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. 2026 Mar 31;17:4665. doi: 10.1038/s41467-026-71277-3

Reversible phase-transformation-induced thermal quenching in Mn(II) chlorides for high-precision information encryption and thermal energy storage

Aibo Li 1, Zongqi Chen 1, Zhengliang Wang 1,✉, Peng Ren 1, Qin Wang 2,✉, Yayun Zhou 3, Qiang Zhou 1,✉, Huaijun Tang 1
PMCID: PMC13201647  PMID: 41917037

Abstract

Photoluminescent materials are widely used in information security applications, yet high-precision optical information encryption remains difficult to achieve. Here we report two monoclinic zero-dimensional organic–inorganic hybrid Mn(II) chloride crystals (C19H42N)2MnCl4 and (C21H46N)2MnCl4. Both show green emission with photoluminescence quantum yields of 85.7% and 89.3%. Upon heating, photoluminescence (PL) is quenched abruptly at 333 and 343 K (ΔT = 10 K) and recovers upon cooling, driven by reversible order–disorder solid–solid phase transitions with phase-transition enthalpies of 140.6 and 160.3 J g–1, respectively. Using two closely spaced PL quenching temperatures, we demonstrate a temperature-window encryption scheme for optical information encryption and anti-counterfeiting, where correct information is revealed only within 333 ≤ T < 343 K. Furthermore, combining latent-heat storage with a temperature-gated PL ON/OFF readout provides a straightforward route to visualized thermal energy storage. This work reveals phase-transition-induced PL quenching and its applications in optical information encryption and visualized thermal energy storage, providing a strategy for designing multifunctional thermo-responsive luminescent materials.

Subject terms: Information storage, Ligands, Organic-inorganic nanostructures

Photoluminescent materials are widely used in information security applications, yet high-precision optical information encryption remains difficult to achieve. Here the authors report two monoclinic zero-dimensional organic–inorganic hybrid Mn(II) chloride crystals and characterize their phase-transition-induced photoluminescence quenching.

Introduction

In the 21st century, human society has entered the information age, where massive volumes of data permeate daily life and social operations1. As counterfeiting becomes more advanced and information-security threats increase, there is an urgent need for reliable encryption technologies2. Traditional encryption schemes, such as software-based cryptographic algorithms3 and nano anti-counterfeiting printing techniques4, often rely on complex equipment, complicated calculations, or professional operations, which limits simple on-site authentication. Therefore, developing innovative technologies with both high confidentiality and responsiveness is of great significance. Optical information encryption based on smart luminescent materials is attractive because it enables real-time visual authentication, simple operation, high-contrast readout, and strong resistance to replication5,6. These materials can exhibit photoluminescence (PL) responsive properties under external stimuli such as light7,8, mechanical force9, chemical substances10, pH11, and temperature12, enabling dynamic and reversible control of optical signals, thus providing a useful basis for secure data storage, identity authentication, and anti-counterfeiting applications13. Nevertheless, many reported luminescent systems still require strict control of excitation conditions, which can lead to slow or ambiguous verification and limited encoding capacity14,15. Therefore, it remains urgent to develop smart luminescent materials for optical information encryption with improved security, adaptability, and practical usability.

Among smart luminescent materials, organic–inorganic hybrid metal halides have attracted considerable interest because their emission is readily tunable16. In particular, organic–inorganic hybrid Mn(II) halides (OIMnHs) are spectrally tunable, cost-effective, and low-toxicity, making them attractive candidates for optical information encryption and anti-counterfeiting17–19. Their luminescence mainly arises from d-d transitions of Mn(II) and is highly sensitive to its coordination environment: Mn(II) in tetrahedral coordination exhibits green luminescence due to a weak crystal field, while Mn(II) in octahedral coordination emits orange or red light because the strong crystal field reduces the energy gap between the 4T1(G) and 6A1(S) states20,21. In recent years, the stimulus-responsive PL properties of OIMnHs have attracted increasing attention and demonstrated application potential in multiple fields17,19. Among them, temperature-responsive OIMnHs possess significant competitiveness in information encryption and anti-counterfeiting due to their thermally tunable luminescent behavior22,23. For example, (DMML)2MnBr4 (DMML = N, N-dimethylmorpholinium), prepared by Lun et al. 24, exhibits bright green luminescence under ultraviolet (UV) light and shows on-off luminescent behavior with temperature changes, enabling temperature-controlled information encryption in the range of 300–450 K. He et al. 25 reported that (TMS)2MnBr4 (TMS = trimethylsulfonium) undergoes luminescence quenching at temperatures slightly below human body temperature, allowing simple PL switching of anti-counterfeiting labels via body heat. However, reported OIMnHs generally suffer from wide temperature response windows, slow response, and poor temperature-dependent luminescence stability, which greatly limit their applications in high-precision anti-counterfeiting scenarios. Therefore, developing temperature-responsive optical materials with narrow response ranges, high stability, and high precision have become a key demand in this field.

In addition to temperature-responsive PL properties, some OIMnHs also show the characteristics of phase change materials (PCMs), and they can undergo thermally induced solid–solid phase transitions accompanied by large enthalpy changes, small thermal hysteresis, and good reversibility26,27. Because these hybrids are structurally tunable, their solid–solid transition temperature and phase transition enthalpy (ΔHtrans) can also be tuned28. In related hybrid halides, these parameters are mainly governed by the alkyl-chain length, the organic–cation type, and the halide identity29,30. Moreover, studies on paraffin-PDPA phase-change hybrids show that latent-heat storage/release is accompanied by changes in fluorescence intensity and color during the phase transition31. However, most studies still rely on combining multiple materials to achieve latent-heat storage and luminescent readout. In contrast, combining latent-heat storage with an intrinsic PL response within a single OIMnHs system remains underexplored and requires further investigation.

In this work, we report two closely related OIMnHs, (C19H42N)2MnCl4 and (C21H46N)2MnCl4, which differ slightly in alkyl chain length yet exhibit highly similar green emission. Both compounds show abrupt PL quenching at 333 and 343 K, respectively, driven by an order–disorder phase transition in the crystal structure. Moreover, the phase transition is reversible upon cooling, with the PL intensity largely recovered over repeated heating–cooling cycles. Accordingly, we demonstrate a narrow temperature-window encryption scheme for optical information encryption and anti-counterfeiting, in which correct information is revealed only within 333 ≤ T < 343 K. Furthermore, the large ΔHtrans accompanying the phase transitions suggests potential for latent-heat storage. By combining their intrinsic temperature-dependent optical modulation properties with thermal energy storage capability, this work highlights the potential for visualized thermal energy storage. These multifunctional properties make the materials promising for thermo-optical integrated applications and provide an experimental basis for the synergistic development of optical information encryption/anti-counterfeiting and advanced energy systems.

Results

Single crystals of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 were obtained by anti-solvent crystallization at room temperature and atmospheric pressure (Supplementary Fig. S1, see the Materials and methods for details). Their crystal structures were first revealed at 293 K using single-crystal X-ray diffraction (SC-XRD). Both kinds of crystals belong to the monoclinic crystal system with the space group C2/c (Fig. 1a, b, Supplementary Tables S1–S9). (C21H46N)2MnCl4 has a little bigger volume (10180.7 Å3) than that of (C19H42N)2MnCl4 (9454.3 Å3), ascribed to its slightly longer alkyl chain (Fig. 1c). Both crystal structures display a well-organized hydrophilic-hydrophobic pattern, where the hydrophobic long alkyl carbon chains stack along one side of the C-axis, while the remaining hydrophilic regions consist of [MnCl4]2– tetrahedra and amino groups, periodically surrounded by the hydrophobic alkyl chains. This arrangement enhances the environmental stability of the organic–inorganic hybrid metal halides and reduces the impact of working conditions on their performance32. The [MnCl4]2– tetrahedra in these structures are separated by organic cations and do not share vertices, edges, or faces, thereby exhibiting a zero-dimensional (0D) structure33. Meanwhile, they are slightly distorted with the different bond lengths of Mn–Cl and bond angles of Cl–Mn–Cl (Supplementary Figs. S2, S3, Supplementary Tables S8, S9). The Mn–Mn distances between the [MnCl4]2– tetrahedra in both crystal structures range from 7.99 to 23.94 Å for (C19H42N)2MnCl4 and from 7.98 to 26.43 Å for (C21H46N)2MnCl4, respectively. This spatial arrangement of [MnCl4]2– tetrahedra effectively increases the distance between Mn–Mn, thereby weakening the Mn–Mn bond and inhibiting nonradiative energy transfer between adjacent [MnCl4]2– centers, which is expected to enhance the photoluminescence quantum yield (PLQY)34. Moreover, the powder X-ray diffraction (P-XRD) patterns of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 further confirm that both compounds exhibit high phase purity and a uniform structure (Supplementary Fig. S4). To further confirm the presence of organic molecules in the crystal structure, (C19H42N)2MnCl4 and (C21H46N)2MnCl4 were subjected to Fourier transform infrared (FTIR) spectroscopy (Supplementary Fig. S5). The characteristic peaks at 2918 and 2850 cm–1 are attributed to the stretching vibrations of C–H bonds in the organic cations. A bending vibration of the C–H bonds in the methyl and methylene groups of the organic cations was observed at 1469 cm–1. The peak at 722 cm–1 corresponds to the rocking vibration of –(CH2)n– (n > 4) in the organic cations35,36. Furthermore, high-resolution mass spectrometry (HRMS) measurements provided further evidence for the identities of the organic cations, and elemental analysis also supported the molecular formulas of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 (Supplementary Fig. S6, Table S10). In addition, elemental mapping images (Supplementary Fig. S7) and energy-dispersive X-ray spectroscopy (EDS) spectra (Supplementary Fig. S8) of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 crystals prove the existence of C, N, Cl, and Mn. Signals from gold (Au) and platinum (Pt) are attributed to the gold and platinum sputter coatings used during the measurement process. No other impurity elements are detected in the samples.

Fig. 1. Crystal structures and spectral properties of two kinds of organic–inorganic hybrid Mn(II) halide.

Fig. 1

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Crystal structures and pictures irradiated by 365 nm UV light of a (C19H42N)2MnCl4 and b (C21H46N)2MnCl4. c Schematic representation of the long alkyl chain ammonium cations, which include the hydrophilic group (purple) and hydrophobic group (green). d Normalized photoluminescence excitation (PLE) spectra of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 at room temperature. e Normalized photoluminescence (PL) spectra of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 at room temperature. f The PL decay curves of (C19H42N)2MnCl4 and (C21H46N)2MnCl4.

To characterize the PL properties of (C19H42N)2MnCl4 and (C21H46N)2MnCl4, their photoluminescence excitation (PLE) and PL spectra are shown in Fig. 1d, e, respectively. Both samples show almost the same PLE spectra with two obvious bands in the range of 350–500nm and PL spectra with one intense band peaking at 532 nm, indicating that Mn2+ in both kinds of crystals exhibits the same luminescent behavior. Bright green light under 365 nm UV irradiation is observed from two samples (Fig. 1a, b), and their International Commission on Illumination (CIE) chromaticity coordinates of (0.287, 0.659) and (0.293, 0.650) almost cover each other (Supplementary Fig. S9), meaning that the green light cannot be distinguished by the naked eye. Supplementary Fig. S10 illustrates a schematic diagram of the emission mechanism of Mn2+ in a tetrahedral weak crystal field, where electrons in the ground state 6A1(S) are excited to different excited states under optical irradiation. Subsequently, the excited-state electron relaxes to the lowest excited state 4T1(G), through several nonradiative processes. At last, they return from the 4T1(G) energy level to the ground state 6A1(S) and emit green light in the process37. Notably, both (C19H42N)2MnCl4 and (C21H46N)2MnCl4 exhibit high PLQY consistent with expectations, measured to be 85.7% and 89.3%, respectively (Supplementary Fig. S11). Their PL decay curves (Fig. 1f) are well fitted into a single-exponential decay equation38. The decay time (τ) values are 1.56 and 1.50 ms, respectively, which is consistent with the green luminescence behavior originating from the 4T1(G) to 6A1(S) transition39,40.

To further elucidate the luminescence mechanisms of (C19H42N)2MnCl4 and (C21H46N)2MnCl4, we performed density functional theory (DFT) calculations of the electronic band structure, total density of states (TDOS), and projected density of states (PDOS)41. Spin polarization was included in all calculations. Supplementary Fig. S12 shows the band gaps of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 with valence band maximum (VBM) and conduction band minimum (CBM) at the same K vector. The band gap of (C19H42N)2MnCl4 is 2.86 eV, and that of (C21H46N)2MnCl4 is 2.85 eV. Meanwhile, combining TDOS and PDOS shown in Supplementary Fig. S13, it is found that the VBM is mainly contributed by the spin-up Mn–3d and Cl–3p states, while the CBM is mainly contributed by the spin-down Mn–3d and Cl–3p states. Furthermore, TDOS and PDOS show that the organic fraction contributes very little to the VBM and CBM, indicating that the organic fraction does not directly contribute to the PL process, and that the green luminescence of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 originates from the [MnCl4]2– tetrahedra corresponding to the d–d (4T1(G) → 6A1(S)) radiative transitions. Furthermore, the flat spectral bands of the VB and CB indicate that no significant intermolecular coupling was found between the [MnCl4]2– tetrahedra, suggesting that [MnCl4]2– can act as an independent luminous center.

Supplementary Fig. S14 shows that the PL spectra of two kinds of crystals measured at different temperatures exhibit similar emission bands. However, when the temperature reaches 333 K, the luminescence intensity of (C19H42N)2MnCl4 decreases sharply and the PL quenching occurs immediately (Supplementary Video 1). Similarly, the PL quenching for (C21H46N)2MnCl4 can be observed at 343 K (Supplementary Video 2). From the temperature-dependent relative PL integral intensity in Fig. 2a, it can be clearly observed that the changing trend of emission intensity with the change of temperature. At the beginning, the anomalous negative thermal quenching observed during the heating process may be explained by the lattice expansion with increasing temperature, which increases the distance between the tetrahedral luminescent centers. The increased Mn–Mn distance weakens the interaction between [MnCl4]2– centers, suppressing non-radiative energy transfer and allowing more energy to be utilized for luminescence, resulting in a significant increase in PL. The emission intensities of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 touch the maximum at 321 and 333 K, respectively. Then, the intensity gradually decreases. In the end, the thermal quenching for both samples occurs at 333 and 343 K, respectively. To investigate the reproducibility and stability of the PL quenching behavior of the two compounds, the samples were heated to 358 K to induce PL quenching, and then cooled to 303 K and held for over 16 h before re-measurement. This heating–cooling cycle was repeated seven times. As shown in Supplementary Fig. S15, the normalized PL spectra reveal no shift in the peak positions for either sample, indicating excellent reproducibility of the PL quenching process. As shown in Supplementary Fig. S16, the relative integrated PL intensities remained high after multiple heating and cooling cycles, demonstrating the good cycling stability of both materials. Interestingly, although (C19H42N)2MnCl4 and (C21H46N)2MnCl4 exhibit similar temperature-dependent PL responses, the PL quenching temperatures of 333 and 343 K do not coincide, creating a narrow temperature response interval of about 10 K. Within this interval, the green emission of (C19H42N)2MnCl4 has disappeared, while (C21H46N)2MnCl4 continues to emit a strong green light. The suitable detectable temperature, high sensitivity to temperature changes, and narrow PL quenching window provide excellent opportunities for (C19H42N)2MnCl4 and (C21H46N)2MnCl4 to be applied in optical information encryption.

Fig. 2. Effect of temperature on the PL properties and structural characteristics of the two samples.

Fig. 2

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a Temperature-dependent relative integral PL intensities of (C19H42N)2MnCl4 and (C21H46N)2MnCl4. b Differential scanning calorimetry (DSC) curves of the two samples. c The false-color images of temperature-dependent powder X-ray diffraction (P-XRD) patterns for the two samples in the range of 11–15°.

To gain further insight into the unusual PL quenching behavior of the two samples induced by temperature, differential scanning calorimetry (DSC) measurements were performed. As shown in Fig. 2b, the DSC curves of (C19H42N)2MnCl4 clearly display an endothermic transition with an onset at around 333 K and an exothermic peak at 300 K. A significant thermal hysteresis phenomenon is observed, which is one of the characteristics of a substance undergoing a first-order reversible solid–solid phase transition42. Similarly, the DSC curves of (C21H46N)2MnCl4 also display a pair of heat absorption and exothermic peaks near 343–353 K and 300–319 K during heating and cooling, respectively, proving the reversible phase transition in (C21H46N)2MnCl4. Combined with the results of their temperature-dependent PL spectra, it is observed that the phase transition temperature in the DSC perfectly matches the temperature of the PL quenching. This indicates that the phase transition is triggered by a temperature-induced structural change, ultimately leading to PL quenching. Furthermore, the ΔHtrans and phase-transition entropy (ΔStrans) of two samples were calculated during the heating process. Specifically, (C19H42N)2MnCl4 exhibited a ΔHtrans of 140.6 J g–1 and a ΔStrans of 417.8 J K–1 kg–1, while (C21H46N)2MnCl4 showed a ΔHtrans of 160.3 J g–1 and a ΔStrans of 459.9 J K–1 kg–1. These ΔHtrans values are comparable to those of high-capacity PCMs that have been reported for thermal energy storage, such as GO–polyurethane solid–solid phase change composites (ΔHtrans ≈ 100 J g–1)43, multiple H-bonding cross-linked supramolecular solid–solid PCMs (ΔHtrans up to 142.5 J g–1)27, and PDPA/paraffin phase-change hybrids (ΔHtrans ≈ 132–160 J g–1)31. The ΔHtrans values of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 are higher than those of mostly reported solid–solid PCMs44, indicating that these Mn(II) hybrids are promising candidates for thermal energy storage. Moreover, because the phase transition is accompanied by a sharp and reversible PL ON/OFF switching, these materials also provide a solid basis for visualized thermal energy storage. Meanwhile, the thermogravimetric analysis (TGA) was performed, as shown in Supplementary Fig. S17, the decomposition temperatures of both substances are higher than 500 K, much higher than their phase transition temperatures, suggesting that the PL quenching is not caused by their decomposition.

We try to demonstrate the phase transition with increasing temperature for both crystal samples by SC-XRD. But the crystallographic data are hard to collect, due to the seriously disordered structure induced by the long alkyl carbon chains at high temperature. So, we collect the P-XRD data at variable temperatures, and the P-XRD patterns are shown in Fig. 2c and Supplementary Fig. S18. At room temperature, (C19H42N)2MnCl4 exhibits strong diffraction peaks at 11.8° and 14.8°. These peaks gradually decrease as the temperature increases, but their positions do not shift significantly, indicating that the substance structure does not undergo significant changes during this process. When the temperature is close to 333 K, the original diffraction peaks almost disappear, and a new peak appears at 12.6°, indicating the generation of new phases. The P-XRD of (C21H46N)2MnCl4 also shows a similar change, except for the temperature at about 343 K. These results are in agreement with those of DSC curves, proving that a thermally induced solid–solid phase transition occurs in the heating process, which causes structural changes and ultimately leads to the PL quenching. More interestingly, it has been confirmed through heating or cooling experiments that the phase transition is reversible. As shown in Supplementary Fig. S19, after cooling to the room temperature for 0.5 h, partial pattern peaks of both samples appear, but their fine structures cannot be observed. As the cooling time exceeds 12 h, most of pattern peaks can be found, which aligns with their initial patterns, proving the slow heat discharging process for both.

As shown in Fig. 3a–d, temperature-dependent Raman scattering measurements were conducted on (C19H42N)2MnCl4 and (C21H46N)2MnCl4 crystals in both low-temperature (LT) and high-temperature (HT) states to elucidate the origin of the thermally induced solid–solid phase transition. Although the two compounds have slightly different long alkyl chain lengths, the patterns of change in their temperature-dependent Raman spectra are nearly identical. In the LT stage, bands near 79 cm–1 and 113 cm–1 are assigned to longitudinal acoustic modes (LAM), while the characteristic band at 252 cm–1 is attributed to a low-frequency vibrational mode of the manganese-chloride (Mn–Cl) bond45–47. The characteristic bands near 1060, 1095, 1127, 1140, and 1166 cm–1 correspond to the mid-frequency region associated with C–C and C–H vibrations48. The band near 1300 cm–1 is attributed to methylene torsion, while the peaks near 1437 cm–1 and 1459 cm–1 correspond to C–H bending vibrations. Additionally, the bands near 2848 cm–1 and 2882 cm–1 are assigned to C–H stretching vibrations29. Under LT conditions, these characteristic peaks were very sharp, indicating that the LT phase is highly ordered, with strong interactions between molecular chains and a high degree of vibrational mode synergy. In the HT phase, molecular thermal motion leads to a loss of intermolecular coupling, resulting in a significant decrease in the intensity of the characteristic peaks; concurrently, the peak shapes became broad and flat, which is often associated with the development of structural disorder49. The changes in the Raman spectra confirm that the phase transition from the LT to the HT phase is accompanied by a transformation from an ordered to a disordered state. This transformation weakens intermolecular interactions, alters electron transitions and PL processes with an increased proportion of non-radiative transitions, ultimately leading to the disappearance of luminescence. Meanwhile, the transition temperature increases from 333 to 343 K with the increasing carbon number of the long alkyl chain from 16 to 18. This pattern is consistent with most reported phase transition temperatures of long alkyl chain metal halides28,30.

Fig. 3. Temperature-dependent Raman spectra of compounds (C19H42N)2MnCl4 and (C21H46N)2MnCl4 in selected wavenumber regions.

Fig. 3

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a 50–300 cm–1, b 1000–1200 cm–1, c 1250–1550 cm–1, d 2750–3050 cm–1.

In addition, to evaluate the practical stability of (C19H42N)2MnCl4 and (C21H46N)2MnCl4, tests were conducted at 313 K and 60% relative humidity (RH). As shown in Supplementary Fig. S20, even after 40 days, no significant change in luminescence intensity can be observed, proving their excellent moisture resistance, which should be due to their long hydrophobic alkyl chain.

By studying the special phenomenon of instantaneous PL quenching triggered by reversible thermodynamic phase transitions in (C19H42N)2MnCl4 and (C21H46N)2MnCl4, an anti-counterfeiting strategy with a narrow-window precise response is proposed. (C19H42N)2MnCl4 and (C21H46N)2MnCl4 can emit similar green light under UV irradiation. (C19H42N)2MnCl4 shows a phase transition-induced rapid PL quenching at 333 K, whereas (C21H46N)2MnCl4 exhibits a phase transition-induced PL quenching at 343 K. This phenomenon provides the basis for their use in optical information encryption. To demonstrate the application advantages of this strategy in optical message encryption and data security, two routes are designed. As shown in Fig. 4a for the first route, under UV light irradiation, the dots composed of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 emit bright green light. The output code of green light is “1”, and the output code of the light quenching is “0”. The detailed process of encrypting and decrypting the information is as follows: when the UV light is turned on, both compounds emit bright green light at temperatures below 333 K, outputting code “1”. By converting the binary code to decimal, the real message is locked, and the invalid message “7777” is displayed. When the temperature rises to 333 K, (C19H42N)2MnCl4 quenching instantaneously, (C21H46N)2MnCl4 exhibits normal luminescence. At this point, the binary code is converted to decimal, revealing the actual message “2025”. When the temperature exceeds 343 K, (C21H46N)2MnCl4 undergoes a rapid PL quenching. The output code “0” is generated, converted to decimal, resulting in invalid information “0000”, and the real message is then locked again. To better demonstrate the advantages of the first route, two types of crystals were ground into powder and placed on the rapid heating platform for warming. The dynamic effect of the design is shown in the Supplementary Video 3. Meanwhile, a temperature-controlled dynamic quick response (QR) code in the second route was also designed using (C19H42N)2MnCl4 and (C21H46N)2MnCl4, as shown in Fig. 4b. The encrypted QR code is displayed green under UV light irradiation; at this time, scanning the QR code yields no information at room temperature. Interestingly, when the temperature is controlled in the range of 333 ≤ T < 343 K, the luminescence for (C19H42N)2MnCl4 is quenched, but (C21H46N)2MnCl4 still emits intense green light. Scanning the QR code on the smartphone displays the information “Yunnan Minzu University” and a campus picture. Continued heating will cause the QR code to disappear, thus protecting the information. As shown in Supplementary Fig. S21, powder obtained by grinding the crystals was used to experimentally evaluate the practical feasibility of the second route. The results show that the experimental performance closely matched the design expectations, confirming the strong potential of the second route for practical implementation. Therefore, the anti-counterfeiting strategy, based on a narrow temperature-window response, provides the required information only within a specific narrow-band temperature range. Any unauthorized attempt to decrypt the code will face the dual challenges: either the temperature is too low to satisfy decryption requirements, or it is slightly too high to destroy the code. Moreover, as shown in Supplementary Fig. S22, after warming and cooling down to room temperature, the substance demonstrates delayed luminescence recovery and does not recover immediately, which two substances showed different PL recovery rates. After cooling naturally for 14 h, their luminescence recovers completely, which is in agreement with their P-XRD patterns (Supplementary Fig. S19). This significantly increases the time required to crack the code, thus improving the security and accuracy of the anti-counterfeiting process. This long recovery time significantly increases the time required to crack the code, thereby enhancing the security and reliability of the anti-counterfeiting process. In addition, the experiments of the rapidly cooling process with dry ice and ice demonstrate that the recovery time can be greatly shortened (Supplementary Videos 4–7), providing an optional rapid-recovery mode for situations where fast re-use of the encrypted patterns is required.

Fig. 4. Two precise temperature-responsive optical information encryption and anti-counterfeiting strategies based on phase-transition-induced PL quenching.

Fig. 4

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a The schematic diagram of the information encryption and decryption process based on the temperature-dependent PL responses of (C19H42N)2MnCl4 and (C21H46N)2MnCl4. b Temperature-responsive quick response (QR) code images under 365 nm UV light. (Illustration of the QR code encryption mechanism: the QR code pattern composed of (C21H46N)2MnCl4 acts as the information layer, while (C19H42N)2MnCl4 forms the encryption layer. The encoded information is decrypted via smartphone detection at specific temperatures (333 ≤ T < 343 K).

Combined with their significant thermal behaviors and PL behavior during the phase transition process, a visual thermal energy storage with a “fast charging and slow discharging mode” was designed in Fig. 5a. During the heat storage process, “PL ON” indicates that heat storage is not yet complete, while “PL OFF” corresponds to fully stored latent heat. This design eliminates the need for additional monitoring equipment and enables real-time visual monitoring of the working state of thermal energy storage devices, effectively reducing the operating cost of the system and providing intuitive and accurate status indication. In addition, the entire process exhibits rapid heat absorption and slow heat release characteristics, adding potential for practical applications. To evaluate the cyclic applicability of the materials, DSC thermal cycling tests were carried out (Fig. 5b, Supplementary Fig. S23). The results show that the DSC cycling curves of samples (C19H42N)2MnCl4 and (C21H46N)2MnCl4 exhibit a high degree of overlap, and the fluctuations in ΔHtrans and ΔStrans are small, indicating that the material has good stability and prospects for visual thermal energy storage application.

Fig. 5. Schematic illustration of visually thermal energy storage enabled by phase-transition-triggered PL switching.

Fig. 5

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a The PL ON/OFF state acts as a visual indicator of the heat-storage and heat-release process associated with the phase-transition latent heat. b The fluctuation of ΔHtrans values of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 after five DSC cycles.

Discussion

In summary, we have synthesized two 0D organic–inorganic hybrid Mn(II) chloride crystals, (C19H42N)2MnCl4 and (C21H46N)2MnCl4, which exhibit bright green PL upon UV excitation. A key point lies in the fact that by fine-tuning the length of the organic cationic alkyl chain, the phase transition temperature can be adjusted from 333 K to 343 K. By taking advantage of the PL quenching induced by the reversible phase transition, we propose a high-precision information-encryption and anti-counterfeiting strategy based on a narrow temperature window (ΔT = 10 K). Experimental demonstrations show that the temperature-window strategy is practical for anti-counterfeiting applications. Moreover, (C19H42N)2MnCl4 and (C21H46N)2MnCl4 combine intrinsic temperature-dependent PL modulation with latent-heat storage in a single system, suggesting potential for visualized thermal energy storage. This study delivers mechanistic insights into the structure-PL relationship across structural phase transitions and provides pioneering guidelines for advancing luminescent materials toward multifunctional applications in high-precision information encryption and visualized thermal energy storage.

Methods

Raw materials

Manganese chloride tetrahydrate (MnCl2·4H2O, 99.0%), N-Hexadecyltrimethylammonium chloride (C19H42NCl, 99.0%, Adamas-beta), octadecyl trimethyl ammonium chloride (C21H46NCl 98.0%, Adamas-beta), N, N-Dimethylformamide (DMF, 99.8%, Adamas-beta), isopropanol (C3H8O 99.8%, Adamas-beta), diethyl ether (C4H10O, 99.5%). MnCl2·4H2O was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. C4H10O was sourced from Chongqing Chuandong Chemical Co., Ltd. All chemicals were used directly without further purification.

Synthesis

(C19H42N)2MnCl4 crystal

In a typical procedure, 2 mmol of C19H42NCl and 1 mmol of MnCl2·4H2O were dissolved in 10 ml of DMF and stirred at 303 K for 5 h, the precursor solution was obtained by filtration. The solution and ether were then sealed in an airtight container and stored. The crystals were slowly precipitated as the ether slowly evaporated and continuously diffused into the precursor solution, consuming the DMF solvent in the process. The obtained crystals were washed three times each with ether, isopropanol to finally obtain light green (C19H42N)2MnCl4 crystals.

(C21H46N)2MnCl4 crystal

Similar to the synthesis of (C19H42N)2MnCl4 crystals, the target crystals were synthesized by using the anti-solvent method by replacing C19H42NCl with C21H46NCl, and the light-green (C21H46N)2MnCl4 crystals were obtained by washing three times with ethyl ether and isopropanol, respectively.

Fabrication and application of thermally controlled dynamic optical encryption codes and QR codes

The dried crystals were ground into fine powder, and their particle size distribution and surface morphology are shown in Supplementary Fig. S24. The pre-designed mold pattern was hollowed out by engraving and fixed onto a rapid heating platform. The powders of (C19H42N)2MnCl4 and (C21H46N)2MnCl4 were placed at designated positions within the mold and slightly pressed to ensure shaping. After removing the mold, the desired optical encryption codes and QR codes were obtained. By controlling the temperature of the heating platform, information was encrypted and decrypted. In the decrypted state, the optical encryption codes were converted from binary to decimal to reveal the true information, while the QR code could be scanned directly with a smartphone to access the encoded content.

Single-crystal X-ray diffraction

Data acquisition of two kinds of single crystals was performed on a SuperNova, AtlasS2 diffractometer with Cu-Kα radiation (λ = 1.54184 Å) using ω–scan technique at 293 K. Using Olex2, the structure was solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least Squares minimisation.

Characterizations

P-XRD was conducted on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The operating voltage, current, scanning rate, angular range, and step size were 40 kV, 40 mA, 10°/min, 10° − 50°, and 0.02°, respectively. Temperature-dependent X-ray diffraction of the samples was measured using a Dandong Tongda TD-3700; the temperature range was 303–348/358 K, and no special atmosphere was used during the test. The morphology and elemental composition were examined using FEI Nova NanoSEM 450 and ZEISS Sigma 300 SEM with an EDS. FTIR spectroscopy was recorded on the NICOLET-IS10 FTIR spectrometer. HRMS measurements were performed on a Waters ACQUITY UPLC I-Class/XEVO G2-XS Qtof-MS system equipped with an electrospray ionization (ESI) source. Elemental analysis (C, H, N) was carried out on a Thermo Scientific FlashSmart elemental analyzer. PL spectrum and PLE spectrum were obtained using an OmniFluo960SP steady-state/transient fluorescence spectrometer. Temperature-dependent emission spectrum, heating–cooling cycle test, and moisture resistance were obtained using a Hitachi F-7000 FL spectrophotometer. PL decay curves were recorded using a Hamamatsu Photonics Quantaurus-Tau C16361-2 fluorescence lifetime spectrometer. PLQY was measured on a Quantaurus-QY Plus C13534-12 UV-NIR absolute PL quantum yield spectrometer (Hamamatsu Photonics). TGA was performed on a NETZSCH STA 2500 simultaneous thermal analyzer heated from room temperature to 773 K under N2 atmosphere at a heating rate of 10 K/min. DSC ramp-up and ramp-down curves were performed using a NETZSCH DSC 214 Differential Scanning Calorimeter from 223–373 K under N2 atmosphere, five heating–cooling cycle tests were conducted under these conditions, with an interval of 36 h between each test. A Renishaw inVia Reflex confocal micro-Raman spectrometer was used to test the temperature-dependent Raman spectra of the samples. The 633 nm laser was selected, and the spectral range of the test was 50–3200 cm–1, and the temperature range of the test was 303–353/373 K, and no special atmosphere was introduced during the test.

Computational methodology

The electronic band structures and PDOS were calculated based on DFT by using the Vienna Ab initio Simulation Package (VASP). The generalized gradient approximation(GGA) of Perdew-Burke-Ernzerhof (PBE) is used to describe the exchange-correlation functional. The cut-off energy for the plane wave basis is set to 600 eV. The k-mesh is forced to be centered on the gamma point, and the k-mesh of 3x3x1 is used. All the structures were fully relaxed (atomic position) up to 10-6 eV /Å force minimization and max force of 0.01 eV/ Å.

Supplementary information

Supplementary Information (2.8MB, pdf)
41467_2026_71277_MOESM2_ESM.pdf (79.8KB, pdf)

Description of Additional Supplementary Files

Supplementary Video 1 (13MB, mp4)
Supplementary Video 2 (15MB, mp4)
Supplementary Video 3 (18.2MB, mp4)
Supplementary Video 4 (4.6MB, mp4)
Supplementary Video 5 (4.8MB, mp4)
Supplementary Video 6 (4.8MB, mp4)
Supplementary Video 7 (1.1MB, mp4)
Transparent Peer Review file (16KB, pdf)

Source data

Source Data (5.4MB, xlsx)

Acknowledgements

This work was financially supported by Yunnan Fundamental Research Program (202301AS070002 to Z.L.W, 202501AT070006 to Q.W) and National Natural Science Foundation of China (22165033 to Z.L.W, 22365034 to Q.Z).

Author contributions

A.B.L.: Methodology, Investigation, Writing - review & editing. Z.Q.C.: Investigation, Writing - review & editing. Z.L.W.: Conceptualization, Methodology, Writing - review & editing, Supervision, Funding acquisition. P.R.: Investigation, Methodology. Q.W.: Conceptualization, Supervision, Funding acquisition. Y.Y.Z.: Conceptualization, Methodology. Q.Z.: Writing - review & editing, Methodology. H.J.T.: Writing - review & editing, Methodology. All authors discussed and edited the paper.

Peer review

Peer review information

Nature Communications thanks the anonymous reviewer(s) for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data supporting the findings of this study are available within the Article and its Supplementary Information. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2359547 and 2405046. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. The Figshare DOI is: 10.6084/m9.figshare.31223542.All data are available from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Contributor Information

Zhengliang Wang, Email: wangzhengliang@foxmail.com.

Qin Wang, Email: wqin@foxmail.com.

Qiang Zhou, Email: q-zhou@ymu.edu.cn.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-026-71277-3.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary Information (2.8MB, pdf)
41467_2026_71277_MOESM2_ESM.pdf (79.8KB, pdf)

Description of Additional Supplementary Files

Supplementary Video 1 (13MB, mp4)
Supplementary Video 2 (15MB, mp4)
Supplementary Video 3 (18.2MB, mp4)
Supplementary Video 4 (4.6MB, mp4)
Supplementary Video 5 (4.8MB, mp4)
Supplementary Video 6 (4.8MB, mp4)
Supplementary Video 7 (1.1MB, mp4)
Transparent Peer Review file (16KB, pdf)
Source Data (5.4MB, xlsx)

Data Availability Statement

The data supporting the findings of this study are available within the Article and its Supplementary Information. Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition numbers 2359547 and 2405046. These data can be obtained free of charge from the CCDC via www.ccdc.cam.ac.uk/data_request/cif. The Figshare DOI is: 10.6084/m9.figshare.31223542.All data are available from the corresponding author upon request. Source data are provided with this paper.

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