Terpyridines (52; Figure  14 ) are known sensors and as such they have been used in various applications including biosensing, metal ion sensing, cation–anion pair sensing, and bio‐imaging. Incorporating terpyridines into polymers, soft materials, and solid supports has helped detect sub‐micromolar levels of metal ions in both organic solvents and aqueous media.^{[ 61 ]} Terpyridines are also electrochromic as demonstrated by Taouil et al.^{[ 75 ]} Terpyridine is further a well‐known ligand for transition metal complexes. This ligand can be functionalized with PAs according to the well‐established synthetic methods (vide supra). These have been exploited by Grätzel et al. for covalently attaching terpyridine to nanocrystalline TiO₂ surfaces for photo‐induced metal‐ligand charge transfer.^{[ 76 ]} The operando redox environment of these sustainable electricity generating devices resembles that of electrochromic devices, making PAs a suitable anchor for the robust covalent attachment of electrochromes to transparent electrodes. Indeed, this was leveraged by Zenkina et al. to chemically bond Fe(II) complexes to ITO with PA functionalized terpyridine ligands (53 and 54). Their self‐assembled monolayers (SAMs) had high contrast ratios (CR = 44%) that were exploited for metal ion detection. They also used the SAMs as scaffolds for preparing monolayers of metal–inorganic complexes. The electrochromism of the robustly immobilized monolayers on metal oxide surfaces could be tailored contingent on the coordinated metal.^{[ 77 ]} The robustness of PAs was demonstrated by multistep metal coordination to increase the thickness of the SAM. Incorporating different metals with unique redox potentials in the SAM enhanced the electrochromism. Each metal could be selectively oxidized/reduced, resulting in discrete and multiple colors by modulating the applied potential. Indeed, colors from red to gray were possible. The advantage of metals compared to their exclusively organic counterparts is an increase in redox stability. As such, metal containing electrochrome can maintain their reversible redox behavior with consistent color changes over an extended number of switching cycles. The electrochromic stability of the SAMs was indeed preserved upward of 400 cycles when switching between the colored and bleached states.

Figure 14.

Terpyridine ligand (52) and its PA functionalized metal complexes 53–55 as per ref. [77]. Photographs of color change and stability of electrochromic devices of 53 and 54 (right panel). Adapted with permission.^{[ 77 ]} Copyright 2017, ACS.

Recently, Magra et al. functionalized terpyridines with PAs to form a covalently bonded monolayer on an ITO electrode. They used a cobalt complex as the electroactive unit in the core and appended the BODIPY fluorophore (55). The monolayer was both electrochromic and electrofluorochromic being the change in photoexcitation fluorescence intensity with applied potential. The fluorescence intensity decreased after the first cycle due to the oxidation of Co(II) to Co(III) and electron transfer to the BODIPY fluorophore. The fluorescence intensity was reversible upon reducing Co(III) to Co(II).^{[ 78 ]}

4.2. Carboxylic Acids

Similar to PAs, carboxylic acids (CAs) can covalently attach to surfaces via their heteroatoms. This functional group has gained wide acceptance to bind dyes to semiconductor surfaces in dye‐sensitized solar cells.^{[ 79} , ^{80 ]} Transition metal complexes with terpyridine and porphyrin as ligands have been extensively functionalized with CAs. Their organic counterparts have also been used for OLEDs. Here, TPA‐functionalized with CA could be bound to ITO for increasing the hole transport capacity and adjusting the electrode work function.^{[ 81 ]} Building upon the success in these different fields, CAs have also been used to covalently bind electrochromes to transparent electrodes. Owing to the weaker carboxylate‐ITO bond, electrochromes are instead covalently attached to TiO₂ particles that are adsorbed on a conductive surface for withstanding the harsh redox environments in operating electrochromic devices. This approach was used by Xavier et al. to covalently attach CA‐functionalized MV to TiO₂. The absorption around 550 nm of the transparent electrode, corresponding to the MV radical cation, increased with an applied potential.^{[ 82 ]}

4.2.1. Metal Complexes

Grätzel et al. designed dual devices that were both electrochromic and photoelectrochromic being the change in emission intensity upon photoexcitation with applied potential. These devices took advantage of both CA and PA‐functionalized electrochromes. They used ruthenium dyes for photoconversion along with methyl viologens and TPAs as the electrochromes (56–65; Figure  15 ). The devices had performances of 90% in change in optical transmission and 3 s response times.^{[ 76 ]} The advantage of integrating multiple carboxylic acids into the electrochrome is an increase in adhesion on the electrode relative to their monofunctionalized counterparts. Indeed, the desorption of bifunctionalized carboxylic acids that were covalently bound to TiO₂ was 180 times slower than its monocarboxylic acid counterpart.^{[ 80 ]}

Figure 15.

Carboxylic acid (CA) and phosphonic acid (PA) functionalized anodic and cathodic electrochromes taken from the ref. [76].

Santa–Notkki et al. developed a combined DSSC and electrochromic device. They used CA functionalized ruthenium complexes (66; Figure  16 ) for harvesting light and enabling the DSSC while PA functionalized MV (34) was used as the electrochrome. The color change of the operating device was from transparent to red at zero voltage and then to black at 1.0 V. The response time of the device was <1 s. However, the performance of the combined device could not be improved beyond these metrics.^{[ 83 ]}

Figure 16.

CA functionalized bipyridine and terpyridine metal complexes adopted from refs. [84, 85, 86].

Ward et al. used CA functionalized ruthenium–dioxolene 67 for anchoring to ATO. The electrochrome changed its optical transmission in the NIR region. Indeed, 67 was covalently attached to the metal‐oxide electrode by CA functionalization. The electrode after anchoring the electrochrome was initially bluish‐gray in color and it turned pink when electrochemically oxidized. The response time for this reversible process was ≈1.5 s.^{[ 84 ]}

A ruthenium complex with an arylamine substituent (68 and 69) was functionalized with CA to serve as a redox‐active center by Zhong et al. Their electrochrome self‐assembled into a stable film on ITO. Owing to the high surface coverage of the electrochrome on the electrode, it had a reversible electrochromism.^{[ 85 ]} The electrochrome further had three‐colored states. By modifying both the distance between the ruthenium complex and the pendant triphenylamine along with the degree of conjugation, the electrochromism could be extended into the NIR. The CR of the films was 40% at both 780 and 1300 nm. Courtesy of CA immobilization of the electrochrome, the η was 150–270 cm² C⁻¹.^{[ 86 ]}

4.3. Siloxanes

Similar to PAs, siloxanes can also bond (Figure  17 ). However, the Si–O–Indium oxide bond is less robust than its corresponding PA. This is in part owing to its susceptibility toward acid‐catalyzed hydrolysis. This aside, Huang et al. systemically studied the role of siloxane functionalized hole transporters such as TPA anchored to ITO. This was to understand how the covalent attachment enhanced the hole transport in OLEDs.^{[ 87 ]} Zenkina et al. also used siloxanes to form SAMs for working electrochromic device electrodes. Here, they studied the electrochromism of transition metal complexes. They used a trichlorosilane anchor 71 to form a mono siloxane layer 72 as a template on the surface. The covalently bonded anchor was subsequently reacted with various metallo‐terpyridines (73). Electrochromic layers (74) were prepared by forming metal–organic complexes of Fe and Ru with the terpyridine ligands. The color of the electrochromic layers could be adjusted by varying the ligands to include pyridine, quinoline, and phenylpyridine (75–78; Figure  18 ). The visible absorption spectra of the SAMs could also be modified by altering the sterics and the electronics of the terpyridine ligands. For example, monoquaternization of the complexes anchored to the surface redshifted the absorption relative to their unquarternized counterparts. The electrochromism of these discrete monolayers had coloration efficiencies varying from 277 to 585 cm² C⁻¹.^{[ 88 ]}

Figure 17.

Covalent attachment of terpyridine electrochromes to transparent metal electrodes via siloxanes according to ref. [88].

Figure 18.

Siloxyl functionalized terpyridine metal complexes for covalent attachment with tetrazine and BODIPY to ITO.^{[ 88} , ⁹⁰ , ⁹¹ , ⁹² , ⁹⁴ , ⁹⁵ , ¹⁰² , ^{103 ]}.

Zenkina et al. further studied in detail the role of both the substrate and the effect of the spacer between the electrochrome and electrode surface on the electrochromic performance. The conductive surface was modified by screen printing various conductive layers such as ITO‐30, ITO‐50, and FTO nanoparticles on ITO/glass and FTO/glass substrates. They incorporated —C—C—, —C = C—, and —C≡C— spacers between both the phenyl‐terpyridine and the pyridine anchor (79–81). The brightness of the layer decreased concomitantly with the increase in electron transfer upon progressing from the aliphatic to the more rigid alkyne spacer. The alkyne afforded greater stability and the greatest change in optical density compared to the other linkers. Whereas the FTO substrate gave the best performance. For example, the Fe(II) alkyne complex appended on FTO had a CR of 69% and η = 3656 cm² C⁻¹. This contrasted with the same electrochrome on ITO‐50 that had a CR of 59% and η = 1932 cm² C⁻¹. The nearly twofold difference in coloration efficiency underlines the choice of electrode in dictating the performance of electrochromic device. The enhancement with the FTO substrate is due to the high specific surface area and the wettability of the electrochromic materials.^{[ 89 ]}

Zenkina et al. also achieved discrete and selective color transitions for the electrochromes by simultaneously depositing two different metal complexes each with different metal centers (75, 82, and 83). For instance, an electrochromic device with both Os and Fe had two distinct color transitions: one corresponding to each metal. The colors changed from orange‐reddish to Wasabi‐green, and to Anzac‐yellow. These hybrid electrochromes had a coloration efficiency of 657 cm² C⁻¹, being enhanced compared to the corresponding individual metal complexes.^{[ 90 ]} Furthermore, they demonstrated the bimetallic hybrid SAMs could also serve as an electrochromic energy storage device.^{[ 91 ]}

The color of the electrochrome could additionally be tuned via post‐synthetic modification of the outer pyridine moiety by N‐quaternization (84‐86). They were able to achieve different shades of green. Quarternization did not affect the electrochromic stability. Rather, it enhanced the coloration efficiency and improved the response times of the corresponding electrochromic devices when using TiO₂ as an electron storage layer. The coloration efficiency of these devices varied from 1018 to 1513 cm² C⁻¹. The bleaching time varied between 1.2 and 3.3 s with coloration times between 0.6 and 1.1 s.^{[ 92 ]} The near sixfold variation of the coloration efficiency variation with the metal containing molecular electrochromes highlights the important role of the ligand. Whereas the ca. sevenfold increase in the coloration efficiency of the metal containing electrochromes over their uniquely organic counterparts underlines how the electrochromic device metric can be increased by including metals. It further serves to illustrate the sensitivity of the coloration efficiency upon the molecular structure.

Zenkina et al. also systemically pursued improving the durability of their electrochromic devices by increasing the number of cycles of switching between the colored and bleached without degradation of either of the color states. This was done by modifying the otherwise flat ITO counter electrode. Increasing the surface area of the counter electrode by printing a layer of ITO nanoparticles reduced the degradation of both the electrode and the gel electrolyte. This, in turn, improved the durability of the electrochromic device, capable of switching between the colored states without fatigue for over 2 × 10⁴ cycles.^{[ 93 ]}

Covalently attaching electrochromes to the ITO electrode via siloxanes was recently pursued by Miomandre et al.^{[ 94 ]} They showed that siloxyl functionalized tetrazine (87) formed a monolayer on the ITO surface and it exhibited both reversible electrochromism and electrofluorochromism. Miomandre et al. further covalently attached a dyad electrochrome (88) consisting of BODIPY and ferrocene to the electrode. A monolayer of the dyad could be formed on the ITO by simply immersing the electrode in a solution of the dyad. The resulting dyad SAM remained electroactive with good electrofluorochromic stability.^{[ 86} , ^{95 ]}

In summary, the covalent attachment of molecular electrochromes to the electrode in electrochromic devices provides the means to organize them into well‐defined structures. Through such organization, the performance of molecular electrochromic devices can be enhanced (Table  1 ) compared to their unattached counterparts. Indeed, the uniform distribution of molecular electrochromes on the electrode such as monolayers concomitant with their immobilization assure, in part, rapid color switching with the kinetics being limited exclusively to rapid electron transfer. Of importance, the electrochromic device stability with consistent coloration and change in optical transmission with repeated switching between potentials can be enhanced with the immobilization of electrochromes on the electrode with device metrics converging with devices enabled by their polymer counterparts.

Table 1.

Summary of electrochromism by the covalent attachment of electrochromes to the electrode.

Electrochromic device structurea)	Anchoring groupa)	Electrochrome	Redox stabilityb), c)	t b /t c [s]d), e)	Coloration efficiency [cm2 C−1]c)	Reference
ITO/MV@TiO2nc/GE/PB@ATOnp	PA	31	8000	0.72/0.6	912	[54]
FTO/MV@TiO2np/GE	PA	34‐36	500	1.5/2.0	330	[18]
MV@TiO2/PTZ@ATO	PA	34, 41	>10 000	<0.25	270	[57]
FTO/MV/PTZ@ATO/FTO	PA	57	>2000d); 5000	0.69/0.45	470	[101]
FTO/TPA@TiO2ns/MV@TiO2/FTO	PA	34, 48		>1/2‐3		[66]
TPA@ATOnp	PA	48, 49	500	0.3/2.6	280	[67]
TPA@ATOnp/MV@TiO2	PA	36, 49		0.6/0.5	440	[68]
Cbz‐MV@TiO2/GE/TPA@TiO2	PA	32, 48	>100 000	<1/1		[69]
MV@TiO2/GE/TPA@TiO2	PA	31, 48	120 000	4.8/3.0		[70]
EDOT@ITOMesoporous	PA	51	100	3/3		[73]
(Fe)TP@ITOnp	PA	53			150	[77]
(Ru)complex‐@TiO2/FTO	CA	69		30 (1st cycle); 20 (≥2nd cycle)	260; 210	[86]
FSnO2/TiO2@MV/TiO2@(Ru)complex/ FSnO2	CA	34, 66		<1		[83]
(Ru)complex@ATO	CA	67		1.5		[84]
ITO‐SPF@Py(Fe)Py/ITO	Si–O	76			585	[88]
(Fe)TP2@ITO‐50/ITO	Py	81		2.2/0.8	1930	[89]
(Fe)TP2@FTO/ITO	Py	81		0.9/0.5	3660	[89]
ITOnp/Os‐Fe‐TP/ITO	Si–O	82		9.3/3.5	657	[90]
ITOnp/(Fe)TPy@TiO2	Si–O	84‐86	3300 (>95%)	1.2‐3.3/0.6‐1.1	1020‐1515	[92]

^a)CA = carboxylic acid; Cbz = carbazole; GE = gel electrolyte; MV = methylviologen; PB = Prussian Blue; TPA = triphenylamine; np = nanoparticles; nc = nanocrystalline; PA = phosphonic acid; PTZ = phenothiazine; Py = pyridine; TPy = terpyridine; Si–O = siloxyl; and SPF = screen printed film.

^b)Number of cycles of reversibly applying the potential.

^c)Blank entries = data not reported.

^d)Coloration (t _c ) and bleaching (t _b) kinetics.

^e)Half device.

5. Surface Modifications

There is a limited number of functional groups that can be covalently bonded to the transparent ITO working electrode in electrochromic devices. Rather than functionalizing the electrochrome for subsequent bonding to ITO via these limited approaches, the working electrode can be modified instead. This has an advantage of integrating a wide range of functional groups that can either covalently or ionically bond the electrochrome. A benefit of electrode functionalization is an increased adhesion and surface coverage of the electrochrome on the electrodes. Functionalizing the electrodes can be done by diazotization. This was used by Mbomekalle et al. to functionalize ITO with arylsulfonates 89 (Figure  19 ). The monolayer of 90 complexed with first‐row transition polyoxometalates (POMs; 91).^{[ 96 ]} The POM with the best electrochromic performance of CR = 40% was FeP₂W₁₇@SiO₂. The performance of the anchored layer was consistent during 150 cycles of redox switching with coloration times of τ = 2.1 s. The film had higher electrochromic stability than its unfunctionalized POMs counterpart that was deposited by physisorption.^{[ 96 ]}

Figure 19.

POM deposited on ITO electrode via covalent modification of the surface by diazotization: 1) reaction of p‐diazosulfobenzoic acid (89) with ITO; 2) complexation of Fe (III) to afford 90; and 3) resulting POM 91 by exchanging water in the coordination sphere. Adapted with permission.^{[ 96 ]} Copyright 2024, ACS.

Electrochromes could further be photocured directly on the electrode to afford insoluble and immobilized polymer films. Zhang et al. used this approach by photopolymerizing rhodamine‐p‐phenylenediamine‐methacrylate (RhNNEs) monomers 92–95 (Figure  20 ) along with comonomers (96 and 97) and a photoinitiator 98.^{[ 97 ]} Incorporating a proton‐coupled electron transfer chromophore and its subsequent polymerization on the ITO surface afforded an electrochromic film of 2 μm thickness. The film has a remarkable stability of 120 days with a contrast cycle life >2100 and CR = 90%. The color changes of the device spanned the entire visible spectra, opening new uses of the electrochrome in displays, encryption technologies, and photolithography.

Figure 20.

Comonomer butyryl methacrylate (96); ethylene glycol dimethacrylate crosslinker (97), and dimethoxy‐2‐phenylacetophenone (98) photoinitiator irradiated at 365 nm with 92–95.

Various approaches to modifying the working electrode of electrochromic devices have been employed for attaching molecular electrodes to the electrode. Working electrochromic devices employing different immobilization strategies maintain their color contrasts even with an extended cycle between the bleached and colored states while the device lifetime is contingent on the electrochrome. However, additional initiatives are required to improve η and increase the CR of acceptable values. 2D polymers such as metal organic frameworks and covalent organic frameworks (COFs) are emerging classes of physisorbed macrostructured electrochromes. They can be formed either by metal‐ligand exchange^{[ 98 ]} or by deposition directly on the electrode.^{[ 99 ]} They are characterized by high electrochemical stability similar to inorganic nanoparticles with the advantage of short τ. While COFs have been less studied, they have the benefit of multiple redox processes and resulting colored states such as ^ecCOF 99 (Figure  21 A).^{[ 100 ]} This COF incorporating 5 showed CR = 20%, τ = 8.2 s and η = 420 cm² C⁻¹ along with multiple electrochromic states albeit with limited electrochromic stability over 200 cycles. Tailoring the COF structure gave rise to three redox processes and subsequently four color states (Figure 21B,C).^{[ 100 ]} These illustrate the intrinsic electrochromic properties of molecular electrochromes can be maintained by incorporating molecular electrochromes into ordered structures that are immobilized on the electrochromic device electrodes. Tuning the pore size of the structure and the layer thickness of parameters will improve the switching kinetics of ordered electrodes while taking advantage of multiple colored states.

Figure 21.

A) Electrochromic COF 99 exhibiting four color states. B) Cyclic voltammogram of 99 in a three‐electrode system. C) Pictures illustrating the change in perceived color of films of 99 on the electrode surface with applied potential. Adapted with permission.^{[ 100 ]} Copyright 2025, Wiley.

6. Outlook and Perspective

Recent advances in the field of molecular electrochromism have been highlighted. Of importance, strategies to anchor molecular electrochromes to the transparent metal oxide coating of transparent electrodes were shown to be a pivotal step toward improving the redox stability of molecular electrochromes. This approach extended the number of cycles that molecular electrochromes can reversibly switch between their neutral and redox‐induced colored states without compromising their coloration efficiency and color contrast. Functionalizing molecular electrochromes with phosphonic acids is the anchor of choice because the robust covalent bond can withstand the harsh redox environment of an operating electrochromic device. This gives rise to consistent device performance even after extended cycling of the applied potential. Different functionalization strategies are emerging for the immobilization of molecular electrochromes and for preventing the migration of the electrochrome during device operation. The covalent attachment of nanoparticles to the electrode and their subsequent bonding of electrochromes further improve the device metrics. This is owing to increased surface area and improved surface coverage of the electrochrome. Replacing ITO with FTO as the device electrode opens the possibility of extending the grafting methods for further improving the electrochromic device performance with the advantage of reduced costs. Collectively, the benefits of molecule electrochrome immobilization include higher CRs and η, shorter response times, higher redox stability, and extended electrochromic device durability compared to their counterparts that are not covalently attached. While the device performance of molecular electrochromes can match those of devices prepared from their electrochromic polymers, molecular electrochromes offer the advantage of discrete property tuning by precisely modifying the molecular structure. This opens new possibilities and applications that can further take advantage of selectively manipulating the optical transmission and multiple redox centers via nanostructured layers that are not possible with polymers. Nanostructured layers with discrete property control further open avenues for using devices in combined roles such as self‐powering electrochromic devices by harnessing energy from sunlight. Such devices can ultimately be used autonomously for long‐term use without the need for wired power sources.

Conflict of Interest

The authors declare no conflict of interest