(C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O with large birefringence induced by coplanar π-conjugated [C₃N₆H₇]⁺ groups

Abstract

Birefringent crystals are vital to modern optical devices as they can effectively modulate the polarization of light. Currently, limited by comprehensive properties such as crystal size and birefringence, the design and synthesis of birefringent crystals are challenging, especially in the short-wavelength spectral region. Herein, (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O were successfully synthesized, and they exhibit large birefringence and short UV cutoff edges (0.37/0.31 at 546 nm and 244/233 nm). Notably, the crystal of (C₃N₆H₇)BF₄·H₂O synthesised using a facile solvent growth method is over 20 mm in length. Theoretical analysis suggests that the parallel arrangement of [C₃N₆H₇]⁺ groups modulated by tetrahedral units induces large optical anisotropy. This work provides promising candidates with exceptional properties for short-wavelength ultraviolet applications.

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Introduction

Birefringent crystals are important optoelectronic functional crystals with specific effects and are widely applied in the preparation of optoisolators, polarizers, rotators, delayers and other passive photoelectric devices.^1–3 Commercial crystals in the ultraviolet region primarily include CaCO₃,⁴ α-BaB₂O₄ (α-BBO)⁵ and MgF₂.⁶ With the rapid development of modern optics, finding new excellent birefringent crystals has become the focus in order to meet the need of diverse applications.

Generally, the selection of functional building units (FBUs) is very crucial because the chemical composition and spatial arrangement of FBU directly influence the properties of crystals. Owing to the large polarization anisotropy, π-conjugated units, as prime candidates, have been proven to be effective for the large birefringence of the crystal. For instance, [BO₃]³⁻,⁷ [CO₃]²⁻,⁸ and [NO₃]⁻,⁹ are beneficial for birefringence performance. Besides, some π-conjugated organic groups with planar configuration, such as [C₂O₄]²⁻, [C₂O₆]²⁻, [C₄O₄]²⁻, [C₆O₆]²⁻, [CO(NH₂)₂], [C(NH₂)₃]⁺, [C₈H₆BrN₂O]⁺ and [HN(CH₂COOH)₂], show high optical anisotropy, which have greater potential to improve birefringence.^10–15 More recently, inspired by the contribution of B₃O₆ groups toward the large birefringence of the α-BBO crystal, π-conjugated organic units with similar geometric planar configurations to B₃O₆ groups were introduced into the crystal structure to improve birefringence performance. Like [H_xC₃N₃O₃]^x−3 (x = 0–3), [C₅H₆ON]⁺, [C₄H₆N₃]⁺ and [C₃N₆H₇]⁺ groups^16–21 have been proven to be ideal FBU for constructing large birefringent crystals. Among them, [C₃N₆H₇]⁺ groups, as excellent birefringent active units, have several advantages. In addition to intrinsic polarization anisotropy, hydrogen bonds formed by the N atoms inside the ring and NH₂ groups outside the ring not only facilitate the coplanar arrangement to improve the birefringent property of the crystal, but are also beneficial to crystal growth to help acquire large-sized crystals. Furthermore, the interatomic distances in the melamine ring are shorter than those in β-BBO, which bring about a greater overlap between the C 2p and N 2p orbitals to generate stronger p_π–p_π interactions and optical anisotropy.²² Thus far, melamine-based compounds with excellent properties have been synthesized, such as 2(C₃H₇N₆)⁺·2Cl⁻·H₂O,²² (H₇C₃N₆)(H₆C₃N₆)ZnCl₃,²³ (C₃N₆H₈)PbBr₄,²⁰ (C₃N₆H₈)SnCl₄,²⁴ (C₃N₆H₇)₂SbF₅·H₂O,²⁵ (C₃N₆H₇)₂SiF₆·H₂O,²⁶ and (C₃N₆H₇)(C₃N₆H₆)HgCl₃.²⁷ According to research findings, the arrangement of [C₃N₆H₇]⁺ groups regulated by different anions can significantly affect crystal properties. It is necessary to introduce suitable anionic units into the crystal configuration to enhance the optical anisotropy of the crystal. However, owing to the narrow band gap between π and π* bonds, it is difficult to balance the birefringence and band gap of the crystal.

To improve this, combining π-conjugated units and non-π-conjugated units should be an effective strategy to design crystals with large optical anisotropy. Generally speaking, π-conjugated units have large hyperpolarizabilities (|β_max|) and polarizability anisotropies (δ) but small HOMO–LUMO gaps. On the contrary, the non-π-conjugated units, such as the tetrahedral [BO₄]⁵⁻ and [PO₄]³⁻ units,^28,29 have wide HOMO and LUMO band gap, but small |β_max| and δ.³⁰ For example, KH₂PO₄ (KDP) has a short UV cutoff edge (174 nm) but shows a small Δn (<0.04 at 1064 nm).³¹ Pan's group proposed the non-π-conjugated BO_4−xF_x (x = 1–4) groups as the preferred units for deep UV materials, which is conducive to obtaining a large band gap and proper birefringence.^2,32 A series of excellent UV/DUV optical materials, such as C(NH₂)₃BF₄,¹² (C₃N₆H₇)₂(B₃O₃F₄(OH)),³³ Li₂B₆O₉F₂,³² LiNaB₆O₉F₂,³⁴ M₂B₁₀O₁₄F₆ (M = Ca, Sr),³⁵ NH₄B₄O₆F,³⁶ CsB₄O₆F,³⁷ Li_0.5Na_0.5AlB₂O₄F₂,³⁸ and Ba₂B₉O₁₃F₄·BF₄,³⁹ has been designed and synthesized. C₃N₆H₇SO₃NH₂ was synthesized by Wang's groups, which has a large experimental bandgap of 5.53 eV and a short UV cutoff edge of 206 nm.⁴⁰ Similar to the [SO₃NH₂]⁻ units, the non-π-conjugated [SO₃CH₃]⁻ units also shows high band gap. For example, Ba(SO₃CH₃)₂ has a very short absorption edge of 159 nm and large band gap of 7.8 eV.⁴¹ Therefore, it is necessary to explore the influence of different anions on the [C₃N₆H₇]⁺ groups arrangement to obtain the large optical anisotropy of the crystal.

Based on the above-mentioned methods, we synthesized two melamine-based compounds, (C₃N₆H₇)BF₄·H₂O (MELBF) and (C₃N₆H₇)SO₃CH₃·H₂O (MELSC) using the π-conjugated organic [C₃N₆H₇]⁺ units and the non-π-conjugated [BF₄]⁻ and [SO₃CH₃]⁻ tetrahedral units. Luckily, centimeter-scale crystal (MELBF) was obtained by a facile solvent method. The crystals were characterized with single crystal X-ray diffraction, powder XRD, infrared spectroscopy, UV-visible transmission spectroscopy and thermal analysis. The results show that crystals have excellent optical properties. Furthermore, theoretical calculations uncover the relationship between the structure and properties of the materials.

Experimental

Synthetic procedures

MELBF was prepared using the recrystallization method. A mixture of C₃N₆H₆ (40 mmol, 5.04 g) and 40% HBF₄ (8 ml) solution was dissolved in deionized water (100 ml). The solution was kept at 80 °C for 4 h, then cooled to obtain centimeter-sized crystals with 47% yield. The polycrystalline powder was obtained by grinding the crystals. The polycrystalline powder was dissolved in water and recrystallized to obtain the crystals for single crystal structure determination. MELSC was prepared using a slow aqueous solution evaporation method. C₃N₆H₆ (10 mmol, 1.26 g) and 4 mL 70% methanesulfonic acid solution were dissolved in 80 mL deionized water. The solution was covered with a plastic film with several small holes and evaporated slowly at room temperature. After two weeks, crystallization was observed at the bottom of the solution. Micrometer-sized crystals were selected for single crystal structure determination. The polycrystalline powder was obtained using the aqueous solution method. A mixture of C₃N₆H₆ (40 mmol, 5.04 g) and 5 mL of 70% methanesulfonic acid solutions were dissolved in deionized water (100 mL). The solution was kept at 65 °C for 4 h and then cooled to obtain the polycrystalline powder with 40% yield.

Structure determination

The single-crystal diffraction data of the selected single crystals were collected using a Bruker D8 Venture X-ray diffractometer with graphite monochromated Mo Kα radiation. The SAINT and SCALE program was used to integrate the diffraction data and digital absorption correction.⁴² SHELXTL program was used to analyze the crystal data.⁴³ PLATON software was used to verify the crystal symmetry.⁴⁴

Powder XRD

The powder XRD pattern was used to verify the phase purity of the crystals. The diffraction data were collected using a Bruker D2 Phaser XRD diffractometer with Cu Kα radiation (λ = 1.5406 Å). The scan 2θ range was 5 to 70° with a 0.02° step and 1 s scan time.

Thermal analysis

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a NETZSCH STA 449F3 Simultaneous Analyzer at a heating rate of 5 °C per minute over a temperature range from 40 to 600 °C.

Infrared (IR) spectrum

The infrared absorption spectra were recorded using a SHIMADZU IRAffinity-1 Fourier transform infrared spectrometer with a resolution of 2 cm⁻¹ in the range of 400–4000 cm⁻¹. Crystal samples and KBr were ground and pressed into transparent plates at a mass ratio of 1 : 200.

UV-Vis-NIR diffuse reflectance spectrum

The UV-Vis-NIR diffuse reflectance spectrum and transmittance spectrum of the title compound were recorded at 200–1600 nm using a SolidSpec-3700DUV spectrophotometer.

Birefringence measurements

Birefringence values were measured using a ZEISS Axio Scope polarizing microscope with a Berek compensator. Small crystals were selected for the measurement at 546 nm. The formula was used to calculate the birefringence: R = |N_e − N_o| = Δn × T, where R represents the optical path difference, Δn represents the birefringence, and T represents the thickness of crystal.

Computational description

Theoretical calculations were carried out using the CASTEP software package based on density functional theory (DFT).^45,46 The Perdew–Burke–Ernzerhof (PBE) functional and generalized gradient approximation (GGA) were chosen.⁴⁷ H 1s¹, B 2s²2p¹, C 2s²2p², N 2s²2p³, O 2s²2p⁴, F 2s²2p⁵ and S 3s²3p⁴ electrons were treated as valence electrons. The kinetic energy, Monkhorst–pack mesh, and pseudo-potentials are 900 eV, 4 × 3 × 2, and norm conserving for MELBF, and 830 eV, 3 × 2 × 2 and norm conserving for MELSC, respectively.

Results and discussion

Crystal growth and chemical phase

By mixing the melamine and tetrafluoroboric acid, colorless block crystals of MELBF were easily obtained by the natural cooling crystallization method. The centimeter-scale crystal was grown and is shown in Fig. 1a. The size of the crystal was 24 mm × 4 mm × 0.17 mm. The crystal exhibits a growth habit characterized by two-directional extension, facilitating its facile cultivation in the form of slender stripes. Experimental and calculated XRD patterns were compared to check the purity of the powder (Fig. 1b and c). The experimental XRD patterns of the target compounds are in agreement with the corresponding calculated XRD patterns, indicating the high purity of the target compounds. In accordance with the Bravais–Friedel–Donnay–Harker (BFDH) theory, the theoretical morphology of MELBF was calculated with the Mercury program (Fig. S1†).⁴⁸

Fig. 1 (a) The as-grown crystals of MELBF. Calculated and experimental XRD patterns for MELBF (b) and MELSC (c).

Crystal structure

Both MELBF and MELSC crystallize in the centrosymmetric triclinic space groups (no. 2) and have similar structural characteristics. Because the B and F atoms have large thermal vibrations at room temperature, the MELBF structure data was collected at 153 K. The data and structural optimization of MELBF and MELSC are listed in Tables S1–S7.† In the MELBF structure, the numbers of independent atoms of C, N, H, B, O and F atoms are 3, 6, 9, 1, 1 and 4, respectively. In the MELSC structure, the numbers of independent atoms of C, N, H, S and O atoms are 4, 6, 12, 1 and 4, respectively. As shown in Fig. 2a, the basic structural units of MELBF are the planar [C₃N₆H₇]⁺ group and tetrahedral [BF₄]⁻ unit. Fig. 2b shows one-dimensional [C₃N₆H₇]⁺ groups chains formed by hydrogen-bonds. Fig. 2c shows multiple parallel [C₃N₆H₇]⁺ chains combined by hydrogen bonds from the [BF₄]⁻ units and [H₂O] units, extending indefinitely within the layer to form two-dimensional planes. The π-conjugated [C₃N₆H₇]⁺ groups are parallelly arranged so that the π electrons are arranged in order, which enhances the optical anisotropy of the structure. The layers are linked together by hydrogen bonding to form a 3D network structure, as shown in Fig. 2d. Similar to the MELBF structure, the basic structural units of MELSC consist of planar [C₃N₆H₇]⁺ groups and tetrahedral [SO₃CH₃]⁻ units. As shown in Fig. 3b, a one-dimensional chain is formed by the hydrogen-bonded [C₃N₆H₇]⁺ groups. Multiple parallel [C₃N₆H₇]⁺ chains extend indefinitely within the layer to form a two-dimensional plane (Fig. 3c). Unlike MELBF, the long chains of [C₃N₆H₇]⁺ are connected with each other by van der Waals force. The layers are connected by hydrogen bonds to form a 3D network structure (Fig. 3d).

Fig. 2 Crystal structure of MELBF. (a) Basic constituent units [C₃N₆H₇]⁺ and [BF₄]⁻. (b) The one-dimensional chain formed by [C₃N₆H₇]⁺ groups. (c) The two-dimensional structure formed by [C₃N₆H₇]⁺ and [BF₄]⁻ groups. (d) Three-dimensional structure.

Fig. 3 Crystal structure of MELSC. (a) Basic constituent units [C₃N₆H₇]⁺ and [SO₃CH₃]⁻. (b) The one-dimensional chain formed by [C₃N₆H₇]⁺ groups. (c) The two-dimensional structure of [C₃N₆H₇]⁺ and [SO₃CH₃]⁻ groups. (d) Three-dimensional structural drawings.

Thermal analysis

The TG and DSC tests were performed to investigate the thermal properties of the materials. As shown in Fig. 4a, the DSC curve of MELBF shows an endothermic peak at 300 °C. The first weight loss between 210 °C and 309 °C was 36.8%. It is inferred that this may be due to the removal of water molecules and BF₃ gas (cal 37.1%). After that, MELBF will experience a continuous loss of weight due to the decomposition of the remaining organic molecules. For the MELSC, the weight losses in the first and second segments were 3.8% and 3.1%, respectively, for a total of 6.9%, probably due to the loss of water molecules (cal 7.5%), while the peaks at 316 °C and 436 °C correspond to the decomposition of the residual product (Fig. 4b).

Fig. 4 The TG-DSC curves for (a) MELBF and (b) MELSC.

Optical properties

The IR spectra of MELBF and MELSC were recorded between 4000 and 400 cm⁻¹ (Fig. S2†). The peaks located at 3301/3447 cm⁻¹ and 1510/1513 cm⁻¹ can be specified as the stretching and deformation vibrations of the N–H bond, respectively. The peak at 3189/3165 cm⁻¹ is considered to be the hydrogen bond vibration of N–H⋯X type bonds, where X means N, O or F. The peaks at 1677/1670 cm⁻¹ as well as the peaks at approximately 1361/1390, 570/553, 607/618 and 782/779 cm⁻¹ demonstrate the NH₂ bending and tensile vibrations in the [C₃N₆H₇]⁺ groups, respectively.⁴⁹

The UV-vis-NIR transmission and diffuse reflectance spectra indicate that MELBF and MELSC have wide transparent regions and short ultraviolet cutoff edges (244 and 233 nm, respectively), as shown in Fig. 5 and 6. The experimental band gaps of MELBF and MELSC are 5.05 and 4.80 eV, respectively (Fig. S3†). The results show that MELBF and MELSC can be used for UV optical materials.

Fig. 5 The UV-vis-NIR transmission spectrum of MELBF.

Fig. 6 The UV-vis-NIR diffuse reflectance spectrum of MELSC.

Experimental and calculated birefringence

The birefringences of MELBF and MELSC were calculated, and the birefringence values of MELBF and MELSC are 0.37 and 0.31 at 546 nm (Fig. 7), respectively. The birefringence of MELBF and MELSC is larger than that of some birefringent crystals, such as (C₃N₆H₇)(C₃N₆H₆)HgCl₃ (0.25@1064 nm),²⁷ (C₃N₆H₇)₂SiF₆·H₂O (0.15@550 nm),²⁶ and (C₃H₇N₆)₆(H₂PO₄)₄(HPO₄)·4H₂O (0.22@1064 nm).⁵⁰ The refractive index difference measurement results show that the retardation values for MELBF and MELSC are approximately equal to 3566 and 2598 nm, respectively. The crystal thicknesses used for the measurements were 11.39 and 12.88 μm, respectively (Fig. 8). The refractive index differences are 0.31 and 0.20 at 546 nm, respectively. The experimental refractive index difference of MELBF is close to the calculated value. However, the experimental refractive index difference of MELSC is slightly lower than the calculated value. The reason may be that the symmetry of the crystal in the triclinic system is low, and it is difficult to obtain crystal samples with proper optical orientation. Meanwhile, the tested samples do not have the optimal optical orientation; thus, the experimental refractive index difference of MELSC crystals is lower than the calculated value.

Fig. 7 The calculated birefringence of MELBF and MELSC.

Fig. 8 Refractive index difference measurements for MELBF and MELSC. MELBF single crystal under the (a) positive (up) and (b) negative (down) compensatory rotation; MELSC single crystal under the (d) positive (up) and (e) negative (down) compensatory rotation; (c and f) the thickness of MELBF and MELSC.

The relationship between optical property and structure

It is widely known that the birefringence of crystals mainly depends on the density of birefringent active units and arrangement in the structure. For the further explanation of the relationship between [C₃N₆H₇]⁺ groups and optical properties, [C₃N₆H₇]⁺ groups’ density of compounds was calculated and their orientations were compared. The [C₃N₆H₇]⁺ densities of MELBF and MELSC were 3.39 × 10⁻³ and 3.94 × 10⁻³ Å⁻³, respectively, lower than most melamine-based compounds (Table S8†). For example, for 2(C₃H₇N₆)⁺·2Cl⁻·H₂O, (C₃H₇N₆)₂Cl₂·H₂O and (C₃H₇N₆)F·H₂O, their [C₃N₆H₇]⁺ densities were 5.58 × 10⁻³, 5.63 × 10⁻³ and 6.00 × 10⁻³ Å⁻³, respectively. Despite the lower density of the [C₃N₆H₇]⁺ groups in MELBF and MELSC, they still exhibit high optical anisotropy, which is primarily attributed to the parallel arrangement of [C₃N₆H₇]⁺ groups in the structure. In order to investigate the influence of the tetrahedral units on the arrangement of the [C₃N₆H₇]⁺ groups, we compared the arrangement of the [C₃N₆H₇]⁺ groups in 18 reported melamine-based crystal structures (Fig. S4†). The results show that the [C₃N₆H₇]⁺ groups are arranged parallel in the crystal structure containing tetrahedral units, such as MELBF, MELSC, (C₃H₇N₆)₂SO₄·H₂O,⁵⁰ and C₃N₆H₇SO₃NH₂.⁴⁰ The band gaps of MELBF and MELSC are 5.05 and 4.80 eV, respectively, higher than that of 2(C₃H₇N₆)⁺·2Cl⁻·H₂O (4.75 eV), (C₃H₇N₆)₂Cl₂·H₂O (4.70 eV), and (C₃H₇N₆)F·H₂O (4.72 eV) compounds.^22,49 To summarize, it indicates that the introduction of non-π-conjugated tetrahedral units into the crystal structure effectively restricts the density of π-conjugated units, increases the bandgap and regulates the parallel arrangement of [C₃N₆H₇]⁺ groups. This experimental result is consistent with the principle of π-conjugated confinement proposed by Chen's group.³⁰

Theoretical calculation

In order to provide deep insights into the structure–property relationship, the electronic band structure, density of states (DOS) and electron density difference (EDD) were calculated using the density functional theory in the CASTEP module, based on first principles calculations. The polarizability anisotropy and HOMO–LUMO gap for the [C₃N₆H₇]⁺, [BF₄]⁻, and [SO₃CH₃]⁻ groups were calculated using the DFT method implemented in the Gaussian09 package at the B3LYP/6-31G level.^51,52 The calculated bandgap using the GGA approach are 4.31 eV for MELBF and 4.30 eV for MELSC (Fig. S5†), lower than the experimental value due to the discontinuity of the exchange correlation energy.³³ The scissor operator with values of 0.74 eV for MELBF and 0.50 eV for MELSC was used to adjust the conduction bands to their actual level based on the experiment band gap of 5.05 and 4.80 eV, respectively.

The optical properties of the compound are determined by electronic transitions close to the Fermi level. The top region of the valence band is dominated by the C 2p, N 2p, O 2p, F 2p and S 3p orbitals, while the bottom of the conduction band is dominated by the C 2p and N 2p states (Fig. 9). The orbitals of C and N overlap in the conduction and valence bands, suggesting the presence of the strong C–N covalent bond in the [C₃N₆H₇]⁺ groups. Moreover, the large overlap between the O 2p, F 2p and N 2p states corresponds to N–H⋯F and N–H⋯O hydrogen bonds.

Fig. 9 Total and partial DOS plots for (a) MELBF and (b) MELSC.

The uniformly distributed π-electron cloud around [C₃N₆H₇]⁺ groups was observed in Fig. 10b and c, which is favourable for large optical anisotropy. The polarization anisotropy and HOMO–LUMO gap of [C₃N₆H₇]⁺, [BF₄]⁻, and [SO₃CH₃]⁻ groups were calculated (Fig. 10a). The results show that the δ of [C₃N₆H₇]⁺ groups is 59.2. Considering the planar arrangement of [C₃N₆H₇]⁺ groups in the crystal structure, it can be inferred that the large birefringence is mainly contributed by the [C₃N₆H₇]⁺ groups. The HOMO–LUMO gap values of [BF₄]⁻ and [SO₃CH₃]⁻ are 15.4 and 8.4, respectively. In addition, the low density of [C₃N₆H₇]⁺ groups allows the large spacing of the π-conjugated [C₃N₆H₇]⁺ groups in the structures of MELBF and MELSC, which is beneficial for increasing the band gap.

Fig. 10 (a) The polarizability anisotropy and HOMO–LUMO gap for the [C₃N₆H₇]⁺, [BF₄]⁻, [SO₃CH₃]⁻ groups. The EDD of MELBF (b) and MELSC (c).

Conclusion

Two UV birefringent crystals, (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O, were synthesized by the simple solvent method. The crystal of (C₃N₆H₇)BF₄·H₂O is over 20 mm in length. (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O exhibit short UV cutoff edge (244/230 nm). (C₃N₆H₇)BF₄·H₂O exhibits high thermal stability due to strong hydrogen bonding in the crystal structure. The experimental band gaps of (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O are 5.05 and 4.80 eV, respectively, higher than that of most melamine-based compounds. According to the first-principles calculation, the birefringence of (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O were calculated to be 0.37 and 0.31@546 nm, respectively. The analyses indicate that the parallel arrangement of planar π-conjugated [C₃N₆H₇]⁺ groups is mainly responsible for the large optical anisotropy of (C₃N₆H₇)BF₄·H₂O and (C₃N₆H₇)SO₃CH₃·H₂O. The synthesis of UV birefringent crystals using organic π-conjugated planar groups with inorganic non-π-conjugated tetrahedra is a promising direction.

Author contributions

Xueling Hou supervised the research and revision of manuscript. Hangwei Jia designed and performed the experiments, wrote the first draft and revised it. Die Xu did the theoretical calculations. Zijian Li, Muhammad Arif and Yansheng Jiang assisted with the experimental work. All authors participated in the discussion.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.†

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22335007, 61835014), the Xinjiang Major Science and Technology Project (2021A01001).

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Footnote

† Electronic supplementary information (ESI) available: The theoretical morphology (Fig. S1), IR spectra (Fig. S2), experiment band structure (Fig. S3), the orientation of [C3H7N6]⁺ groups in reported compounds (Fig. S4), electronic band structure (Fig. S5), Tables S1−S7: crystallographic data, atomic coordinates, displacement parameters, selected bond lengths and angles, hydrogen bonds. CCDC 2344610 and 2344611. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi01061b

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