Syntheses, Crystal Structures, SHG Response and Purple Luminescent Property of Tetra(isothiocyanate) Mn(Ⅱ) and Substituted Benzyl Triphenylphosphonium Cations

1. INTRODUCTION

Noncentrosymmetric (NCS) molecular materials have received much interest because of their potential applications in the areas of nonlinear optical (NLO) materials, ferroelectrics, piezoelectric, etc. Among these NCS materials, NLO crystal material with distinct SHG (second harmonic generation) response is one of the important optoelectronic information functional materials, and nonlinear optical elements in modulation switch, remote communication, information processing and other fields show a trend of accelerating development^[1]. For instance, a NLO crystal material coupling other physical property such as photo-luminescent property, SHG responses ferroelectric, etc. will make them bear multi-function and have many practical applications^{[2, 3]}.

There is considerable interest in utilizing the crystal engineering techniques and the principles of supramolecular chemistry to prepare molecular materials with specific structural, optical, conductive, and magnetic properties, in which some non-covalent interactions such as weak p···π or π···π stacking interactions and H-bonding interactions are employed to obtain some new materials. On one hand, ligands containing heteroaromatic rings with large π···π stacking interactions construct all kinds of supramolecules. Moreover, it can be an important part of light-emitting materials to effectively enhance the fluorescence response of the compounds^[4-7]. On the other hand, inorganic complex anions with Cl^-, Br^-, CN^- and SCN^- are proved to be very useful and potential building block for organic-inorganic hybrid materials^{[8, 9]}. Besides, manganese(Ⅱ) compounds have attracted great interest due to their interesting structural diversity and potential applications in light-emitting materials. Mn(Ⅱ) complexes with excellent luminescent properties have attracted extensive attention due to their low cost and low toxicity. They have potential applications in photoluminescence, electroluminescence, luminescent sensors and biomarkers^[10]. However, the inorganic-organic hybrid materials containing tetra (isothiocyanate) Mn(Ⅱ) anion and heteroaromatic rings organic cation are scarcer.

Keep these in mind, we choose benzyltriphenylphosphine and thiocyanate as ligands, assembled with Mn(Ⅱ) salt in a mixed solution, and obtained a novel compound 2(BPP)⁺· [Mn(NCS)₄]^2-. As we expected, compound 1 exhibits strong SHG responses and excellent fluorescence properties. Here we described its crystal structure, luminescent properties and strong SHG response which may be influenced by the change of substituted group on the phenyl ring of benzyl group in the cations, and shows that it is an excellent fluorescent and nonlinear material.

2. EXPERIMENTAL

2.1 Materials and physical techniques

All reagents and solvents employed in this experiment were obtained from commercial sources and used directly without further purification. PL emission spectra were measured at room temperature on a spectra fluorophotometer (JASCO, FP-6500). At room temperature, Ultraviolet-visible (UV/Vis) diffuse reflectance spectroscopy of 1 was measured by a Shimadzu (Tokyo, Japan) UV-2550 spectrophotometer in a range of 200~800 nm. BaSO₄ was used as the 100% reflectance reference. The powder crystals of 1 were used for the measurement. The elemental analyses were measured on a Vario EL ⅡI elemental analyzer. Thermogravimetric analysis (TGA) measurements were performed on a TA-Instruments STD2960 system from 298 to 1100 K. Powder X-ray diffraction (PXRD) data were recorded on a Rigaku D/MAX 2000 PC X-ray diffraction instrument. The PXRD diffraction was measured with CuKα radiation (λα₁ = 0.1540598 nm, λα₂ = 0.1544426 nm) under the generator voltage (40 kV) and tube current (40 mA) by using continuous scan type from 5.0 to 50.0° at room temperature.

2.2 Synthesis of the title complex

Compound 1 was prepared by the conventional solution method (Scheme 1). MnCl₂·4H₂O (1 mmol, 0.1258 g) and KSCN (4 mmol, 0.3887 g) were stirred in methanol (15 mL) solvent for 45 minutes. The precipitate was filtered out and the clear solution was taken. Then the methanol solution of benzyltriphenylphosphonium chloride (2 mmol, 0.7774 g) was dropped into the clear solution and stirred for 30 minutes. The obtained filtrate was volatilized slowly at room temperature. For 1, yield, 0.01832, 78% based on BPP. Calcd. (%) for 1: C, 65.25; H, 4.43; N, 5.64. Found (%) for 1: C, 65.44; H, 4.67; N, 5.55. IR (KBr, cm^-1, s for strong, m medium, w weak): 3410 (W), 3100 (W), 2825 (W), 2050 (s), 1400 (m), 1487(m), 1588(m), 1125 (m), 750 (m), 500 (m). As shown in Fig. S2, powder X-ray diffraction (PXRD) patterns were collected at room temperature apparatus, and the fitting results of compound 1 can be well matched, proving that compound 1 is a pure phase.

Scheme1

Scheme1.  Preparation of compound 1

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2.3 Crystal structure determination

Single crystal of 1 was obtained directly from the above preparation. The single-crystal X-ray diffraction studies were performed with a Bruker Smart Apex Ⅱ single crystal diffractometer operating with a graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The crystal was kept at 299.9 K during data collection. Using Olex2^[11], the structure was solved with the SHELXS structure solution program by direct methods and refined with the SHELXL refinement package using least-squares minimization^[12]. All non-hydrogen atoms except the guest molecules were refined by full-matrix least-squares techniques with anisotropic displacement parameters and the hydrogen atoms were geometrically fixed at the calculated positions attached to their parent atoms, and treated as riding atoms^[13]. Crystal data for 1, space group Cc with a = 11.0435(11), b = 23.857(2), c = 20.012(2) Å, V = 5083.0(8) ų, Z = 4, μ(MoKα) = 0.527 mm^-1, C₅₄H₄₄MnN₄P₂S₄, M_r = 994.05, D_c = 1.299 g/cm³, F(000) = 2060 and GOOF = 1.038. For 1, 44587 reflections measured (6.40°≤2θ≤55.26°), 11772 unique (R_int = 0.0427, R_sigma = 0.0514) which were used in all calculations. The final R = 0.0505 (I > 2σ(I)) and wR = 0.1199 (all data). The Flack parameter was 0.00(3) (Fig. 1a). The selected bond lengths and bond angles for 1 are given in Table 1. The hydrogen bond parameters are shown in Table S1.

Table 1

Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for Compound 1

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Bond	Dist.		Bond	Dist.
Mn(1)–N(1)	2.046(7)		N(4)–C(4)	1.131(8)
Mn(1)–N(2)	2.078(7)		P(1)–C(5)	1.790(5)
Mn(1)–N(3)	2.039(7)		P(1)–C(6)	1.820(5)
Mn(1)–N(4)	2.043(7)		P(1)–C(8)	1.791(5)
Angle	(°)		Angle	(°)
N(4)–Mn(1)–N(1)	114.5(3)		N(3)–Mn(1)–N(1)	105.9(3)
N(2)–Mn(1)–N(4)	110.2(2)		N(2)–Mn(1)–N(1)	105.0(3)
N(2)–Mn(1)–N(3)	110.76(9)		N(3)–Mn(1)–N(4)	110.2(3)

3. RESULTS AND DISCUSSION

3.1 Description of the structure

As shown in Table 1, compound 1 crystallizes in monoclinic system, space group Cc. Each asymmetric unit consists of one [Mn(NCS)₄]^2- anion and two [BPP]⁺ cations. Fig. 1(a) depicts the coordination environment of the Mn(Ⅱ) atom with atomic numbering scheme. Every Mn²⁺ ion binds to four N (N(1), N(2), N(3), N(4)) atoms of thiocyanate, and the [Mn(NCS)₄]^2– anion presents seriously distorted tetrahedral coordination geometry^{[14, 15]}. For [Mn(NCS)₄]^2– anions, the average Mn–N bond length is 2.05 Å, the N–Mn–N bond angles fall in the 105.000~114.500° range, and the N–C–S bond angles of four NCS^– groups is averaged by 178.500(6)°. In addition, the average C–N and C–S bond distances are 1.13 and 1.60 Å, respectively, similar to those analogues containing [Mn(NCS)₄]^2– anions^[33-35]. The nearest Mn···Mn distance between two adjacent anions is 11.34 Å^[16]. For each [BPP]⁺ cation, the average C–P bond length is 1.80 Å, and the C–P–C bond angles are in the range of 108.50~113.9°, so the four C atoms together with the P atom form a regular tetrahedron.

Figure 1

Figure 1.  (a) Coordination environment of 1 and (b) the 1D chain of [Mn(NCS)₄]^2-anions through intermolecular S···S interactions between anions. Symmetry codes: A: x, 1–y, 1/2+z; B: x, y, 1+z; C: x, 1–y, 3/2+z

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The most interesting fact is that the [Mn(NCS)₄]^2- anions form a linear chain through S···S interaction (Fig. 1b), and the S···S distance of 3.412 Å is shorter than the sum of van der Waals radii of two sulfur atoms, indicating a strong supramolecular interaction^{[17, 18]}. It is also worth noting that the C–H···S hydrogen bond is found between the S(3) atom of the anion and the adjacent cation (Fig. 2a): C(16B)– H(16B)···S(3A) (symmetry code: A = x, –y, –1/2+z, B: 1+x, y, z) with C(16B)···S(3A) to be 3.488 and H(16B)···S(3A) being 3.254 Å^{[19, 20]}. As shown in Fig. 2b, there are also many π···π stacking interactions between neighboring [BPP]⁺ cations in complex 1, and the distance is 3.554 Å. The [BPP]⁺ cations filled in two chains of [Mn(NCS)₄]^2- anions through S···S interaction. From the stacking diagram along the a axis (Fig. S3), it can be seen that the thiocyanate radicals are connected like an irregular hexagon, which encapsulates the organic phosphorus in the cavity and enhances the stability of the structure. Additionally, a comparison between 1 and the [BzTPP]₂[Zn(NCS)₄] compound previously reported reveals that when the cation is identical^[21], while the metal ion of the anion changes from Zn(Ⅱ) to Mn(Ⅱ), the crystal system and space group are still the same, but the shortest S···S, M···P and M···M (M = Mn or Zn) distances between the adjacent anions and cations, the stacking mode and the weak interactions of the cations and anions are significantly different^[20].

Figure 2

Figure 2.  (a) C–H···S hydrogen bonds between the S(3) atom of the anion and the adjacent cation and (b) π···π interaction in compound 1

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3.2 Luminescent property and solid-state UV/Vis diffuse reflectance spectroscopy

The photo-luminescent properties of 1 were investigated in solid state at room temperature (Fig. 3). Under 248 nm excitation, 1 shows a strong emission band at 396 nm. As documented, the complex 2(BzTPP)⁺[Zn(NCS)₄]^2- has a broad luminescence peak in the range of 327~361 nm, with a maximum of 356 nm^[21]. Obviously, when the anion [Zn(NCS)₄]^2- changes to [Mn(NCS)₄]^2-, the maximum emission peak shifts to an Einstein shift of 40 nm, indicating that [Mn(NCS)₄]^2- plays an important role in the fluorescence emission of the compounds, and the magnitude of the peak value is attributed to the ligand metal transition^{[10, 22]}. What's more, the C–Hd···S hydrogen bonds between the [Mn(NCS)₄]²⁻ anion and the [BPP]⁺ cation improved the emission intensity^[21]. Interestingly, 1 also exhibits interesting semiconductor properties. In order to deeply investigate the mechanism, solid-state UV/Vis diffuse reflectance spectroscopy was performed at room temperature. As shown in Fig. 3b, an intense absorption occurs at the band edge onset of 320 nm. The corresponding optical band gap can be defined as 4.0 eV according to Tauce equation (Fig. S4, inset). This value is within the band gap of most known organic-inorganic hybrid perovskites (3.5~4.5 eV)^[36].

Figure 3

Figure 3.  (a) Fluorescence spectrum of compound 1 at room temperature. (b) UV/Vis absorption spectrum of 1. The inset shows the Tauc plot, and the estimated band gap is 4.0 eV

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3.3 SHG effect analysis

The compounds with the second-order nonlinear optical properties must be chiral or non-centric^[23-28]. Because the title compound adopts space group Cc, we study its nonlinear optical properties^[27]. The second harmonic generation (SHG) measurements show that compound 1 has nonlinear optical activity and its SHG response is 2.25 times that of standard potassium dihydrogen phosphate (KDP). The reason can be attributed to the coordination of N from thiocyanate to a metal left, thus resulting in the donation of the lone pair of electrons on the N atoms to the metal left and the formation of an excellent donor acceptor (D-A) system^{[23, 31, 32]}. Furthermore, the C–H···S hydrogen bonds between the [Mn(NCS)₄]²⁻ anion and the [BPP]⁺ cation can effectively mediate the push-pull strength. In addition, the mechanism of SHG formation may be attributed to the formation of a distorted tetrahedron formed by the coordination of thiocyanate with manganese ions, and the polarization effect cannot be counteracted^[28], which makes the complex as a potential secondorder nonlinear optical material.

Figure 4

Figure 4.  SHG signals of compound 1

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3.4 Thermal analysis

In order to study the thermal stability of compound 1, thermal analysis (TGA) was carried out in N₂ (100 mL/min) at a heating rate of 10 ℃·min^-1 from room temperature to 1100 K. As shown in Fig. 5, complex 1 has high thermal stability below 573 K. Above 431 K, the DTA curve of 1 shows an obvious rising peak which may be caused by the high nitrogen content and high energy^[31]. When the temperature is higher than 573 K, the compound loses weight, corresponding to the release of organic ligands. The thermal stability of complex 1 is higher than that of similar compounds due to the strong S⋅⋅⋅S interaction and abundant C–H⋅⋅⋅S and π⋅⋅⋅π interactions.

Figure 5

Figure 5.  TGA diagram for compound 1

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4. CONCLUSION

In conclusion, a new manganese compound 2(BPP)⁺·[Mn(NCS)₄]^2- (1) was synthesized by the simple solution method. It has strong purple luminescent property with wavelength 396 nm. In addition, SHG measurements show that 1 has nonlinear optical activity and its SHG response is 2.25 times that of standard potassium dihydrogen phosphate (KDP), which is promised with great potential to the development of new supramolecular fluorescence and nonlinear materials.

References

1. [1]

   Liu, C.; Mei, D. J.; Cao, W. Z.; Yang, Y.; Wu, Y. D.; Li, G. B.; Lin, Z. S. Mn-Based tin sulfide Sr₃MnSn₂S₈ with a wide band gap and strong nonlinear optical response. J. Mater. Chem. 2019, 7, 1146–1150.
   

2. [2]

   Tang, Y. Z.; Yu, Y. M.; Xiong, J. B.; Tan, Y. H.; Wen, H. R. Unusual high-temperature reversible phase-transition behavior, structures, and dielectric-ferroelectric properties of two new crown ether clathrates. J. Am. Chem. Soc. 2015, 137, 13345–13351.  doi: 10.1021/jacs.5b08061
   

3. [3]

   Chen, S. P.; Wang, C. F.; Zhou, H. T.; Tan, Y. H.; Wen, H. R.; Tang, Y. Z. Ferroelectric property, switchable dielectric, and excellent second harmonic generation response in a homochiral organic salt: l-prolinammonium 1-adamantane carboxylate. Cryst. Growth Des. 2018, 18, 6117–6122.  doi: 10.1021/acs.cgd.8b00994
   

4. [4]

   Antorrena, G. .; Palacio, F.; Castro, M.; Pellaux, R.; Decurtins, S. Thermal and magnetic study of mixed-metal oxalate-bridged 2D magnets. J. Magn. Magn. Mater. 1999, 196, 581–583.
   

5. [5]

   Hollingsworth, M. D. Crystal engineering: from structure to function. Science 2002, 295, 2410–2413.  doi: 10.1126/science.1070967
   

6. [6]

   Galet, A.; Muñoz, M. C.; Gaspar, A. B.; Real, J. A. Architectural isomerism in the three-dimensional polymeric spin crossover system {Fe(pmd)₂[Ag(CN)₂]₂}:   synthesis, structure, magnetic properties, and calorimetric studies. Inorg. Chem. 2005, 24, 8749–8755.
   

7. [7]

   Wernsdorfer, W.; Aliaga-Alcalde, N.; Hendrickson, D. N.; Christou, G. Exchange-biased quantum tunneling in a supramolecular dimer of single-molecule magnets. Nature 2002, 416, 406–409.  doi: 10.1038/416406a
   

8. [8]

   Yang, E. C.; Zhao, H. K.; Ding, B.; Wang, X. G.; Zhao, X. J. Four novel three-dimensional triazole-based zinc(Ⅱ) metal-organic frameworks controlled by the spacers of dicarboxylate ligands:   hydrothermal synthesis, crystal structure, and luminescence properties. Cryst. Growth Des. 2007, 7, 2009–2015.  doi: 10.1021/cg070356n
   

9. [9]

   Hu, R. F.; Zhang, J.; Kang, Y.; Yao, Y. G. A fluorescent Zn-benzotriazole 2D polymer with (6, 3) topology. Inorg. Chem. Commun. 2005, 8, 828–830.  doi: 10.1016/j.inoche.2005.06.017
   

10. [10]

    Chen, J. L.; Guo, Z. H.; Luo, Y. S.; Qiu, L.; He, L. H.; Liu, S. J.; Wen, H. R.; Wang, J. Y. Luminescent monometallic Cu(I) triphenylphosphine complexes based on methylated 5-trifluoromethyl-3-(2΄-pyridyl)-1, 2, 4-triazole ligands. New J. Chem. 2016, 40, 5325–5332.  doi: 10.1039/C5NJ03529E
    

11. [11]

    Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, 339−341.  doi: 10.1107/S0021889808042726
    

12. [12]

    Sheldrick, G. M. SHELXS-97, Program for the Solution of Crystal Structures. University of Göttingen, Germany 1997.

13. [13]

    Yu, Y. Z.; Chang, S. Y.; Han, X.; Chen, G. X.; Xuan, Y. W.; Wu, X. L.; Wang, F. Hydrothermal synthesis, crystal structure and luminescence property of a 2D manganese(Ⅱ) coordination polymer. Chin. J. Struct. Chem. 2019, 38, 147–154.
    

14. [14]

    Chen, X.; Dai, S. L.; Cheng, Z. P.; Liang, L. B.; Han, S.; Liu, J. F.; Zhou, J. R.; Yang, L. M.; Ni, C. L. Syntheses, crystal structures, and magnetic properties of two hybrid materials self-assembly from tetra(isothiocyanate)cobalt(Ⅱ) anion and substituted benzyl triphenylphosphinium. Synthesis and Reactivity in Inorganic, Metal-Organic, and Nan-Metal Chemistry 2012, 42, 987–993.  doi: 10.1080/15533174.2012.680114
    

15. [15]

    Chen, H. J.; Zhang, L. Z.; Cai, Z. G.; Yang, G.; Chen, X. M. Organic-inorganic hybrid materials assembled through weak intermolecular interactions. synthesis, structures and non-linear optical properties of [4, 4΄-bipyH₂][M(NCS)₄] (M = Mn²⁺, Co²⁺ or Zn²⁺; 4, 4΄-bipy = 4, 4΄-bipyridine). J. Chem. Soc. Dalton Trans. 2000, 14, 2463–2466.
    

16. [16]

    Bleiholder, C.; Werz, D. B.; Koppel, H.; Gleiter, R. Theoretical investigations on chalcogen-chalcogen interactions:   what makes these nonbonded interactions bonding? J. Am. Chem. Soc. 2006, 128, 2666–2674.  doi: 10.1021/ja056827g
    

17. [17]

    Liu, X. M.; Lu, X. Z.; Fu. F.; Liu, B.; Hu, H. M.; Gao, Q. C.; Wang, J. W.; Xue, G. L. Synthesis and crystal structure of a novel charge transfer salt, (TMT-TTF)₄[HPMo₁₂O₄₀]. J. Mol. Struct. 2005, 751, 17–21.  doi: 10.1016/j.molstruc.2005.05.003
    

18. [18]

    Bleiholder, C.; Gleiter, R.; Werz. D. B.; Köppel, H. Theoretical investigations on heteronuclear chalcogen-chalcogen interactions:   on the nature of weak bonds between chalcogen lefts. Inorg. Chem. 2007, 46, 2249–2260.  doi: 10.1021/ic062110y
    

19. [19]

    Steiner, T. The hydrogen bond in the solid state. Angew. Chem. Int. Ed. 2002, 41, 48–76.  doi: 10.1002/1521-3773(20020104)41:1<48::AID-ANIE48>3.0.CO;2-U
    

20. [20]

    Chen, X.; Chen, W. Q.; Han, S.; Liu, J. F.; Zhou, J. R.; Yu, L. L.; Yang, L. M.; Ni, C. L.; Hu, X. L. Two hybrid materials self-assembly from tetra(isothiocyanate)cobalt(Ⅱ) dianion and substituted benzyl triphenylphosphinium. J. Mol. Struct. 2010, 984, 164–169.  doi: 10.1016/j.molstruc.2010.09.022
    

21. [21]

    Ye, H. Q.; Xie, J. L.; Yu, J. Y.; Liu, Q. T.; Dai, S. L.; Huang, W. Q.; Zhou, J. R.; Yang, L. M.; Ni, C. L. Syntheses, crystal structures, luminescent properties of two new molecular solids with tetra(isothiocyanate)zinc(Ⅱ) and substituted benzyl triphenylphosphonium cations. Synthetic Metals. 2014, 197, 99–104.  doi: 10.1016/j.synthmet.2014.09.006
    

22. [22]

    Yang, H. L.; Chen, F.; He, X.; Li, Y.; Zhang, X. Q. Synthesis, crystal structure, thermal stability, luminescence and magnetic property of a new Mn(Ⅱ) coordination polymer. Chin. J. Struct. Chem. 2018, 37, 1834–1841.
    

23. [23]

    Zhou, H. T.; Yang, K.; Liu, Y.; Tang, Y. Z.; Wei, W. J.; Shu, Q.; Zhao, J. J.; Tan, Y. H. In situ [2 + 3] cycloaddition synthesis, crystal structures, strong SHG responses and fluorescence properties of three novel Zn coordination polymers. Chin. Chem. Lett. 2019, 31, 841–845.
    

24. [24]

    Fan, X. W.; Liu, Y.; Tang, Y. Z.; Wei, W. J.; Zhang, J. C.; Luo, Z. Y.; Wang, C. F.; Tan, Y. H. High-temperature reversible phase-transition behavior, switchable dielectric and second harmonic generation response of two homochiral crown ether clathrates. Chem. Asian J. 2019, 14, 2203–2209.  doi: 10.1002/asia.201900512
    

25. [25]

    Tang, Y. Z.; Zhou, M.; Huang, J.; Tan, Y. H.; Wu, J. S.; Wen, H. R. In situ synthesis and ferroelectric, SHG response, and luminescent properties of a novel 3D acentric zinc coordination polymer. Inorg. Chem. 2013, 52, 1679–1681.  doi: 10.1021/ic302411r
    

26. [26]

    Gao, J. X.; Xiong, J. B.; Xu, Q.; Tan, Y. H.; Liu, Y.; Wen, H. R.; Tang, Y. Z. Supramolecular interactions induced chirality transmission, second harmonic generation responses, and photoluminescent property of a pair of enantiomers from in situ [2 + 3] cycloaddition synthesis. Cryst. Growth Des. 2016, 16, 1559–1564.  doi: 10.1021/acs.cgd.5b01684
    

27. [27]

    Liu, Y.; Zhou, H. T.; Chen, S. P.; Tan, Y. H.; Wang, C. F.; Yang, C. S.; Wen, H. R.; Tang, Y. Z. reversible phase transition and switchable dielectric behaviors triggered by rotation and order-disorder motions of crowns. Dalton Trans. 2018, 47, 3851–3856.  doi: 10.1039/C8DT00003D
    

28. [28]

    Yang, K.; Yang, C. S.; Dong, X. X.; Tan, Y. H.; Tang, Y. H.; Wei, W. J. Two rare-earth molecular ferroelectrics with high curie temperatures, large spontaneous polarization, switchable second harmonic generation effects, and strong photoluminescence. Chemistry-A European Journal 2020, 26, 5887–5892.  doi: 10.1002/chem.202000188
    

29. [29]

    Yang, K.; Dong, X. X.; Xu, Q.; Tan, Y. H.; Tang, Y. Z. Nopinic acid is an unprecedented homochiral single-component organic ferroelectric. Appl. Mater. Today 2020, 20, 100687.  doi: 10.1016/j.apmt.2020.100687
    

30. [30]

    Fan, H. L.; Lin, C. S.; Chen, K. C.; Peng, G.; Li, B. X.; Zhang, G.; Long, X. F.; Ye, N. (NH₄)Bi₂(IO₃)₂F₅: an unusual ammonium-containing metal iodate fluoride showing strong second harmonic generation response and thermochromic behavior. Angew. Chem. Int. Ed. 2020, 59, 5268–5272.  doi: 10.1002/anie.201913287
    

31. [31]

    Li, F. F.; Lu, L. P. A new Mn(Ⅱ) coordination polymer: synthesis, structure and magnetic property. Chin. J. Struct. Chem. 2019, 38, 1814–1822.
    

32. [32]

    Shi, C.; Ma, J. J.; Jiang, J. Y.; Hua, M. M.; Xu, Q.; Yu, H.; Zhang, Y.; Ye, H. Y. Large piezoelectric response in hybrid rare-earth double perovskite relaxor ferroelectric. J. Am. Chem. Soc. 2020, 142, 9634–9641.
    

33. [33]

    Savard, D.; Leznoff, D. B. Synthesis, structure and light scattering properties of tetraalkylammonium metal isothiocyanate salts. Dalton Trans. 2013, 42, 14982–14991.  doi: 10.1039/c3dt50974e
    

34. [34]

    Li, Q.; Shi, P. P.; Ye, Q.; Wang, H. T.; Wu, D. H.; Ye, H. Y.; Fu, D. W.; Zhang, Y. A switchable molecular dielectric with two sequential reversible phase transitions: [(CH₃)₄P]₄[Mn(SCN)₆]. Inorg. Chem. 2015, 54, 10642−10647.  doi: 10.1021/acs.inorgchem.5b01437
    

35. [35]

    Neumann, T.; Jess, I.; Näther, C. Crystal structures of bis[4-(dimethylamino)-pyridinium] tetrakis(thiocyanato-κN) manganate(Ⅱ) and tris[4-(dimethylamino)pyridinium] pentakis-(thiocyanato-κN)manganate(Ⅱ). Acta Cryst. 2018, E74, 15–20.
    

36. [36]

    Wang, C. F.; Fan, X. W.; Tan, Y. H.; Wei, W. J.; Tang, Y. Z. High-temperature reversible phase transition and switchable dielectric and semiconductor properties in a 2D hybrid [(C₃H₁₂N₂O)CdCl₄]_n. Eur. J. Inorg. Chem. 2019, 2907–2911.