Water‐Operable Photochromic Perovskite‐Like Materials for Rewritable Anti‐Counterfeiting and Display

ABSTRACT

Stimuli‐responsive solid‐state photochromic materials are attractive for information encryption and smart displays, yet purely organic systems often suffer from high water solubility, while fully inorganic counterparts are constrained by cost‐effective and scalable fabrication. These limitations highlight the challenge of developing water‐stable, water‐operable photochromic materials for underwater information writing and wet‐environment anti‐counterfeiting. Herein, we report a water‐stable 1D perovskite‐like lead chloride hybrid, (AeBpy)₂Pb₃Cl₁₀ , constructed with a viologen‐based organic dication (AeBpy²⁺: N‐(2‐aminoethyl)‐4,4′‐bipyridinium). The structure features a negatively charged 1D inorganic backbone electrostatically balanced by AeBpy²⁺, a structural motif characteristic of low‐dimensional organic–inorganic perovskite‐like materials. Upon light irradiation, efficient photoinduced electron transfer enables rapid photochromism, with coloration occurring within 10 s and complete thermal decoloration achieved at 160°C within 10 min, supporting robust photo‐writing/erasing cycles. Importantly, the photochromic response remains operable directly in water, enabling aqueous information writing and on‐demand thermal erasing. This work provides a viable materials design strategy to address the challenge of water‐stable and water‐operable photochromic systems, enabling underwater information writing, wet‐environment anti‐counterfeiting, and rewritable displays.

A water‐stable 1D lead chloride hybrid, (AeBpy)₂Pb₃Cl₁₀ , is developed using viologen dications (AeBpy²⁺: N‐(2‐aminoethyl)‐4,4′‐bipyridinium). It features rapid, reversible photochromism via photoinduced electron transfer (PET), shifting from colorless to blue under UV light. Its exceptional structural stability allows for reliable information writing and encryption directly in water, advancing the design of robust, underwater smart displays.

1. Introduction

Stimuli‐responsive solid‐state functional materials have attracted increasing attention due to their promising applications in information storage, anti‐counterfeiting, and smart display technologies [1, 2, 3]. Among them, photochromic materials [4, 5, 6] are particularly appealing because of their noncontact operation, high spatial resolution, and reversible optical responses. For practical deployment in real‐world scenarios involving rain exposure, high humidity, or direct water contact, photochromic materials used in solid‐state information carriers, such as trademarks, banknotes, security documents, and labels, must be able to operate stably in aqueous environments for applications including underwater information writing and wet‐environment anti‐counterfeiting. Although some photochromic materials may still exhibit coloration upon partial dissolution in water, such dissolution often causes leaching and loss of spatial confinement, resulting in image blurring or destruction and rendering encoded information difficult to read; moreover, material dissolution can compromise the reversibility of the photochromic process, hindering effective erasing and repeated use. However, existing photochromic systems generally suffer from intrinsic limitations depending on their material class. Organic photochromic molecules, such as spiropyrans [7], diarylethenes [8], and viologen derivatives [9], often exhibit high water solubility, where dissolution and hydrolysis severely compromise their long‐term performance. In contrast, inorganic photochromic materials [10, 11, 12], including transition‐metal oxides [13] and chalcogenide‐based systems [14], are commonly associated with limited compatibility with large‐area manufacturing, which hinder their practical scalability and widespread deployment. As a result, developing photochromic materials that are not only compatible with cost‐effective, large‐area fabrication but also exhibit high water stability and enable direct, sustained operation in aqueous environments remains a critic challenge.

Organic–inorganic hybrid perovskite materials are generally characterized by low‐cost fabrication, solution‐processable synthesis, and compatibility with large‐area preparation, making them particularly appealing for practical applications [15, 16, 17]. Structurally, such low‐dimensional hybrid materials typically consist of negatively charged inorganic frameworks coupled with positively charged organic cations that occupy the interstitial space to maintain charge balance and structural stability [18, 19, 20]. Therefore, low‐dimensional hybrid perovskites offer highly flexible structural tunability through rational design of both inorganic frameworks and organic cation, providing a versatile platform for integrating photochromic functionality within a single material system. However, even when improved water stability has been achieved in some low‐dimensional perovskite systems, their operation under aqueous conditions remains highly restricted. Most reported materials can tolerate moisture or short‐term water exposure, yet they are rarely capable of open, sustained operation directly in water, where continuous contact with water molecules and dissolved species often disrupts optical responsiveness and/or structural integrity. Moreover, despite the structural versatility of organic–inorganic hybrid perovskites, the functional role of organic cations is often limited to structural templating or charge compensation, while their potential to impart intrinsic photochromic activity has rarely been explored. In most reported photochromic perovskite systems, the optical response originates primarily from the inorganic framework or extrinsic dopants, rather than from the organic components themselves [21]. Systematic investigations into the incorporation of photoactive organic cations as integral functional units within the perovskite lattice remain scarce, particularly in the context of achieving stable and reversible photochromic behavior under aqueous conditions.

To address this issue, an effective strategy is to employ highly conjugated organic cations, which reduce water affinity and, through ordered π–π stacking, strengthen Coulombic interactions with the inorganic sublattice, thereby enhancing the overall water stability. Meanwhile, pyridinium (viologen‐type) moieties not only provide the necessary positive charges to balance the negatively charged inorganic backbone but also serve as intrinsic photoactive units capable of reversible redox chemistry, enabling photochromism via photoinduced electron transfer. This dual role of viologen‐based cations fundamentally improves water stability while simultaneously imparting light‐responsive behavior to the hybrid framework. Herein, we report a water‐stable one‐dimensional perovskite‐like structure of (AeBpy)₂Pb₃Cl₁₀ incorporating a viologen‐based organic cation bearing ethylamine substituents. The structure features a negatively charged 1D inorganic backbone electrostatically coupled with positively charged organic cations, which is characteristic of low‐dimensional perovskite‐like materials. AeBpy²⁺ cations assemble into infinite 1D π‐stacked arrays along the a axis via continuous π–π interactions between adjacent bipyridinium units. This configuration consists of a negatively charged low‐dimensional inorganic backbone electrostatically coupled with positively charged organic cations, a structural motif characteristic of low‐dimensional perovskite‐like materials. Such a rational molecular design not only confers remarkable water stability but also facilitates efficient photoinduced electron transfer (PET) under light irradiation, giving rise to a pronounced and reversible photochromic response, with a distinct color change from colorless to deep blue observable within 10 s of light exposure. Notably, the photoinduced colored state can be completely erased by thermal treatment at 160°C within 10 min, restoring the material to its original state and enabling a typical photo‐writing/thermal‐erasing process. Benefiting from the combined merits of aqueous stability, rapid light‐triggered coloration, and thermally driven reversibility, this system allows the construction of water‐operable photochromic anti‐counterfeiting and display platforms, in which information can be written by light directly in water and erased on demand by heat. Overall, this work addresses the challenge of achieving water‐stable and water‐operable photochromic materials capable of reliable operation under aqueous and humid environments, by establishing a rational design strategy based on low‐dimensional perovskite‐like hybrid frameworks.

2. Results and Discussion

2.1. Single‐Crystal structure

Single‐crystal x‐ray diffraction analysis reveals that (AeBpy)₂Pb₃Cl₁₀ crystallizes in the monoclinic space group P2₁/n (CCDC 2525273). Detailed crystallographic data are summarized in Tables S1–S3 in the Supporting Information. As shown in Figure 1a, the compound adopts a low‐dimensional organic–inorganic hybrid architecture composed of a negatively charged inorganic sublattice and positively charged viologen‐based organic dications (AeBpy²⁺). The inorganic component consists of one‐dimensional chain‐like motifs constructed from edge‐sharing [PbCl₆] octahedra, forming a continuous anionic backbone (Figure 1b). These inorganic chains are spatially isolated from one another and charge‐balanced by the intercalated AeBpy²⁺ cations, which reside in the interstitial regions between adjacent chains. The organic AeBpy²⁺ dications assemble into one‐dimensional molecular chains through pronounced π–π stacking interactions between neighboring bipyridinium units, as illustrated in Figure 1c. Notably, the π‐stacked organic cations align preferentially along the a axis, with centroid‐to‐centroid distances in the range of 3.47–3.69 Å, indicative of moderate π–π interactions. This directional π stacking enforces an ordered alignment of the organic cations along the chain direction, which contributes to enhanced structural integrity and lattice stability [22, 23]. Thermogravimetric analysis further reveals that the hybrid material exhibits good thermal stability, with no significant weight loss observed up to approximately 265°C (Figure S1), indicating that the organic–inorganic framework remains thermally robust well above the temperature required for thermal bleaching. Overall, the cooperative integration of edge‐sharing inorganic PbCl₆ chains and the orderly arranged π‐stacked viologen dications gives rise to a well‐organized low‐dimensional perovskite‐like hybrid structure, in which electrostatic interactions and π–π stacking synergistically stabilize the lattice and underpin the robust solid‐state architecture.

FIGURE 1.

Crystal structure of (AeBpy)₂Pb₃Cl₁₀ . (a) Overall organic–inorganic hybrid architecture viewed along the crystallographic a axis. (b) Enlarged view of the inorganic sublattice, illustrating the one‐dimensional chain constructed from edge‐sharing [PbCl₆] octahedra. (c) Packing arrangement of the AeBpy²⁺ organic cations, highlighting the infinite one‐dimensional π–π stacking along the a axis.

2.2. Photochromic Behavior

Figure 2a presents the macroscopic photochromic behavior of (AeBpy)₂Pb₃Cl₁₀ under alternating light and thermal stimuli. In the pristine state, the crystalline sample appears colorless. Upon UV light irradiation, the material undergoes a rapid and pronounced color change to deep blue. The colored state can be completely erased by subsequent thermal treatment at 160°C, restoring the original colorless appearance. This reversible color switching process can be repeatedly cycled by alternating UV irradiation and heating, demonstrating a typical photo‐writing/thermal‐erasing behavior at the macroscopic level. The clear and distinct color contrast between the two states provides intuitive visual evidence for the robust and reversible photochromic response of this hybrid material. The evolution of the UV–vis absorption spectra under UV (Xe‐lamp, λ < 420 nm) irradiation provides direct insight into the photochromic behavior of (AeBpy)₂Pb₃Cl₁₀ . In the pristine state (0 s), the sample exhibits weak absorption tail extending into the visible region, consistent with its colorless appearance. As shown Figure 2b, upon UV irradiation, new absorption features gradually emerge and intensify, most notably a band centered at approximately 405 nm and a broad visible absorption extending from 500 to 750 nm with a maximum around 610 nm, which are characteristic of viologen radical species [24]. With increasing irradiation time from 10 s to 5 min, the intensities of both the 405 nm and 610 nm absorption bands continuously increase, indicating the progressive accumulation of photoinduced colored species. Notably, the absorption spectra recorded after 15 min and 20 min of irradiation nearly overlap, suggesting that the generation of photoinduced species reaches a saturated state under the applied irradiation conditions. This saturation behavior indicates that the photochromic process approaches completion within approximately 15 min, beyond which further irradiation does not lead to additional spectral changes (Figure 2c). Consistent with the optical evolution, electron paramagnetic resonance (EPR) spectroscopy provides direct evidence for the formation of photoinduced radicals: a pronounced signal at g = 2.003 emerges upon irradiation, confirming the generation of paramagnetic viologen‐radical [25] AeBpy^•+, and the signal disappears after thermal treatment at 160°C, indicating efficient radical annihilation during the bleaching process. Collectively, the coupled UV–vis and EPR results corroborate a reversible light‐induced radical formation/thermal recombination mechanism underlying the photochromic switching of this hybrid material.

FIGURE 2.

Photochromic behavior. (a) Reversible color change of (AeBpy)₂Pb₃Cl₁₀ . (b) Time‐dependent UV–vis absorption spectra under UV irradiation. (c) Electron paramagnetic resonance spectra recorded in the colorless state, the photo‐colored state, and after thermal bleaching. (d) PXRD patterns of simulated, as‐prepared, colored, and decolored (AeBpy)₂Pb₃Cl₁₀ . (e) Reversible and stable photochromic cycling behavior. A Xe lamp equipped with a reflective cutoff filter was used as the irradiation source, allowing UV light with wavelengths shorter than 420 nm to pass.

Powder x‐ray diffraction (PXRD) patterns collected before and after photoinduced coloration show excellent agreement with the simulated pattern derived from single‐crystal data (Figure 2d). The PXRD patterns of the as‐prepared, colored, and decolored samples remain essentially unchanged, indicating that no structural change occurs during the reversible photochromic process. Furthermore, the robustness of the photochromic response was evaluated through repeated photo‐writing/thermal‐erasing cycles, as illustrated in Figures 2e and S2. The absorbance variation between the colored and bleached states remains highly consistent over successive cycles, indicating excellent reversibility of the photochromic switching process. Notably, no obvious attenuation in absorbance contrast is observed during repeated cycling, demonstrating stable optical output upon successive photochromic operations. These results suggest that the light‐induced radical formation and subsequent thermal bleaching processes are highly reversible, enabling reliable and repeatable photochromic switching behavior in this hybrid system.

2.3. Electronic Structure Analysis

Density functional theory (DFT) calculations reveal that (AeBpy)₂Pb₃Cl₁₀ is a wide‐bandgap material with a calculated bandgap of approximately 3.43 eV (Figure 3a), in good agreement with the experimental optical bandgap of ∼3.2 eV obtained from the Tauc plot (Figure 3b) [26]. Notably, the electronic band structure exhibits additional electronic states located within the bandgap region. These intermediate states should not be regarded as the conduction band minimum; instead, projected density of states (PDOS) analysis clearly indicates that they predominantly originate from the low‐lying LUMO levels of the viologen‐based organic cations. The presence of these viologen‐derived intragap states gives rise to sub‐bandgap optical absorption, which experimentally manifests as an Urbach tail [27, 28] extending below the fundamental absorption edge. Such behavior reflects the strong electron‐accepting nature of the viologen moieties, characterized by their intrinsically low LUMO energy levels. Upon light irradiation, photoexcited electrons from the inorganic Pb–Cl framework can be efficiently transferred and trapped by the viologen‐derived intragap states, leading to partial filling of these sub‐bandgap levels. This electronic‐structure picture is further supported by x‐ray photoelectron spectroscopy (XPS) measurements (Figure 2c–e). Upon coloration, the C 1s and, more prominently, the N 1s spectra exhibit clear changes in the relative contributions of the fitted components, consistent with an altered electronic environment of the viologen (pyridinium) units upon electron uptake. The Cl 2p and Pb 4f signals display no discernible change before and after irradiation spectra (Figure S3).

FIGURE 3.

Electronic structure and chemical‐state evolution. (a) Calculated band structure and projected density of states (PDOS). (b) Tauc plot showing the optical bandgap. (c–e) High‐resolution XPS spectra of C 1s, N 1s, and Cl 2p in the colorless and photo‐colored states.

Taken together, the combined DFT/PDOS analyses and XPS results support a photoinduced electron transfer process in (AeBpy)₂Pb₃Cl₁₀ , in which the inorganic Pb–Cl sublattice can act as an electron donor, while the viologen‐based organic cations function as electron‐accepting sites. The photoinduced population of viologen‐related intragap states leads to an enhancement of sub‐bandgap absorption, which is consistent with the gradual development of visible absorption bands and the corresponding color change from colorless to deep blue. The involvement of viologen radical species is further supported by the appearance of characteristic absorption features together with reversible EPR signals upon light irradiation. Subsequent thermal treatment promotes charge recombination, effectively depopulating these intragap states and restoring the original optical transparency.

2.4. Photochromic Behavior in Air and Under Aqueous Conditions

Figure 4a highlights the practical photochromic imaging capability and water‐operable nature of (AeBpy)₂Pb₃Cl₁₀ . Patterned coloration can be readily achieved under UV irradiation in air (Figure 4b), demonstrating clear and well‐defined image formation based on the photochromic response. The photochromic patterning resolution reaches 4 LP mm⁻¹, as shown in Figure S4. Numerous stimulus‐responsive perovskite systems have been explored for anti‐counterfeiting and information storage applications [29, 30, 31, 32]. However, examples of photochromic perovskite materials that remain stable and operable in aqueous environments remain limited. Notably, (AeBpy)₂Pb₃Cl₁₀ maintains comparable photochromic imaging behavior when directly operated in water. As shown in Figures 4c and S5, distinct color contrast can be generated upon UV irradiation while the sample is fully immersed, indicating that the photoresponsive process remains effective under continuous aqueous contact. The water contact angle of (AeBpy)₂Pb₃Cl₁₀ remains nearly constant (∼30°) over time, indicating stable droplet–surface contact (Figure S6). The crystal remains transparent and undergoes photochromic coloration under irradiation in water (Figure S7), while PXRD patterns after water immersion confirm that the crystal structure remains intact (Figure S8). In contrast, the precursors of (AeBpy)₂Pb₃Cl₁₀ do not exhibit any photochromic behavior in water (Figure S9), indicating that the photochromic response disappears once the crystal structure collapses. Guided by this observation, we further investigated the photochromic stability of (AeBpy)₂Pb₃Cl₁₀ in various aqueous environments. Stable photochromic responses are maintained in domestic water (pH ≈ 6.5) as well as in 0.5 M and 1 M NaCl solutions (simulating saline environments), with the colored state persisting for more than 8 h without noticeable fading, demonstrating excellent water stability and resistance to complex aqueous conditions (Figures S10 and S11). The ability to sustain stable coloration and image readability during prolonged water immersion distinguishes this system from most reported photochromic materials and underscores its suitability for water‐operable photochromic applications.

FIGURE 4.

Water‐operable photochromism. (a) Schematic illustration of the UV‐induced formation of viologen radicals and the reversible color change in water. (b and c) Pictures of samples colored by ultraviolet radiation in air and water, respectively.

3. Conclusion

In summary, we have developed a water‐operable low‐dimensional perovskite‐like hybrid, (AeBpy)₂Pb₃Cl₁₀ .The hybrid structure combines a wide‐bandgap inorganic backbone with electron‐accepting AeBpy²⁺ cations, giving rise to pronounced and reversible photochromic behavior. Upon light irradiation, rapid coloration from colorless to deep blue is achieved through photoinduced electron transfer to the viologen units, accompanied by the formation of viologen radical species, as evidenced by characteristic UV–vis absorption bands and EPR signals. Thermal treatment efficiently erases the colored state, enabling reliable photo‐writing/thermal‐erasing cycles with excellent optical reversibility. Importantly, the photochromic response remains fully operative under aqueous conditions, allowing direct light‐induced coloration in water and retaining functionality after prolonged immersion. The combination of water tolerance, rapid optical switching, and stable cycling performance highlights the unique advantage of employing redox‐active organic cations in low‐dimensional perovskite‐like hybrids. This work provides a viable design strategy for constructing water‐compatible photochromic materials and opens new opportunities for applications in aqueous anti‐counterfeiting, information encryption, and rewritable display technologies.

4. Experimental Section

All experimental details are available in the Supporting Information.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: chem71107‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that supports the findings of this study are available in the Supporting information of this article.