Organic electrochromic molecules: synthesis, properties, applications and impact

Introduction

Electrochromism is a phenomenon by which a single species is capable of undergoing an electron-transfer in which a color change can be observed between the two different redox states. These electron transfers are also known as redox reactions, where a species can gain an electron to become reduced (n-type or electron-accepting species) or conversely lose an electron to become oxidized (p-type or electron-donating species). Generally, to observe electrochromism, a potential slightly above the redox potential of the species is applied and maintained. Species that become colored during these redox reactions are often referred to as an electrophore or electrochrome [1]. Electrophores most commonly see application in electrochromic devices, which the main discussion herein highlights new molecular species with interesting electrochromic properties for applications in electrochromic devices and beyond. However, due to their stable and reversible redox states, the value of these electrophore species lies beyond their electrochromism in application such as, but not limited to, smart materials, energy storage, memory materials, and biological labels.

Of these applications, electrochromic devices are some of the simplest and first devices in which electrochromic species have seen applications to demonstrate their redox properties. In terms of electrochromic devices, several definitions can be used to describe the efficiency of a particular species [2].

Color describes the photophysical property attributed to the light directly entering the eye on the visible range (420–700 nm) of the electromagnetic spectrum.

Contrast ratio is the quantitative measurement of the intensity of the color change. It is described as the ratio of the light diffusely reflected through the colored state of the display (R_x) and the intensity of light reflected from the bleached state (R₀).

This property is often measured at a single wavelength and in the case of a species with two distinctly different, highly colored states, the contrast ratio is perceived as poor and would be wavelength dependent.

Coloration efficiency η describes the absorbance change as a function of the charge (potential) applied per unit area. In general, coloration efficiency can be correlated to the molar extinction coefficient of a species, where a species with a higher molar extinction coefficient will exhibit a higher coloration efficiency.

Write-erase efficiency is the percentage of coloration that can be converted back to its original state (color or bleached), ultimately describing the change in absorbance. In an ideal electrochromic device, the efficiency should be as close to 100% as possible and represents the quality of design and construction of a device (in addition to the molecular properties of the active species).

Response time is the time required to reach the colored state from the original bleached state, normally on the order of ms for some of the most efficient devices.

Cycle life describes the process of cycling between bleached and colored states of a device until it reaches physical limitations or loses efficiency due to chemical side reactions.

While the definitions above are for the characterization of a final electrochromic device, from an academic or R&D point-of-view it is important to first understand the molecular properties of the electrophore.

Cyclic voltammetry and differential pulse voltammetry are the most popular characterization methods to determine the electrochemistry of species. This is a fundamental characterization technique in order to understand the electrical device efficiency, and the voltage required to power a device and observe elchrochromism. Cycling through the species’ redox potential can be used to determine the molecular stability of a species and its potential for device applications.

UV-vis spectroscopy and spectroelectrochemistry are used to analyze the absorption properties of the species with respect to the electromagnetic spectrum. Spectroelectrochemistry is a particularly powerful tool in characterizing the coloration of the electrophore as a function of charge (potential) input. Monitoring a single wavelength while cycling through the colorless to colored state can be used to probe the species’ coloration efficiency at a molecular level.

Finally, electron paramagnetic resonance (EPR) and density functional theory (DFT) can be used as supplemental tools in understanding the nature of the redox states from a molecular point of view and probe their overall stability.

Brief history of electrochromism

The term “electrochromism” was first suggested by Platt in 1961 and described to be analogous to “thermochromism” and “photochromism” [3]. It was described to be a shift in the absorption and emission spectra of certain dyes in the presence of a strong electric field and was a theoretical concept based of experimental evidence made previously. Platt proposed that this phenomenon would be expected for conjugated organic dyes of the dipolar linear-chair length, such as merocyanines due to the change in their relative energy between two resonance states.

Although the term “electrochromism” was coined in the 1960s, the phenomenon was observed over 100 years earlier. One of the earliest “electrochromic” species observed was tungsten(III) oxide (WO₃) in as early as 1815, which changed color from pale yellow to blue under reducing conditions [4]. Another early example was Prussian blue (hexacyanoferrate) used for photography in 1842 where photoinduced electron transfer was the driving force for electrochromism. Later on, electro-oxidation of Prussian blue was carried out using iron to generate images on photographic paper [5].

Where e⁻ (hv) represents an electron from water or another ambient donor [6].

Although these early electrochromic examples contain transition metal elements, the focus of this manuscript will be on organic molecules. The Dutch division of Philips was the first company to utilize commercial development of electrochromism with an aqueous organic viologen (popular electrochromic species discussed below) [7]. In 1977, viologens began to see application in automatic, dimming mirrors and viologen is to date, to the best of my knowledge, the only electrochromic organic molecule used in commercial applications (several organic polymers exist but are not the focus of this manuscript) [8].

Electrochromism as an educational tool

One of the beauties of electrochromes, in addition to their brilliant colors, is their usefulness as an educational tool in understanding chemical concepts found in everyday products. Several well-known electrochromic species can be accessed through established and simple synthetic means that can be carried out at an undergraduate level. Examples of educational electrochromic experiments have been previously established [9], [10] but there has been one example recently published that has been able to merge the synthetic aspects of organic chemistry with the characterization aspects of physical chemistry. Pouan et al. describe N,N′-bis(cycteine)pyromellitic diimide (BCPD) as a suitable candidate for this purpose (Fig. 1a) [11]. Imides have been extensively studied in the field of organic materials and their photoelectrochemistry has been well established [12]. These species are highly electron-accepting in which the imide carbonyl group can undergo reduction, resulting in the benzyl-type radical intermediate accompanied by a distinct color change (Fig. 1b). By characterization of the electrochemical properties of BCPD, the distinct and reversible color change can be observed, which at an advanced level can be correlated to the structural and electronic properties of the species. With the knowledge of the reduction potential, it is easy to expand such an experiment to understanding the redox chemistry between two chemical species. This was exemplified by adding a drop of BCPD solution to an activated surface of aluminum (sanded surface) to observe the colored radical species (Fig. 1c). The experiment also included a mock device design in which FTO (fluorine doped tin oxide) coated glass was placed in a solution of BCPD to observe the production of the radical species on the surface upon application of potential, demonstrating the fundamental function in an electrochromic device (Fig. 1d). This experiment, along with other well know electrochromes provides a necessary tool for understanding real world applications of fundamental research, exemplifying applications such as smart windows, electrochromic displays, or sunglass.

Fig. 1:

Synthesis, electrochemistry, and electrochromism of BCPD. Adapted with permission from (Ref. [11]). Copyright (2018) American Chemical Society.

Other applications

While the focus of this manuscript is to highlight advances in electrochromic organic molecules, it is important to note that these species can be multifunctional and provide utility in other organic electronics. The value of these species lies in their switchable nature, in particular for the context of this manuscript, their redox-switchable properties. Molecules that feature several stable and reversible redox-states have recently shown promise for application in energy storage, memory devices, and quantum science.

One of the more traditional applications of electrochromic molecules has been for use as smart windows [13]. The governing principle is that an electrochromic switchable layer is sandwiched between two panes of glass or a single pane is coated with an electrochrome with an insulating gap to the second, non-coated pane of glass. For this application, a species with dual-band electrochemical modification is ideal, such that it undergoes multiple electrochemical modes with unique absorption of the electromagnetic spectrum. For example, one redox state would absorb the near infrared (NIR) region of the electromagnetic spectrum, which on a hot day would keep a building cool without the need for extensive energy consumption on air-conditioning. A second redox state would only absorb the visible region of the electromagnetic spectrum, dimming the incoming light while allowing for the building to stay warm on a cold day (Fig. 2). In recent years there have been several; reviews published in the field of smart windows [14], [15].

Fig. 2:

Example of a dual-band smart window [16].

A species with good electrochromic properties is typically accompanied with exceptional electrochemical properties and highly stable redox-states that are favorable for energy storage in the form of organic radical batteries or redox-flow batteries. In an organic radical battery, each organic molecule undergoes chemically reversible oxidation or reduction to be classified as either the p-type electrode or n-type electrode, respectively, upon charging of the battery [17]. Upon discharge of the battery, the redox-active species are able to return to their original redox state (Fig. 3a). Schubert et al. have published an extensive review on the topic of organic radical batteries and their performance compared to traditional Li-ion batteries (see Ref. [17]).

Fig. 3:

(a) Schematic representation of an all-organic battery. Adapted with permission from (Ref. [17]). Copyright (2016) American Chemical Society. (b) General schematic of an organic redox-flow battery. Reprinted by permission from Macmillan Publishers Ltd: Nature Reviews Materials (Ref. [18]), copyright 2016. (c) Illustration of a redox-active molecular switch [21]. (d) Net redox outcome for photoredox transformations. Adapted with permission from (Ref. [22]). Copyright (2016) American Chemical Society.

While organic radical batteries address immediate applications in everyday technologies, they are not a viable strategy for addressing the large energy storage demand for harvesting renewable energies. Instead redox-flow batteries have surfaced as the alternative technology, where they utilize a redox-active species in a fuel cell-type architecture, achieving much higher capacities (Fig. 3b). In recent years, several organic examples have been published, including the popular electrochrome viologen (discussed below) as viable species for redox-flow batteries [18], [19], [20].

Memory devices and data storage arises from the reversible redox behavior of several electrochromes where they can be electronically stimulated between two distinct states, resulting in an “on” and “off” state (Fig. 3c) This is also known as molecular machines or molecular switches, which also have potential applications beyond data devices (see Ref. [21] for a review).

Finally, electrochemically active species pose as useful tools in organic photochemistry [22]. The combination of highly reversible redox properties, along with an accompanied absorptive colored state in one redox form offers the potential for “metal-free” alternatives to catalytic reactions (Fig. 3d). Nicwicz et al. published a comprehensive review of organic photoredox species and their catalytic performance recently to highlight these features (see Ref. [22]).

Of these vast applications, one particular family of compounds offers the potential as versatile candidates. They are viologens, famed for their highly stable redox states and brilliant, observable color changes between the different redox states. Viologen and their recent molecular advances will be the primary focus of this manuscript with emphasis on their observable color changes and potential for multicolored electrochromic devices. Motifs related to viologen and other electrochromic molecules for future technological applications will also be briefly highlighted.

Viologen: the face of organic electrochromism

Viologens are the most intensely studied species of organic electrochromes [23]. They are the product of N,N′-diquaternization of 4,4′-bipyridine, which results in a highly electron-accepting n-type species (Fig. 4). Viologens were first reported by Michaelis in the early 1930s, where the term viologen was also first introduced [24]. It wasn’t until the 1980s, when extensive publications and reviews surfaced highlighting the broad spectrum of application for viologens. Most applications are largely based on the viologens’ ability to undergo highly reversible redox chemistry giving rise to three differently colored oxidation states (dependent on N-substituents and counter anion) and the fact that the bipyridinium radical is among the most stable known organic radicals [23]. During the early development of viologens, their redox behavior was employed for biological applications, where they were eventually used as an herbicide due to their potent electron-accepting properties and redox stability [24].

Fig. 4:

Synthesis and redox chemistry of methyl viologen.

As mentioned above, viologens are dicationic species and can exist in three different redox states; dicationic, monocationic radical, and neutral species (Fig. 4). Of the most stable forms is the ground state dicationic species and typically offers the least number of features as they are typically either a crystal or powder and in their pure state are optically transparent in the visible region (some dicationic viologens are colored due to the charge transfer between a coordinating anion and the dicationic core; chlorides result in yellow colored salts and iodides result in scarlet colored salts) [25]. While dicationic viologens are very robust, they can be hydrogenated, which occurs more readily after aromaticity is broken upon the first addition of H₂. Additionally, in the presence of hydroxide in a methanol solution, the hydroxide will act as a better base than 4,4′-bipytridine and can demethylate viologen [26].

The first redox potential of methyl viologen in water is −0.689 V vs. SCE, one of the lowest for an organic molecule and is fully reversible [25]. The radical cation of methyl viologen in dilute solutions is an intense blue color with a molar extinction coefficient of ~13 000 L mol⁻¹ cm⁻¹, which is attributed to a change transfer between the (formally) +1 and 0 valence nitrogen atoms even though in EPR the radical appears to be delocalized over the entire bipyridyl core [25]. Under intermolecular interactions in concentrated solutions, viologen will form a dimeric species that will give a hypsochromic shift in absorption resulting in a violet colored species [6]. Viologen radicals are highly stable species and can be prepared as air-stable solids but oxidize back to the dication when in solution. The color of the radical species can be tuned through the R substituents, giving a wide array of different colors with the potential development of full color electrochromic displays (modified viologen examples will be discussed in the latter portion of this manuscript). In addition to the electrochromic properties of viologens, the high stability of the radical cation merits the dicationic form as an efficient radical scavenger producing an efficient radical trap in the form of the radical cation [27].

While the radical cation is very stable, it still has a strong electron affinity that gives rise to the second redox step of viologen, resulting in a neutral species (also accessible via two-electron reduction from the dicationic species). Similar to the radical, the neutral species is a highly colored red/orange species depending on the substituents. The neutral species is typically of less interest in the context of viologen compared to the radical species and isn’t commonly mentioned in extensive reports of viologen radicals.

Of the viologen family, methyl viologen has been most extensively researched in the context of electrochromic devices. However, the write-erase efficiency of an aqueous methyl viologen electrochromic device is very low since both the dicationic and radical species are highly soluble, permitting diffusion and poor switchability [28]. One solution to this problem that has been employed is to tether the active electrochrome to an immobilized polymeric surface or electrode [28]. This strategy is also viable for any electrochrome discussed herein and the focus will therefore be on the properties of small molecules rather than polymeric species. Additionally, the incorporation of long alkyl chains on the nitrogen substituents can facilitate for a solution-to-solid electrochrome and thus improved device efficiency [25]. Much of the viologen research has been focused on producing different device architectures rather that modification of the electrochrome itself, several selected examples are shown in Fig. 5 [29]. While viologens have been well established as electrochromic species, they also have several drawbacks. One of the biggest is the preservation of the colored radical state, which without constant voltage, returns to its colorless dicationic state through self-erasing mechanisms. Another problem of viologens is that they require a large degree of functionalization to reach a molecular motif where the reduced states are insoluble and have good write-erase efficiencies [30]. Modifications to the viologen core is a more recent strategy that has seen success in tuning the properties of these species that can lead to tunable electronics, new colored states, and more stable radical species.

Fig. 5:

Selected examples of viologens used in electrochromic devices (anions omitted for clarity) [29].

Expanding the viologen library

There are several strategies that can be employed in order to modify viologen to tune its properties and potential applications. One of the simplest is to vary the R substituent between alkyl, benzyl, or aryl substituents (Fig. 6). This is a useful strategy for enhancing the physical properties of viologen while maintaining its electronic properties, however it does not lend itself to new classes of molecules with very unique properties. Other strategies that lead to tunable electronic properties include; introducing spacers between the two pyridyl units, bridging of the two pyridyl units with heteroatoms, and embedding the viologen moiety in extended frameworks (Fig. 10).

Fig. 6:

General functionalization of viologen and potential structural modifications (anions omitted for clarity).

Introducing spacers in viologen can greatly enhance the scope of electronic and photophysical properties while still maintaining the stable redox behavior of the viologen system and its potential applications as electrochromes. One particular example worth noting was reported in 2012 by Khodorkovsky et al. utilizing three structurally unique viologens (Fig. 7) [31]. Each electrochrome displayed a unique color that could be controlled through the benzyl phosphoric acid group, which not only behaved as an anchor for the dye but was bulky enough to prevent the cationic radical dimerization. By preventing dimerization in the radical state, the true color for each viologen species could be observed. Bridging the pyridyl units with a thiophene moiety, resulted in tuning of the radical color from the traditional viologen blue to a vibrant red. Using the electronic properties and redox potentials of these viologen, transmission spectra was used to determine the potential at which maximum color change would be observed for the devices. Three electrochromic windows were prepared using nanoporous titania on FTO coated glass and 0.1 M solution of LiClO₄ in propylene carbonate as an electrolyte (Fig. 7). While the viologen and device design lead to electrochromic devices with color true to the single radical cation, the overall cell design resulted in semi-transparent devices. Due to the electrochrome loading, a contrast ratio was not provided even though there was a distinct color change visible to the naked eye. The development of an RGB electrochromic device is essential to the design of new fully colored display technologies that can be realized with the development of new colored electrochromes.

Fig. 7:

Core modified viologen and their respective electrochromic devices. Reprinted from Ref. [31], Copyright (2011), with permission from Elsevier.

Another strategy for structural modification of viologen has been through the introduction of a bridging unit via the 3,3′ positions of the viologen backbone. While bridged viologens have been known since 1985, it wasn’t until recently that more extensive molecular characterization has emerged [32], [33]. The introduction of heteroatoms adds additional value to the viologen properties by the introduction of new electronic and optical properties, as well as a new site for further functionalization and modification. One of the earliest species synthesized was the sulfur-bridged viologen (Fig. 8), which showed similar electrochromic properties to the parent methyl viologen and was only investigated for its photosensitization properties and no electrochromic discussion was reported [32], [33].

Fig. 8:

Structure of sulfur-bridge and phosphoryl-bridged methyl viologens.

In 2011, Baumgartner et al. introduced the phosphoryl-bridged viologen, which would allow for several new modified viologens to be introduced in the following years (Fig. 8) [34]. The distinct pyramidal geometry of the phosphorus atom gives rise to a stabilized LUMO orbital (for a detailed phosphole review see Ref. [35]) resulting in a species that can more readily accept an electron than its parent methyl viologen. While the phosphoryl-bridged viologen had similar electrochromic properties to the parent methyl viologen, the reduction threshold for the species was an impressive 500 mV lower than the first reduction of methyl viologen. Additionally, the second reduction showed a similar trend and was 520 mV lower for the phosphoryl-bridged viologen compared to methyl viologen. From an energetic standpoint, the development of an electrochrome whose activation potential is significantly reduced can provide the opportunity for developing electrochromic displays that require less electrical input to achieve their respective color change and thus more efficient devices or access to multiple colored states. While the introduction of heteroatoms provides tunable properties of the viologen electrochrome it also introduces new synthetic challenges. Notably, the basicity and nucleophilicity of the nitrogen atoms was significantly reduced through the introduction of the phosphoryl-bridge. For example, 4,4′-bipyridine is readily methylated by methyl iodide where in the phosphoryl-bridged analogue there is no methylation by methyl iodide, even under elevated reaction temperatures. In order to gain full access to the viologen species, a stronger methylating agent, methyl triflate, was necessary.

Benzylation of the phosphoryl-bridged bipyridine also posed similar synthetic challenges, where neat reaction conditions under prolonged reaction time (3 days) and elevated temperatures (50–60°C) were required to cleanly isolate diquaternized species (Fig. 9a) [36]. This functionalization was accelerated by using microwave synthesis and proven successful for a variety of benzyl substituents. Even this synthetic method showed limitations with the introduction of highly electron-withdrawing substituents, in particular 2,3,4,5,6-pentafluorobenzyl, where even under prolonged microwaves conditions, only the monosubstituted species was isolated. While these substituents result in non-conjugated addition to the viologen, the reduction potentials of the new series could be tuned and correlated relatively well to Hammet parameters for the N-substituents. Unfortunately, these species did not result in new colors for the radical species compared to the methylated counterpart. This was supported through EPR spectroscopy and DFT calculations where it was observed that the radical remained isolated on the viologen backbone. However, to the best of my knowledge, this work resulted in the first example of electrochromic devices of a bridged viologen species. The proof-of-concept devices reported were a simple solution-based design constructed from two pieces of FTO coated glass. First-generation devices were used to show coloration of the species without the need for a supporting electrolyte in the solution to observe the radical species coloration (Fig. 9b). This feature has the potential to result in simplified electrochromic devices made of a single electrochromic material and conducting electrodes. Electrochromic switching studies in an electrochemical cell showed good coloration efficiency, however due to the cell thickness and diffusion effects a write-erase efficiency was not obtained for this phosphoryl-bridged viologen species. A second-generation electrochromic device was designed to illustrate device switching through modification of the original proof-of-concept cell design to demonstrate the visual switching the viologen (Fig. 9c).

Fig. 9:

Synthesis of benzyl-substituted phosphoryl-bridged viologens (a), first-generation solution-based electrochromic device (b), and second-generation electrochromic device to show switching (c) [36].

Following the introduction of benzyl-substituted phosphoryl-bridged viologens, an extension to aryl-substituted species was developed to broaden the library of electrochromic molecules [37]. Once again, the reduced nucleophilicity of the nitrogen atoms posed synthetic complications where the traditional Zincke salt of viologen could to be accessed and thus an alternative synthesis was developed. A modified Mesmeyanov [38] reaction utilizing hypervalent iodonium salts was employed to successfully arylate the phosphoryl-bridged bipyridine (Fig. 10). This new synthetic method was also exemplified to work well on 4,4′-bipyridine to achieve the desired viologen in a single synthetic step. By introducing conjugated substituents, the electrochemistry was modified such that the radical cation could be accessed at 100–200 mV lower of a potential than that of the benzyl analogues and the neutral state by 200–300 mV lower. As is observed with most viologen species the aryl-substituted phosphoryl-bridged showed complete electrochemical reversibility between the three redox states. Spectroelectrochemistry of the synthesized species resulted in newly colored radical species of green and violet for phenyl and thienyl substituents, respectively. The neutral species also showed new colored states of pale yellow and ochre color for phenyl and thienyl substituents, respectively. Once again, proof-of-concept devices using these new viologens were demonstrated to show their potential as electrochromes (Fig. 10). Interestingly, while in the benzylated species only the first redox species color could be observed with this device design, in the case of the arylated viologens both redox state colors could be observed. This feature highlights the importance of electronic tuning to reduce redox potentials and the development of practical electrochromic devices. The new colors were well supported by EPR and DFT calculations where it was observed that the radical delocalization was extended to the aryl-substituent in addition to the viologen core. Molecular design to access a wide variety of colors is essential to the development of full color electrochromic devices.

Fig. 10:

Synthesis and electrochromic behavior of aryl-substituted phosphoryl-bridged viologens [37].

Several other heteroatom-bridged viologens have recently surfaced in literature, however they have been the focus of energy storage and sensitizer applications over their potential as electrochromic applications (selected species shown in Fig. 11; for a review see Ref. [39]).

Fig. 11:

Structure known of heteroatom-bridged viologens no discussed here. Anions omitted for clarity.

Another modification to viologens is to embed them into a larger carbon-based framework where their favorable redox properties can be utilized in conjunction with the carbon framework properties. One particular interesting subcategory of viologens are helquats, viologen embedded helicenes [40]. The chiral helical topology of helicenes leads to new physical and optical properties [4]. Helicenes with the embedded viologen have been recently synthesized by Weber et al. by quaternization with trimethyloxonium tetrafluoroborate (Fig. 12a) [41]. Unfortunately, these particular species were only studied in their cationic state and further work is required to understand their potential electrochrome properties. Teply et al. also reported a helquat with internally quaternized nitrogen atoms have also been reported, developing a new system of viologens (Fig. 12b) [42]. This particular system tethered an electron-donating unit onto the electron-withdrawing helquat core resulting in highly colored dicationic species due to the intramolecular charge transfer between donor and acceptor fragments. While the target application of these species was as nonlinear optical chromophores, they could also lead to interesting structural designs for new electrochromes.

Fig. 12:

Structures of helicene embedded viologen, also known as helquats.

Even though viologens have been well established as electrochromes since the 1980s, the recent advances modifying the structure here are vital in future development of either newly color viologens or more redox efficient viologens for electrochromic devices.

Beyond viologen: recent developments in electrochromic small molecules

While viologens have been one of the most extensively studied organic electrochromes, there are several other species with electrochromic properties and applications. Some historical examples and their electrochromic colors are highlighted in Fig. 13 [2].

Fig. 13:

Chemical structure of some well-established organic electrochromes (radical color depicted by color of the molecule and redox-active core structure bolded).

In recent years new electrochromic molecules have been introduced. For example, the incorporation of an azulene moiety with a porphyrinoid core has shown interesting redox properties, much like the parent porphyrins [43], [44]. The incorporation of thiophene or furan within an azulene containing porphyrinoid resulted in brilliant colored redox species with good electrochemistry. The thiophene analogue changed colors from dark red to purple to navy blue, while the furan analogue change color rom wine-red to orange to orange pink between the neutral, monocationic radical and dicationic states, respectively (Fig. 14). These two electrochromes highlight the significant electronic and color impact the incorporation of different heteroatoms can introduce in an electrochrome and thus the ability to tune colors for electrochromic devices.

Fig. 14:

Thiophene- and furan-based porphyrinoids with their respective redox color changes show over the structure.

Triphenylamine can be easily oxidized to its radical state producing a stable colored radical species when the para position of the phenyl ring is protected through substitution (unsubstituted position will result in dimerization of the triohenylamine), leading to another class of electrochromes [45], [46]. Their application in electrochromic polymers and materials has been well investigated and more recently small molecule examples have surfaced in literature. One example from Chen et al. is the combination of triphenylanimes with diimides, which resulted in electrochromic molecules that could be electropolymerized to give a differently colored electrochrome (Fig. 15a) [47]. Although this particular example utilizes the unsubstituted para position of the triphenylamine to produce polymers, para-substitution can lead to stable redox-active electrochromic monomers.

Fig. 15:

Triphenylamine-based electrochromes, their respective redox color changes (a). Reprinted from Ref. [47], Copyright (2017), with permission from Elsevier. The development of NIR devices from triarylamine based electrochromes (b). Adapted with permission from (Ref. [48]). Copyright (2018) American Chemical Society.

More recently, triphenylamine derivatives have been utilized to design high contrast electrochromic devices capable of going from colorless to all-black [48]. Capodilupo et al. reported dibenzofulvene-bridged arylamines (Fig. 15b) that result in mixed valence compounds capable of absorption into the NIR range of the electromagnetic spectrum due to the intervalence charge transfer (IV-CT). By changing the exocyclic fulvene substituent, the electrochromic properties of this new library of molecules could be effective tuned. In contrast to some other 3-center mixed valence systems, these triarylamine-fulvenes result in three distinguishable redox processes. While all species reported showed absorption into the NIR upon oxidation, the 3-center species did not show bleaching at higher voltages. Due to the intense color change, the devices fabricated had very high contrast ratios but did require prolonged coloration times of 10s where bleaching was faster, reported at only a few seconds. The electrochromic color change from colorless to black, with absorption in the NIR, makes these species strong candidates for application in smart windows.

As highlighted above, tetrathiafulvenes have also seen application as electrochromic materials. More traditionally these have been tethered into a polymeric backbone and could be cycled thousands of times in an electrochromic device [49], [50]. Expanding the tetrathiafulvalene scaffold (similar to adding spacers into the viologens mentioned above) provides the opportunity to develop new electrochromes. Three classes of tetrathiafulvalenes/ditetrathisfulvalenes were synthesized by Mukhopadhyay et al. with bridging anhydrides or imides of varying core size (Fig. 16) [51]. Once again, the combination of electron-donating and electron-withdrawing fragments results in mixed valence species and multistate electrochemical properties. Each of the three classes could be chemically oxidized with FeCl₃, where the emergence of the colored radical species could be monitored via UV-vis resulting in absorption bands into the NIR. As with the previous example, these NIR bands were a function of the mixed valence states of the compounds due to IV-CT. The multistate redox properties and electrochromic behavior lend them as another structural motif for electrochromic devices, particularly in the popular application of smart windows.

Fig. 16:

Chemical structures of selected tetrathiafulvalenes/ditetrathisfulvalenes fused to anhydrides and imides.

Finally, pyran-pyrylium species offer interesting electrochromic properties as oxidation leads to a radical species capable of undergoing reversible C–C fond formation [52]. In this example, phenylmethylenepyran was synthesized and the switching properties were probed by spectroelectrochemistry (Fig. 17a). The electrochemistry showed relatively fast redox behavior between the bond formation and bond breaking, translating to potentially good reversibility in a device. Both the NO₂ and OMe species showed relatively good electrochromic reversibility over several cycles, however there is evidence of formation of the neutral bispyran species resulting in poor life cycles. This study demonstrates a unique class of electrochromes where the colored state could become immobilized in solution due to structural differences, leading to viable candidates for electrochromic devices.

Fig. 17:

Synthesis (a) and staructure (b) of discussed pyran-pyrylium electrochromes.

Expanding the pyrylium species to a pyrylium-carbene hybrids were recently reported by Hansmann et al. as tunable, redox-switchable species [53]. A simple one step synthesis resulted in a series of tunable species where it was found that pyran stability increased with more electron deficient carbenes (Fig. 17b). All compounds featured two reversible redox steps where the radical cation and neutral species displayed distinct color changes between each state. It was observed that the carbene moiety did not only tune the electronic properties but had a strong influence on the photophysical properties of each redox state. All of the radicals were stable indefinitely under inert condition but also for several minutes to hours under air, which is highly desirable for potential application in more robust electrochromic devices.

Conclusion and outlook

The goal of this manuscript was to introduce new classes of organic electrochromic molecules through the diverse structural examples discussed herein. Understanding structure-property relationship at a molecular level is pivotal for the development of new electrochromes. While the examples discussed here are only at the academic level of electrochromic devices, several have shown promise as viable candidates for a new generation of functional devices. Alternative applications, such as smart windows, are currently more sought after then the development of full color electrochromic displays and thus understanding the progression of electrochromes is essential to accessing molecules and materials with potential large-scale commercial applications.

However, all hope is not lost for poor performing electrochromes, whether they do not absorb the desired portion of the electromagnetic spectrum or have poor color switching properties. As was mentioned above, stable and reversible redox behavior has a broad scope of applications from energy storage to memory devices. Tuning of the electronics to desirable potentials is vital for applications as battery materials, while access to a redox-active colored state could see application in photoredox catalysis. The future success of the examples highlighted in this manuscript will lie in their applications across a broad scope of organic electronics. Several of the examples discussed here have already gone beyond electrochromic devices and these discussions can be found in the reviews mentioned in addition to the discussion presented here.