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• In recent years, organic-inorganic metal halides (OIMHs) have received intensive and continuous attention owing to their unique photoluminescence and electroluminescence properties in various fields such as solar cells, light-emitting diodes (LEDs), biological imaging, anti-counterfeiting labeling, scintillators, and sensors [1–10]. Among them, lead (Pb)-based metal halides have shown remarkable potentials in liquid crystal backlight displays and solid-state lighting due to their excellent optical properties (high photoluminescence quantum yield, narrow emission linewidth, tunable optical bandgap), outstanding defect tolerance and simple synthesis procedures [11,12]. Despite the great advances of Pb-based metal halides, they still suffers from two major problems, i.e., the poor stability and the potential toxicity of the heavy metal Pb, which have become the biggest obstacles to their large-scale commercial applications [13,14]. Therefore, the development of new environmentally friendly, efficient, and stable metal halide materials with excellent luminescence property has become an imminent task [15–18].

  Manganese (Mn)-based OIMHs offer not only the key advantages of Pb-based materials, such as high emission quantum efficiency, facile synthesis, and low cost, but also exhibit long-lived phosphorescence emission, friction luminescence, and low toxicity. These attributes make them ideal alternatives to Pb-based metal halide fluorescent materials [19–22]. In general, Mn-based OIMHs can achieve green and red emission by changing the crystal field strength. According to the crystal field theory, the coordination of Mn²⁺ ions with organic molecules has a great influence on the optical properties. Due to the change of coordination environment, D-state leaps from Mn²⁺ ions ⁴T₁–⁶A₁ can relax with different emission colors [23–26]. The octahedrally coordinated Mn(Ⅱ) with higher field strengths exhibits low-energy red emission, while the tetrahedrally coordinated Mn(Ⅱ) with lower field strengths exhibits high-energy green emission [27,28].

  Human eyes are more sensitive to green and can even distinguish slight changes in hue [29]. Under this circumstance, it is crucial to design high-performance green emitters with the desired emission wavelength and bandwidth, high photoluminescence quantum yield (PLQY), and excellent thermal stability [30,31]. Generally, the OIMHs always display broad emission spectra due to the strong electron-phonon coupling in their soft and deformable lattices [32]. However, Mn-based OIMHs generally maintain relatively fixed emission characteristics. This is because the luminescence properties of Mn²⁺ ions are primarily governed by their coordination environment and exhibit relatively low sensitivity to lattice distortion [33]. That is, modulating polyhedral distortion through strategic selection of organic ligands allows for precise control over emission peak positions in Mn-based OIMHs. Additionally, choosing different organic cations can control the inter-Mn tetrahedron distances, thereby suppressing excitation energy transfer between adjacent Mn²⁺ centers and enhancing the overall luminescent efficiency. Therefore, by selecting different organic cations to fine-tune the structural distortion of Mn-coordinated tetrahedron and their distance, ideal green light emission can be achieved.

  Inspired by the polyhedron regulation in OIMHs, we selected 1-methylpiperidine C₆H₁₃N and 4‑chloro-1-methylpiperidine C₆H₁₂CIN as organic ligands to combine with Mn²⁺ ions, synthesizing two 0D compounds, [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄. Structural and luminescence analysis results indicate that both compounds contain isolated [MnBr₄]^2- tetrahedron, leading to bright green emission. The structural transformation caused by changes in organic cations not only enables fine-tuning of the emission color but also increases the distance between [MnBr₄]^2- tetrahedron, effectively suppressing excessive energy dissipation and yielding a nearly doubled PLQY (from 38.8% to 74.5%). Further experimental and theoretical analyses reveal that, compared to [C₆H₁₄N]⁺, the introduction of [C₆H₁₃NCl]⁺ increased the H···Cl interactions, significantly improving the environmental stability of [C₆H₁₃NCl]₂MnBr₄. Under ambient conditions (30%−60% RH and 10–23 ℃), the crystallinity of [C₆H₁₃NCl]₂MnBr₄ remains unchanged after being placed for 4 months. Moreover, [C₆H₁₃NCl]₂MnBr₄ exhibits an emission color closer to the standard green light compared to [C₆H₁₄N]₂MnBr₄. Our work provides insights into the polyhedral regulation of luminescence properties in OIMHs and offers guidance for the design and optimization of novel, efficient, and stable metal halide luminescent materials.

  Two single-crystal compounds were obtained through a cooling crystallization method by selecting C₆H₁₃N and C₆H₁₂CIN cations, respectively. The bulk crystals of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ appear green and transparent under natural light. Both of them exhibit bright green light emission under 365 nm excitation (Fig. S1 in Supporting information). Single-crystal data analysis (Table S1 in Supporting information) revealed that [C₆H₁₄N]₂MnBr₄ belongs to the monoclinic crystal system with a centrosymmetric space group of P2₁/c (Fig. 1, left), while [C₆H₁₃NCl]₂MnBr₄ belongs to the orthorhombic crystal system with a polar space group of Pna2₁ (Fig. 1, right). The PXRD patterns of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ match well with their simulated XRD patterns, confirming the high purity of these synthesized compounds (Fig. S2 in Supporting information). Detailed analysis of the single-crystal structures indicates that both compounds exhibit a typical 0D crystal structure. In this structure, four Br⁻ ions coordinate with one Mn²⁺ ion to form an isolated [MnBr₄]^2- tetrahedral unit. The cations ([C₆H₁₄N]⁺ or [C₆H₁₃NCl]⁺) not only separate the tetrahedral units within the structure but also serve to maintain charge balance. Due to the introduction of chlorine atoms, the Mn-Mn distance increases from 8.061 Å to 8.668 Å.

  Figure 1

  

  Figure 1.  Schematic demonstration of the crystal structures of [C₆H₁₄N]₂MnBr₄ (left) and [C₆H₁₃NCl]₂MnBr₄ (right).

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  In order to study the mechanism of luminescence, photoluminescent excitation (PLE), photoluminescent (PL) and ultraviolet-visible (UV–vis) absorption spectra were collected based on the two compounds. As a result, the PLE spectrum of [C₆H₁₄N]₂MnBr₄ reveals multiple absorption transitions at 277, 292, 363, 376, 437, 453, and 470 nm (Fig. 2a). These transitions correspond to the following Mn²⁺ electronic transitions: ⁶A₁(S) → ⁴A₂(F), ⁶A₁(S) → ⁴T₁(F), ⁶A₁(S) → ⁴E(D), ⁶A₁(S) → ⁴A₁, ⁴E(G), ⁶A₁(S) → ⁴T₂(G), and ⁶A₁(S) → ⁴T₁(G), respectively [34]. The absorption spectrum of [C₆H₁₄N]₂MnBr₄ (Fig. 2b) also confirms the existence of these transition absorptions. The optical band gap of [C₆H₁₄N]₂MnBr₄ is calculated to be approximately 2.38 eV based on the Kubelka–Munk (K–M) function. Furthermore, under excitation at 363 nm, [C₆H₁₄N]₂MnBr₄ exhibits narrowband green emission at 533 nm with a full width at half maximum (FWHM) of 62 nm. The emission peak positions under different excitation wavelengths (Fig. S3a in Supporting information) remain nearly unchanged, confirming that [C₆H₁₄N]₂MnBr₄ has a consistent green emission source originating from a single luminescent center. Furthermore, the PL decay curves of [C₆H₁₄N]₂MnBr₄ (Fig. S4a in Supporting information, λ_ex = 363 nm, λ_em = 533 nm) exhibit a single exponential decay profile. The fitted millisecond-scale lifetime (0.27 ms) matches the characteristic radiative transition of Mn²⁺ ions, directly proving that the 533 nm emission arises from Mn²⁺ centers [35].

  Figure 2

  

  Figure 2.  (a) PLE and PL spectra of [C₆H₁₄N]₂MnBr₄. (b) UV–vis absorption spectrum and optical band gap of [C₆H₁₄N]₂MnBr₄. (c) PLQY of [C₆H₁₄N]₂MnBr₄. (d) PLE and PL spectra of [C₆H₁₃NCl]₂MnBr₄. (e) UV–vis absorption spectrum and optical band gap of [C₆H₁₃NCl]₂MnBr₄. (f) PLQY of [C₆H₁₃NCl]₂MnBr₄.

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  Although the PLE spectrum of [C₆H₁₃NCl]₂MnBr₄ is roughly the same as that of [C₆H₁₄N]₂MnBr₄, the PL emission characteristics of [C₆H₁₃NCl]₂MnBr₄ are significantly different from that of [C₆H₁₄N]₂MnBr₄, with the PL peak center wavelength shifted to 522 nm and the FWHM reduced to 48 nm (Fig. 2d). In addition, the light absorption range of [C₆H₁₃NCl]₂MnBr₄ is slightly broader than that of [C₆H₁₄N]₂MnBr₄ (Fig. 2e). Its optical band gap is calculated to be approximately 2.25 eV based on the K-M function [36]. The excitation dependent spectra and the fitting results of PL decay lifetime of [C₆H₁₃NCl]₂MnBr₄ also prove that the emission comes from Mn²⁺ ion (Figs. S3b and S4b in Supporting information). The blue shift in peak position and the narrowing of the FWHM can be attributed to the chlorination effect, which causes [C₆H₁₃NCl]⁺ to occupy a larger space compared to [C₆H₁₄N]⁺, leading to an enhanced ordering in the [C₆H₁₃NCl]₂MnBr₄ crystal. This results in an increase in the Mn-Mn distance and enhanced ligand electronegativity, which intensifies the splitting of Mn²⁺ ion energy levels, thereby increasing the excitation energy and causing the blue shift. At the same time, the interaction and energy dissipation between Mn²⁺ ions are reduced, leading to a more concentrated emission and a narrower FWHM. Additionally, the reduction in the distortion of the [MnBr₄]^2- tetrahedron, which weakens the electron-phonon coupling, also contributes to the blue shift and narrowing of the FWHM (Fig. S5 in Supporting information).

  In addition to the change in PL characteristics, the PLQY of [C₆H₁₃NCl]₂MnBr₄ nearly doubled compared to [C₆H₁₄N]₂MnBr₄, increasing from 38.8% to 74.5% (Figs. 2c and f). This significant change in PLQY can be attributed to the effect of different cations on crystal structure. Specifically, compared to [C₆H₁₄N]⁺, the substitution of a hydrogen atom with a chlorine atom in [C₆H₁₃NCl]⁺ causes a change in the electron cloud, making [C₆H₁₃NCl]⁺ occupy a larger space. This change leads to an increase in the nearest Mn-Mn distance from 8.061 Å to 8.668 Å. The degree of overlap between electron clouds decreases, and the dipole-dipole interaction and the symmetry-guided spin-exchange interaction are weakened, resulting in a reduction in the energy transfer process between adjacent Mn²⁺ centers [37]. This is equivalent to reducing concentration quenching without the need to use dopant activator ions, which is beneficial for achieving a higher PLQY. Additionally, a double-peak phenomenon appears in Fig. 2c. This is due to the significant distortion of the [MnBr₄]^2- tetrahedron in [C₆H₁₄N]₂MnBr₄, which creates a complex local lattice environment and enhances electron-phonon coupling (manifested as a large S value), promoting the stable formation of self-trapped excitons (STEs). These STEs recombine through different energy valleys or phonon-assisted channels, generating a secondary emission peak above 600 nm in addition to the intrinsic peak, resulting in a double peak. In [C₆H₁₃NCl]₂MnBr₄, the introduction of chloride atoms alters the interaction between the cations and [MnBr₄]^2- anions, suppressing excessive tetrahedral distortion. This weakens the electron-phonon coupling (a small S value), making it difficult to stabilize multi-channel recombination of STEs, and only the intrinsic single peak is observed.

  To further explore the electron-phonon coupling of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄, we investigated their temperature-dependent PL in a range of 80–300 K (Figs. 3a and d). As the temperature increased, the PL intensities of both [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ gradually decreased. This phenomenon is due to the fact that the non-radiative recombination centers become active upon heating and capture more photogenerated carriers, thus reducing the probability of radiative transitions. Moreover, compared with [C₆H₁₃NCl]₂MnBr₄, the PL peak position of [C₆H₁₄N]₂MnBr₄ exhibited an obvious blue shift, and the FWHM of the emission band increased significantly with the increase in temperature. Generally speaking, in a 0D structure, the blue shift is mainly caused by the decrease in crystal field strength resulting from the thermal expansion of the host lattice [38]. The compound [C₆H₁₄N]₂MnBr₄ exhibits more pronounced thermal expansion due to differences in the cation structures and interactions. The [C₆H₁₄N]⁺ cations primarily interact through weak van der Waals forces, resulting in a highly flexible structure. Upon heating, thermal motion easily breaks these weak interactions, causing a significant expansion of the lattice. In contrast, the [C₆H₁₃NCl]⁺ cations experience stronger interactions, facilitated by the chloride atoms. These interactions enhance both the cation-cation and cation-anion bonds, thus restricting the lattice and minimizing thermal expansion. Consequently, [C₆H₁₄N]₂MnBr₄ shows significant thermal expansion, which leads to a blue shift in its luminescence. In addition, the increase in temperature also leads to more excited electrons diffusing to higher vibrational energy levels and then undergoing radiative transitions back to the ground state from these different energy levels, thereby widening the FWHM of the emission band.

  Figure 3

  

  Figure 3.  (a) Temperature-dependent PL spectra of [C₆H₁₄N]₂MnBr₄. (b) FWHMs of [C₆H₁₄N]₂MnBr₄ as a function of temperature under heating conditions. (c) PL intensities of [C₆H₁₄N]₂MnBr₄ as a function of 1/T under heating conditions. (d) Temperature-dependent PL spectra of [C₆H₁₃NCl]₂MnBr₄. (e) FWHMs of [C₆H₁₃NCl]₂MnBr₄ as a function of temperature under heating conditions. (f) PL intensities of [C₆H₁₃NCl]₂MnBr₄ as a function of 1/T under heating conditions.

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  Next, we analyzed two key values related to temperature. First, the Huang-Rhys factor S is determined by fitting the below formula [39].

  $ \operatorname{FWHM}(T)=2.36 \sqrt{S} \hbar \omega_{\text {phonon }} \sqrt{\operatorname{coth} \frac{\hbar \omega_{\text {phonon }}}{2 K_{\mathrm{B}} T}} $

  (1)

  where FWHM(T) is the full width at half maximum at the experimental temperature, S is electron-phonon coupling, ℏω_phonon is the phonon frequency, K_B is the Boltzmann constant, and T is the experimental temperature. Based on the fitting of Eq. 1, the Huang-Rhys factors (S) for [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ are calculated to be 15.85 and 7.89, respectively (Figs. 3b and e). A comparison reveals that the chlorination effect leads to a significant reduction in the electron-phonon coupling in [C₆H₁₃NCl]₂MnBr₄. This is due to the fact that chlorination makes the [MnBr₄]^2- tetrahedral structure more regular, reducing the localized phonon modes induced by lattice distortion, weakening the electron-phonon energy exchange, and consequently leading to a significant decrease in the S value from the PL spectrum fitting. This confirms the correlation between structural distortion and coupling strength. This result also supports the inference that the weakening of the electron-phonon coupling contributes to the blue shift in the emission peak and the narrowing of the FWHM. The reduction in electron-phonon coupling can be attributed to the larger space occupied by [C₆H₁₃NCl]⁺, which enhances the structure ordering and reduces the distortion of the [MnBr₄]^2- tetrahedron, thereby decreasing the interaction between electrons and lattice vibrations [40,41]. The relationship between PL intensity and temperature was further fitted using the Arrhenius equation:

  $ I(t)=I_0 /\left(1+\mathrm{A} e^{-E_{\mathrm{b}} / T K_{\mathrm{B}}}\right) $

  (2)

  where I(t) is the PL intensity at the experimental temperature, I₀ is the PL intensity at 0 K, A is a constant, K_B is the Boltzmann constant, E_b is the exciton binding energy. By applying Eq. 2 for fitting, the fitted E_b value of [C₆H₁₄N]₂MnBr₄ is 88.88 meV (Fig. 3c), and the fitted E_b value of [C₆H₁₃NCl]₂MnBr₄ is 59.01 meV (Fig. 3f). Both of these values are higher than the room temperature thermal energy (~25 meV), indicating that both materials can stably form excitons. A larger E_b value means that excitons do not easily dissociate before radiative recombination, thus increasing the probability of radiative recombination and facilitating efficient luminescence. [C₆H₁₃NCl]₂MnBr₄ exhibits a higher quantum yield because, on one hand, the introduction of chlorine atoms increases the Mn-Mn distance, reducing energy dissipation between adjacent tetrahedra. On the other hand, the weaker interaction between electrons and lattice vibrations reduces the non-radiative recombination probability and increases the radiative recombination probability. In contrast, [C₆H₁₄N]₂MnBr₄, although having higher exciton stability, has enhanced electron-phonon coupling, which increases the potential non-radiative transition pathways, thus limiting its quantum yield.

  To further clarify the PL mechanism, we investigated their electronic structures and energy transition mechanisms. Firstly, we analyzed the band structures of [C₆H₁₄N]₂MnBr₄ (Fig. 4a) and [C₆H₁₃NCl]₂MnBr₄ (Fig. 4b) adopting the density functional theory (DFT) calculations. For [C₆H₁₄N]₂MnBr₄, the valence band maximum (VBM) and the conduction band minimum (CBM) are not located at the same K-point, which indicates that this material has an indirect band gap. This means that phonons are required to participate in the recombination of electrons and holes for luminescence, so its luminous efficiency will be affected to some extent. However, for [C₆H₁₃NCl]₂MnBr₄, the VBM and CBM are located at the same K-point, demonstrating an obvious direct band gap characteristic. This implies that when the material absorbs energy, electrons can directly transit from the valence band to the conduction band, making it easier to generate photons.

  Figure 4

  

  Figure 4.  (a) Electronic band structure of [C₆H₁₄N]₂MnBr₄. (b) Electronic band structure of [C₆H₁₃NCl]₂MnBr₄. (c) Schematic diagram of the energy transfer process of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄. (d) Density of states of [C₆H₁₄N]₂MnBr₄. (e) Density of states of [C₆H₁₃NCl]₂MnBr₄.

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  Furthermore, we can also find that the band gap of [C₆H₁₃NCl]₂MnBr₄ (1.86 eV) is smaller than that of [C₆H₁₄N]₂MnBr₄ (2.24 eV). This is because the electron-withdrawing atom Cl significantly stabilizes the conduction band energy of the compound. The increase in the band gap caused by lattice expansion is less than the decrease in the band gap caused by the stabilization of the conduction band energy of the compound. The calculated band gaps are all lower than the experimentally obtained ones because standard DFT calculations usually underestimate the band gaps of materials. As shown in Figs. 4d and e, both the VBM and CBM of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ are mainly composed of Mn-d and Br-p orbitals. Therefore, the D-d transition is the main optical transition process in both materials (Fig. 4c).

  The environmental and thermal stability of metal halides are critical determinants for their practical implementation in optoelectronic devices. To systematically evaluate these properties, we conducted comprehensive stability assessments on both [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄ compounds. Thermogravimetric analysis (Fig. S6 in Supporting information) demonstrates distinct thermal stability profiles: [C₆H₁₄N]₂MnBr₄ maintains structural stability below 175 ℃, while the chlorinated compound [C₆H₁₃NCl]₂MnBr₄ exhibits enhanced thermal resistance, showing negligible decomposition up to 212 ℃. Complementary environmental stability tests under ambient conditions (30%−60% RH and 10–23 ℃) further highlight improvement of stability. As shown in Figs. 5a and d, [C₆H₁₄N]₂MnBr₄ suffers severe moisture-induced degradation after four months' air exposure, whereas [C₆H₁₃NCl]₂MnBr₄ retains its morphological integrity. This observation is corroborated by XRD analysis (Fig. S7 in Supporting information), where the chlorinated compound [C₆H₁₃NCl]₂MnBr₄ maintains phase purity comparable to its pristine state, confirming superior crystallinity stability. The substantial improvement in both thermal and environmental stability through chlorine incorporation. Chlorinated cations establish stronger intermolecular interactions with the [MnBr₄]^2- framework, effectively stabilizing the crystal lattice against thermal decomposition. Furthermore, the enlarged molecular volume and enhanced hydrophobicity of chlorinated cations create a protective surface barrier. This dual mechanism physically isolates the [MnBr₄]^2- centers from environmental reactants (e.g., H₂O, O₂), thereby suppressing oxidation and hydrolysis pathways. Such structural engineering provides a generalizable strategy for developing stable metal halides.

  Figure 5

  

  Figure 5.  (a) Photos of fresh [C₆H₁₄N]₂MnBr₄ powder (left) and photos of [C₆H₁₄N]₂MnBr₄ powder (right) after being exposed to air for four months. (b) Photos of the devices based on [C₆H₁₄N]₂MnBr₄ (left) and [C₆H₁₃NCl]₂MnBr₄ (right). (c) PL spectra of LED based on [C₆H₁₄N]₂MnBr₄ driven at different powers. (d) Photos of fresh [C₆H₁₃NCl]₂MnBr₄ powder (left) and photos of [C₆H₁₃NCl]₂MnBr₄ powder (right) after being exposed to air for four months. (e) CIE coordinates of LED based on [C₆H₁₄N]₂MnBr₄ (blue star), [C₆H₁₃NCl]₂MnBr₄ (red star) and green standard (black circle). (f) PL spectra of LED based on [C₆H₁₄N]₂MnBr₄ were driven at different powers.

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  Building upon these stability enhancements, we fabricated light-emitting diodes (LEDs) to evaluate the materials' optoelectronic potential. Fig. 5b presents comparative images of operational devices under ambient and driven (3.2 V) conditions. Chromaticity analysis reveals a significant spectral shift upon chlorine substitution: The CIE coordinates transition from (0.34, 0.61) for [C₆H₁₄N]₂MnBr₄ to (0.23, 0.69) for the chlorinated compound (Fig. 5e), approaching the standard green emission (0.21, 0.71) specified in the CIE 1931 chromaticity diagram. Notably, both materials exhibit excellent color stability under varying drive powers (Figs. 5c and f), with emission intensities showing linear correlation to input power (a critical characteristic for high-brightness applications). The corresponding fitting plots are shown in Fig. S8 (Supporting information). The combination of enhanced environmental resilience and stable electroluminescent performance positions these Mn-based OIMHs as promising candidates for solid-state lighting technologies.

  Moreover, to investigate the molecular mechanisms underlying the luminescence enhancement and stability of the chlorinated compound, we conducted a systematic Hirshfeld surface analysis on their organic cations. Both materials share identical [MnBr₄]^2- tetrahedron, enabling focused comparison of cation-driven interactions. Fig. 6 displays surfaces color-mapped by intermolecular distances: red (less than van der Waals radii sum), white (near van der Waals radii sum), and blue (greater than van der Waals radii sum). In the [C₆H₁₄N]⁺ system, the H···H contact dominates the intermolecular interactions with a contribution of 63.4%, far exceeding the H···Br (36.5%) and other weak interactions (H···Mn only 0.1%) (Fig. 6a). This high proportion of H···H contacts significantly enhances the Coulombic repulsion between the cations, leading to a more loosely packed lattice structure.

  Figure 6

  

  Figure 6.  Hirshfeld surface and fingerprint plots for (a) [C₆H₁₄N]⁺ in [C₆H₁₄N]₂MnBr₄, (b) [C₆H₁₃NCl]⁺ in [C₆H₁₃NCl]₂MnBr₄.

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  After introducing the chlorine atom, the H···H contact ratio in [C₆H₁₃NCl]⁺ decreases to 42.7%, while a new H···Cl interaction appears, contributing 21.9%. This reorganization of interactions creates a more balanced attractive-repulsive network: H···Br (32.2%) and H···Cl cooperatively enhance the cation-[MnBr₄]^2- host-guest attraction, while the optimized H···H ratio (42.7% vs. 63.4%) effectively alleviates the cation repulsion (Fig. 6b). X-ray refinement analyses reveal that chlorination substantially reduces the Mn–Br bond length deviation from 0.069 Å to 0.012 Å (Fig. S5). The specific bond lengths are shown in Tables S2 and S3 (Supporting information). This structural optimization arises from the strong electronegativity of Cl (Pauling χ = 3.14) [42], which enhances electrostatic coupling between organic cations and the [MnBr₄]^2- framework via electronic polarization effects, thereby suppressing Jahn-Teller distortions in the Br coordination environment. This suppressed Jahn-Teller distortion correlates with tighter molecular packing and uniform force distribution, fostering robust hydrogen bonding and van der Waals networks. The resultant defect-minimized crystalline architecture directly accounts for the observed enhancements in both environmental stability and fluorescence quantum yield.

  In summary, we have successfully designed two 0D Mn-based OIMHs ([C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄) and effectively improved the luminescence performance and stability of [C₆H₁₃NCl]₂MnBr₄ through the chlorination effect. Both of them exhibit green light emission under 363 nm excitation, which is attributed to the D-d transition (⁴T₁−⁶A₁) of the tetrahedral coordinated Mn²⁺ center. The presence of the Cl atom leads to an increase in the occupied space of [C₆H₁₃NCl]⁺, thereby suppressing excitation energy transfer between adjacent Mn²⁺ centers, resulting in an approximately one-fold increase in PLQY (from 38.8% to 74.5%). Additionally, [C₆H₁₃NCl]₂MnBr₄ demonstrates superior environmental stability due to the increased H···Cl interactions. Moreover, [C₆H₁₃NCl]₂MnBr₄ possesses CIE color coordinates (0.23, 0.69) closer to the NTSC green standard compare to [C₆H₁₄N]₂MnBr₄. Our work not only provides new insights into the relationship between the structure and optical properties of manganese-based metal halides but also offers a promising material for next-generation optoelectronic applications.

  Accession codes

  CCDC 2444704−2444705 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

  CRediT authorship contribution statement

  Xiulian Cai: Writing – original draft, Methodology, Data curation. Qilin Wei: Software. Yaping Gong: Software. Tong Chang: Writing – review & editing, Methodology, Funding acquisition, Conceptualization. Tongtong Kou: Methodology. Ziyang Song: Validation. Zengshan Yue: Data curation. Huanxin Su: Methodology. Yuanxiang You: Investigation. Chengmin Ji: Conceptualization. Xinxin Han: Software. Shiguo Han: Writing – review & editing, Funding acquisition. William W. Yu: Writing – review & editing, Project administration, Funding acquisition, Conceptualization.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22205233, 62405165, 62374104), Taishan Scholar Foundation of Shandong Province (No. tsqn202408058), Basic and Applied Basic Research Foundation of Guangdong Province (No. 2025A1515012289), the Natural Science Foundation of Fuzhou City (No. 2024-Y-020), China Postdoctoral Science Foundation (No. 2024M751789), Postdoctoral Innovative Projects of Shandong Province (No. 202400321), and the Shandong Provincial Natural Science Foundation (No. ZR2024QB208).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.111515.

  
  
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  Figure 1  Schematic demonstration of the crystal structures of [C₆H₁₄N]₂MnBr₄ (left) and [C₆H₁₃NCl]₂MnBr₄ (right).

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  Figure 2  (a) PLE and PL spectra of [C₆H₁₄N]₂MnBr₄. (b) UV–vis absorption spectrum and optical band gap of [C₆H₁₄N]₂MnBr₄. (c) PLQY of [C₆H₁₄N]₂MnBr₄. (d) PLE and PL spectra of [C₆H₁₃NCl]₂MnBr₄. (e) UV–vis absorption spectrum and optical band gap of [C₆H₁₃NCl]₂MnBr₄. (f) PLQY of [C₆H₁₃NCl]₂MnBr₄.

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  Figure 3  (a) Temperature-dependent PL spectra of [C₆H₁₄N]₂MnBr₄. (b) FWHMs of [C₆H₁₄N]₂MnBr₄ as a function of temperature under heating conditions. (c) PL intensities of [C₆H₁₄N]₂MnBr₄ as a function of 1/T under heating conditions. (d) Temperature-dependent PL spectra of [C₆H₁₃NCl]₂MnBr₄. (e) FWHMs of [C₆H₁₃NCl]₂MnBr₄ as a function of temperature under heating conditions. (f) PL intensities of [C₆H₁₃NCl]₂MnBr₄ as a function of 1/T under heating conditions.

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  Figure 4  (a) Electronic band structure of [C₆H₁₄N]₂MnBr₄. (b) Electronic band structure of [C₆H₁₃NCl]₂MnBr₄. (c) Schematic diagram of the energy transfer process of [C₆H₁₄N]₂MnBr₄ and [C₆H₁₃NCl]₂MnBr₄. (d) Density of states of [C₆H₁₄N]₂MnBr₄. (e) Density of states of [C₆H₁₃NCl]₂MnBr₄.

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  Figure 5  (a) Photos of fresh [C₆H₁₄N]₂MnBr₄ powder (left) and photos of [C₆H₁₄N]₂MnBr₄ powder (right) after being exposed to air for four months. (b) Photos of the devices based on [C₆H₁₄N]₂MnBr₄ (left) and [C₆H₁₃NCl]₂MnBr₄ (right). (c) PL spectra of LED based on [C₆H₁₄N]₂MnBr₄ driven at different powers. (d) Photos of fresh [C₆H₁₃NCl]₂MnBr₄ powder (left) and photos of [C₆H₁₃NCl]₂MnBr₄ powder (right) after being exposed to air for four months. (e) CIE coordinates of LED based on [C₆H₁₄N]₂MnBr₄ (blue star), [C₆H₁₃NCl]₂MnBr₄ (red star) and green standard (black circle). (f) PL spectra of LED based on [C₆H₁₄N]₂MnBr₄ were driven at different powers.

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  Figure 6  Hirshfeld surface and fingerprint plots for (a) [C₆H₁₄N]⁺ in [C₆H₁₄N]₂MnBr₄, (b) [C₆H₁₃NCl]⁺ in [C₆H₁₃NCl]₂MnBr₄.

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