Multicolor light emission in manganese-based metal halide composites

In recent years, incorporating metal framework into hybrid organic–inorganic metal halides (OIMHs) has been proven as an effective method to achieve different luminescence colors with high efficiency.^1–8 Among various OIMHs for future lighting and display, manganese (Mn)-based OIMHs show great promise due to easy processibility and flexible photoluminescence tunability.^9,10 By changing the coordination environment of the Mn (II) center, the emission can be tuned from green to red. In addition, the emission from the Mn (II) center can be extremely efficient, exemplified by recent achievements that the photoluminescence quantum yields (PLQY) of tetrahedral Mn-based OIMHs reach up to 90%.¹¹ 

These advantages open up great opportunities for Mn-based OIMHs as next-generation light emitters. While the metal-centered d–d [⁴T₁(G)–⁶A₁] radiative transition conventionally enables light emission from green to red in Mn-based OIMHs,^11–18 a missing component is blue emission, which is important for constructing white light and for developing multicolor lighting.^19,20

In order to overcome this limitation, metal doping has been introduced to tune emission colors via choosing specific metals, resulting in adjustable bandgap and high PLQY.^21–24 However, doping has been limited to a few metal candidates, possibly due to the difficulty to incorporate the metal dopants in Mn–halide frameworks. In addition, the doped metals also complicate the molecules environment, resulting in difficulties to understand the structure–property relationship and rational material design. Therefore, we are motivated to seek for more efficient and convenient methods to develop blue emission into Mn-based OIMHs.

In the OIHM system, organic ligand and metal–halide framework are spatially separated, providing feasibility to construct two independent emission centers within the crystal structure. Specifically, in Mn-based OIMHs, the Mn–halide octahedral or tetrahedral is responsible for efficient red or green emission as an independent emission center.^25–27 In order to construct an additional blue emission center, the organic ligands outside Mn–halide units provide great feasibility owing to the large tunability of organic nature in the hybrid system. Following this route, we incorporate a blue emissive ligand methylbenzylamine (MBA) into Mn-based OIMHs to realize multicolor emission. A perovskite-like octahedral Mn-based OIMH with general formula AMX₃ is synthesized, which shows two independent emission centers: one from Mn-X structures and one from blue emissive organic ligands. The photoluminescence (PL) and photoluminescence excitation (PLE) spectra indicate that the two emission centers show different response to excitation wavelength. Specifically, a white light emission is obtained under the excitation of 330 nm and the emission color can be tuned by changing the excitation wavelength. Our work thus not only realizes multicolor emission but also opens up great potential for Mn-based OIHMs as anti-counterfeiting materials.^28–38 

By simply mixing MnCl₂ [manganese (II) chloride] and MBACl (methylbenzylamine chloride) in alcohol solvent and keeping at room temperature for one week, a flake-like transparent OIMH single crystals can be achieved, as shown by the optical photos in Fig. S1. The good crystallinity of the crystals can also be revealed by the stick-like structure from scanning electron microscope (SEM) images in Fig. S2. As determined by single-crystal x-ray diffraction (SCXRD), OIMHs shows a chemical formula of MBAMnCl₃(2H₂O), which possesses the zero-dimensional (molecular) crystal structure with octahedrons [Fig. 1(a)] and is also consistent with energy dispersive x-ray spectroscopy (EDS) measurements (Fig. S3).

MBAMnCl [with chemical formula of MBAMnCl₃(2H₂O)] consists of the orthorhombic phase and P2₁ space group, with the lattice parameters of a = 6(2) Å, b = 36.409 Å, and c = 6.377 Å. The isolated octahedral [MnX]²⁻ is surrounded by large MBA organic molecules. This unique structure enables zero-dimensional (molecular) Mn-based composites to possess a long Mn–Mn distance. The angles between two adjacent Cl⁻ and Mn²⁺ (Cl–Mn–Cl) range from 92.09° to 100.74°. The angles between two adjacent O²⁻ and Mn²⁺ (O–Mn–O) range from 74.39° to 86.94°. The angles between two adjacent Cl⁻, O²⁻, and Mn²⁺ (O–Mn–Cl) range from 84.10° to 99.35°. The bond distances between two Cl⁻ and Mn²⁺ vary from 2.391 to 2.527 Å. The bond distances between two O²⁻ and Mn²⁺ vary from 2.189 to 2.409 Å.

X-ray diffraction (XRD) patterns indicate high purities of our MBAMnCl crystals. According to XRD patterns in Fig. 1(b), the MBAMnCl and precursor MBACl show quite different feature peaks. Specifically, the peaks of the former one locate at 13.2°, 15.5°, 20°, and 24°, while the peak of the latter one locates at 4.7°. This comparison indicates high purity of our MBAMnCl without impurities from MBACl. High purity of OIHM is also confirmed by the powder XRD patterns, which are consistent with simulated SCXRD patterns in Fig. S4. Thus, we can safely explore the photophysical properties of MBAMnCl crystals without further purification.

Now, we characterize the PL spectrum of MBAMnCl crystals with different excitation wavelengths. Under the excitation wavelength of 270 nm, the PL shows a typical red emission with a dominant peak at 658 nm and full width at half maxima (FWHM) of 108 nm in Fig. 1(c). This red emission coincides with the emission feature from reported Mn-based octahedral structures. Interestingly, when the excitation wavelength is increased to 330 nm, a blue-emitting component locating at around 470 nm appears in the PL of MBAMnCl crystals. This blue emitting component, together with the Mn-based red emitting component, makes the overall spectrum very broad (ranging from below 400 nm to over 800 nm). As the excitation wavelength increases to 380 nm, the ratio of blue emitting component dominants. In order to reveal the significant emission color change with different excitation wavelengths, the corresponding CIE coordinates are plotted in Fig. 1(d). At the excitation of 270 nm, a red emission with CIE coordinates of (0.59, 0.33) is obtained. At the excitation of 380 nm, a blue emission with CIE coordinates of (0.26, 0.29) is obtained. Surprisingly, at the excitation of 330 nm, a white emission with CIE coordinates of (0.33, 0.35) is realized.

We exclude the possibility that the blue emission originates from the Mn octahedral units by comparing the absorption and PLE results. The red emission is believed to originate from octahedral structure luminescence of manganese (II) composites.^15–18 Such characteristics are confirmed by similar absorption spectra [in Fig. S5(a)] and PLE spectra at fixed emission of 650 nm [Fig. S5(b)]. In both spectra, the feature peaks of MBAMnCl locate at wavelengths of around 280, 370, and 430 nm, which agree well with the electronic transitions between the ground and excited states of the Mn²⁺ ion.^25–27 Thus, the origin of the red emission results from the Mn–Cl octahedral structure, suggesting that the blue emission originates from an independent emission center other than the octahedron. Furthermore, PL lifetime results in Fig. 2(a) show longer PL lifetime of red emission than blue emission in MBAMnCl, also indicating that the blue and red emissions originate from two distinct states.

Since red emission is from the Mn–Cl unit, we hypothesize that the blue emission might originate from organic compounds. In order to verify our hypothesis, PL characteristics of the organic precursor MBACl are measured. As shown in Fig. 2(b), the dominated blue emission locates at around 415 nm with the FWHM of 185 nm. As a comparison, we can observe that MBAMnCl shows obvious red shift, further demonstrating that the blue emission does not originate from the precursor MBACl. However, the similar blue emission property of MBACl and MBAMnCl suggests their intrinsic connection. By comparing the PLE of the blue emission between MBAMnCl (with emission at 450 nm) and MBACl (with emission at 415 nm) in Fig. 2(c), we find that they possess almost the same PLE patterns, which are totally different from the PLE pattern of red emission in MBAMnCl. This comparison indicates that the contribution of blue emission in MBAMnCl comes from the organic component-MBA.

In order to further confirm our hypothesis, we change the halide composition from Cl to Br (bromide), which is a Mn-based composite that we have reported before.³⁹ As shown in Fig. 3(a), the wavelength dependent PL spectra exhibit mixed green and red emission, with peaks at 530 and 650 nm, respectively. These two emission colors originate from mixed Mn tetrahedral and octahedral units. Interestingly, when we zoom in the PL spectrum in the range from 390 to 450 nm, we notice a blue emission in composite MBAMnBr {with the chemical formula of (MBA)₂[MnBr₄] or MBA[MnBr₃(EtOH)]}. It locates at around 425 nm with the FWHM of 40 nm, and the intensity of the blue emission increases with increasing excitation wavelength.

Similar to the case of MBAMnCl, we exclude the possibility of organic precursor leading to blue emission in our composite by comparing the powder x-ray diffraction (XRD) patterns of MBAMnBr and MBABr [manganese (II) bromide] in Fig. S6 (refer to Fig. 1 in Ref. 39). These two patterns show quite different feature peaks. Specifically, the feature peaks of MBAMnBr locate at 7.4° and 8.9°, while the feature peak of MBABr locates at 10° and 14°. This comparison indicates the high purity of our MBAMnBr without impurities from MBABr. By comparing PL of the ligand MBABr (Fig. S7) with that of MBAMnBr in Fig. 3(a), we find that MBABr has similar blue emission region locating at around 480 nm. In addition, the PLE of blue emission and red emission in MBAMnBr show different patterns, indicating that they come from different emission centers (Fig. S8). In contrast, almost the same PLE patterns between MBAMnBr at emission of 425 nm and MBABr at emission of 440 nm are observed in Fig. S8, which further confirms that organic component MBA contributes to the blue emission in Mn-based composites.

To rationalize similar blue emission characteristics between organic halide salts and Mn-based composites, we replot the schematic diagram of crystal structures of these composites (refer to Fig. 4 in Ref. 39), with an emphasis on the hydrogen bonding between organic component (MBA) and halogens (Cl or Br) [Figs. 3(b) and 3(c)]. A substructure of MBA-halide is clearly visible in the figure, similar to organic salts MBACl and MBABr, serving as an additional blue emission center in the Mn-based composites. As a result, both PL emission color and PLE characters are similar between the new emission centers and MBACl (or MBABr).

To further confirm that the emission center of MBACl is introduced successfully, we compare the Fourier-transform infrared (FTIR) spectra between organic salt MBACl and OIHM MBAMnCl. At the low-frequency region, almost the same peak patterns are observed, especially the peaks at 1622, 1337, and 874 cm⁻¹, which can be indexed to the antisymmetric bending mode δ_as(NH₃⁺), the stretching mode υ(C-N), and the out-of-plane bending mode δ_r(NH₃⁺) of MBA-Cl, respectively (consistent with the previous report).^40,41 Similar peak patterns of FTIR between MBAMnCl and MBACl demonstrate that the emission center MBA exists in our Mn-based composites, conforming the results of PL, PLE, and PXRD. At the same time, we also notice the difference at the high-frequency region, where the stretching mode υ(NH₃⁺) of ammonium ions at 2800–3000 cm⁻¹, the largest amplitude in the spectrum of MBACl salt, is largely weakened and shifted to low wavenumbers in the MBAMnCl composites compared to that in the MBACl salt. The reason is ascribed to the addition of Mn-based octahedrons or tetrahedra in the structure, which interrupts parts of the bonding effect between MBA and Cl. Similar FTIR result for MBAMnBr is shown in Fig. S9.

Traditional anti-counterfeiting technology usually uses monochromatic or dichromatic phosphors to print patterns; in recent years, doping or mixed phosphors with different emissions are also used in optical anti-counterfeiting technologies.^42–51 However, it is still limited by issues such as the deployment of multiple components and the selectivity of doping. Therefore, in order to meet increasing commercial requirements on anti-counterfeiting, it is urgent to develop new environmentally friendly materials with simple components, multi-color emissions, excellent stability, and easy processability. Here, we used a one-step solution method to obtain multi-color emissions anti-counterfeiting material based on only one single component. It can be seen in Fig. S10(a) that there is a very large color span under different excitation wavelengths for the material. Among these colors, the most obvious color difference locates in CIE coordinates of (0.59, 0.33) with a red emission, CIE coordinates of (0.26, 0.29) with a blue emission, and CIE coordinates of (0.33, 0.35) with a white emission. Therefore, MBAMnCl shows three distinct colors under three different excitation wavelengths, making it promising for commercial high-end anti-counterfeiting applications.^52–54 In addition, only cheap UV lights with different emission wavelengths are needed for the identification of our anti-counterfeiting materials. Our material also shows excellent stability after simple encapsulation. As shown in Fig. S10(b), the optical characteristics of three-color emission can still be maintained after encapsulation for one and a half years in the air. Moreover, our material can be dissolved in alcohol solution easily and applied to different substrates (including flexible ones) with different shapes (through shadow masks) using the simple spraying technology.

In order to demonstrate this concept of real anti-counterfeiting application, we dissolve the material MBAMnCl into alcohol solution and then spray it on the flexible polyethylene terephthalate (PET) substrate with our university logo (Fig. 4). Under UV light of different excitation wavelengths, the logo exhibits distinctly different colors: red logo at 270 nm, blue logo at 380 nm, and white logo at 340 nm. Therefore, after meeting the necessary criteria of uniqueness, stability, and detectability of the anti-counterfeiting materials, we believe that our material has a great potential to be applied in high-end anti-counterfeiting technology.

We successfully achieve an additional blue luminescence center in Mn-based composites, through selecting specific organic component methylbenzylamine (MBA) to form another emission center. This blue emission is fundamentally different from green and red emission in other Mn-based composites, which result from Mn–halide frameworks. The coexistence of different luminescence centers in our Mn-based composites is confirmed by photoluminescence (PL) and PL excitation (PLE) results. As a result of different PLE responses of different emission centers, we can tune the resulting emission color with selecting different excitation wavelengths. Our new approach can be generalized to other materials, e.g., by changing halogens from Cl into Br. Distinct multi-color emission of our Mn-based composites demonstrates their promise for applications in the anti-counterfeiting technology. As such, our results provide a novel strategy for full-spectral emission in Mn-based OIMHs and lay a solid foundation for a range of new applications.

All chemicals (α-methylbenzylamine (MBA, 99%), hydrochloric acid (48 wt. % in H₂O, ≥99.99%), manganese (II) chloride (MnCl₂, 99.9%), and ethanol (C₂H₆O, 99.7%)) were commercially purchased from Sigma-Aldrich and used without further purification.

Fabrication of MBAMnCl₃(2H₂O) single crystals. MBA (1 mmol), manganese (II) chloride (1 mmol), and hydrochloric acid (1 mmol) were dissolved in ethanol solution (30 ml), respectively. By slow evaporation of ethanol at room temperature for about one week, transparent crystals were obtained. The yield of all Mn-based organic–inorganic halide materials is about 60%–70%. The stoichiometric equations for material synthesis are as follows:

Fabrication of university logo films. 100 mg of the crystal of composite MBAMnCl₃(2H₂O) was dissolved in 350 μl ethanol solutions. Then, the mixed solutions were heated at 60 °C and stirred for 30 min. Logo films were obtained by spraying the solutions on PET substrates through shadow masks, followed by annealing at 70 °C for 10 min.

The XRD patterns of the products were recorded with X'Pert PRO x-ray diffractometer using Cu Kα1 irradiation (λ = 1.5406 Å). The ultraviolet–visible absorption spectra were measured with PerkinElmer model Lambda 900. Steady-state photoluminescence spectra were recorded with a 405 nm laser as excitation and an Andor spectrometer (Shamrock sr-303i-B, coupled to a Newton EMCCD detector) as detection. Scanning electron microscope (SEM) images and energy dispersive x-ray spectroscopy (EDS) analysis were performed using a LEO 1550 SEM operated at 18 kV accelerating voltage, with an Oxford Instruments X-Max 80 mm² SDD detector. Attenuated total reflectance (ATR-FTIR) mode was used for the FTIR spectroscopy characterizations. All spectra were recorded via a PIKE miracle ATR accessory with a diamond prism in a Vertex 70 spectrometer (Bruker) using a DLaTGS detector at room temperature.

The single-crystal x-ray diffraction data for DP-60 and DP-150 were collected at 298 K by using Cu Kα radiation on a Bruker D8 VENTURE single crystal x-ray diffractometer (SCXRD) equipped with a kappa geometry goniometer. Data reductions and absorption corrections were performed with the APEX3 suite. Structures were solved by a direct method using the SHELXL-97 software package. The crystal structure was refined using full-matrix least squares based on F2 with all non-hydrogen atoms anisotropically defined. Hydrogen atoms were placed in calculated positions by means of the “riding” model. The details about data collection, structure refinement, and crystallography are summarized in Table S1.

See the supplementary material for detailed descriptions of crystal morphology and information, EDS, XRD, absorption, and FTIR.

The authors acknowledge the support from Knut and Alice Wallenberg Foundation (No. Dnr KAW 2019.0082) and the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009–00971). W.C. was supported by the China Scholarship Council (CSC). F.G. is a Wallenberg Academy Fellow.

The authors have no conflicts to disclose.

Weidong Cai: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Investigation (lead); Methodology (lead); Validation (lead); Visualization (lead); Writing – original draft (lead). Chaoyang Kuang: Data curation (supporting); Methodology (supporting). Tianjun Liu: Conceptualization (supporting); Data curation (supporting). Yuequn Shang: Conceptualization (supporting); Data curation (supporting). Jia Zhang: Conceptualization (supporting); Data curation (supporting). Jiajun Qin: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Supervision (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Feng Gao: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that supports the findings of this study are available within the article and its supplementary material.