Synthesis, crystal structure, optical properties and bactericidal activity of a new organic-inorganic hybrid: Benzyl triethylaminium tetrachloromanganate(II)

Abstract In this work, a lead-free organic-inorganic hybrid crystal with dual functions of green-emission and bactericidal activity, benzyl triethylaminium tetrachloromanganate(II) ([BzTEA]2[MnCl4]), was synthesized by the slow evaporation solution growth technology and characterized by single-crystal and power X-ray diffraction, IR spectrum, Scanning Electron microscopy, and PL spectrum. The compound crystallized in the monoclinic space group P21/c. The thermal study revealed that the material showed good thermal stability and can be used for applications below 222.1 °C. The material had excellent optical properties as it fluoresced green under 350 nm UV light. The compound exhibited favorable bactericidal activity against Staphylococcus aureus and Escherichia coli. Graphical abstract Research Highlights An inorganic-organic hybrid salt [BzTEA]2[MnCl4] was synthesized and characterized. The salt shows green fluorescence emission at 534 nm in a solid state at room temperature. The salt exhibited favorable thermal stability and could be used in applications below 222.1 °C. The salt had an optical band gap of 1.66 eV and was a typical semiconductor material with good optical properties. The as-synthesized salt exhibits good antibacterial activities against S. aureus and E. coli.


Introduction
In recent years, luminescent materials have shown great potential for application in the fields of optoelectronics, biomedicine, sensors, and so on [1][2][3][4], and have received more and more attention.However, the existing luminescent materials were not easy to produce and were mostly based on precious metals such as europium, cerium, etc, and there are certain limitations in application [5][6][7][8][9][10][11].Therefore, the research has turned to organic-inorganic hybridized halides.Among them, the organic-inorganic metal halides have excellent luminescence, conductivity, and other properties, which can replace some of the traditional rare-earth luminescent materials [12][13][14][15][16][17][18][19][20][21].The organic-inorganic hybridized manganese halides have low cost, low toxicity, high stability, and have obvious advantages over the traditional lead and copper [22][23][24].Compared with most manganese halides, manganese chloride has good magnetic and luminescent properties [25][26][27].Based on previous studies, such as [BzTMA] [33], all of which exhibit excellent optical properties.In particular, [BzTEA] 2 [MnBr 4 ] is an ideal luminescent material with a good luminous efficiency and a quantum yield of 97.8%.It is obtained by modifying the alkyl group of TEA þ with a benzyl group [30].Therefore, in an endeavor to further expand this field, we combined the [BzTEA] þ cation with the [MnCl 4 ] 2− anion to acquire a new type of optical crystal [BzTEA] 2 [MnCl 4 ] featuring antimicrobial properties.The crystal structure was characterized by using infrared spectroscopy.The morphology of the crystal was visualized by scanning electron microscopy.The optical properties of the crystal were studied by UVvis diffuse reflectance and fluorescence spectra.Minimum Inhibitory Concentration (MIC) and diameter of the inhibition circle were used to explore the antimicrobial activity of the crystals against Escherichia coli and Staphylococcus aureus.

Synthesis of [BzTEA] 2 [MnCl 4 ]
[BzTEA] 2 [MnCl 4 ] was prepared via two-step synthesis route as described in Scheme 1.In the first step, benzyl triethylaminium chloride ([BzTEA]Cl) was synthesized by mixing a stoichiometry of BzCl and TEA in hot acetone.Typically, BzCl (0.63 g, 5 mmol) and TEA (0.51 g, 5 mmol) were dissolved in 30 ml of acetone, and the resulting solution was refluxed for 24 h under vigorous stirring condition.The white viscous solid was obtained by filtration, washed with acetone and ether, dried at 70 � C for 12 h in a vacuum oven.Yield: 75.4%.The [BzTEA] þ cation was confirmed by the electrospray ionization mass spectrometry (ESI-MS) as shown in Fig. S1.In the second step, MnCl 2 �4H 2 O (0.40 g, 2 mmol) and [BzTEA]Cl (0.91 g, 4 mmol) were dissolved in 50 ml methanol, and 3 ml hydrochloric acid was added to provide an excess of chloride ions for responsibility to ensure the formation of [MnCl 4 ] 2− anion.A clear green solution was obtained by refluxing the mixed solution for 6 h.The solution was concentrated slowly to a small volume and crystallized in the refrigerator.The blue-green crystals were obtained after a week.Yield: 80.4%.Anal.Calc.for C 26 H 24 MnN 2 Cl 4 : C, 55.64; H, 4.31; N, 4.99%; Found: C, 55.72; H, 4.41; N, 4.88%.

Characterization
Elemental analyses (C, H, and N) were recorded on a 240 C Perkin-Elmer instrument.FT-IR spectrum was recorded in the 4000-400 cm −1 range on a Nicolet Avatar 360 spectrophotometer with the sample in KBr.UV-Vis diffuse reflection spectroscopy was recorded on a Shimadzu UV-2550 spectrophotometer in the wavelength range of 800-200 nm.The Powder XRD data were carried out on a Rigaku-Ultima IV X-ray diffractometer with Cu Ka radiation (k ¼ 1.5406 Å) and a fixed power source (40 kV, 40 mA).Scanning electron microscopy (SEM) and elemental mapping were performed on a Phenom ProX.The solid-state emission spectrum was recorded using a Hitachi F-7000 fluorescence spectrophotometer.The electrospray ionization mass spectrometry (ESI-MS) was measured on an AB SCIEX, API3200.The X-ray structural data were collected on a Bruker Smart APEX diffractometer equipped with a CCD area detector.Graphite-monochromatic Mo-Ka radiation (k ¼ 0.71073 Å) X-ray source was utilized and the data were recorded in x-scan mode.The cell parameters were retrieved in SMART software [34] and all observed reflections were refined using SAINTPlus [35].The structure was solved by the direct method and the least squares refinement was done via the SHELXTL program package [36].

Antibacterial activity determination
The antibacterial activities on Escherichia coli and Staphylococcus aureus were carried out in terms of minimum inhibitory concentration (MIC) and diameter of the inhibition circle [37,38].The two bacteria were incubated on Luria-Bertani (LB) medium at 37 � C for 24 h.A gradient dilution of the samples (in mmol�L −1 ) was adopted for the 2fold dilution method, and the bacterial concentration was 1.2 � 10 6 CFU�mL −1 .The diameter of the ring of inhibition was measured by the plate filter paper method, with the concentration of the sample being 0.05 mol�L −1 .

Description of crystal structure
The crystal structure of [BzTEA] 2 [MnCl 4 ] is depicted in Fig. 1.The detail of crystal data, experimental conditions, and structure refinement are listed in Table 1.
[BzTEA] 2 [MnCl 4 ] was crystallized in the monoclinic crystal system with space group ), suggesting that the ethyl group had a wider site resistance than the methyl group, which affected the bond angle.As indicated in Fig. 2, there was a C − H���p weak interaction [33,39] between the adjacent [BzTEA] þ cations, and the distance of the C(23) atom to the centroid of the benzene ring was 3.448 Å.In the single-nucleus [MnCl 4 ] 2− anion geometry,   2 and 3.As shown in Fig. 3, a C − H���Cl hydrogen bond [31,33]

Thermal stability
The results of [BzTEA] 2 [MnCl 4 ] thermogravimetric analysis (TG) and differential thermal analysis (DTA) are shown in Fig. 7.As could be seen from the TG curves in Fig. 7, there was no significant weight loss between room temperature and 222.1 � C,  indicating that [BzTEA] 2 [MnCl 4 ] was stable up to 222.1 � C. The range of stabilization was not as good as that of tetrabromanganese but was sufficient for most situations [28].The decomposition of [BzTEA] 2 [MnCl 4 ] proceeded in two stages.The first decomposition started at 222.1 � C and ended at 462.5 � C with a loss of 78.46% of the material.The second loss occurred between 462.5 and 760.8 � C, with 20.1% of the material removed.Therefore, the prepared material was completely stable before 222.1 � C [42].The peaks in the DTA thermogram curves at 267.1, 305.5, 393.3, and 746.5 � C (blue lines) were due to the decomposition of the salt at different stages, which was consistent with the TG analysis.

IR spectrum
The infrared spectrum of [BzTEA] 2 [MnCl 4 ] is shown in Fig. 8.The characteristic absorption band at 3053 cm −1 was the C − H stretching vibration of the benzene ring [33].The bands at 2992 cm −1 and 2855 cm −1 were attributed to the C − H stretching vibration absorption peaks of methyl and methylene [43].The skeleton stretching vibration of the benzene ring appeared at 1631 cm −1 .The flexural vibration peaks of methylene were located at 1485 and 1457 cm −1 .The absorption peak of the in-plane bending vibration in CH 3 was located at 1347 cm −1 .The bands at 1024 cm −1 and 1153 cm −1 attributed to the C − N stretching vibration absorption peaks of the single substituted benzene ring.The characteristic absorption bands at 675 cm −1 and 748 cm −1 were the C − H bending vibration absorption peaks of the single substituted benzene ring [44].

UV-Vis diffuse reflection spectroscopy
The optical properties of [BzTEA] 2 [MnCl 4 ] were investigated by UV-vis diffuse reflectance spectroscopy (Fig. 9).Uv-vis diffuse reflection spectrum of [BzTEA]  [MnCl 4 ] 2− anion [32].As shown in Fig. 9(b), the Tauc equation of [BzTEA] 2 [MnCl 4 ] with an absorption cutoff wavelength of 527 nm corresponding to an optical band gap of 1.66 eV, The band gap value was similar to that of most metal halides, which belonged to typical semiconductor materials, indicating that the crystal had good optical properties [33].

PL spectrum
In order to better analyze the luminescence characteristics of [BzTEA] 2 [MnCl 4 ], its emission spectrum in the visible range (400-700 nm) was analyzed as shown in Fig. 10 and the inset shows the photos of the crystals in natural condition and under UV light.It is seen that the emission spectrum showed the highest intensity near 534 nm under 350 nm UV light, which corresponded to the electronic transition of Mn 2þ from the ground state of 6 A 1 to the excited state of T d in the [MnCl 4 ] 2− tetrahedron.Furthermore, the cause of these transformations may be the expected conformational transformation within the tetrahedral crystal field.There was no doubt that it came from the 4 T 1 / 6 A 1 radiation transition, consistent with the green emission signature of Mn 2þ [29,46].Compared with [BzTEA] 2 [MnBr 4 ] (521 nm), the fluorescence spectrum of [BzTEA] 2 [MnCl 4 ] showed a redshift, which indicated that different anions would affect the luminous properties of Mn 2þ [30].The fluorescence spectra of [BzTEA] 2 [MnCl 4 ] showed a blue shift relative to [Bz(Me) 3 N] 2 [MnCl 4 ] (547 nm), which indicated that different cations also affected the luminescence properties of manganese tetrachloride [29].

Antibacterial properties
The inhibitory activity of the compounds against Escherichia coli and As can be easily found, the inhibitory activities of [BzTEA] 2 [MnCl 4 ] against Escherichia coli and Staphylococcus aureus were slightly higher than that of [BzTEA]Cl, demonstrating that the introduction of manganese tetrachloride anion enhanced its inhibitory activity to some extent.The antibacterial activity against Escherichia coli and Staphylococcus aureus were close to those the previously reported [33], revealed that the formulated [BzTEA] 2 [MnCl 4 ] was an desirable candidate antimicrobial agent.

Conclusion
In this paper, a new organic-inorganic hybrid material, benzyl triethylamine tetrachloromanganate ([BzTEA] 2 [MnCl 4 ]), was synthesized.The compound consisted of two [BzTEA] þ cations and one [MnCl 4 ] 2− anion and crystallized in the monoclinic space group P2 1 /c.The electrostatic interaction, C − H���p interaction and C − H���Cl hydrogen bonds in the crystal were helpful in stabilizing and formed a three-dimensional network structure.The crystal morphology of [BzTEA] 2 [MnCl 4 ] with a rough surface, uneven edge, and blocky accumulation was obtained by the scanning electron microscopy.Thermogravimetric analysis showed that the material was stable up to 222.1 � C. The emission wavelength of the crystal was measured at 534 nm by ultraviolet spectrophotometry, which was in the green wavelength range.It was also consistent with the fluorescence color of the crystal emitted under ultraviolet light.The fluorescence experiment verified that the experimental product had excellent fluorescence properties.It came from the 4 T 1 / 6 A 1 radiation transition, which was consistent with the green emission characteristics of Mn 2þ , and the fluorescence experiment accorded with the principle.The as-prepared hybrid material showed strong antibacterial activity against Staphylococcus aureus and Escherichia coli.Therefore, it could be concluded from this study that the organic-inorganic hybrid material was a new type of multifunctional material that can be used in the fields of luminescence and antibacterial.
of the crystal along the b-axis is displayed in Fig.4.It was seen that the electrostatic interaction, C − H���p interaction between organic [BzTEA] þ and inorganic [MnCl 4 ] 2− anions and C − H���Cl hydrogen bonds between the [BzTEA] þ cations were helpful in the forming of a three-dimensional supramolecular network structure[31,33].
Staphylococcus aureus was investigated by means of minimum inhibitory concentration (MIC) and diameter of the inhibition circle, which are displayed in Figs.(S2 and S3), severally.The diameter of the circle of inhibition (cm) and the minimum inhibitory concentration (MIC) are described in Table 4.It could be observed that [BzTEA]Cl and [BzTEA] 2 [MnCl 4 ] exhibited good inhibitory activity against Escherichia coli and Staphylococcus aureus at the same dose.The MIC of [BzTEA]Cl was approximately 25 mmol�L −1 while the MIC of [BzTEA] 2 [MnCl 4 ] was about 12.5 mmol�L −1 .
1/2 .Cl were from 104.79 � to 114.81 � with an average value of 109.50 � , in agreement with those of some organic-inorganic hybrids formerly reported containing [MnCl 4 ] 2− ion [40,41].Compared with [Bz(Me) 3 N] 2 [MnCl 4 ] [29], the average Mn − Cl bond length in tetrahedral [MnCl 4 ] 2− ion was significantly greater than that (2.369 Å) and the average Cl − Mn − Cl bond angle coincided with that (109.49� ) in [Bz(Me) 3 N] 2 [MnCl 4 ] [29], being demonstrated that different cations take some effects on the conformation of manganese tetrachloride.The smallest Mn-Mn distance in [BzTEA] 2 [MnCl 4 ] was 9.505 Å, larger than that (8.485 Å) in [Bz(Me) 3 N] 2 [MnCl 4 ] [29], which is attributed to the effect of greater steric hindrance of [BzTEA] þ than [Bz(Me) 3 N] þ .The main bond length and bond angle parameters are indicated in Tables the Mn(II) ion was coordinated by four Cl atoms to form a twisted tetrahedral coordination geometry.The bond lengths between Mn − Cl were in a narrow range from 2.396 Å to 2.414 Å with an average value of 2.405 Å.The bond angles of Cl − Mn −

Table 4 .
The inhibition zone diameter values (cm).