Stepwise structural transformation in hybrid antimony chloride for time-resolved and multi-stage informational encryption and anti-counterfeiting

Abstract

The growing issue of counterfeiting has driven an increasing demand for advanced anti-counterfeiting technologies. Stepwise structural transformations among numerous compounds offer a promising approach to achieving high-level anti-counterfeiting methods, such as multi-step and time-resolved techniques. However, research in this area is still in its early stages. In this work, we report the first example of the stepwise structural transformation from ASbX₄ (A = cation, X = halide) to A₂SbX₅ and further to A₃SbX₆. The compounds are [Bzmim]₄[Sb₄Cl₁₆] (1, Bzmim = 1-benzyl-3-methylimidazolium), [Bzmim]₂SbCl₅ (2) and [Bzmim]₃SbCl₆ (3). 1, 2, and 3 exhibit distinct photoluminescent properties: non-emission, red emission peaking at 600 nm, and green emission peaking at 525 nm, respectively. Consequently, stepwise structural transformations enable stepwise luminescent switching from an “off” to multi-“on” states. Moreover, the switching mode can be adjusted from “off-on” to “off-on¹-on²” by simply changing the reactant ratio. Using SbCl₃ ethanol solutions as invisible ink, multi-step and time-resolved information encryption and anti-counterfeiting were demonstrated by combining [Bzmim]Cl solution, UV light, time, and temperature as developers. The tunable composition and photoluminescent response modes of these IOMHs position them as excellent candidates for high-level information encryption and anti-counterfeiting applications. This work sheds light on the potential for developing advanced informational encryption technologies.

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Introduction

Information encryption is crucial in fields such as economics and the military. The advancement of cracking technologies has raised the bar for developing new encryption materials and techniques.¹ Among these, photoluminescent (PL) materials are particularly suitable for high-level information encryption and anti-counterfeiting due to their invisibility under normal conditions, rendering them uncopiable by conventional photocopiers.^1–6 To further extend the application of PL materials in information encryption and anti-counterfeiting, introducing time as a variable in the decryption process can significantly enhance security by increasing the difficulty of cracking encrypted information. For instance, information encoded using fluorescent and phosphorescent materials can be selectively read by observing different time intervals after switching excitation light off.^1,3–5 The combination of tunable PL properties, such as excitation wavelength and lifetime, enables multiple changes in information, facilitating higher-level encryption and anti-counterfeiting through increased complexity.¹ While extensively studied PL lifetime-based encryption methods demonstrate potential, differences in the response rates of stimuli-responsive PL materials, which alter their luminescent properties under external stimuli such as light, heat, or gases,^7,8 also offer time-dependent PL changes. For example, photo-induced structural transformation rate controlled by hydrogen bonding have been applied to time-resolved anti-counterfeiting.⁹ However, most PL changes in stimuli-responsive materials occur only once. Achieving multi-step PL changes in such materials could greatly enhance information encryption levels, though research in this area remains limited.

Luminescent inorganic–organic hybrid metal halides (IOMHs) have recently garnered significant attention for their great advantages of chemical flexibility and straightforward preparation.^10–16 Represented by the general formula A_aM_bX_c (where A is an organic cation, M is a metal ion, and X is a halide anion), IOMHs offer versatile design possibilities by varying cations, metal ions, halide anions, or their ratios.^11–15 Their ionic structures, characterized by weak interactions between cations and anions, contribute to structural resilience and high chemical reactivity. For example, Pb-,¹⁷ Sn-,¹⁸ Bi-,¹⁹ Cu-,^20,21 Mn,²² and Sb-based^23–25 IOMHs exhibit sensitivity to light, moisture, and other stimuli. Such responses, often caused by changes in the A/M/X ratio, enhance the applicability of IOMHs in encryption and anti-counterfeiting due to their PL properties being closely tied to composition and structure.^26–32 Notable studies include Ma et al.'s report on the light-induced decomposition of Sn-IOMHs with the release of SnBr₂, leading to PL “off-on” switching,¹⁸ and Zang et al.'s work demonstrating ethanol-triggered (C₄NOH₁₀)Cl release from Mn-IOMHs.²² Song et al. reports the release of (C₁₉H₁₈P)I induced structural transitions from (C₁₉H₁₈P)₂CuI₃ to (C₁₉H₁₈P)₂Cu₄I₆ for dynamically tunable emissions.²⁰ Our group has previously shown the controlled release of [PP14]Br from [PP14]₂PbBr₄ under heat.³³ These controlled release rates directly influence luminescent switching rate, enabling time-resolved luminescent information change. Importantly, IOMHs exhibit the potential for multi-step structural transformations due to the structural diversity of M_bX_c anions. For instance, Lei et al. reported stepwise structural changes from [Ph₃EtP]₂Sb₂Cl₈ to [Ph₃EtP]₂SbCl₅·EtOH, and further to [Ph₃EtP]₂SbCl₅, resulting in a luminescent switching from non-emission to yellow-emission and finally to red-emission.²⁴ Despite these advancements, research on multi-step structural transformations and controlled component release in IOMHs remains in its infancy. Exploring stimuli-responsive IOMHs with multi-step structural and PL changes, combined with controlled release rates of AX and MX_n components, is essential for achieving high-level encryption techniques, such as time-resolved and multi-stage information encryption. This represents a promising yet underexplored avenue for advancing information encryption and anticounterfeiting technologies.

Herein, we report for the first time the stepwise structural transformation from ASbX₄ to A₂SbX₅ and further to A₃SbX₆. The compounds are [Bzmim]₄[Sb₄Cl₁₆] (1, Bzmim = 1-benzyl-3-methylimidazolium), [Bzmim]₂SbCl₅ (2), and [Bzmim]₃SbCl₆ (3). Compound 1 exhibits no emission at room temperature, while 2 and 3 display red and yellow-green emissions under 390 nm UV light, respectively. These compounds can be selectively synthesized by varying the ratio of [Bzmim]Cl to SbCl₃. The conversion time from 1 to 2, driven by the release rate of SbCl₃, can be tuned by adjusting the reactant ratio. As a result, a reactant ratio-controlled PL switching behavior was observed. When the [Bzmim]Cl to SbCl₃ ratio is less than 2, an “off-on” PL switching occurs, with the switching time decreasing as the ratio increases. When the ratio exceeds 2, an “off-on¹-on²” switching is achieved, where the emission initially changes to red (1 to 2) and subsequently to yellow-green (2 to 3) upon further heat treatment. Using SbCl₃ solutions as invisible inks and [Bzmim]Cl solutions as developers, advanced encryption techniques were successfully demonstrated. This unique stepwise structural transformation behavior enables high-level information encryption and anti-counterfeiting applications based on the time-resolved and multi-stage PL changes, expanding the application scope of IOMHs in the field of information encryption and anti-counterfeiting.

Results and discussion

Structures and PL properties of the compounds. As shown in Fig. 1, the [Sb₄Cl₁₆]⁴⁻ cluster contains 16 Cl⁻, including two μ³-Cl (Cl1) and four μ²-Cl (Cl5 and Cl7) atoms, and the Sb–Cl distances range from 2.400 to 3.226 Å. Each [Sb₄Cl₁₆]⁴⁻ cluster can be divided into two [SbCl₄]⁻ anions, two SbCl₃ molecules, and two Cl⁻, based on the typical Sb–Cl bond lengths, which mostly fall within the range of 2.1 to 2.8 Å.^23,24,34,35 In contrast, Sb–Cl bonds longer than 2.8 Å are considered secondary bonds. The secondary bonds result in weaker interactions compared to normal Sb–Cl bonds. In addition, both Sb1 and Sb2 exhibit distorted octahedral coordination geometries due to the larger bond length of secondary bond than normal bond. Unlike other zero-dimensional (0D) IOMHs, where the [MX_y]ⁿ⁻ units are well-separated by counter cations,^10,12–14 the neighboring [SbCl_n]³⁻ⁿ units in 1 are connected by bridging Cl⁻ by secondary bonds. Besides the covalent and ionic bonds, abundant supramolecular interactions, including C–H⋯Cl hydrogen bonds and C–H⋯π interactions (as shown in Fig. S1†), contribute to the formation of a 3D supramolecular network structure.

Fig. 1 (a) Crystal structure of 1 viewed along a axis. (b) Structure of [Sb₄Cl₁₆]⁴⁻ cluster in 1. (c) Crystal structure of 2 viewed along c axis. (d) Crystal structure of 3 viewed along a axis. H-atoms are omitted for clarity.

Compounds 2 and 3 also feature 0D ionic structures, with cation-to-anion ratios of 2 : 1 and 3 : 1, respectively. The anions in 2 and 3 are [SbCl₅]²⁻ and [SbCl₆]³⁻, respectively. The Sb atoms exhibit a pyramidal penta-coordination geometry in 2 and an octahedral coordination geometry in 3 as shown in Fig. 1. In contrast to 1, the [SbCl_n]³⁻ⁿ anions in 2 and 3 are fully separated from adjacent anions by the surrounding [Bzmim]⁺ cations. Like 1, supramolecular interactions are abundant in 2 and 3, including C–H⋯Cl hydrogen bonds, π⋯π and C–H⋯π interactions, contributing to the formation of a 3D supramolecular network structure as shown in Fig. S2 and S3.†

Luminescent switching and accordingly structural transformation

As is well known, antimony(III) chloride units often act as the luminescent centres in Sb-IOMHs.^36–40 However, 1 is non-emissive at room temperature (RT), but exhibits red emission with a peak at 660 nm when cooled to 200 K (Fig. 2a), with a Stokes shift of 311 nm. At RT, the weak emission observed at 443 nm can be attributed to the emission of the [Bzmim]⁺ cation.^41,42 As shown in Fig. 2b and c, 2 emits red light with a peak at 600 nm under excitation of UV light, while 3 emits yellow-green light with a peak at 525 nm. The excitation spectra reveal the absorption bands of 2 and 3 are respectively at 396 and 342 nm, corresponding to the ¹S₀ → ³P₁ transitions of Sb³⁺.⁴³ The Stokes shifts for 2 and 3 are 204 nm and 183 nm, respectively, which are significantly smaller than that of 1 at 200 K. We also measured the PL quantum yield (PLQY) of 2 and 3 at RT. Under excitation wavelength of 375 nm, their PLQY values were determined to be 23% and 86%, respectively, aligning well fitted curves reveal that the PL lifetime of 2 and 3 at RT are both 2.3 μs. The μs-level lifetime aligns well with the previous report, further confirming that the PL of the materials origin from the energy transition between ³P₁ and ¹S₀ of Sb³⁺.^36–40

Fig. 2 Excitation (dotted lines) and emission (full lines) spectra of 1 (a), 2 (b), and 3 (c). (d) Images of single crystals of 1 under 390 nm UV light at different times. (e) Images of thin films of 2 (red one) and 3 (green one) under 390 nm UV light. (f) A comparation of the PXRD pattern between simulated and as-made sample of 1 before and after exposed in ambient condition.

To further study the mechanism behind the luminescent properties of the compounds, the distortion degrees of 1 and 3 were calculated using the formula below because their Sb³⁺ atoms have a same octahedral coordination geometry.

where d is the average Sb–Cl bond length and d_n are the six individual Sb–Cl bond lengths.⁴⁴ The average distortion degree of 1 and 3 is 0.0126 and 1.37 × 10⁻³, respectively. In Sb-IOMHs, photoexcitation is accompanied by the transition of the s² ion from its ground state to an excited state with higher symmetry. Consequently, the degree of distortion in the environment of the Sb³⁺ in the ground state plays a critical role: the smaller the distortion degree, the lower the electron excitation energy required to transform the structure into the excited state (Fig. S5†). Conversely, as the distortion increases, the electron excitation energy also increases due to the slight shift of the excited-state parabola, as illustrated in Fig. S5.†^36,43,45 The ground state parabola may intersect with the excited state one in IOMHs with high distortion degree, leading to a lower luminescent quenching temperature, as observed in 1. Furthermore, this shift in the excited-state parabola contributes to an increased Stokes shift, explaining why 1 exhibits a larger Stokes shift compared to 3.

The distances between metal ions in hybrid ns² metal halide materials can also influence their PL properties.⁴⁶ The distances between neighbouring Sb atoms in 1 are 4.279, 4.252, and 4.560 Å, with the shortest value of 4.252 Å. For 2 and 3, the minimum distances between Sb ions are 8.893 and 9.513 Å, respectively. These distances significantly influence the PL properties of the compounds through the concentration quenching effect.^10,47 Upon excitation, a migration process of the excitation energy through the lattice to quenching sites would occur,⁴⁷ which is energy transfer from [SbCl_n]³⁻ⁿ to [SbCl_n]³⁻ⁿ in title materials. The separation degree between Sb ions plays a pivotal role in this migration process, as the possibility of the process increases with the decreasing Sb⋯Sb distance.⁴⁷ Consequently, the Sb⋯Sb distance of 4.252 in 1 results in non-emission property at RT. The migration process can be hampered at lower temperature, leading to the PL emission of 1 at 200 K. For 2 and 3, the larger distances between Sb⋯Sb ions result in well-separated SbCl_n³⁻ⁿ, enabling these compounds to exhibit favourable PL properties at RT.

Interestingly, when the single crystals of 1 was stored under ambient conditions, it was observed that the crystal turned light yellow after 7 days, accompanied by a luminescent change from non-emission to red emission, as shown in Fig. 2d. The single crystal was then ground into powder, and powder X-ray diffraction (PXRD) analysis was performed to identify the product. The PXRD pattern matched well with the simulated pattern of 2, suggesting that the transformation from 1 to 2 occurred (Fig. 2e). Based on compositional and structural analysis of 1 and 2, the transformation is likely due to the release of SbCl₃ from 1. As described earlier, the [Sb₄Cl₁₆]⁴⁻ cluster can be considered as comprising two [SbCl₄]⁻ anions, two SbCl₃ units, and two Cl⁻ ions. The SbCl₃ units within 1 are weakly bonded to the surrounding [Bzmim]⁺, [SbCl₄]⁻, and Cl⁻, allowing them to be released from the structure. Upon release of SbCl₃, the remaining Cl⁻ ions bond with the [SbCl₄]⁻, forming [Bzmim]₂SbCl₅ (2), along with a change of PL properties. The framework of the single crystal of 1 remained intact after the structural transformation, highlighting its soft nature. The transformation time from 1 to 2 was found to depend on the degree of openness of the reaction system. In a closed system, the transformation from non-emissive white powder to red-emissive yellow powder occurred much more slowly than in an open system. As shown in Fig. S6,† the transformation of the powder of 1 in an open system was completed within ∼3 hours, whereas it required ∼15 hours in a closed system. Considering the volatility of SbCl₃ at RT, the unequal chemical potential between 1 and the surrounding air likely serves as the driving force for the transformation. Table 1 summarizes reported cases of AX or MX_n release-induced structural transformations in IOMHs, underscoring the novelty of this finding. In addition to the SbCl₃-release-induced structural transformation from 1 to 2, it was discovered that immersing the powder of 2 in liquid ethanol for several days resulted in luminescence quenching. The PXRD pattern shown in Fig. S7† confirms that this phenomenon is caused by the reformation of 1, likely due to the extraction of [Bzmim]Cl from 2. This finding indicates that the structural transformation between 1 and 2 is reversible. A similar reversible phenomenon was observed between 2 and 3. As shown in Fig. 2f, a reversible structural transformation between 2 and 3, accompanied by PL switching between red and yellow-green emission, was noted. This transformation is attributed to the extraction and re-insertion of [Bzmim]Cl within the [Bzmim]_n−3SbCl_n structure.²⁶ The extraction of [Bzmim]Cl from 3 can be triggered by exposure to moisture environment or liquid ethanol. Conversely, the re-insertion process can be achieved by either heating the [Bzmim]Cl@2 mixture to a temperature below the melting point of 3 (410 K) or placing the mixture in a dryer at RT. Combining with the structural transformations from 2 to 1 in liquid ethanol environment, ethanol is effective in stepwise extracting [Bzmim]Cl from the [Bzmim]_n−3SbCl_n, enabling the stepwise structural transformations and luminescence switching among the three compounds.

Table 1 Summary of the structural transformation in IOMHs

Initial compound	Final compound	Stimulus	Released compound	Reversibility	Ref.
C4N2H14^2+ = N,N′-dimethylethylenediaminium ion, MA^+ = methylamine ion, C4NOH10^+ = protonated morpholine, PP14^+ = N-butyl-N-methylpiperidinium, DEDMA^+ = diethyldimethyl ammonium, TPA^+ = tetrapropyl ammonium, C22H24P^+ = butyltriphenyl phosphonium, C19H18P^+ = methyltriphenyl phosphonium, Ph3EtP^+ = ethyltriphenyl phosphonium.
(C4N2H14)SnBr4	(C4N2H14Br)4SnBr6	Light	SnBr2	No	18
MAPbI3	MA4PbI6	H2O	PbI2	Yes	17
(C4NOH10)5Mn2Cl9·C2H5OH	(C4NOH10)2MnCl4	Heat	(C4NOH10)Cl	No	22
[PP14]2[PbBr4]	[PP14]9[Pb3Br11][PbBr4]2	Heat	[PP14]Br	Yes	33
[DEDMA]2SbCl5	[DEDMA]3Sb2Cl9	H2O	[DEDMA]Cl	Yes	48
[Bzmim]3SbCl6	[Bzmim]2SbCl5	H2O	[Bzmim]Cl	Yes	26
(TPA)CuBr2	(TPA)2Cu4Br6	H2O	(TPA)Br	Yes	21
(C22H24P)2SbCl5	(C22H24P)2Sb2Cl8	Ethanol	(C22H24P)Cl	Yes	23
(C19H18P)2CuI3	(C19H18P)2Cu4I6	MeOH	(C19H18P)I	Yes	20
[Ph3EtP]2Sb2Cl8	[Ph3EtP]2SbCl5·EtOH	EtOH	SbCl3	No	24
[Bzmim]4[Sb4Cl16]	[Bzmim]2SbCl5	Air	SbCl3	Yes	This work

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Ratio-controlled PL switching rate and mode in Sb-IOMHs

Since the title compounds can all be synthesized from SbCl₃ and [Bzmim]Cl, the final product can be precisely controlled by adjusting the reactant ratio. Combined with the structural transformations among the three compounds, it is anticipated that altering the reactant ratio leads to controllable stimuli-responsive behavior driven by these potential structural transformations, thereby enabling tunable PL switching mode. To investigate the effect of reactant ratio on the resulting PL switching behavior, a series of ethanol solutions of [Bzmim]Cl and SbCl₃ were prepared and mixed in plastic vials. The SbCl₃ solution was added first, followed by the addition of [Bzmim]Cl in varying molar ratios as shown in Fig. 3a (see the experimental section for details). Immediately upon mixing, non-emissive white powder was generated, confirmed to be compound 1 by PXRD analysis as shown in Fig. S8.† The ethanol evaporated from the system within a few minutes, and the white powders of 1 gradually turned light yellow and began emitting red light under ambient conditions. The transformation time strongly correlated with the molar ratio of [Bzmim]Cl to SbCl₃; higher ratios resulted in shorter transformation times. For instance, powders with the highest [Bzmim]Cl/SbCl₃ ratio of 4 fully exhibited red emission within 0.5 hours, whereas those with the lowest ratio of 1 required approximately 3 hours to fully display red emission (Fig. 3a). PXRD patterns confirmed that 2 was the dominant product in all cases, responsible for the observed red luminescence. Interestingly, when the [Bzmim]Cl/SbCl₃ ratio exceeded 2.5, an additional emission color transition was observed. Upon heating the powders at 343 K, the red emission switched to yellow-green. PXRD analysis confirmed the formation of compound 3, which was responsible for the yellow-green luminescence (Fig. S8†). Fig. 3b illustrates the stepwise structural transformations from SbCl₃ to [Bzmim]₄[Sb₄Cl₁₆], [Bzmim]₂SbCl₅, and finally to [Bzmim]₃SbCl₆.

4SbCl₃ + 4[Bzmim]Cl → [Bzmim]₄[Sb₄Cl₁₆] (fast) (1)

[Bzmim]₄[Sb₄Cl₁₆] ↔ 2[Bzmim]₂SbCl₅ + 2SbCl₃ (slow) (2)

[Bzmim]₂SbCl₅ + [Bzmim]Cl ↔ [Bzmim]₃SbCl₆ (slow) (3)

Fig. 3 (a) Digital pictures of a series of powders after mixing the reactants (under 390 nm UV light). (b) Demonstration of the reaction processes after mixing the reactants (light green ball: Cl atom; dark green ball: Sb atom).

The stepwise structural transformation and corresponding multi-step PL switching provide valuable insights into the reaction mechanism underlying the synthesis of Sb(III) based IOMHs. When the ratio of [Bzmim]Cl to SbCl₃ decreases, excess SbCl₃ remains in the reaction system. The limited availability of Cl⁻ ions prevents the formation of isolated anions such as [SbCl₅]²⁻ and [SbCl₆]³⁻. Consequently, the tetrameric structure of [Sb₄Cl₁₆]⁴⁻-based material is formed (formula (1)), which exhibits non-emissive properties at RT. Over time, compound 1 gradually transforms into the red-emissive compound 2, as represented in formula (2). This transformation is driven by the release of SbCl₃, which reacts with [Bzmim]Cl when excess [Bzmim]Cl is present in the system, ultimately forming [Bzmim]₂SbCl₅ (2) until the [Bzmim]Cl is completely consumed. Although the reaction involves SbCl₃ release, the process resembles the insertion of [Bzmim]Cl into 1 to produce 2. When the [Bzmim]Cl/SbCl₃ ratio exceeds 2, further [Bzmim]Cl insertion occurs into 2, resulting in the formation of compound 3 after heat treatment. This stepwise transformation causes the emission color of the powders to switch from red to yellow-green. Thus, the insertion of [Bzmim]Cl into SbCl₃ proceeds in a stepwise manner as shown in Fig. 3b, enabling the sequential formation of 1, 2, and 3. The reverse processes, involving the extraction of [Bzmim]Cl, can also be achieved in a stepwise fashion. Placing the green-emissive powder of 3 in a moist environment triggers the initial extraction of [Bzmim]Cl, resulting in a PL color switch back to red, corresponding to compound 2. Subsequent addition of liquid ethanol completes the transformation to the non-emissive compound 1, as the PL switches from red to non-emission.

Time-resolved and multi-steps information encryption based on Sb-IOMHs

The reactant ratio-controlled PL switching, coupled with tunable switching rates, supports time-resolved information encryption and advanced anticounterfeiting strategies. To meet the demand for confidential information encryption, selecting a suitable composite for invisible ink is crucial. Typically, one of the components used in the synthesis of IOMHs can serve as the invisible ink, while another component acts as the developer. For example, Li et al. reported the transformation of an invisible Pb-MOF (metal–organic framework) into a composite of luminescent MAPbBr₃ nano crystals and Pb-MOF by employing Pb-MOF as the invisible ink and MABr as the developer.⁴⁹ Similarly, Xia et al. demonstrated a MABr-triggered information decryption process using PbBr₂ containing lanthanide MOFs as invisible ink.⁵⁰ Sun et al. explored full-color information encryption by combining halide salts with CsPbCl₃ quantum dots as the invisible ink and developer, respectively.⁵¹ They also introduced selective information display through additional water treatment as a developer. In another study, they used composites of Cs₄PbBr₆ and CsPbCl₃, as well as CsPbBr₃ nanoplatelets, as invisible inks for multi-functional encryption.⁵² However, the high compositional tunability of such materials family has rarely been utilized for information encryption and anti-counterfeiting. The controllable reaction rates and PL. switching behaviors of the [Bzmim]_n−3SbCl_n series materials offer unique potential for time-resolved and multi-step information encryption. This introduces “time” as an additional developer, providing an innovative pathway for enhancing the security and functionality of anti-counterfeiting measures and confidential information storage.

Fig. 4a illustrates the details of the invisible ink, developers, and the fabrication strategy for the multi-step and time-resolved information encryption process. Initially, confidential messages written with varying concentrations of SbCl₃ ink are completely invisible under both visible light and UV light. Upon applying [Bzmim]Cl ink at a specific concentration to the entire paper, red-emissive information gradually appears under a UV lamp, corresponding to the structural transformation from compound 1 to 2. The true information can be concealed within letters like “H”, “HI”, “HIA”, or “HIAM”. Heating the paper at high temperatures induces the transformation from compound 2 to 3, revealing the letters “HI” and “AM”, designed to confuse potential adversaries with false messages. The authentic information remains hidden unless [Bzmim]Cl ink with the correct concentration is applied, and the message is observed at the appropriate time and temperature.

Fig. 4 (a) Demonstration of the multi-step and time-resolved decryption processes of information, the insert image shows the invisible ink and developers. (b) Digital pictures of the filter paper coated by [Bzmim]Cl and SbCl₃ solutions with different concentrations (under 390 nm UV light) at different time. (c) Digital pictures of the written letter on a filter paper before and after decryption under 390 nm UV lamp at different time. (d) Digital pictures of the paper before and after heating with the words written by [Bzmim]Cl and SbCl₃ inks (under 390 nm UV light).

As a proof of concept, a series of circular filter papers (5 mm in diameter), coated with invisible SbCl₃ inks of different concentrations, were prepared and arranged as shown in Fig. 4b. The PL “off” and “on” states are respectively defined as “0” and “1” in a binary system. Upon applying [Bzmim]Cl solution of uniform concentration, the information transitions to “41 601” after 5 minutes, “48 097” after 10 minutes, and “65 535” after 30 minutes. This demonstrates time-resolved information changes driven by the concentration-dependent rate of structural transformation. Interestingly, in contrast to the observations in plastic vials, the filter paper experiments showed that red emission first appeared in samples with the highest SbCl₃ concentration, rather than those with the highest [Bzmim]Cl/SbCl₃ ratio (Fig. 4b). This discrepancy may be attributed to the overwhelming presence of [Bzmim]Cl in the reaction system on the filter paper. As a result, the reaction rate was primarily governed by the amount of SbCl₃ available, rather than the reactant ratio. This phenomenon was further observed in another experiment, where a letter was written on paper using SbCl₃ solutions at specific concentrations (details provided in Fig. 4c). Initially, the letter “L” becomes visible under UV light by emitting red luminescence. An adversary attempting to decode the information may stop at this stage, mistaking it for the true message. However, as time progresses, the letter transitions to “C” and eventually to “O”, revealing the actual message. Notably, the decryption process is single-use. Any attempt to uncover the hidden information leaves an irreversible mark, providing an additional layer of security by alerting the user that the information decryption has been tampered with.

For the multi-stage information encryption and anti-counterfeiting demonstration, the word “disagree” was selected to be written on the paper (Fig. 4d, detailed in the experimental section). The correct decryption process requires the following steps: (1) drop a [Bzmim]Cl solution of a specific concentration onto the paper; (2) heat the paper to a specific temperature and (3) observe it under a UV lamp. After the first step, the word “disagree” becomes visible under UV light, emitting red luminescence. However, this is false information. To reveal the true message, a secondary developer (heat) must be applied. Upon heating at 80 °C for 30 min, the true message, “agree”, emerges under UV light. The concentration of the [Bzmim]Cl solution plays a critical role in this process. If the concentration is too low, there will be insufficient [Bzmim]Cl on the paper to trigger the structural transformation, leaving the information hidden. Conversely, if the concentration is too high, an excess of [Bzmim]Cl on the paper leads to all the letters emitting yellow-green light under UV illumination after heating, still displaying the false information. The interplay between the concentration of developer and the structural transformation is crucial for revealing the correct information at the appropriate stage, adding complexity and security to the decryption process. Recently, there are some advances in multi-stage informational encryption and anti-counterfeiting using single-component multimode luminescent metal halides.^53–55 For example, Xia et al. demonstrated multiple encryption using excitation-dependent multi-excitonic emission in ATPP₂SnCl₆:Sb³⁺.⁵³ They also integrated self-trapped excitons, dopants, and defects within a single-component luminescent material to achieve multimode luminescence for multilevel anti-counterfeiting.⁵⁴ By contrast, our study presents a novel approach based on controllable structural transformations among multiple components. This approach facilitates tunable multimode luminescence through multi-component transformations, surpassing the commonly reported structural transformations between two components.

Conclusions

In conclusion, a reversible structural transformation from [Bzmim]₄[Sb₄Cl₁₆] to [Bzmim]₂SbCl₅ and further to [Bzmim]₃SbCl₆ was reported, resulting in a PL switching among non-emission, red-emission and yellow-green emission. This demonstrates the first example of multi-step reversible structural transformations from ASbX₄ to A₂SbX₅ and further to A₃SbX₆ in IOMHs. Additionally, reactant ratio-controlled PL switching modes were investigated, transitioning from a simple “off-on” mode to a novel “off-on¹-on²” mode. Using SbCl₃ ethanol solutions of varying concentrations as invisible ink, and the combination of [Bzmim]Cl solution, UV light, time, and temperature as developers, multi-step, time-resolved information encryption and anticounterfeiting was achieved. The tunable composition and PL-responsive behavior of these IOMHs position them as excellent candidates for high-level information encryption and anticounterfeiting applications. This work highlights the potential of stimuli-responsive IOMHs in advancing information protection technologies and paves the way for exploring new functional materials in this field.

Author contributions

Zeping Wang: conceptualization, data curation, formal analysis, funding acquisition; investigation, methodology, writing – original draft, writing – review & editing; Xiaoying Huang: funding acquisition, project administration, supervision, writing – review & editing.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper and/or the ESI.† X-ray crystallographic data for [Bzmim]₄[Sb₄Cl₁₆] (CCDC 2415044†) is available in https://www.ccdc.cam.ac.uk/.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the financial supports from the National Nature Science Foundation of China (No. 22205236 and 92261115) and the Natural Science Foundation of Fujian Province (No. 2022J05089).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2415044. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi00256g

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