Switchable circularly polarized luminescent Mn-based hybrid metal halides

Abstract

Hybrid metal halides with stimuli-responsive circularly polarized luminescence (CPL) have risen to prominence benefiting from their facile preparation and versatile chiral applications. Despite recent rapid developments, the ability to craft hybrid metal halides with strong and switchable CPL remains a challenging issue. This report presents the synthesis of chiral zero-dimensional (0D) hybrid metal halides demonstrating robust and switchable CPL activity. Specifically, a pair of 0D chiral hybrid metal halides (R-/S-MPZ)MnBr₄ and the corresponding racemic counterparts (rac-MPZ)MnBr₄ were synthesized by introducing R-/S-/rac-MPZ units (R-/S-/rac-MPZ = R-/S-/rac-2-methylpiperazine) as organic cations. The synthesized compounds achieved stable and efficient CPL emission with a notable luminescent anisotropy factor (g_lum) of up to 1.12 × 10⁻². Remarkably, reversible CPL can be manipulated to achieve dependence on the variation of environmental humidity and temperature. The findings open up a promising pathway to controllably tune chiroptical behavior of metal halides with large and switchable CPL to develop new security optical coding and hidden communication.

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Shuang Pan

Shuang Pan is an associate professor at the College of Chemistry and Materials Engineering, Wenzhou University. She received her PhD from Fudan University in 2018. From 2016 to 2019, she was a visiting student/postdoctoral fellow in Prof. Zhiqun Lin's group at the Georgia Institute of Technology. Her research interests mainly focus on the precise construction of chiral optically active materials and organic/inorganic hybrid materials. She has developed a universal strategy for large-scale, high-throughput film fabrication and explored potential applications in areas such as optoelectronic devices, flexible electronics and energy conversion.

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Introduction

Stimuli-responsive luminescent materials represent a unique class of substances capable of undergoing reversible alterations in their luminescent characteristics upon application of external stimuli (e.g., light, temperature, electric fields, or moisture).^1–3 Their distinct attributes position them with broad-ranging applications such as information decryption, anti-counterfeiting measures, and sensor technologies.^4,5 As research on stimuli-responsive luminescent materials has advanced, there has been a growing recognition for the importance of incorporating circularly polarized luminescence (CPL) into this field.^6,7 While traditional luminescent materials rely solely on modulation of emission color and intensity, stimuli-responsive CPL materials introduce another dimension—chirality, enabling the realization of more complex and diverse optical response patterns. However, despite making some progress, there are still numerous challenges in modulating stimuli-responsive CPL materials, including intricate chemical synthesis processes,^8,9 concerns regarding material toxicity,^10–12 and a deficiency in understanding the mechanisms for control.¹³ Therefore, research into developing stimuli-responsive CPL materials with refined performance control methods and deeper exploration of stimuli-responsive mechanisms is crucial to propel the continuous advancement and application of this field.

Recently, impressive progress has been made in the development of hybrid metal halides due to their facile synthesis and exceptional optical properties such as bright photoluminescence, long carrier lifetimes and color tunability.^14,15 Notably, CPL-active low-dimensional metal halide materials can be facilely created by incorporating chiral organic molecules as cations to construct crystal frameworks with intrinsic chirality.^16,17 More intriguingly, due to their inherent ionic nature, metal halides are highly sensitive to minor changes in the surrounding environment, leading to alterations in the crystal phase, structural dimensions, and metal coordination modes. The integration of stimuli-responsive characteristics within chirality has the potential to introduce a degree of adjustability to CPL active systems, potentially resulting in intriguing switchable chiroptical properties and multi-level anti-counterfeiting applications. For instance, a recently reported novel zero-dimensional (0D) hybrid lead–tin bromide enantiomer displays a luminescent anisotropy factor (g_lum) of ±3.0 × 10⁻³, allowing water-dependent switchable CPL emissions with tunable colors.¹⁸ Similarly, chiral antimony chlorides with a g_lum of 1.2 × 10⁻³ undergo structural transformation under dimethylformamide vapor and can revert under heating conditions.¹⁹ Besides, a pair of manganese-based metal halides, (R-/S-BrMBA)₃MnBr₅, could achieve reversible CPL chirality and color switching via temperature variation.¹⁰ However, stimuli-responsive chiral halide metals reported to date suffer from inadequate g_lum values. Therefore, it is of crucial significance to construct stimuli-responsive luminescent chiral metal halides with high g_lum values,²⁰ as well as expanding multi-layer encryption anti-counterfeiting applications.

In this study, we presented the synthesis and characterization of a series of low toxicity constituents 0D hybrid metal halide (R-/S-/rac-MPZ)MnBr₄ (R-/S-/rac-MPZMB) single crystals (SCs). Through the incorporation of chiral R-/S-MPZ molecules into the [MnBr₄]²⁻ manganese bromide inorganic units, a pair of chiral metal halides R-MPZMB and S-MPZMB emitting distinct green light were obtained, with photoluminescence (PL) peaks centered around 530 nm. The highest observed g_lum for R-MPZMB SCs was determined to be 1.12 × 10⁻². Leveraging the outstanding CPL emission properties, circularly polarized light-emitting diodes (CP-LEDs) utilizing R-/S-MPZMB SCs were fabricated, showcasing proficient discrimination of polarized emission states and emitting green luminescence. Through density functional theory (DFT) computations, more insights into the electronic structures and optical properties of these metal halides were obtained. Our findings suggested effective induction of [MnBr₄]²⁻ inorganic anions by these chiral organic cations through hydrogen bonding, with resultant CPL activity arising from d–d transition emission of distorted [MnBr₄] inorganic units. Intriguingly, under controlled moisture and thermal conditions, R-/S-MPZMB materials demonstrated reproducible fluorescence and CPL color modulation, serving as versatile and reversible CPL emission sources. Ultimately, a suite of anti-counterfeiting applications was devised, integrating printed patterns, thereby showcasing the versatile utility of chiral lead-free metal halides across the realms of anti-counterfeiting and luminescence.

Results and discussion

The synthesis of bulk R-/S-/rac-MPZMB SCs was achieved through the application of slow solvent evaporation crystallization techniques within hydrobromic acid solutions, as outlined in detail in the Methods section. This involved the deliberate selection of R-/S-/rac-MPZ as organic units. The optical characteristics of the acquired single crystals under daylight conditions are shown in Fig. S1 (ESI†). Fig. 1a showcases that crystal structures of the chiral manganese hybrids R-MPZMB and S-MPZMB exhibit enantiomorphism and can be classified within the chiral P2₁ space group. In this structural arrangement, each Mn atom is surrounded by four Br⁻ ions, forming tetrahedral [MnBr₄]² units. These tetrahedrons are distinctly isolated from one another within the crystal lattice, separated by large organic units (MPZ), resulting in a 0D structural configuration at the molecular level. The MPZ units combined with the [MnBr₄]²⁻ tetrahedra through non-covalent interactions such as Coulomb force and weak N–H⋯X (X = Br) hydrogen bonds. In contrast to the arrangement of chiral manganese hybrids, the racemic counterpart comprises [MnBr₄]²⁻ units separated by alternating stacking of R-MPZ units and S-MPZ units, which lead to the crystallization of rac-MPZMB in the achiral space group P2₁/n (Fig. S2a, ESI†). The crystallographic structures were confirmed via single crystal X-ray diffraction (SCXRD; Table S1, ESI†). The powder X-ray diffraction (PXRD) patterns of these powders ground from the corresponding single crystals had a strong correlation with the simulation data extracted from their SCXRD, as illustrated in Fig. 1b and Fig. S2b (ESI†). The deviation peak observed around the 15° in the PXRD patterns of R-/S-MPZMB was likely attributed to the inherent susceptibility of powder. This similarity in diffraction patterns with other piperazine bromides was also observed (Fig. S3, ESI†). Furthermore, the scanning electron microscopy (SEM) images in Fig. S4 (ESI†) illustrated the morphology of the corresponding R-/S-/rac-MPZMB powder samples, revealing a homogeneous distribution of elements such as Mn, N, and Br, as confirmed by energy-dispersive X-ray spectroscopy (EDS). Exemplified by R-MPZMB, the EDS analysis demonstrated an atomic ratio of Mn to Br approximately at 1 : 4, consistent with elemental composition of the target compound. Additionally, the high chemical purity of these obtained batches was evident from the X-ray photoelectron spectra (XPS; Fig. S5, ESI†), which display distinctive peaks at 652 and 640.2 eV, attributed to Mn 2P_1/2 and Mn 2P_3/2, respectively, affirming the presence of Mn²⁺ in the R-/S-MPZMB and rac-MPZMB single crystals.^21,22 Thermogravimetric analysis (TGA) conducted on R-, S-, and rac-MPZMB powders aimed to assess their chemical thermal stability, a critical property particularly in organic–inorganic hybrid metal halides. As illustrated in Fig. 1c and Fig. S6 (ESI†), these compounds have remarkable thermal robustness, with the initial decomposition point surpassing 300 °C corresponding to the breakdown of organic ammonium salts and some carbides.²³ Such exceptional thermal stability of the material suggested promising potential for chiroptoelectronic applications.²⁴

Fig. 1 (a) The crystal structure diagrams of R-/S-MPZMB SCs. (b) The experimental part and the simulation part of the XRD pattern of R-/S-MPZMB. (c) The TGA spectrum of R-MPZMB. (d) CD spectra and corresponding absorption spectra of R-, S-MPZMB. (e) The corresponding g_abs-factor values of R-/S-MPZMB.

To explore the chiroptical properties of the enantiomorphic hybrid metal halide compounds, circular dichroism (CD) spectra were conducted. The CD spectra (Fig. 1d) of obtained R-/S-MPZMB samples exhibited distinct mirror symmetries, showcasing peaks at wavelengths around 350 nm, 380 nm and 445 nm, respectively. Notably, these peaks closely aligned with the absorption peaks observed at 355 nm, 375 nm and 455 nm, respectively. The CD signature of R-/S-MPZMB underwent a change near the exciton absorption edge, referred to as a Cotton effect.²⁵ The absence of these chiroptically active transitions corresponding to the R-/S-MPZ units, which exhibited lower absorption values than 250 nm (as depicted in Fig. S7, ESI†), suggested the successful transfer of chirality from the chiral ligands to the inorganic sublattice. In addition, the Fourier transform infrared (FTIR) spectra of the R-/S-/rac-MPZ (organic amine), R/S/rac-MPZBr₂ (R-/S-/rac-methylpiperazine bromide) and metal halide R-/S-/rac-MPZMB are presented in Fig. S8 (ESI†). The distinct N–H symmetry stretching band peaks were evident at wavenumbers around 3073 nm⁻¹ for R-/S-/rac-MPZMB SCs. The N–H stretching vibration absorption peak at around 3211 cm⁻¹ (higher than 3073 cm⁻¹ of R/S-MPZMB) of R-/S-/rac-MPZ may be attributed to the reduced hydrogen bonding of N–H⋯Br.²⁶ This observation signified that the robust interactions between the hydrogen ions in NH₂⁺ of the organic unit and inorganic unit [MnBr₄]²⁻ through N–H⋯Br hydrogen bonding.^27,28 In comparison, although the racemic compound shared absorption peaks within the absorption range of chiral enantiomers, it did not exhibit any chiroptical activity (Fig. S9, ESI†). Accordingly, the anisotropy factor of CD absorption (g_abs) was obtained to evaluate their chiroptical property (Fig. 1e). For R- and S-MPZMB at the absorption peak of 360 nm, the g_abs values were −1.63 × 10⁻³ and 1.28 × 10⁻³, respectively. Similarly, at the absorption peak of 455 nm, the g_abs values were −1.5 × 10⁻³ for R-MPZMB and 1.14 × 10⁻³ for S-MPZMB.

Luminescent manganese(II) complexes have been extensively studied due to their excellent photophysical properties.²⁹ The photoluminescence excitation (PLE) and photoluminescence (PL) spectra of these as-prepared R-/rac-MPZMB samples were also examined to delve into their optical properties (Fig. 2a and Fig. S10, ESI†). The PLE spectra of R-MPZMB demonstrated peak absorptions corresponding to electronic transitions from the ground to excited states of the Mn²⁺ ion, which was consistent with the UV-Vis absorption spectra (Fig. 1d). Such peaks corresponded to electronic transitions from the ⁶A₁ ground state of the Mn²⁺ cation to various excited states ⁴T₁(P) (366 nm), ⁴E(D) (380 nm), [⁴A₁(G),⁴E] (438 nm), ⁴T₂(G) (456 nm), and ⁴T₁(G) (474 nm) (Fig. S10, ESI†).²⁹ Under the action of the ground state transition, a dominant PL peak emerged at 530 nm, emitting a bright green light (as shown in the inset diagram of the R-MPZMB single crystal sample under 365 nm UV light in Fig. 2a). Interestingly, the rac-MPZMB single crystals exhibited a distinct unique emission pattern, showcasing PL peaks at around 530 nm and 630 nm, respectively. As shown in Fig. 2b, the chiral enantiomers showed the average PL decay lifetime of 71 μs (S-MPZMB) and 73 μs (R-MPZMB) at 530 nm, while the rac-MPZMB have shorter fluorescence decay lifetimes of 5.9 μs at 630 nm and 4.73 μs at 530 nm. The photoluminescence quantum yields (PLQYs) were measured and gave average values of 12% and 10% at 530 nm for R-MPZMB and S-MPZMB, respectively, while values for rac-MPZMB were below 1% at both 530 nm and 630 nm. In order to further understand the luminescent mechanism and verify the accuracy of these optical band gaps, the density functional theory (DFT) calculations were used to obtain the density of states (DOS) and the band structure for these single crystals. As depicted in Fig. 2c, a subtle band fluctuation of the conduction band (CBM) and valence band (VBM) within the Brillouin zone with a localized character was presented for R-MPZMB SC, arising from the isolated [MnBr₄]²⁻ species with no obvious intermolecular coupling.³⁰ The minimum of CBM was situated at the A point, while the maximum of VBM was found at the Y₂ point, indicating that R-MPZMB single crystals existed in the form of an indirect bandgap, with the predicted bandgap measuring approximately 1.98 eV, lower than the experimental value of 3.29 eV (Fig. S11, ESI†). This could be attributed to the common underestimation of band-gap values of semiconductors via DFT calculations.³¹ Similarly, the density functional theory (DFT) calculation results showcased an indirect bandgap of 2.72 eV in rac-MPZMB (Fig. S12a, ESI†), with the VBM located at the A point and the CBM at the Y₂ point. This value was also lower than the experimental value of 3.52 eV (Fig. S12b, ESI†). Fig. 2d and Fig. S12c (ESI†) showed the calculated total state density and orbital-resolved partial density of state (PDOS) for R-MPZMB and rac-MPZMB SCs, respectively. In these structures, the contributions from Br 4p and Mn 3d orbitals were significant, at the top of the VBM and the bottom of the CBM. In the charge transfer process between Mn and Br, the d–d transition dominated as the main optical transition between the VBM and CBM. The organic part played a minor role in lattice formation, particularly at the edge of the band.³¹

Fig. 2 (a) Normalized steady-state PL (λ_ex = 365 nm), PLE spectra and digital images under 365 nm UV irradiation (inset) of R-MPZMB and rac-MPZMB SCs. (b) Time-resolved PL decay of the PL emission at 530 nm for R-/S-MPZMB, as well as emission at 530 nm and 630 nm for rac-MPZMB. (c) The electronic band structure, (d) total and orbital-projected PDOS of R-MPZMB. (e) CPL spectra and (f) the corresponding g_lum values of R-/S-MPZMB.

To explore the CPL potentiality, we conducted CPL spectrum measurements of R-/S-MPZMB SCs at ambient temperature (Fig. 2e). In our observations, a distinctly opposite direction with a prominent peak at approximately 545 nm was noted in the hybrid enantiomers. Furthermore, the wavelength of CPL aligned remarkably well with their PL emission, providing confirmation of the chiroptical properties of enantiomers in the excited state. Subsequently, the luminescent anisotropy factor, g_lum, considered a crucial parameter, was introduced to provide a more comprehensive evaluation of the CPL property. It is defined by the equation g_lum = 2 × (I_L − I_R)/(I_L + I_R), where I_R and I_L represent the intensities of the right and left CPL, respectively.³² As illustrated in Fig. 2f, the maximum g_lum for R-/S-MPZMB were 2.13 × 10⁻² and 2.29 × 10⁻², respectively, surpassing that of many 0D chiral metal halides constructed via hydrogen bonding interactions.^{10,16,33–35} These chiral metal halide-based LED devices with the ability to emit polarized light in two circularly polarized states were prepared and demonstrated, each with varying intensity (Fig. S13, ESI†).^24,32 Based on the test results, the circular polarization degree (P) of the CP-LED device was calculated using the formula P = (I_σ⁺ − I_σ⁻)/(I_σ⁺ + I_σ⁻), where I_σ⁺ and I_σ⁻ represent the σ⁺ and σ⁻ intensity, respectively.³⁶ Distinct responses of R-/S-MPZMB SCs CP-LEDs and a calculated P ≈ |0.3|, showcasing the effective optical selectivity between left-handed circularly polarized light (L-CPL) and right-handed circularly polarized light (R-CPL), were obtained.⁵

Intriguingly, the flexible crystal structure and inherent ionic composition in low-dimensional chiral metal halide present opportunities for intelligently manipulating the structure–property relationship in reaction to external stimuli.³⁷ When the R-MPZMB powder with bright green emission (peak at 525 nm) was exposed to the atmospheric environment (i.e., humidity influence), the green luminescence gradually disappeared, leading to quenched powders (i.e., left panel, inset in Fig. 3a; denoted R-MPZMB@WET stage). Upon subjecting these R-MPZMB@WET powders to a certain heating condition (100 °C, 20 min), they transitioned from non-luminescence in the visible range to exhibit orange-red emission (broad peak at 625 nm) under UV irradiation (denoted R-MPZMB@RED stage; middle panel), which eventually reverted back to green emission (denoted R-MPZMB@GREEN stage; right panel) with a further increase in temperature (150 °C, 10 min). When the heat source was removed, a rapid transition from R-MPZMB@GREEN to R-MPZMB@RED stage occurred, confirming the reversibility of this discoloration process. Continuing to place the R-MPZMB@RED sample in the atmospheric environment for a period of time, the powder was quenched again in air. Based on the corresponding XRD patterns, we preliminarily confirmed that these quenched powders at R-MPZMB@WET stage underwent decomposition into chiral piperazine halides and manganese bromide (Fig. S14, ESI†). To illustrate the continuous structural variation in the process, we conducted temperature-dependent PXRD analysis (30–150 °C, Fig. 3b). As the temperature increased from 30 °C, the diffraction peaks at 14.5°, 29°, 29.4°, and 30° gradually diminished, while a series of peaks emerged within the estimated ranges of 14.1° and 23.8° to 26.4°, potentially indicating the formation of a new structure. After further heating from 100 °C to 150 °C, the diffraction peaks of 11.5°, 12.7°, 24.1°, 24.75°, 30.2° and 31.5° gradually shift towards larger angles. Unfortunately, due to the instability of the single crystal during high-temperature testing, we cannot obtain single crystal data for comparison with the powder data. Nevertheless, continuous structural changes can be confirmed from temperature-dependent PXRD data.

Fig. 3 (a) The PL spectra of R-enantiomer samples (light orange line: R-MPZMB@WET; orange line: R-MPZMB@RED; green line: R-MPZMB@GREEN). The inset photographs depict that R-MPZMB@WET (left panel) transformed into R-MPZMB@RED (middle panel) when heated at a temperature of 100 °C for 20 min, eventually to R-MPZMB@GREEN (right panel) when the sample was heated at a temperature of 150 °C for 10 min. (b) The temperature-dependent PXRD patterns of R-MPZMB@WET from 30 °C to 150 °C. (c) Pseudocolor map of PL spectra during one heating cycle, (d) changes of PL intensity and peak position of 10 heating cycles from R-MPZMB@WET to R-MPZMB@RED from room temperature to 100 °C. The corresponding CPL spectra of (e) R-/S-MPZMB@RED sample. (f) Pseudocolor map of PL spectra during one cooling cycle, (g) changes of PL intensity and peak position of 10 cooling cycles from R-MPZMB@GREEN to R-MPZMB@RED from 150 °C to 100 °C in the air (70 ± 5% RH) at RT. The corresponding CPL spectra of (h) R-/S-MPZMB@GREEN sample.

The luminescence discoloration of Mn metal halides is primarily influenced by temperature or moisture, as indicated in previous reports.^{10,13,18,21,38,39} To identify the key stimulus, a controlled experiment was conducted in a glove box filled with dry N₂ to eliminate the influence of air (Fig. S15a and b, ESI†). Notably, upon removal of the heat source in dry N₂, the green emission continued to maintain, instead of transition back to R-MPZMB@RED stage observed during the cooling process in a humid environment. Only when removed from the glove box and exposed to a relatively high humidity environment, the powder underwent a complete transformation into an orange-red emission under UV irradiation within a mere 2 minutes, indicating the inseparable role of water in the discoloration process (Fig. S15c, ESI†). Besides, the powder from R-MPZMB@GREEN to R-MPZMB@RED stage was monitored using Fourier-transform infrared spectroscopy (FTIR). As illustrated in Fig. S16 (ESI†), the O–H stretching vibration (3385 cm⁻¹) and bending vibration (1609 cm⁻¹) of corresponding water molecules continuously strengthened with the increasing cooling time, indicating a consistent rise in water content within the system. These observations further confirmed the significance of water in the photoluminescence transition process. Thus, the continuous structural evolution observed during in situ temperature changes was preliminarily suggested to be attributed to the gradual removal of water as temperature increased. Taken together, the reversible color-changing mechanism was postulated as follows. Under excessive humidity conditions, R-MPZMB powders gradually quenched to the R-MPZMB@WET stage, comprising byproducts like piperazine halide and manganese halides. It seems that the majority of adsorbed water molecules evaporated at 100 °C, and [MnBr_x] ions can recombine with the dissociative MPZ units. Referring to prior reported literature, it is hypothesized that [MnBr_x] clusters may combine with H₂O to form octahedral inorganic units, leading to the initially observed orange-red fluorescence.¹⁸ As the temperature rose further, the water content was further removed and the [MnBr_x] returned to the tetrahedral coordination pattern [MnBr₄]²⁻, leading to green fluorescence characteristics. Fig. 3c illustrates the gradual evolution of sample from R-MPZMB@WET to R-MPZMB@RED within 20-minutes of heating at 100 °C. As plotted in Fig. 3d, after 10 cycles between R-MPZMB@WET and R-MPZMB@RED, the PL emission intensity and the peak position of R-MPZMB@RED could be well maintained. The corresponding CPL spectra of R- and S-MPZMB@RED samples exhibited a broad peak at approximately 640 nm, reaching its maximum value of g_lum at approximately −7.9 × 10⁻³ and 8.3 × 10⁻³, respectively (Fig. 3e). Similarly, Fig. 3f demonstrates the progressive shift of R-enantiomers sample from R-MPZMB@GREEN to R-MPZMB@RED to within a 10 min period of cooling in air (70 ± 5% relative humidity (RH)) at RT. Going through 10 cycles cooling period between R-MPZMB@RED and R-MPZMB@GREEN, the PL emission intensity R-MPZMB@GREEN was slightly down with maintained peak position (Fig. 3g). Correspondingly, the CPL spectrum of the R- and S-MPZMB@GREEN revealed an emission peak at approximately 545 nm, achieving a g_lum of about −3.5 × 10⁻³ and 6.3 × 10⁻³, respectively (Fig. 3h). The dynamic nature of this CPL fluorescence alteration presents intriguing possibilities for implementing controlled CPL luminescence applications. A series of cyclic stability experiments of the S-enantiomer were similar as shown in Fig. S17 and S18 (ESI†).

Utilizing the moisture-triggered phase conversion, accompanied by a noticeable color change, R-MPZMB proved to be a valuable asset in large-scale anti-counterfeiting applications. Fig. 4a and Fig. S19 (ESI†) delineate the procedural steps involved in fabricating the intended pattern using a straightforward screen-printing technique. Two types of precursor inks were prepared, including a pure metal halide precursor DMF solution (i.e., R-MPZMB ink), and a metal halide-polymer composite ink formed by mixing the metal halide precursor solution with a PVP polymer at a mass ratio of 1 : 4 (i.e., R-MPZMB@PVP ink). As part of the printing process, the ink permeated into the mesh of the screen pattern due to pressure applied by the squeegee, thereby depositing a printed ink drop onto the glass surface beneath the printing plate. The two different ink components can be sequentially printed onto the same pattern through two printing cycles with different molds, facilitating the effortless formation of a desirable patterned film on the surface. In the schematic illustration at the bottom right of Fig. 4a, the letter “Z” was printed using pure R-MPZMB ink, while the letters “W” and “U” were printed using R-MPZMB@PVP ink. Fig. 4b and c illustrate the distinct performance variations in patterns created using R-MPZMB ink and R-MPZMB@PVP ink under external field stimulation. The pattern utilizing R-MPZMB exhibited reversible PL switching, transitioning from a non-emissive state to an orange-red emission and further to a green emission with rising temperature (Fig. 4b). Conversely, the pattern employing R-MPZMB@PVP sustained an orange-red emission without transitioning to green emission (Fig. 4c). This situation could be attributed to the hydrophilic nature of PVP, potentially hindering the ongoing dehydration of metal halides even at elevated temperatures, thereby impeding further structural transformations and maintaining their red-light characteristics. The FTIR spectroscopy was used to compare the ink with and without PVP under the same annealing conditions, as shown in Fig. S20 (ESI†). Compared with R-MPZMB ink, the absorption peak of O–H stretching vibration (3348 nm⁻¹) related to H₂O was stronger for R-MPZMB@PVP ink, indicating that some moisture was still retained after annealing. This result corresponded to the results of our practice. The detailed process of information encryption and decryption was showcased in Fig. 4d. Initially, the dots and letters emitted no fluorescence, serving as the starting point for information encryption. Upon exposure to a temperature of 100 °C, all dots and letters emitted orange-red light (third panel in Fig. 4d), presenting false information as the first stage of decryption. Further heating to 150 °C caused the R-MPZMB-made dots and letters to transition to a vibrant green (indicated by dashed boxes in fourth panel in Fig. 4d), while the R-MPZMB@PVP-made dots and letters maintained their orange-red hue, constituting the second decryption key. Consequently, only those with access to the decryption key can accurately interpret true information, such as identifying two green dots and the letter “Z”. Following decryption, the green dots spontaneously revert to orange-red and then become colorless upon exposure to water intrusion, effectively concealing the information again and leaving no trace of the decoding process. The implemented dual-mode encryption–decryption technology ensures a high level of security, effectively preventing any potential information leakage.

Fig. 4 (a) Schematic diagram of the screen printing and the encryption and decryption of anti-counterfeiting patterns made with R-MPZMB and R-MPZMB@PVP inks. The fluorescent images of the patterns printed by (b) R-MPZMB and (c) R-MPZMB@PVP under humid conditions at room temperature, 100 °C and 150 °C. (d) Fluorescent images of decrypted and encrypted anti-counterfeiting patterns of letters “WZU” and dots made with R-MPZMB (labelled by dashed boxed) and R-MPZMB@PVP (labeled outside dashed boxed) inks under UV light.

Conclusions

In conclusion, we have synthesized novel chiral 0D lead-free manganese halides, R-/S-MPZMB SCs, exhibiting dazzling green emission and an outstanding g_lum value of 1.12 × 10⁻². Experimental results and theoretical calculations indicated that the CPL activity of these chiral metal halides originated from the asymmetric isolated inorganic unit [MnBr₄]²⁻, with the asymmetry induced by the introduction of chiral organic cations R/S-MPZ units. Furthermore, these single crystal samples exhibited reproducible fluorescence and CPL color-changing behavior under stimuli such as humidity and heat. Lastly, leveraging the excellent CPL emission activity and stimulus-tunable color-changing behavior of these chiral single crystals, we have constructed green CP-LED devices with good performance and patterned prints with anti-counterfeiting functionality, thereby expanding the application scope of low-dimensional chiral lead-free metal halide materials in multifunctional anti-counterfeiting and luminescence domains.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank the financial support from the National Natural Science Foundation of China (62104170, 62474125 and 22109120), the Zhejiang Provincial Natural Science Foundation of China (LY23F040001) and the Major Scientific and Technological Innovation Project of Wenzhou City (Grant No. ZG2024040).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc01823k

‡ Equal contribution.

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