English

• 0.   Introduction

  Multifunctional molecular materials with bistable switching behaviors have long been at the forefront of materials research, driven by their promising applications in next-generation information processing, high-density data storage, and related technologies, particularly in the current era of rapid information expansion^[1-4]. Among the diverse candidates explored, organic-inorganic hybrid materials stand out owing to their structural diversity, facile self-assembly, and synthetic tunability^[5-7]. Their structures can be readily modulated by external stimuli, including heat, light, electric fields, magnetic fields, and pressure, giving rise to switchable electronic, magnetic, and optical properties^[8-12]. Nevertheless, the rational design of multifunctional materials that operate within a targeted temperature range and exhibit synergistic dielectric, magnetic, and luminescent responses remains a substantial challenge.

  Low-dimensional hybrid metal halides of the general formula A₂BX₄ (A=organic ammonium cation, B=divalent metal ion, X=halide anion) have been widely investigated as a versatile platform for such functionalities^[13-17]. In these systems, the organic moiety typically undergoes structural rearrangement in response to external stimuli, while the inorganic component governs the intrinsic physical properties. Notably, phase transitions near the switching temperature can induce abrupt changes in multiple physical channels^[18-21]. Among this family, zero-dimensional (0D) compounds are particularly attractive due to their flexible and tunable structures, which provide an ideal playground for developing high-performance multifunctional materials^[22-24]. Additionally, most of these materials are lead-free, further broadening their application potential. Mn²⁺ ions impart rich physical characteristics to the hybrids, including magnetism, luminescence, and dielectric switching. To date, it has been feasible to integrate two or even three functionalities into low-dimensional manganese-based halides that undergo structural phase transitions^[25-26]. The photoluminescence in these compounds arises primarily from spin-orbit coupling of Mn²⁺, which induces significant mixing between the ground-state (t_2g)³(e_g)² and excited-state (t_2g)⁴(e_g)¹ configurations^[27-30]. Meanwhile, the unpaired d electrons mediate magnetic interactions, and the switchable dielectric properties render them excellent candidates for dielectric switching devices^[31-33]. Consequently, low-dimensional manganese-based halides offer a promising and feasible route toward multifunctional molecular materials. Despite our group′s previous success in developing a series of hybrid crystals that undergo first-order structural phase transitions and exhibit multiple synergistic switching responses^[34-37], understanding of how to precisely control the phase-transition temperature in this system, as well as the underlying mechanisms by which the phase transition affects the magnetic, dielectric, and luminescent properties, remains limited.

  Following this strategy, we successfully synthesized a 0D manganese-based hybrid (TMBE)₂[MnBr₄] (1) using (2-bromoethyl)trimethylammonium bromide ((TMBE)Br) and manganese bromide, which showed a high phase transition temperature of 413 K. Differential scanning calorimetry (DSC) and dielectric measurements reveal that 1 undergoes first-order phase transitions arising from order-disorder changes of TMBE⁺ cations. Compared with (TMCE)₂[MnBr₄] (TMCE⁺=(Et₃NCH₂Cl)⁺) that we previously reported^[23], the Br-substituted 1 possesses a much higher phase transition temperature, owing to reinforced intermolecular noncovalent interactions and an elevated cation flipping energy barrier. Variable-temperature luminescence spectra reveal significant intensity changes near the phase transition temperature of 344 K, accompanied by obvious thermal hysteresis. 1 represents a rare example that integrates luminescent, dielectric, and magnetic synergistic switchable responses in a single-phase molecular material, indicating its great potential in multifunctional switching applications and offering an alternative approach for designing target-temperature multifunctional molecular materials.

  1.   Experimental

  1.1   Materials and instruments

  All solvents and materials are commercially available and used without further purification. A Rigaku Synergy single-crystal diffractometer was used to obtain diffraction data via ω scan mode with Mo Kα radiation (λ=0.071 073 nm). The obtained diffraction data were directly analyzed using OLEX2 software, and structural refinement was further performed with the SHELXT program. The powder X-ray diffraction (PXRD) patterns of 1 were measured by the Rigaku D/MAX diffractometer with Cu Kα radiation (λ=0.154 nm), operating at 40 kV and 15 mA, over a 2θ range of 5°-50° at a scanning rate of 10 (°)·min^-1. The experimental peak of the compound was consistent with the simulated peak, indicating that the compound is pure (Fig.S1, Supporting information). The thermogravimetric analysis (TGA) measurement of 1 was performed using a PerkinElmer TGA 8000. DSC measurement of 1 was carried out with a NETZSCH DSC 214 Polyma instrument. The heating/cooling process of the compound was carried out in a nitrogen environment at a rate of 20 K·min^-1. The dielectric constant (ε=ε′-iε″) was measured using a TH2828A variable temperature dielectric meter. The test frequency was 10 kHz, and the applied voltage was 1.0 V. The magnetization rate of 1 was measured by the MPMS-XL7 SQUID magnetometer. The FLS-980 fluorescence spectrophotometer (Edinburgh Instruments, UK) was employed to measure the excitation spectra, emission spectra, fluorescence lifetimes, and quantum yield.

  1.2   Synthesis of compound 1

  MnBr₂·4H₂O (2 mmol) and choline bromide (4 mmol) were weighed and dissolved in concentrated HBr (5 mL) solution. The mixture was then placed on a heating table (50 ℃) to evaporate and crystallize. Finally, greenish-yellow crystals were obtained after a week. Elemental analyses Calcd. for (C₅NH₁₃Br)₂MnBr₄(%): C, 16.95; H, 3.70; N, 3.95. Found(%): C, 16.93; H, 3.72; N, 3.91.

  2.   Results and discussion

  2.1   Thermal analyses

  DSC was performed to characterize the reversible phase transitions of 1 (Fig.1). During the heating process, two endothermic peaks appeared at 344 and 413 K, while during cooling, two exothermic peaks appeared at 335 and 389 K. The nearly equal enthalpy changes in heating and cooling processes confirm the reversible nature of these phase transitions. Quantitative analysis of peak areas verifies two closely spaced structural phase transitions at 344 and 413 K. Accordingly, 1 experienced thermodynamically reversible phase transitions in the range of 310-430 K with thermal hysteresis widths of 9 and 24 K, respectively, characteristic of first-order transitions. The corresponding enthalpy changes (ΔH) were 1.849 and 3.934 kJ·mol^-1, and entropy changes (ΔS) were 5.37 and 9.52 J·mol^-1·K^-1. Based on the Boltzmann equation ΔS=Rln N, the calculated values of N were 1.92 and 3.14, respectively, indicating increased structural disorder after phase transitions. Notably, compared with (TMCE)₂[MnBr₄], the maximum phase transition temperature increases from 364 to 413 K, corresponding to an increment of 49 K. TGA showed that 1 exhibited high thermal stability, with decomposition starting at approximately 487 K (Fig.S2).

  Figure 1

  

  Figure 1.  DSC data of compound 1 from 312 to 432 K

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  2.2   Single-crystal structure analysis and Hirshfeld analysis

  Single-crystal X-ray diffraction data were collected at 278 K because high-temperature data show severe degradation. 1 crystallizes in the orthorhombic crystal system with the Pca2₁ space group (Table S1). The asymmetric unit contains one [MnBr₄]^2- anion and two TMBE⁺ cations, all in well-ordered states (Fig.2a). The Mn—Br bond lengths range from 0.249 8 to 0.251 3 nm, and Br—Mn—Br bond angles vary from 107.43(8)° to 111.93(9)° (Table S2), indicating [MnBr₄]^2- tetrahedron is slightly distorted compared to the standard tetrahedron, which is also observed in (TMCE)₂[MnBr₄]. Similarly, the separated [MnBr₄]^2- anion together with the discrete organic TMBE⁺ cations also form a 0D structure, which is stabilized by interionic electrostatic interaction, C—H…Br and Br…Br interaction (Fig.2b and S3). However, as summarized in Table 1, 1 exhibits remarkably compressed interatomic distances relative to (TMCE)₂[MnBr₄]. Specifically, the nearest Mn…Mn distance, the shortest halogen distance, and the minimum C—H…Br bond length (C…Br) for 1 are 0.796, 0.379, and 0.283 nm, all shorter than those of (TMCE)₂[MnBr₄], whose corresponding structural parameters are 0.836, 0.403, and 0.293 nm, respectively. These structural variations primarily modulate intermolecular interactions, thereby inducing corresponding changes in phase transition behavior and physical properties. This phenomenon has not only been well documented in the literature but has also been confirmed by our subsequent experiments^[38-41].

  Figure 2

  

  Figure 2.  Molecular structure of compound 1 at 278 K: (a) asymmetric structural unit; (b) packing diagram

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  Table 1

  

  Table 1.  Some parameters of (TMBE)₂[MnBr₄] (1) and (TMCE)₂[MnBr₄]

  下载: 导出CSV

  Parameter	(TMBE)2[MnBr4]	(TMCE)2[MnBr4]
  d(Mn…Mnnearest) / nm	0.795 54	0.863 35
  d(X…Brshortest)* / nm	0.378 67	0.403 27
  d(C…Br) / nm	0.283-0.291	0.293-0.327
  Effective volume / nm3	0.361 09	0.353 22
  dnorm	0.496 4	0.529 4
  Tp, max / K	413	364
  * X=Cl/Br.

  To further confirm the occurrence of a reversible phase transition in 1, the well-ground powder was used to determine the variable-temperature PXRD. As shown in Fig.3, the PXRD patterns of 1 did not change at 303 and 323 K, but when the temperature rose to 343 K, some changes were observed in the PXRD pattern. New diffraction peaks emerged at 11.42°, 13.04°, 13.36°, and 18.74°, indicating that a structural phase transition might be occurring at this point. At 363 K, the diffraction peaks at 13.8°, 14.84°, 17.42°, 18.08°, 19.54°, 23.9°, 25.04°, 29.96°, 35.88°, 37.7°, and 42.38° disappeared. When the temperature further increased to 403 K, no obvious changes occurred in the PXRD pattern, suggesting that the first structural phase transition was completed at 363 K. At 423 K, the two diffraction peaks near 13.26° merged into a single peak. Meanwhile, the intensities of the diffraction peaks between 20° and 30° diminished or even vanished, and new diffraction peaks emerged at 22.96° and 26.58°, showing that the PXRD pattern underwent a second transformation. With the temperature further rising, there were no additional changes observed in the PXRD pattern, indicating that compound 1 had completed its second transformation at 423 K. The emergence of new peaks, as well as the changes in the diffraction peaks, suggests that the structure of compound 1 has changed and confirms the thermal reversibility of the phase transition in 1. These observations are consistent with the results obtained from DSC measurements.

  Figure 3

  

  Figure 3.  Variable-temperature PXRD patterns of compound 1

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  To reveal the effect of halogen substitution on intermolecular interactions, the Hirshfeld surface and 2D fingerprint plot were calculated using Crystal Explorer (Fig.4). The average d_norm value and effective volume of 1 are 0.496 4 and 0.361 09 nm³, respectively. Compared with (TMCE)₂[MnBr₄], whose d_norm value and effective volume are 0.529 4 and 0.353 22 nm³, the larger effective volume indicates enhanced steric effect, while the smaller d_norm value suggests stronger intermolecular interactions between organic cations and inorganic anions (Fig.S4 and Table 1). These interactions increase the energy barrier for phase transitions and thus elevate the phase transition temperature, which is consistent with DSC and dielectric measurements.

  Figure 4

  

  Figure 4.  (a) Hirshfeld surface and (b) 2D fingerprint plot of compound 1

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  2.3   Dielectric properties

  Temperature-dependent dielectric measurements were performed to characterize the phase transitions. At 100 kHz, the dielectric constant (ε′) showed a step-like anomaly near 340 K, increasing rapidly from 8.56 to 11.08 during heating. With further heating, ε′ gradually rose to 14.06 and then increased abruptly to 16.12 at approximately 406 K, finally reaching 17.03. During cooling, ε′ dropped sharply from 16.17 to 13.36 at 406 K, followed by minor anomalies at 344 and 316 K (Fig. 5a). These reversible dielectric anomalies were clearly observed, consistent with DSC results and confirming reversible phase transitions. Although we have not obtained a high-temperature single-crystal structure, we can infer from the previously reported Mn-based hybrid metal halides (e.g., (TMCE)₂[MnBr₄]) that the phase transitions may be related to the rearrangement and disorder of organic cations. Particularly, frequency-dependent measurements (0.5 kHz-1 MHz) in Fig. 5b showed that ε′ decreased with increasing frequency while the anomaly positions remained unchanged. With the substitution of Cl^- with Br^- in the (TMXE)⁺ cation, the temperature evolution of dielectric constants further verifies the structural origin of dielectric switching.

  Figure 5

  

  Figure 5.  (a) Dielectric curve of compound 1 at 100 kHz; (b) ε′ values of 1 at different frequencies

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  2.4   Magnetic properties

  The magnetic properties of 1 were investigated using temperature-dependent magnetic susceptibility measured from 2 to 300 K under 1 000 Oe. The χ_mT value at 300 K was 4.417 cm³·mol^-1·K, which slowly decreased to 4.204 cm³·mol^-1·K at 15 K and then dropped rapidly to 3.862 cm³·mol^-1·K at 2 K (Fig.6a). The slight decrease at low temperatures indicates weak antiferromagnetic interactions between Mn²⁺ ions. Fitting with the Curie-Weiss law gave 4.413 cm³·mol^-1·K of C and -2.035 K of θ (Fig. 6a, inset). Field-dependent magnetization at 2-10 K reached 4.909μ_B at 7 T, nearly the theoretical saturation value of 5μ_B for Mn²⁺, which is attributed to zero-field splitting (ZFS) rather than significant antiferromagnetic coupling (Fig. 6b). Fitting using Eq.1 by the PHI program yielded 0.12 cm^-1 of D, 2.001 of g, and 1.62×10^-5 cm³·mol^-1 of TIP (temperature-independent paramagnetism):

  $ \widehat{H}=D\left[{\widehat{S}}_{z}^{2}-\frac{s\left(s+1\right)}{3}\right]+g{\mu }_\text{B}\widehat{S}\bullet H $

  (1)

  where $ \widehat{H} $ is the effective spin Hamiltonian, D is the axial zero-field splitting parameter, $ {\widehat{S}}_{z} $ is the operator for the z-component of the spin angular momentum, s is the total spin quantum number of the system, g is the effective electron g-factor, $ \widehat{S} $ is the total spin vector operator, and H is the applied external magnetic field.

  Figure 6

  

  Figure 6.  (a) Temperature dependence of χ_mT for compound 1; (b) Field dependence of the magnetization strength of 1; (c) Temperature dependence of χ_mT during heating/cooling

  Inset: temperature dependence of χ_m^-1 for 1.

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  High-temperature magnetic susceptibility was further measured during heating and cooling (Fig.6c). The χ_mT value increased gradually to 4.451 cm³·mol^-1·K at 357 K and then decreased abruptly to 4.441 cm³·mol^-1·K during heating. During cooling, it decreased to 4.431 cm³·mol^-1·K at 347 K and then increased abruptly to 4.436 cm³·mol^-1·K. Similar to the thermally responsive magnetic switching observed in (TMCE)₂[MnBr₄], 1 exhibited comparable magnetic switching behavior, demonstrating magneto-structural coupling. The subtle χ_mT changes exclude spin-state transitions, and large Mn…Mn distances rule out strong magnetic coupling. Accordingly, the reversible magnetic switching originates from structural phase transitions, which induce subtle changes in Mn…Mn distances, Br—Mn—Br angles, and spatial reconstruction.

  2.5   Luminescence response

  Compound 1 emitted intense green luminescence under UV irradiation (Fig.S5, inset). The excitation spectra showed four peaks at 278, 293, 364, and 377 nm, corresponding to ⁶A₁(S)→⁴A₂(F), ⁶A₁(S)→⁴T₁(F), ⁶A₁(S)→⁴E(D), and ⁶A₁(S)→⁴T₂(D) transitions of Mn^{2+ [42-44]}. A single emission peak at 514 nm was observed under different excitation wavelengths, which is similar to that of the Mn-based halides reported previously (Fig.7a). The CIE coordinates (0.190 9, 0.698 4) confirm the green emission (Fig.S5). In addition, the photoluminescence quantum yield (PLQY) of 1 was 27.71% under ambient conditions (Fig.S6), and its lower PLQY relative to (TMCE)₂[MnBr₄] originates from the shorter Mn…Mn distance (Table 1)^[45]. The fluorescence lifetime was 0.234 ms at room temperature (Fig.7c).

  Figure 7

  

  Figure 7.  (a) Excitation spectra and emission spectra of compound 1; (b) Temperature-dependent emission spectra of 1; (c) Fluorescence lifetime of 1

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  Variable-temperature luminescence was measured from 290 to 400 K (Fig.7b and S7). The emission intensity gradually decreased with increasing temperature and dropped rapidly near the phase transition temperature due to enhanced lattice vibration and non-radiative relaxation. The intensity recovered during cooling, demonstrating favorable thermal switching behavior. These results reveal that 1, designed by cation halogen substitution, possesses excellent synergistic dielectric, magnetic, and luminescent responses, with tri-functional behaviors identical to those of (TMCE)₂[MnBr₄].

  3.   Conclusions

  In summary, a zero-dimensional organic-inorganic manganese-based hybrid (TMBE)₂[MnBr₄] with high-temperature phase transitions has been successfully constructed via halogen substitution of organic cations. Reversible order-disorder phase transitions are confirmed by DSC and dielectric measurements, with the highest transition temperature reaching 413 K. Hirshfeld surface analysis reveals that enhanced intermolecular interactions increase the phase transition energy barrier. (TMBE)₂[MnBr₄] exhibits integrated switchable dielectric, magnetic, and green luminescent responses synergistically triggered by structural phase transitions, which provides a new paradigm for the exploration of advanced multifunctional molecular materials.

  
  Supporting information is available at http://www.wjhxxb.cn
  
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  Figure 1  DSC data of compound 1 from 312 to 432 K

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  Figure 2  Molecular structure of compound 1 at 278 K: (a) asymmetric structural unit; (b) packing diagram

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  Figure 3  Variable-temperature PXRD patterns of compound 1

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  Figure 4  (a) Hirshfeld surface and (b) 2D fingerprint plot of compound 1

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  Figure 5  (a) Dielectric curve of compound 1 at 100 kHz; (b) ε′ values of 1 at different frequencies

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  Figure 6  (a) Temperature dependence of χ_mT for compound 1; (b) Field dependence of the magnetization strength of 1; (c) Temperature dependence of χ_mT during heating/cooling

  Inset: temperature dependence of χ_m^-1 for 1.

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  Figure 7  (a) Excitation spectra and emission spectra of compound 1; (b) Temperature-dependent emission spectra of 1; (c) Fluorescence lifetime of 1

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  Table 1.  Some parameters of (TMBE)₂[MnBr₄] (1) and (TMCE)₂[MnBr₄]

  Parameter	(TMBE)2[MnBr4]	(TMCE)2[MnBr4]
  d(Mn…Mnnearest) / nm	0.795 54	0.863 35
  d(X…Brshortest)* / nm	0.378 67	0.403 27
  d(C…Br) / nm	0.283-0.291	0.293-0.327
  Effective volume / nm3	0.361 09	0.353 22
  dnorm	0.496 4	0.529 4
  Tp, max / K	413	364
  * X=Cl/Br.

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