Ultrafast Self-Assembly of Efficient Blue-Excitable Hybrid Manganese Bromide Microcrystals for Wide-Gamut Display Backlights

Abstract

Blue-excitable lead-free metal halide microcrystals (MCs) with high efficiency and narrow emission are highly desired for next-generation wide-gamut displays. Herein, an ultrafast self-assembly strategy is developed to prepare hybrid manganese bromide MCs with tunable alkyl-chain lengths. With increasing alkyl-chain length, the emission intensity of the obtained MCs first improves significantly and then deteriorates, accompanied by a gradual narrowing of the full width at half maximum (FWHM) from 46.37 to 41.97 nm. The optimized [(C₂H₅)₄N]₂MnBr₄ MCs exhibit strong narrow-band green emission at 518 nm with a FWHM of 43.30 nm and a high photoluminescence quantum yield of 98.70% under 455 nm excitation. This superior emission performance is ascribed to cooperative weak interactions, an appropriate Mn-Mn distance, and highly localized electronic transitions within isolated [MnBr₄]²⁻ tetrahedra. Moreover, the [(C₂H₅)₄N]₂MnBr₄ MCs show moderate thermal stability, retaining 52.03% of their initial emission intensity at 413 K. By integrating [(C₂H₅)₄N]₂MnBr₄ with K₂SiF₆: Mn⁴⁺ red phosphors on a blue chip, a white light-emitting diode with a high luminous efficacy of 134.60 lm/W and a wide gamut of 112.6% NTSC is achieved, demonstrating the great promise of ultrafast-assembled hybrid manganese bromides for efficient and wide gamut display backlighting.

1. Introduction

Wide-color-gamut displays are attracting increasing attention in high-definition visual technologies, including liquid-crystal displays and mini-LED backlights [1]. In phosphor-converted white light-emitting diodes (WLEDs), the emission bandwidth and emission position of phosphors by blue light excitation directly determine the color purity and achievable color gamut of backlight modules [2]. Narrow-band green phosphors, such as β-SiAlON:Eu²⁺ [3], La₀.₈₂₇Al₁₁._9−xMn_xO₁₉.₀₉ [4], and CsPbBr₃ [5], together with narrow-band red phosphors, including K₂SiF₆: Mn⁴⁺ [6], K₂TiF₆: Mn⁴⁺ [7] and CsPbI₃ [8], have been widely explored as color-conversion emitters for wide-gamut display backlights, thereby enabling displays with higher color saturation and improved optical efficiency. However, conventional rare-earth or transition-metal-activated inorganic green phosphors generally require high-temperature solid-state synthesis, which increases energy consumption and limits scalable, low-cost production. In contrast, green-emitting CsPbBr₃ suffers from the inherent toxicity of Pb and potential instability. Therefore, lead-free green emitters that combine high photoluminescence efficiency, narrow emission, facile synthesis, and good thermal stability are highly desirable for next-generation wide-gamut displays.

Zero-dimensional hybrid Mn²⁺ metal halides, in which isolated [MnX₄]²⁻ tetrahedra are spatially separated by bulky organic cations, have recently emerged as promising narrow-band green emitters owing to their Mn²⁺ d–d transitions, excitability by blue light, and environmental friendliness [9,10]. For instance, Fan et al. [11] reported (BPTA)₂MnBr₄ (BPTA = 3-Bromopropyltrimethylammonium) single crystals (SCs) with a FWHM of 49 nm and a photoluminescence quantum yield (PLQY) of 73.5%, demonstrating that suitable organic-cation selection can regulate the separation and photophysical behavior of the emissive Mn²⁺ centers. Zhang et al. [12] developed C₁₆H₃₈N₂MnBr₄ SCs by structural regulation of the Mn²⁺ coordination environment and Mn···Mn separation, achieving high PLQY of 76% under 460 nm excitation. Duan et al. [13] reported [Et(Ph)₃P]₂MnBr₄ (Et(Ph)₃P = ethyltriphenylphosphonium) SCs with a 523 nm green emission, a FWHM of 50 nm, and a high PLQY of 94.4% under 450 nm excitation. The corresponding WLED device achieved a luminous efficacy (LE) of 121.3 lm/W and a wide color gamut of 108.7% NTSC. These studies indicate that blue-light excitation can be achieved through diverse organic-cation frameworks, and is modulated by the local Mn²⁺ coordination symmetry, spatial isolation of the inorganic tetrahedra, and nonradiative relaxation processes.

Despite their promising performance as blue-excitable green emitters, hybrid manganese bromides are commonly obtained as single crystals through cooling or antisolvent crystallization [14,15], processes that typically require long growth periods and strict synthetic conditions. To overcome these limitations, mechanochemical method, precipitation method and self-assembly method have been developed for the facile synthesis of hybrid Mn²⁺ metal halide microcrystals (MCs). [16,17,18] For example, Meng et al. [19] synthesized (C₂₂H₂₂O₂P)₂MnBr₄ MCs by ball-milling, achieving a PLQY of 96.1% under UV light excitation. Zhou et al. [18] reported (C₂₀H₂₀P)₂MnBr₄ MCs prepared via ultrafast self-assembly, which exhibited a PLQY of 93.83% under 450 nm radiation. Nevertheless, the development of highly efficient blue-excitable Mn²⁺ metal halide MCs for high-performance wide-gamut WLEDs remains challenging.

Herein, we report an ultrafast self-assembly strategy for preparing efficient blue-excitable hybrid manganese bromide MCs with different alkyl-chain length, including in [(CH₃)₄N]₂MnBr₄, [(C₂H₅)₄N]₂MnBr₄, and [(C₃H₇)₄N]₂MnBr₄. By regulating the alkyl-chain length of the organic cations, the emission behavior and thermal stability of the hybrid manganese bromides are systematically investigated. The optimized [(C₂H₅)₄N]₂MnBr₄ MCs exhibit intense narrow-band green emission under blue-light excitation with a near-unity PLQY of 98.70%. The relationships among alkyl-chain length, weak intermolecular interactions, Mn···Mn separation, photoluminescence behavior, and thermal quenching are further discussed. Finally, a WLED device is fabricated by integrating [(C₂H₅)₄N]₂MnBr₄ green MCs and commercial K₂SiF₆: Mn⁴⁺ red phosphors with a 450 nm InGaN chip, achieving high LE of 134.60 lm/W and a wide color gamut of 112.6% NTSC. This work provides a simple and efficient strategy for designing lead-free hybrid manganese bromide MCs toward high-efficiency wide-gamut display backlighting.

2. Results and Discussion

2.1. Synthesis of [(C₂H₅)₄N]₂MnBr₄ MCs via Ultrafast Self-Assembly Process

[(C₂H₅)₄N]₂MnBr₄ MCs were synthesized through an ultrafast self-assembly process, as illustrated in Figure 1a. Briefly, (C₂H₅)₄NBr and MnBr₂·4H₂O with a molar ratio of 2.25 were dissolved in 10 mL of ethanol under stirring. A green emissive suspension was immediately observed under UV-light irradiation, indicating the rapid formation of [(C₂H₅)₄N]₂MnBr₄ MCs. The as-prepared [(C₂H₅)₄N]₂MnBr₄ exhibits an irregular morphology (Figure 1b). The Mn and Br elements with the molar ratio of 1:4.16 are uniformly distributed within the particles by energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 1c), suggesting a slightly Br-rich chemical environment in the obtained MCs.

The diffraction peaks of the MCs prepared by the ultrafast self-assembly process are in good agreement with the simulated pattern of [(C₂H₅)₄N]₂MnBr₄ (CCDC 2042083). The Rietveld refinement gives R_wp, R_p, and χ² values of 7.31%, 5.54%, and 2.88, respectively, confirming the reliability of the refined structure (Figure 1d). [(C₂H₅)₄N]₂MnBr₄ crystallizes in the tetragonal P-4(_)2₁m space group (No. 113). The refined lattice parameters are a = 13.3703 Å, b = 13.3703 Å, c = 14.4230 Å, V = 2578.313 ų and Z = 2, as summarized in Table S1. In this structure, the bulky (C₂H₅)₄N⁺ cations separate the [MnBr₄]²⁻ tetrahedra, giving rise to a large Mn···Mn distance of 9.454 Å.

To understand the ultrafast self-assembly behavior of [(C₂H₅)₄N]₂MnBr₄ MCs in ethanol, the interactions between (C₂H₅)₄N⁺ and [MnBr₄]²⁻ were analyzed using Independent Gradient Model based on Hirshfeld partitioning (IGMH). Figure 1f presents the δ_g versus sign(λ₂)ρ scatter plot and the corresponding isosurfaces of [(C₂H₅)₄N]₂MnBr₄. The green regions with sign(λ₂)ρ values close to zero indicate weak van der Waals interactions. The IGMH results suggest that the interactions between (C₂H₅)₄N⁺ and [MnBr₄]²⁻ tetrahedra are mainly associated with van der Waals interactions and non-classical C–H···Br hydrogen bonds [20].

Furthermore, Hirshfeld surface analysis and two-dimensional fingerprint plots were used to quantitatively evaluate the intermolecular contacts, as shown in Figure S1 and Figure 1g. Here, d_i represents the distance from the Hirshfeld surface to the nearest atom inside the surface, whereas d_e denotes the distance to the nearest atom outside the surface [21]. The fingerprint plots show that H···H contacts account for 54.6% of the total Hirshfeld surface, while H···Br/Br···H contacts contribute 44.2%. These results confirm the presence of abundant H···H and H···Br/Br···H interactions in [(C₂H₅)₄N]₂MnBr₄. The weak interactions, particularly the non-classical C–H···Br hydrogen bonds, may play an important role in facilitating the rapid assembly of [(C₂H₅)₄N]₂MnBr₄ MCs.

In addition, the Gibbs free energy change (ΔG) for the reaction in ethanol was calculated according to the following equation:

ΔG = ∑ G_product − ∑ G_reactant

(1)

The calculated ΔG is −42.49 kcal/mol (Table S2), which is lower than that reported for the synthesis of (C₂₀H₂₀P)₂MnBr₄ under a similar reaction system [18]. The negative ΔG value indicates that the formation of [(C₂H₅)₄N]₂MnBr₄ can proceed spontaneously in ethanol.

2.2. PL Properties of Hybrid Manganese Bromide with Different Alkyl-Chain Length

The PL excitation (PLE) and PL spectra of [(CH₃)₄N]₂MnBr₄, [(C₂H₅)₄N]₂MnBr₄, and [(C₃H₇)₄N]₂MnBr₄ MCs synthesized through an ultrafast self-assembly process are shown in Figure 2a. The excitation spectra of MCs with different alkyl-chain length show similar features, with four distinct excitation bands at 200–250 nm, 250–330 nm, 330–415 nm and 415–510 nm. Among them, the excitation bands in 415–510 nm are relatively intense, indicating effective excitation under blue-light irradiation. Specifically, the excitation peaks at 472, 455, 437, 391, 378 and 365 nm are attributed to the characteristic spin-forbidden d–d transitions of Mn²⁺, corresponding to ⁶A₁(⁶S) → ⁴T₁(⁴G), ⁶A₁(⁶S) → ⁴T₂(⁴G), ⁶A₁(⁶S) → [⁴A₁(⁴G), ⁴E(⁴G)], ⁶A₁(⁶S) → ⁴T₂(⁴D), ⁶A₁(⁶S) → ⁴E(⁴D) and ⁶A₁(⁶S) → ⁴T₁(⁴P), respectively. Under 455 nm irradiation, the MCs exhibit strong narrow-band green emission. As the alkyl-chain length increases, the emission maximum of the MCs exhibits a gradual blue shift from 521 nm to 514 nm, accompanied by a narrowing of the FWHM from 46.37 nm of [(CH₃)₄N]₂MnBr₄ MCs to 41.97 nm of [(C₃H₇)₄N]₂MnBr₄ MCs (Figure 2b). Remarkably, [(C₂H₅)₄N]₂MnBr₄ MCs show the strongest emission among the prepared MCs, with a PLQY of 98.70% (Figure 2c and Figure S2). The Commission Internationale de l’Éclairage (CIE) chromaticity coordinates of MCs with different alkyl-chain lengths are A (0.20, 0.70) of [(CH₃)₄N]₂MnBr₄, B (0.17, 0.71) of [(C₂H₅)₄N]₂MnBr₄ and C (0.15, 0.69) of [(C₃H₇)₄N]₂MnBr₄, respectively, which are close to the NTSC standard green coordinate of (0.21, 0.71), highlighting the potential of these hybrid manganese bromide green emitters for display applications.

To understand the emission mechanism, the adjacent Mn-Mn distance is studied. The Rietveld refinement results of [(C₃H₇)₄N]₂MnBr₄ MCs (Figure S3 and Table S3) show the shortest Mn-Mn distance of 9.655 Å. Combing with the previously reported [22,23], the shortest Mn-Mn distances in the green MCs increase in the order of [(CH₃)₄N]₂MnBr₄ < [(C₂H₅)₄N]₂MnBr₄ < [(C₃H₇)₄N]₂MnBr₄, indicating that elongation of the alkyl chain leads to greater spatial separation of the isolated [MnBr₄]²⁻ tetrahedra. As the alkyl-chain length of the organic precursor increases, the enlarged Mn-Mn distance weakens the interactions and excitation-energy migration among adjacent [MnBr₄]²⁻ tetrahedra, thereby suppressing concentration quenching and contributing to the efficient green emission [24,25,26]. Nevertheless, an overly large Mn···Mn separation may introduce nonradiative channels that consequently decrease the emission intensity [27].

The photoluminescence emission intensity is not only related to the Mn-Mn distance but is also strongly affected by defect states. Figure 2d shows the PL decay curves of MCs with different alkyl-chain lengths. These decay curves can be fitted by a single-exponential function, as follows: [22]

I(t) = I₀ + A exp(−t/τ)

(2)

where the I(t) and I₀ are the fluorescence intensity at time t and 0, respectively. The PL decay curves of MCs can be well fitted by a single-exponential function, indicating that the emission predominantly originates from a single type of emissive center, which is consistent with Mn²⁺-centered d–d transitions. The calculated lifetimes of the [(CH₃)₄N]₂MnBr₄, [(C₂H₅)₄N]₂MnBr₄, and [(C₃H₇)₄N]₂MnBr₄ MCs are 369.95, 402.42 and 380.00 μs, respectively, confirming that the emission center arises from the d–d transitions of Mn²⁺.

The radiative and nonradiative decay rates from the PLQYs and PL lifetimes were further calculated using k_r = Φ/τ and k_nr = (1 − Φ)/τ. The calculated k_nr values are approximately 4.84 × 10², 3.23 × 10¹, and 5.18 × 10² s⁻¹ for the [(CH₃)₄N]₂MnBr₄, [(C₂H₅)₄N]₂MnBr₄, and [(C₃H₇)₄N]₂MnBr₄, respectively. The corresponding radiative rates are approximately 2.22 × 10³, 2.45 × 10³, and 2.11 × 10³ s⁻¹. Therefore, the [(C₂H₅)₄N]₂MnBr₄ exhibits not only the highest radiative rate but, more importantly, a nonradiative decay rate that is more than one order of magnitude lower than those of the [(CH₃)₄N]₂MnBr₄ and [(C₃H₇)₄N]₂MnBr₄. These results demonstrate that near-unity PLQY of [(C₂H₅)₄N]₂MnBr₄ arises from effective suppression of nonradiative deactivation.

Figure S4a,b presents the PLE contour maps and selected PLE spectra of [(C₂H₅)₄N]₂MnBr₄ MCs, monitored at different emission wavelengths. The PLE spectra of MCs show similar profiles at different emission wavelengths, and no obvious shift in the excitation peaks is observed. In addition, the PL contour maps and selected PL spectra of [(C₂H₅)₄N]₂MnBr₄ MCs under different excitation wavelengths are shown in Figure S4c,d. The PL spectra of phosphors retain similar spectral shapes under different excitation wavelengths, and their emission peaks exhibit no noticeable shift. Temperature-dependent PL decay measurements further were studied, as shown in Figure S5. The PL decay curves can be approximately fitted using a single-exponential function with fitted lifetimes of 391.03, 393.00, 402.42, and 412.09 μs at 103, 203, 303, and 403 K, respectively. These results demonstrate that the emission of [(C₂H₅)₄N]₂MnBr₄ MCs originates from the radiative d–d transitions of Mn²⁺ rather than defect-related emission.

The effect of the alkyl-chain length on the photoluminescence properties of the green MCs was further discussed based on crystal-field theory for the 3d⁵ electronic configuration of Mn²⁺, as shown in Figure S6. These results show that the crystal-field strength follows the order of [(CH₃)₄N]₂MnBr₄ > [(C₂H₅)₄N]₂MnBr₄ > [(C₃H₇)₄N]₂MnBr₄, which indicates that the crystal-field strength gradually decreases with increasing alkyl-chain length. The decrease in crystal-field strength with increasing alkyl-chain length is consistent with the observed blue shift in Mn²⁺ emission. Meanwhile, the reduced Racah parameter B suggests enhanced Mn–Br covalency and a stronger nephelauxetic effect, which may contribute to the narrowing of the emission bandwidth.

To gain insight into the photoluminescence mechanism of [(C₂H₅)₄N]₂MnBr₄, Density functional theory (DFT) calculations were performed to investigate its band structure and electronic structure. Figure 2e,f show the calculated band structure and density of states of [(C₂H₅)₄N]₂MnBr₄ respectively. [(C₂H₅)₄N]₂MnBr₄ exhibits a direct bandgap of 2.67 eV which is close to the optical bandgap of 2.37 eV estimated from the absorption spectrum (Figure S7). Compared with [(CH₃)₄N]₂MnBr₄, [(C₂H₅)₄N]₂MnBr₄ possesses a smaller bandgap, which is beneficial for facilitating radiative recombination. In addition, the energy bands near the valence-band maximum (VBM) and conduction-band minimum (CBM) in the Brillouin zone are relatively flat, which can be attributed to the structural isolation of the [MnBr₄]²⁻ tetrahedra by bulky organic cations. the VBM of [(C₂H₅)₄N]₂MnBr₄ is mainly composed of hybridized Mn 3d and Br 4p orbitals, whereas the CBM is dominated by Mn 3d orbitals. The orbital hybridization between Br 4p and Mn 3d through Mn–Br coordination helps relax the spin-forbidden selection rule of the Mn²⁺ d–d transitions [12]. Moreover, the (C₂H₅)₄N⁺ cationic orbitals exhibit a large bandgap and show negligible coupling with Mn and Br orbitals, indicating that the emission of [(C₂H₅)₄N]₂MnBr₄ mainly originates from the d–d transitions within the [MnBr₄]²⁻ tetrahedra. The electronic isolation of the [MnBr₄]²⁻ tetrahedra further promotes the highly localized nature of these d–d transitions.

2.3. Stability of Hybrid Manganese Bromide MCs

Thermal stability of [(C₂H₅)₄N]₂MnBr₄ and [(C₃H₇)₄N]₂MnBr₄ MCs were studied by temperature-dependent PL measurements, as shown in Figure 3. PL maps and spectra of [(C₂H₅)₄N]₂MnBr₄ and [(C₃H₇)₄N]₂MnBr₄ MCs reveal a progressive decrease in emission intensity with increasing temperature, owing to enhanced thermally activated nonradiative recombination. Meanwhile, both MCs show a gradual blue shift in the emission peak upon heating, which is associated with thermally induced lattice expansion. Notably, [(C₂H₅)₄N]₂MnBr₄ MCs can retain 52.03% of its initial PL intensity at 413 K relative to that at 293 K (Figure 3c), which is inferior to commercial β-SiAlON:Eu²⁺ and NaK₂Li[Li₃SiO₄]₄:Eu [28,29]. However, [(C₃H₇)₄N]₂MnBr₄ MCs retain only 2.88% (Figure 3f), indicating pronounced thermal quenching. In comparison with [(CH₃)₄N]₂MnBr₄ [22], these results demonstrate that elongation of the organic alkyl chain leads to a gradual deterioration in the thermal stability of hybrid Mn²⁺ bromide MCs.

To clarify the correlation between organic alkyl-chain length and thermal stability, Hirshfeld surface analysis of [(CH₃)₄N]₂MnBr₄ and [(C₃H₇)₄N]₂MnBr₄ was carried out, as shown Figure S8 and Table S4. The main intermolecular contacts in [(CH₃)₄N]₂MnBr₄ are H···H and H···Br/Br···H interactions, accounting for 43.4% and 56.1%, respectively. For [(C₃H₇)₄N]₂MnBr₄, the corresponding contributions are 64.1% and 33.7%, respectively. These results indicate that elongation of the organic alkyl chain substantially reduces the proportion of H···Br/Br···H contacts from 56.1% to 33.7%, weakening the hydrogen-bonding interactions in the crystal lattice and consequently deteriorating the thermal stability.

The influence of increasing alkyl-chain length on the spatial arrangement of the isolated [MnBr₄]²⁻ tetrahedra, the organic–inorganic intermolecular interactions, and the resulting photoluminescence properties are summarized in Figure S9. With increasing alkyl-chain length, the spatial separation between neighboring tetrahedra increases, whereas the contribution of H···Br/Br···H contacts decreases from 56.1% to 33.7% The enlarged Mn···Mn separation weakens Mn²⁺–Mn²⁺ coupling and suppresses excitation-energy migration. In parallel, the gradual decrease in crystal-field strength accounts for the observed blue shift and narrowing of the Mn²⁺ emission band. The PLQY exhibits a non-monotonic dependence on alkyl-chain length, increasing from 82.09% to 98.70% and then decreasing to 80.32%. The near-unity PLQY of [(C₂H₅)₄N]₂MnBr₄ MCs is attributed to an appropriate Mn···Mn separation, organic–inorganic interactions and local crystal-field strength, as well as fewer nonradiative relaxation.

2.4. The Application of WLED Based on [(C₂H₅)₄N]₂MnBr₄ MCs

The as-prepared [(C₂H₅)₄N]₂MnBr₄ MCs, serving as the green emitter, were combined with commercial K₂SiF₆: Mn⁴⁺ red phosphors and integrated with a 450 nm InGaN LED chip to fabricate a WLED device. Figure 4a shows the EL spectrum of the WLED recorded at a driving current of 20 mA. The device exhibits bright white emission with CIE chromaticity coordinates of (0.3151, 0.3329), which are close to the standard white-light coordinates of (0.33, 0.33). The WLED also shows a color rendering index (R_a) of 46.1 and a correlated color temperature (CCT) of 6353 K. Benefiting from the efficient green emission of [(C₂H₅)₄N]₂MnBr₄ MCs under blue-light excitation, the fabricated WLED achieves a high LE of 134.60 lm/W, outperforming many reported WLEDs based on hybrid manganese halides (Table S5), such as 117.17 lm/W of (C₂₀H₂₀P)₂MnBr₄ based on WLED [18], 121.3 lm/W of [Et(Ph)₃P]₂MnBr₄ based on WLED [13], and C₁₆H₃₈N₂MnBr₄ based on 133 lm/W of WLED [12]. As shown in Figure 4b, the gamut of the [(C₂H₅)₄N]₂MnBr₄-based WLED is calculated to be 112.6% of the NTSC standard, confirming its wide-gamut emission characteristics and promising potential for LCD backlight applications. Furthermore, the current-dependent EL performance of the [(C₂H₅)₄N]₂MnBr₄-based WLED was investigated, as shown in Figure 4c. The EL spectral profile remains nearly unchanged as the driving current increases, while the emission intensity gradually increases. When the current is increased to 200 mA, the LE remains as high as 104.70 lm/W (Figure 4d), indicating the favorable current tolerance of the [(C₂H₅)₄N]₂MnBr₄-based WLED device.

3. Experiment and Characterization

3.1. Materials

Manganese (II) bromide tetrahydrate (MnBr₂·4H₂O, 99%, Aladdin, Shanghai, China), tetramethylammonium bromide ((CH₃)₄NBr, 98%, Aladdin, Shanghai, China), tetraethylammonium bromide ((C₂H₅)₄NBr, 98%, Aladdin, Shanghai, China), tetrapropylammonium bromide ((C₃H₇)₄NBr, 98%, Aladdin, Shanghai, China), and ethanol (C₂H₅OH, ≥99.7%, Sinopharm, Beijing, China) were used without further purification.

3.2. Synthesis of Hybrid Manganese Bromide MCs via Ultrafast Self-Assembly Method

For the synthesis of [(C₂H₅)₄N]₂MnBr₄ MCs, 2.25 mmol of (C₂H₅)₄NBr and 1.00 mmol of MnBr₂·4H₂O were dissolved in 10 mL of ethanol under stirring for 500 rpm. A green suspension formed rapidly within 10 s at room temperature. The suspension was collected, washed thoroughly with ethanol, and dried at 60 °C for 6 h to obtain [(C₂H₅)₄N]₂MnBr₄ MCs. [(CH₃)₄N]₂MnBr₄ and [(C₃H₇)₄N]₂MnBr₄ MCs were synthesized using the same procedure, except that (C₂H₅)₄NBr was replaced with (CH₃)₄NBr or (C₃H₇)₄NBr, respectively.

3.3. Preparation of WLED Device

To fabricate the WLED device, [(C₂H₅)₄N]₂MnBr₄ MCs, K₂SiF₆: Mn⁴⁺ phosphors, and UV-curable silicone were thoroughly mixed at a mass ratio of 1:0.8:2. The obtained phosphor–silicone composite was subsequently coated onto a 450 nm blue InGaN chip and cured under UV irradiation for 15 min.

3.4. Characterization

The morphology and elemental distributions of MCs were characterized by scanning electron microscopy (SEM, ZEISS Gemini 300, Oberkochen, Germany) equipped with EDS. The structure data of hybrid manganese bromide MCs were recorded by powder X-ray diffraction (XRD, Bruker D8 Advance, Billerica, MA, USA) and Rietveld refinement by GSAS 3.4.5 package with the EXPGUI interface [30]. PL properties of MCs were measured using a fluorescence spectrometer (Edinburgh FS5 v2, Scotland, UK). Absorption spectrum of [(C₂H₅)₄N]₂MnBr₄ MCs were recorded on a UV–vis spectrophotometer (PerkinElmer Lambda 750, Waltham, MA, USA). The electroluminescence (EL) characteristics of the LED devices were evaluated using a spectroradiometer (Hopocolor HP8000, Hangzhou, China).

3.5. Absolute PLQY Measurements

The absolute PLQY of the hybrid manganese bromide MCs were measured using an Edinburgh Instruments FS5 v2 spectrofluorometer equipped with an SC-30 integrating-sphere module. The PLQY was calculated by

$$PLQY={\displaystyle \frac{E_s-E_{r,\mathit{scaled}}}{S_r-S_s}}$$

(3)

where E_s is the integrated emission intensity of the sample, E_r,scaled is the scaled reference background within the emission region, and Sr and Ss are the integrated excitation-scattering intensities of the reference and sample, respectively.

3.6. Theoretical Calculations

Noncovalent interactions in [(C₂H₅)₄N]₂MnBr₄ were analyzed using the IGMH implemented in Multiwfn 3.8 [31] and the generated isosurfaces were rendered with VMD 1.9.3 [32]. Hirshfeld surface analysis of the hybrid manganese bromide MCs was performed using CrystalExplorer21 [33]. DFT calculations were further carried out with CASTEP 6.0 to obtain the electronic band structure and bandgap of [(C₂H₅)₄N]₂MnBr₄. A periodic structural model with the composition C₆₄H₁₆₀N₈Mn₄Br₁₆, comprising a total of 252 atoms, was constructed for the calculations. The Brillouin zone was sampled using a 2 × 2 × 2 Monkhorst–Pack k-point mesh. The exchange–correlation interactions were described within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional [34,35]. Spin polarization was included to account for the magnetic characteristics of Mn²⁺ ions. The plane-wave cutoff energy, self-consistent-field convergence criterion and maximum residual force were set to 500 eV, 1.0 × 10⁻⁵ eV and 0.02 eV Å⁻¹, respectively.

4. Conclusions

In summary, blue-light-excitable hybrid manganese bromide MCs were developed through an ultrafast self-assembly strategy. The alkyl-chain length of the organic cation plays an important role in regulating the optical properties and thermal stability of the hybrid manganese bromides. With increasing alkyl-chain length, the emission peak gradually blue-shifts from 521 nm to 514 nm, while the FWHM narrows from 46.37 to 41.97 nm. Among the synthesized samples, [(C₂H₅)₄N]₂MnBr₄ MCs have the optimal performance, with a near-unity PLQY of 98.70%, which can be attributed to appropriate Mn···Mn separation, organic–inorganic interactions, and suppression of nonradiative relaxation. The optimized MCs retain 52.03% of their initial PL intensity at 413 K. A WLED fabricated by combining [(C₂H₅)₄N]₂MnBr₄ green MCs with K₂SiF₆: Mn⁴⁺ red phosphors on a 450 nm InGaN chip achieves an LE of 134.60 lm/W and a wide color gamut of 112.6% NTSC. The as-prepared WLED has excellent current tolerance, with its electroluminescence intensity increasing monotonically as the driving current rises from 20 mA to 200 mA, while retaining a high LE of 104.70 lm/W at 200 mA. This work establishes clear relationships among alkyl-chain length, Mn···Mn separation, intermolecular interactions, and luminescence performance, providing valuable design principles and a scalable ultrafast self-assembly strategy for developing efficient, thermally stable, lead-free hybrid manganese halide emitters for wide-gamut displays.