Recent progress in polyoxometalate–viologen photochromic hybrids: structural design, photochromic mechanism, and applications

Abstract

Polyoxometalate–viologen hybrids are a class of photochromic materials with variable structures and interesting application performance. In recent years, some progress has been made in polyoxometalate–viologen photochromic hybrids through rational structural design, realizing the development from thin film to crystalline state. In this review, we summarize recent progress on polyoxometalate–viologen hybrid photochromic materials, especially in structural design, photochromic mechanism, and applications such as photocatalysis, ultraviolet detection, detection of amines and inkless erasable printing.

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Li Li

Dr Li Li received her PhD degrees from the Department of Chemistry of Northeast Normal University. She currently holds a master tutor position in Materials Science and Engineering, Henan Polytechnic University. Her research directions mainly focus on metal–organic frame materials (MOFs) assemblies and applications: photochromism, photocatalysis, smart detection, anti-counterfeiting, etc.

Yu Yang-Tao

Yang-Tao Yu is currently a master’s degree candidate in Li Li's group at the School of Materials Science and Engineering of Henan Polytechnic University. His current research interest is the design and application of novel inorganic–organic hybrid photochromic materials.

Hong Zhang

Prof. Hong Zhang worked as a Postdoc at Yale University in America from 2004, and worked as a Professor visiting the North Carolina State University in America from 2006 before joining Florida State University (FSU) as a visiting scholar. She is currently a Professor at Northeast Normal University and has authored or co-authored over 100 peer-reviewed papers. Her current research interests include the development of functional coordination polymers and polyoxometalate chemistry for photochromism and catalysis: deep desulfurization, photochromic films, ink-free printing, amines detection, smart windows, smart fabric, etc.

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1. Introduction

Photochromic materials with reversible color changes have received tremendous scientific attention for smart windows, molecular switching, data storage, anticounterfeiting and so on.^1–15 Polyoxometalates (POMs) are a class of very important photochromic materials due to their good redox properties and photoactivity, and readily undergo photoinduced electron transfer to form Mo/W(V)-containing heteropolyblue.^16–29 In general, purely inorganic POMs have poor photochromic behavior and are difficult to synthesize, while purely organic photochromic materials have poor fatigue resistance and high temperature resistance despite their fast response and rich color,^30,31 so it has been difficult to find new photochromic materials in pure organic or inorganic systems. By contrast, organic–inorganic hybrid photochromic materials can not only maintain or even enhance the properties of the respective components, but also generate new properties based on the synergistic effect between the components, which is more attractive to researchers.^32–36 In 2010, Dolbecq et al. published an interesting comprehensive review emphasizing the importance of organic–inorganic hybrids and proposing the potential applications of POMs as photochromic materials.³⁷

Among the known POM-based photochromic hybrids, the common organic ligands such as alkylammonium, multicarboxylic acid and sulfonium cations have been widely studied, and exhibit highly tunable photochromic properties with strong color changes.^38–58 However, the color change in these systems is due to photoreduction of Mo/W⁶⁺ ions through electron transfer inside the POM moiety under UV light, and the organic counter ion only acts as a stabilizer of the reduced POM.^59–62 In recent years, with the in-depth study of viologen-based photochromic compounds,^63–77 researchers have found that electron-deficient viologen ligands are capable of hybridizing with electron-rich POMs; this can not only form hybrid materials with diverse structures, but also bring more outstanding redox properties, photoactivity and thermal stability.^64,78–80 Currently, the viologen ligands used to construct POM-based photochromic hybrids have the following advantages: (1) viologen ligands, whose coordination sites are varied, can be easily modified, they can directly coordinate with the POMs or coordinate with transition metal ions first and then combine with POMs; (2) viologens not only act as counter ion but also take part in the coloration process, and can tune and optimize the photochromic property of POMs; (3) the combination of POMs with viologens can generate new properties in different fields such as photocatalysis, ultraviolet detection and so on.

POM–viologen hybrids as an important class of photochromic materials have been of wide interest for their excellent properties and application potential. Most early research on POM–viologen hybrid chromic materials focused on photo/electro chromic films.^81–89 For example, in 2009, Xu's research group prepared a color-changing composite film containing POMs and viologen by using a layer-by-layer self-assembly method.⁸⁸ It was not until 2010 that Sun et al. synthesized and reported the first case of POM–viologen photochromic crystalline material, namely 4,4′-bipyridine polymolybdate single crystal supramolecular compound,⁹⁰ and POM–viologen photochromic crystalline materials are gradually becoming more attractive, not only because they facilitate in-depth exploration of structure–property relationships, but also because the synergy and complementarity between components can generate new properties for applications such as amine detection and ink-free printing.^78,91–94 Subsequently, multi-stimuli-responsive materials have also been explored, which greatly promoted the ongoing development of POM–viologen hybrid photochromic materials.^94–96

In the last few years, a series of research works on POM–viologen hybrids has been done by our group. The main purpose of this review is to combine these works with other research in the field to summarize the rules for the structure design, photochromic mechanism and properties (Fig. 1), which can serve as a guide for the synthesis of new materials in the future.

Fig. 1 Schematic illustration of polyoxometalate–viologen hybrids with color type and applications.

2. Structural design

As is well known, the structural design and synthesis of new POM–viologen photochromic materials has always been a challenge because of many factors including pH, the selection of solvent, the metal salt, reaction time, and most importantly, anionic structures and organic ligands determining the structure and property of the compound. In POM–viologen hybrid materials, the self-assembly form mainly includes the following three types: (1) viologen exists as counter cation; (2) viologen first coordinates with transition metal ions, and then introduces POMs; (3) viologen directly coordinates with the POM metal center. In this section, the three existing modes of viologen in POM-based hybrid photochromic materials will be introduced one by one.

2.1. Viologen exists as counter cation

As anionic metal–oxygen clusters, POMs are good inorganic photochromic materials, while viologen and its derivatives are often positively charged electron-deficient compounds, so the unique positive charge characteristic of viologen can be used as a counter cation combining with electron-rich POMs.^96,97 This kind of viologen is mostly in the form of double protonation, single protonation and alkyl substitution. At the same time, because viologen and POMs interact through electrostatic attraction, the distance between the two is relatively close. It is easier to generate C–H⋯O and N–H⋯O bonds building two-dimensional or three-dimensional topological structures by self-assembly, and also provide a way of electron transfer when stabilizing compounds.

In 2010, the first case of POM–viologen crystalline photochromic hybrids (4,4′-bipyridine)Mo₇O₂₂·H₂O was reported by Sun et al.⁹⁰ The compound was obtained by hydrothermal synthesis comprising protonated 4,4′-bipyridine cations and polyoxomolybdate anions [Mo₇O₂₂]²⁻ and crystal water, which are bound to each other by hydrogen bonding, electrostatic attraction and intermolecular forces, forming a two-dimensional network (Fig. 2a). The synthesis of this compound also opened the curtain of POM–viologen photochromic crystalline hybrid materials and began to attract more and more attention. For example, using C₁₄H₁₁N₄O viologen cations and ammonium molybdate, we synthesized a novel neutral coordination compound (C₁₄H₁₁N₄O)₂[Mo₈O₂₆].⁷⁹ The compound is composed of two isolated C₁₄H₁₁N₄O cations and a [Mo₈O₂₆]⁴⁻ anion (Fig. 2b), while the C₁₄H₁₁N₄O monomer is hydrogen-bonded to the polyoxomolybdate by C–H⋯O and N–H⋯O. At the same time, to verify the difference between viologen and alkylammonium in the POM-based photochromic materials, taking the reported (H₂pipz)₃[Mo₈O₂₇] (containing no viologen) and (C₁₄H₁₁N₄O)₂[Mo₈O₂₆] as important examples, the photochromic mechanism of the compounds was studied in detail by using X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), UV/Vis diffuse reflectance spectrum, IR, PXRD etc. The results reveal that the viologen ligand can not only stabilize POMs, but also participates in the photochromic process, while the organic counterion H₂pipz only acts as a stabilizer to reduce POM. In addition, viologen can modulate the photochromic properties of polyoxomolybdate, showing a color modulation effect (Fig. 3b). There is a significant difference in the color change of the compound before and after photoirradiation, compared with the traditional monochromatic change. Such a new toning method may be helpful to improve the properties of the photochromic polyoxomolybdate family.

Fig. 2 Molecular structure: (a) (4,4′-bipyridine)Mo₇O₂₂ (cited from ref. 90). (b) (C₁₄H₁₁N₄O)₂[Mo₈O₂₆] (cited from ref. 79).

Fig. 3 ESR spectra of (H₂pipz)₃[Mo₈O₂₇] (a) and (C₁₄H₁₁N₄O)₂[Mo₈O₂₆] (b) (cited from ref. 79).

Nowadays POM–viologen hybrid crystalline photochromic materials combining multiple color-changing capabilities and exploring multifunction applications have become a research hotspot.^98–103 For example, Guo et al. successfully prepared a new two-dimensional hybrid material EV[Mo₉O₂₈] (EV²⁺ = ethyl viologen cation) by introducing the ethyl viologen, a well-known electron acceptor, into the polyoxomolybdate layered structure.⁹⁸ As shown in Fig. 4a, the polyoxomolybdate layer is isolated by EV²⁺, and the inorganic and organic components are linked by hydrogen bonds and CH⋯π interactions. The compound has dual color-changing properties: photochromic and thermochromic (Fig. 4b) with an ultralong-lived charge separation under illumination and heating, possibly due to the dense crystal packing mode that prevents contact with oxygen molecules. In the process of light/heat-induced discoloration, the generated Mo⁵⁺ and EV˙⁺ radical promote the electron transfer in the inorganic and organic components, respectively, resulting in a significant increase in the electrical conductivity of the compound.^104,105 This work provides an alternative way to manipulate the electronic properties of 2D semiconductors via external stimuli rather than changing chemical composition, and has reference significance for the later research.^106–113

Fig. 4 (a) Supramolecular structure and interaction force of EV[Mo₉O₂₈]. (b) Photochromic and thermochromic phenomenon (top) and I–V plots at 298 K (bottom, 300 W, ca. 110 mW cm⁻²) (cited from ref. 98).

2.2. Viologen first coordinates with transition metal ions, and then introduces POMs

In this section, the existence form of viologen in POM–viologen hybrid photochromic materials is to coordinate with transition metal ions first and then combine with POMs. That is, transition metal ions with special electronic structure need to first coordinate with viologen and then use the unsaturated coordination sites to coordinate with the oxygen at the end of the POMs.^114,115 The construction of such POM–viologen hybrid materials requires 4.4′-bipyridine to modify the coordination site, and viologen ligands containing carboxylic acid and pyridine groups are currently widely used. This type of structure helps to extend the wavelength response range, and may enable good photocatalytic activity in the visible and NIR light.

In 2019, the Fu research group used a self-assembly and bottom-up design strategy to introduce phosphotungstate clusters into the main chain of the electron-deficient copper–viologen framework by copper substitution and ligand linkage.⁹³ The compound is [Cu₂(H₂O)₃(CPBPY)₂(CuHPW₁₁O₃₉)]·7H₂O and the [HPW₁₁O₃₉]⁶⁻ anion is connected to the copper center by a covalent bond, forming a double chain connected by copper–viologen (Fig. 5). Supported by the design of the copper–viologen complex network structure, the compound exhibits an absorption range from UV to NIR with excellent photocatalytic activity. Because of the introduction of transition metal ions, a strong absorption band at 500–1500 nm can be clearly found by UV-Vis-NIR diffuse reflectance spectroscopy, which is due to the charge transfer between metal and organic ligands and the d–d leap of copper ions.^116–119 Therefore, the introduction of transition metals into the POM anion cluster is beneficial for achieving the expansion of the light absorption range of POMs.

Fig. 5 Structure of polyoxometalate–viologen hybrid photochromic materials with the introduction of transition metals (the compound is [Cu₂(H₂O)₃(CPBPY)₂(CuHPW₁₁O₃₉)]·7H₂O). (a) The structural unit. (b) The spatial packing arrangement with CuHPW₁₁O₃₉⁴⁻ anions (cited from ref. 93).

2.3. Viologen coordinates directly with the POM metal center

According to the conclusion proposed by Marcus,¹²⁰ we can know that the closer the distance between the electron donor and the electron acceptor, the faster the electron transfer rate between them. But the dispersion of the salts in a solution will not favor the electron transfer. In order to obtain ultra-sensitive photochromic materials, we found that the assembly of viologen ligand and POMs in the coordination mode is an applicable method.

In 2019,⁸⁰ we synthesized two new covalently bonded POM–viologen crystalline photochromic hybrid materials [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (Bpyen = 1,2-bis(4,4′-bipyridinium)ethane) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (Pbpy = 1,1′-[1,4-phenylenebis-(methylene)]bis(4,4′-bipyridinium)) (Fig. 6). Covalently bonded hybrid materials are very rare in the system of POM-based photochromic materials.^121–125 We think that the successful synthesis of these two compounds was related to the correct choice of POMs, viologen and pH adjustment of solution. Firstly, (NH₄)₆Mo₇O₂₄·4H₂O readily transformed into γ-[Mo₈O₂₆]⁴⁻ with the open Mo sites under hydrothermal conditions. Secondly, the ligands of the above two viologens contain two terminal pyridyl N atoms, which can coordinate with the open Mo sites. Finally, the 1 mol L⁻¹ hydrochloric acid was used to adjust the mixed solution so that its pH was in the acidic range. Compared with other photochromic compounds that have been reported, the photoreaction rates of 1 and 2, respectively, are 5 s and 1 s (Fig. 7). These phenomena indicate that the covalent bond between viologen and POM components does benefit the electron transfer.

Fig. 6 Molecular structures of [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (a) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (b) (cited from ref. 80).

Fig. 7 Photochromic phenomenon of [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (left) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (right) (cited from ref. 80).

In short, the different self-assembly forms between POMs and viologen ligands will have different effects on the hybrid materials including photoresponse rate, reversibility and fatigue resistance. In addition, it can produce different properties; for example, viologen first coordinates with transition metal ions, and then introduces POMs, which has potential photocatalytic properties. Viologen coordinating directly with the POM's metal center can as well ink-free printing medium due to fast photoresponse rate. Therefore, we summarized the above three assembly forms of POMs and viologen in the hope of providing some help for the synthesis of POMs–viologen hybrid photochromic materials in the future.

3. Photochromic mechanism

As promising candidates for photoresponsive materials, study of the inherent photochromic mechanism is very important. In POM–viologen hybrid photochromic materials, the electron transfer (ET) mechanism is widely utilized, which includes two basic objects: electron donor and electron acceptor, and the electron-deficient viologen ligand usually plays the role of electron acceptor. For example, in 2017, Gao and co-workers prepared a novel solar UV-sensitive photochromic film (denoted as PVP/PEI/EuW₁₀/AV²⁺) composed of POMs and viologen on quartz substrate by the drop-casting method.¹²⁶ Under the irradiation of solar ultraviolet light, the photochromic film was transformed to the blue state, visualized by the naked eye, with good fatigue resistance (Fig. 8), and it showed no changes under solar visible light, giving it considerable potential for novel portable durable solar UV detection devices. In this paper, the inferred photochromic mechanism was that it is due to the electron transfer from PVP carbonyl group to viologen ligands.

Fig. 8 (a) Changes in the emission intensity at 600 nm (red line, irradiation intensity at 254 nm) and absorption intensity at 610 nm (black line) of the photochromic hybridized film over 8 cycles. (b) Photographs of the hybridized film in sunlight at different times of the day in the local area (Jiamusi). The exposure time was 10 min (cited from ref. 126).

Compared with film materials, crystalline materials are more helpful for studying the mechanism of photochromism. In 2015, Gao's group developed a novel crystalline photochromic and luminescent switchable material, [(AV²⁺)(p-AV)(EuW₁₀O₃₆)]_n·2nH₂O (p-AV = poly(1-ethyl-1′-butyl-4,4′-bipyridinium), that can respond quickly after UV irradiation (Fig. 9). They think the coloration mechanism originates from an electron-transfer process and viologen ligands as electron acceptor. In the process of photochromism, POMs just act as photosensitizers.⁹⁶

Fig. 9 (a) Molecular structure of [(AV²⁺)(p-AV)(EuW₁₀O₃₆)]_n·2nH₂O; (b) UV-Vis absorption spectra of compound. (c) Time-dependent photoluminescence spectra (emission: 254 nm), its recovery spectra on exposure to humid air (95%, 5 min) or on heating at120 °C for 10 min. (d) The different cycles (emission intensity: 601 nm, emission: 254 nm) (cited from ref. 96).

However, the interaction between viologen ligands and POMs and the effects of different viologen ligands on POMs have not been well understood. So, the photochromic mechanism based on POM–viologen hybrids requires further in-depth study. To realize this aim, we chose two viologen ligands with different electron-withdrawing ability and synthesized two crystalline compounds [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (1) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (2).⁸⁰ In this work, we explored the photochromic mechanism and structure–property relationship, and drew the following two conclusions: (1) the photochromic mechanism is electron-transfer from O_Mo–O atoms to viologen and Mo(VI); (2) the viologen ligands with weak electron-withdrawing ability favor electron-transfer to Mo(VI).

We take compound [(Bpyen)₂(Mo₈O₂₆)]·2H₂O as an important example to analyze conclusion 1. The in situ electron spin resonance (ESR) experiments showed two distinct signals at g values of 2.0039 and 1.9514 (Fig. 10a), which, respectively, corresponded to the viologen free radicals and Mo(V). From the above conclusion, we can infer that viologen and POMs act as electron acceptors. However, there are two types of oxygen atom from H₂O and γ-[Mo₈O₂₆]⁴⁻, and both are potential electron donors. Combining the facts that the decolored sample after losing water still changes color and the shortest O_Mo–O⋯N⁺ distance is 2.6453(4) Å, we think that O_Mo–O from γ-[Mo₈O₂₆]⁴⁻ should be the electron donor. Electrostatic potential (ESP) surfaces further prove the above conclusion (Fig. 11a). By comparing the ESR spectra of the identical mole quantities, we found that the signal peak of Mo(V) is obviously stronger than [Pbpy]˙⁺ radicals in [(Pbpy)₂(Mo₈O₂₆)]·4H₂O. Moreover, the absorption band around 598.8 nm of Mo(V) is also clearly stronger than [Pbpy]˙⁺ radicals at 470.5 nm and above 800 nm in the UV-vis spectra (Fig. 12b). These data showed that electrons of the O_Mo–O atoms in [(Pbpy)₂(Mo₈O₂₆)]·4H₂O are mainly transferred to the Mo(VI) upon irradiation. The LUMO level of (Pbpy) is higher than that of (Bpyen) (Table 1), which implies that the electron-withdrawing ability of Pbpy is weaker than that of Bpyen and explains the reason why electrons tend to transfer to Mo(VI) in [(Pbpy)₂(Mo₈O₂₆)]·4H₂O. That is to say, the weak electron-withdrawing ability of the coordinated ligand favors photoreduction of Mo(VI) to Mo(V). These discoveries will help in improving performance for the well-known photochromic polyoxomolybdate family.

Fig. 10 (a) The in situ electron spins resonance (ESR) spectra with equal molar quantity of [(Bpyen)₂(Mo₈O₂₆)]·2H₂O. (b) The in situ electron spins resonance (ESR) spectra with equal molar quantity of [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (cited from ref. 80).

Fig. 11 Electrostatic potential (ESP) surfaces of compounds [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (a) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (b) (cited from ref. 80).

Fig. 12 UV-Vis absorption spectra of compounds [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (a) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (b) (cited from ref. 80).

Table 1 Comparison of different parameters of [(Bpyen)₂(Mo₈O₂₆)]·2H₂O (1) and [(Pbpy)₂(Mo₈O₂₆)]·4H₂O (2) (cited from ref. 80)

	1	2
a TD-DFT (B3LYP/6-31G+g(d,p)).
The shortest OMo–O⋯N^+ distance (Å)	2.6453(4)	2.6540(5)
Dihedral angle (Å) between two pyridyl groups of the coordinated 4,4′-bipyridinium	50.1(1)	0.9(2)
Face-to-face π⋯π distance (Å) between pyridine rings	No	3.8378(3)
Ligand, LUMO energy (eV)	Bpyen, −9.02	Pbpy, −8.19
Oscillator strength f (radical)^a	0.003 ([Bpyen]˙^+)	0.006 ([Pbpy]˙^+)

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4. Applications

As a new type of intelligent material, photochromic materials have been widely used in dyes, glasses, inks etc.^127–141 However, the exploration of photochromic materials should not be confined only to their color-changing properties, so in recent years, combining the photochromic phenomenon with other properties to explore new multifunctional materials has attracted a lot of attention.

POM–viologen hybrids as an important class of photochromic materials not only retain the advantages of POMs and viologen, but also produce new properties through the rational design of POMs and viologen ligands. The reasons are as follows: (1) viologen compounds have a fast photoresponse ability, visible color change and good reversibility, while POMs as inorganic semiconductor materials have a large gap between their well-defined HOMO and LUMO.¹⁴² Therefore, the possibility arose for the preparation of photochromic hybrid materials with a large energy gap preventing absorption in the visible and infrared regions, which can be used for UV detection; (2) the POM–viologen hybrid materials have an excellent reversibility and fast response ability under external stimuli such as light, solvents and organic amines, which brings some new opportunities for application and development in inkless erasable printing, detection of amine toxic substances, anion/cation detection and so on. The following is an introduction to the research progress on the application of POM–viologen hybrid photochromic materials. We hope our work can provide some help for the design of multifunctional POM–viologen hybrid photochromic materials.

4.1 Photocatalytic degradation

Photocatalytic technology is a simple, efficient and energy-saving green environmental protection technology, which plays an important role in the remediation of water pollution and the control of organic pollutants.^143–146 POMs can form peroxy complexes or catalytically active metal ions in the highly oxidized state, and thus have been widely used as effective oxidants for the removal of organic pollutants. However, their high solubility in aqueous solution and low stability under catalytic conditions mean they have low recoverability and recovery rates in the practical application process.^147–153 Although these problems have been solved by immobilizing POMs on solid carriers (such as TiO₂, SiO₂ and organic polymers), there are still some other problems such as leaching of POMs, narrow absorption bands, inhomogeneous sites and unclear structures.¹⁵⁴ Therefore, the design and synthesis of efficient photocatalysts with clear structures and precise distribution of catalytic active sites remains a difficult research task.

In recent years, the crystalline organic–inorganic composite hybrids have provided a new possibility for the combination of the redox properties of organic polyoxymethylene with metal–organic frameworks. This is an effective way to stabilize and improve traditional POM fragments.^155–162 In 2018, Fu and co-workers designed and constructed a highly efficient photocatalyst based on the broadband solar response molecular system [Cu₂(CPBPY)₄(H₂O)₂][PW₁₂O₄₀][OH]·6H₂O (CPBPY=N-(3-carboxyphenyl)-4,4′-bipyridinium) (Fig. 13).⁹² With the metal–viologen framework as media, the complex can lengthen the absorption spectrum from UV to the NIR region, especially in the NIR region, which enables the photocatalyst to degrade wastewater effectively. As shown in Fig. 13c, the photodegradation of methylene blue (MB) solution under different light irradiation conditions was investigated. The MB solution was observed to be completely degraded in 30 min under full spectrum light irradiation. Under visible and NIR light irradiation, around 98.2% of MB was degraded within 60 min. But for the contrast reaction without catalyst, the degradation rate of MB solution as shown in Fig. 13d is obviously lower. The above results indicate that the addition of electron-rich POM to metal–viologen frameworks is a promising strategy to explore more full-spectrum POM catalysts.

Fig. 13 (a) Coordination modes of the Cu ion in [Cu₂(CPBPY)₄(H₂O)₂][PW₁₂O₄₀][OH]·6H₂O. (b) The packing arrangement with encapsulated PW₁₂O₄₀³⁻ anions. (c) Photocatalytic degradation of MB. Inset: UV-Vis absorption spectrum of MB solution during NIR light photodegradation. (d) Degradation rates of MB (the contrast reaction without catalyst) (cited from ref. 92).

Based on the above research results, in 2019, using self-assembly and bottom-up design strategies, the same research group introduced the electron-rich phosphotungstate clusters with photocatalytic activity into the backbone of an electron-deficient copper–viologen framework; the compound is [Cu₂(H₂O)₃(CPBPY)₂(CuHPW₁₁O₃₉)]·7H₂O.⁹³ This designed catalytic molecular system has a high photooxidative ability for the degradation of dye pollutants, and has a photostimulated response effect in ultraviolet, visible light, and even near-infrared light. As shown in Fig. 14a, the MB solution can be completely degraded within 20 min under full-spectrum light irradiation. Within 40 min of visible light irradiation, its degradation rate is about 99.3%. Incredibly, the degradation rate is close to 100% after 80 min of NIR light irradiation. In contrast, photodegradation rates are almost negligible in the absence of catalysts (Fig. 14b). At the same time, the stability of the compound is quite good, and its photocatalytic activity does not decrease significantly after 5 rounds of degradation of MB by irradiation with various light sources. In addition, it also exhibited highly efficient photodegradation activity towards the organic dyes rhodamine B and methyl orange (Fig. 14c and d). In this study, the POMs/CP linkage framework with broad-spectrum photocatalytic activity is reported for the first time, which opens up a promising way for future research on efficient full-spectrum activated POM catalysts.

Fig. 14 Demonstration of the comparative effect of catalytic degradation with or without catalyst: (a) Degeneration rates of MB. (b) Degeneration rates of MB without catalyst. (c) Degeneration rates of RhB with catalyst and the blank reaction without catalyst. (d) Degeneration rates of MO with catalyst and the blank reaction without catalyst (cited from ref. 93).

4.2 Ultraviolet detection

The ultraviolet radiation brought by sunlight is a kind of natural stimulation radiation that human beings cannot avoid. Objectively speaking, moderate solar ultraviolet rays have certain benefits, such as promoting the body's calcium absorption, effectively killing of the body's surface bacteria to strengthen skin resistance and prevent anemia, etc. But transitional solar ultraviolet radiation can cause considerable harm to the human body, such as accelerating skin aging and skin inflammation, even leading to skin cancer, and it also causes eye discomfort and other adverse problems.^163–167 So especially in hot summer and in certain environments, it is important to determine the UV intensity quickly and easily.¹⁶⁸ Therefore, it is attractive and challenging to develop a simple, portable and efficient UV sensor for daily life.

In recent years, we have found that viologen-based photochromic materials have good reversibility, fast optical response rate and visible color change, and have potential as novel ultraviolet light detectors. However, to develop an efficient UV light detector, the color or spectrum of the visible light region needs to be unchanged to ensure good detection capability in the UV light region.^126,167 Therefore, to solve this problem, the materials designed for synthesis should be considered as having a large energy gap, thus preventing them from absorption response phenomena in the visible and infrared regions. As we know, POMs have a large energy difference between their well-defined highest occupied molecular orbital energy level (HOMO) and lowest unoccupied molecular orbital energy level (LUMO).¹⁴² So, by introducing POMs, the energy level of viologen compounds can be changed. The HOMO and LUMO of the synthesized POM–viologen hybrid material produce a large energy difference, so that the compound with a larger energy gap can realize the possibility of reducing the visible light response to stimuli.

In 2019, our group reported a new type of polyoxometalate–viologen hybrid crystalline photochromic material (Pbpy)(Me₂NH₂)₃[PW₁₁ZnO₄₀] (Pbpy = 1,1′-[1,4-phenylenebis-(methylene)] bis(4,4′-bipyridinium)).⁹¹ As shown in Fig. 15a, the asymmetric unit consists of one polyoxoanion, one viologen cation and three Me₂NH²⁺ ions to achieve the purpose of charge balance. In this compound, the three-dimensional structures have been also constructed through electrostatic interactions and strong intermolecular hydrogen bonds due to viologen cations, Me₂NH²⁺ ions and polyoxoanions coming into contact with each other (Fig. 15b). Meanwhile, the π⋯π interactions between the viologen cations in the compound may be useful for the improvement of the stability of the photoinduced generated viologen radicals,^169–175 thus improving the light response speed of the material. When irradiated with ultraviolet rays in the atmospheric environment, we can observe with the naked eye that the color of the compound quickly changes from yellow to blue, but the irradiated samples can be completely decolorized after heat treatment at 130 °C for 2 h in air, which indicates that the discoloration and fading process of the material is reversible (Fig. 15d). In addition, the compound does not change color under visible light, but can show significantly different color changes under different intensities of ultraviolet light (Fig. 15c). This characteristic can be useful for UV detection. Moreover, a thin film was prepared based on the material by the drop casting method on a quartz substrate and, as shown in Fig. 15f, the film showed a considerable difference in color change within 2 minutes when it was exposed to sunlight for different times on a sunny day. Therefore, the POM–viologen hybrid photochromic material has great application potential as a portable solar ultraviolet detection device that can be observed and recognized by the naked eye and used for daily protection in practical life.

Fig. 15 (a) The asymmetric unit of (Pbpy)(Me₂NH₂)₃[PW₁₁ZnO₄₀]. (b) Polyhedral representation of the three-dimensional structure by hydrogen bonds connecting in compound. (c) UV-Vis absorption spectra. (d) Photochromic phenomenon. (e) A plot of absorption changes at 609 nm as a function of irradiation time at different UV intensities (mW cm⁻²). (f) The portable compound film is used to detect the solar UV intensity at different time periods (Changchun) (cited from ref. 91).

4.3 Amines detection

Amines are known to be toxic and can be easily inhaled or ingested, causing great harm to the human body, such as inhibiting the nervous system and cardiovascular system, causing damage to the skin or causing cancer.^176–178 Therefore, it is necessary to develop effective means to detect toxic amines.

Viologen compounds as photochromic materials have attracted more and more attention. The electron-deficient viologen cation (V²⁺) has the ability to form single-electron radicals through electron transfer from suitable electron donors, and amines happen to be good electron donors, so introducing viologens into a metal organic framework (MOF) can make it possible to visualize the detection of amines.^7,179 In recent years, there have been some examples of using MOF materials to detect amines, but most of the reports are based on the enhancement of lanthanide ions, fluorescence quenching and aromatic fluorophores, and there are few gas-chromic MOF materials or multifunctional materials.^180–187

In 2021, Wang et al. synthesized three POMs–viologen hybrid compounds by introducing viologen ligands into the POMs system,⁷⁸ namely [Ag′(bmypd)_0.5(β-Mo₈O₂₆)_0.5], [Ag′₂(bypy)₄(HSiW₁₂O₄₀)₂]·14H₂O, and [Ag′(bypy)(γ-Mo₈O₂₆)_0.5] (Fig. 16). These three synthetic photochromic compounds have outstanding photoresponse properties and all exhibit amine vapour color reactions. When exposing the compound to saturated amine vapour, the crystals undergo different color changes. For example, when the compound [Ag′(bmypd)_0.5(β-Mo₈O₂₆)_0.5] was exposed to NH₃ for 3 minutes, the color of the compound changed from light yellow to green. After it was placed in ethylenediamine (en), DMF and diethanolamine (DEA) steam atmosphere for 10 min, the color of the compound underwent a noticeable change that was visible to the naked eye. However, the color change of the compound was not obvious after it was placed in a dimethylacetamide (DMAC) or DEA environment for 1 h (Fig. 17). Therefore, these compounds can be applied to the detection of toxic organic amine substances, especially for the detection of NH₃ in a standard environment. So far, there have been few studies on the combination of viologen and POMs to construct new photochromic materials and amine detection materials, most of which are some viologen-based metal–organic complexes with good photochromic properties and amine detection properties. The above research group showed the great potential of POM–viologen hybrid photochromic materials for the rapid and effective detection of amine toxic gases.

Fig. 16 (a) The unit cell diagram of [Ag′(bmypd)_0.5(β-Mo₈O₂₆)_0.5]. (b) The 2D layer of [Ag′(bmypd)_0.5(β-Mo₈O₂₆)_0.5]. (c) The unit cell diagram of compound [Ag′₂(bypy)₄(HSiW₁₂O₄₀)₂]·14H₂O. (d) The 1D supramolecular chain of compound [Ag′₂(bypy)₄(HSiW₁₂O₄₀)₂]·14H₂O. (e) The unit cell diagram of compound [Ag′ (bypy)(γ-Mo₈O₂₆)_0.5]. (f) The 2D topological mesh of compound [Ag′(bypy)(γ-Mo₈O₂₆)_0.5] (cited from ref. 78).

Fig. 17 Amine detection effect picture of [Ag′(bmypd)_0.5(β-Mo₈O₂₆)_0.5] (a) and [Ag′(bypy)(γ-Mo₈O₂₆)_0.5] (b) (cited from ref. 78).

However, in the detection of toxic amines, how to quickly and conveniently analyze and determine the category and concentration of toxic amines is still a huge challenge, and it is of great significance to human life and health and rapid treatment. In 2022,¹⁸⁸ Gao and co-workers synthesized and reported a class of highly sensitive POM–viologen crystalline photochromic hybrid materials with selective detection for ethylenediamine (H₂AV)[H₂(P₂W₁₈O₆₂)]·9.5H₂O (AV = N,N′-bis(δ-aminopropyl)-4,4′-bipyridinium)(Fig. 18). The complex has a special selective response to EDA gas, from white to dark blue (Fig. 19a). Regarding other amines, they may not react with the complex or the reaction is not obvious due to electron push effect or size effect. In order to directly reflect its sensitivity in detecting concentration changes, the complex was placed in EDA gas of different concentrations. It was found that the color of the complex changed from blue to dark blue with the increase of concentration (Fig. 19b and c). It is worth noting that detection by visual resolution and UV-vis spectroscopy can only deliver visual and rapid response to EDA gases at concentrations above 30 ppb. Therefore, Gao et al. adopted the Raman spectrum detection method, effectively solving the above problems, and through photochromism and the Raman signal to EDA gas achieved a super sensitive response, improving the detection limit of EDA, up to 0.1 ppb. This is of great significance for the accurate detection and identification of toxic amines, and also broadens the application and development of POMs–viologen hybrid materials.

Fig. 18 (a) The combined ball–stick and weak interactions between the AV²⁺ monomer and [P₂W₁₈O₆₂]⁶⁻ (a) and 2D framework of (H₂AV)[H₂-(P₂W₁₈O₆₂)]·9.5H₂O (b) (cited from ref. 188).

Fig. 19 (a) Color changes of the complex after treatment with different solvents: methanol (MeOH), ethanol (EtOH), acetonitrile (MeCN), triethanolamine (TEOA), triethylamine (TEA), NH3, 1,3-diaminopropane (DAP), 1,5-pentane-diamine (PDA), and ethylenediamine (EDA). (b) Color changes under different concentrations of EDA gas. (c) Changes of UV-visible absorption spectra under different concentrations of EDA gas (cited from ref. 188).

4.4 Inkless and erasable printing

Photochromic materials are able to change their original color on external stimulation and do not require dye for staining, so these materials have been used in the field of erasable and inkless printing.^189–197 Compared with traditional printing technology,^198–201 the development of ink-free erasable printing technology could reduce the excessive dependence of the printing industry on ink, and at the same time alleviate the environmental pollution and damage caused by excessive use of ink and excessive cutting down of trees. Therefore, the development of inkless erasable printing materials is of great help to improve the environmental pollution problem. However, not all photochromic materials can be used as inkless erasable printing materials, which not only need to have good color retention after photochromism for people to use for a period of time, but also require the printed content to erase itself without the influence of any external factors. This reversible self-erasing function enables the same sheet of paper to be used multiple times, thereby avoiding the one-time use of paper in printing, making printing not only cost-effective but also more environmentally friendly.

In 2019, we reported a new type of POM–viologen hybrid crystalline photochromic material (Pbpy)(Me₂NH₂)₃[PW₁₁ZnO₄₀].⁹¹ The compound has obvious color change from pale yellow to blue under ultraviolet light. The irradiated sample could be completely decoloured after annealing at 130 °C for 2 h in air or after storage in the dark in air for more than 2 d. Due to the sensitivity of the light response, good reversibility and fatigue resistance, the synthesized compounds can be used as ink-free erasable printing materials. As shown in Fig. 20, in order to study the effect of inkless printing, the crystalline compounds are ground into powders, immersed in ethanol, and then sonicated for 40 minutes to form suspensions.^202,203 The resulting suspension is then evenly dripped onto a large piece of filter paper and air-dried at room temperature. The pattern template ready for printing is covered with the upper layer of the filter paper. After 30 seconds of UV light irradiation, the template is removed from the upper layer of filter paper, we can see that the pattern to be printed has been clearly printed on the paper, and the pattern is presented in blue. The printed content can be read and used for 11 days in a normal environment, and can be printed and used repeatedly, which can fully meet people's needs for daily short-term use (Fig. 21).

Fig. 20 Photographs of a pattern printed without ink under light irradiation (cited from ref. 91).

Fig. 21 Photograph of a flower pattern printed inkless on paper and the effect of preserving the pattern at room temperature: (a) Photograph of content printed on one coated paper with a flower motif. (b) Printing of the content after 20 h. (c) Printing of the content after 3 days. (d) Photograph of the paper after printing for the 11 days (cited from ref. 91).

5. Conclusions and outlook

In this review, we have summarized the research progress of POM–viologen hybrid photochromic materials with a focus on the structural design, photochromic mechanism, and applications.

Due to the easy modification of viologen ligands, the self-assembly form mainly includes viologen as counter cation, viologen first coordinating with transition metal ions, and then introducing POMs, viologen directly coordinating with the POMs. In addition, the synergetic feature between viologen and POMs, for instance, competing electron transfer process, new knowledge on photochromic mechanism, and appealing applications have also been provided. This paper for the first time reviews POM–viologen hybrid photochromic compounds and systematically discloses their underlying structure–property relationships, which will shed useful insights into how to rationally design and synthesize new POMs–viologen hybrid photochromic compounds with improved performances. Below, we give a brief overview of future prospects in this area.

5.1 structural design and photochromism

As is well known, the structural design and performance modulation of novel POM–viologen hybrid photochromic materials have been a challenge because of many factors, including the concentration and molar ratio of the starting materials, reaction time, pH, nature of the solvent, and, most importantly, the anion structure and the organic components that determine the structure and properties of the compound. For example, the covalently bonded neutral coordination molecule is easily formed by combining polyoxomolybdate and viologen with terminal pyridyl N atoms. The POM–viologen MOF photochromic materials are easily constructed by keggin polyanions such as SiW₁₂O₄₀⁴⁻, PW₁₂O₄₀³⁻, PMo₁₂O₄₀³⁻, bipyridinium derivatives with terminal carboxylate, and metal ions.

The photochromic property of POMs can be tuned using the different electron-withdrawing abilities of viologen ligands. Therefore, the viologen ligands with strong electron-donating groups, such as –CH₃, –C(CH₃)₃, etc., favor photoreduction of Mo(VI) to Mo(V). The viologen ligands with strong electron-withdrawing groups, such as –NO₂, –CN, –F, etc., are conducive to the transfer of electrons to viologen.

5.2 Application prospects

As a new type of stimulation-responsive intelligent material, POM–viologen hybrid photochromic materials have some advantages, such as good stability, strong photosensitivity and outstanding reversibility, showing potential applications in photocatalysis, ultraviolet detection, detection of amines, inkless and erasable printing and so on. At present, the future application and development of POM–viologen hybrid photochromic materials will mainly focus on the following aspects: optimization of the performance of POM–viologen hybrid photochromic materials, including light response rate, fatigue resistance, and exploration of new application potential; for example, heterogeneous photocatalytic organic reactions limited by the low-efficiency carrier separation of the currently available photocatalytic materials. The addition of electron-deficient viologen helps the compound in the fast and continuous consumption of photogenerated electrons, which may be promising in facilitating the separation of electron–hole pairs to improve photocatalytic performance.

In brief, POM–viologen hybrid photochromic materials have been developed in recent years. However, to date there are many application performances waiting to be explored; we hope that our work is helpful for researchers of photochromic materials in the future, and provides some ideas in the development of POMs–viologen photochromic hybrids.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Universities of Henan Province of China (NSFRF220404), Key Research Project of Higher Education Institutions of Henan Province of China (23A430025) and Science and technology project of Henan Province (NSFRF230617).

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