Development,characterization and optical activity evaluation of three new benzimidazole compounds

2.1 Materials and physical measurements

All chemicals and solvents are of A.R. grade (≥99%) and used without further purification. Distilled water was used for all procedures. IR spectrum were recorded on a Shimazu IR435 spectrometer as KBr disk (4000–400 cm^-1). Elemental analyses of C, H and N were performed using a Perkin-Elmer 240 elemental analyzer. The purity of the bulk microcrystalline materials obtained from the syntheses was checked by Powder X-ray diffraction analyses. Powder X-ray diffraction (PXRD) data were measured on a Philips X’ Pert Pro MPD X-ray diffractometer (Cu-Kα= 1.5418 Å) with an X’ Celerator detector. UV-vis diffuse reflectance spectra (DRS) were recorded with the aid of a Cary 5000 UV-vis infrared spectrophotometer. Photoluminescent measurements of compounds 1-3 in the solid state were conducted on a HITACHI F-4600 spectrophotometer and the data were collected at room temperature.

2.2 Synthesis of compounds

{[L₁ZnBr₂](CH₃OH)₂} (1). ligand L₁ (0.0108 g), ZnBr₂ (0.00675 g) and adipic acid (0.0044 g) were dissolved in 4 mL methanol. Added ligand L₁ to the solution of metal salt, stired for 5 minutes, if there is precipitation, then filtered into the vial, sealed the bottle with plastic wrap, and placed in a quiet place. But adipic acid is not involved in coordination in 1. The yellow block crystals yield is 3%. Anal. Calc. (%) for 1: C, 40.05; H, 3.97; N, 16.99%. Found: C, 41.02; H, 3.54; N, 16.81%. FW = 659.71; IR(KBr): 3243.4(s), 1619.8(w), 1597.8(w), 1441.6(s), 1287.6 (w), 1140.2(m), 748.6(s), 579.5(w) cm^–1.

{[L₁Co(CH₃COO)CH₃O]·CH₃OH} (2). ligand L₁ (0.0108 g), Co(NO₃)₂·6H₂O (0.0146 g) and sodium acetate (0.0041 g) were dissolved in 4 mL methanol. Added ligand L₁ to the solution of metal salt, stired for 10 minutes. Remove the magneton, then sealed the bottle with plastic wrap, and placed in a quiet place. The purple bulk crystals were prepared of 5%. Anal. Calc. (%) for 2: C, 60.12; H, 5.43; N, 10.79%. Found: C,61.12; H, 5.41; N, 11.09%. FW = 519.45; IR(KBr): 3409.67(m), 1608.33(s), 1452(m), 1365.7(s), 1288.7(w), 1155.6(w), 1022.7(w), 724(m) cm^-1.

{[L₁(NO₃)₂]} (3). ligand L₁ (0.0108 g), Cr(NO₃)₂·6H₂O (0.02 g) and adipic acid (0.0073 g) were dissolved in 4 mL methanol. Add ligand L₁ to the solution of metal salt, stir for 10 minutes. Remove the magneton, then sealed the bottle with plastic wrap, and placed in a quiet place. Cr does not participate in coordination. In addition, this crystal isn’t soluble in water. The olorless block crystals were prepared of 7%. Anal. Calc. (%) for 3: C, 56.89; H, 4.34; N, 18.09%. Found: C,56.91; H, 4.30; N, 18.12%. FW = 464.43. IR(KBr): 3036.6(s), 1626.4(m), 1397.6(m), 1309.6(s), 1212.8(m), 1036.8(m), 882.8(m), 754.7(s), 616.6(m) cm^-1.

2.3 X-ray crystallography study

Crystallographic data for the compounds were collected at 100(2) K on a Bruker APEX-II area-detector diffractometer equipped with graphite-monochromatized Mo-Kα radiation (wavelength: 0.71073Å). Their structures were solved by direct method and expanded using Fourier techniques. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms were assigned with common isotropic displacement factors and included in the final refinement by using geometrical constraint. The structures were refined with full-matrix least-squares techniques on F² using the OLEX-2 program package [14]. Table 1 summarizes the crystal data of supramolecular compounds 1-3. Table 2 lists the selected bond lengths and bond angles of the supramolecules 1-3.

A powdered sample (10 mg) of compounds 1-3 was added to solution (3 mL) of M(NO₃)_c (M^c+ = Ba²⁺, K+, Cr³⁺, Zn²⁺, Fe³⁺, Ni²⁺, Pb²⁺, Co²⁺, Al³⁺, La³⁺ (0.01 mol/L). Fluorescence experiment were conducted after stiring for 30 min [15]. In the liquid state, the different pH on the fluorescence changes of compounds 1-3 was recorded via preparing a series of B-R buffer solutions with a series of pH 2-12.

2.5 Adsorption Measurements of compounds 1-3

We researched the adsorption property of compounds 1-3 on used the same organic dyes (MB MO RHB). The steps were carried out following in the literature methods [16].

2.6 Photocatalysis properties of compounds 1-3

Photocatalytic technology can firsthand apply solar energy as light source to drive the reaction, which has been extensively used in the environment to remove out pollutants, so it has make a big difference in the removal of toxic and harmful pollutants [17–24]. We researched the photocatalytic degradation property of compounds 1-3 on used the same organic dyes (MB MO RHB). The steps were carried out following in the literature methods [25].

3.1 Description of crystal structure

{(CH₃OH)₂} (1). Complex 1 is triclinic and P-1 space group and contains one Zn(II) atom, one L₁, two Br anions and two CH₃OH molecules (Fig. 1a). The unit cell parameters were a = 9.0416(4) Å, b = 9.9434(5) Å, c = 14.9665(7) Å, and α= 92.591(4)° β= 91.222(4)° γ= 108.840(4)°. The core-metal ion Zn(II) chooses the four-coordination pattern, two Br atoms (Br₁ and Br₂) and two N atoms (N₁ and N₃) coordinate with Zn automatically to assemble a tortile tetrahedral framework. The Zn-N and Zn-Br bond lengths are in normal ranges of 2.0164(20)–2.3907(4) and 2.3907(4)–2.4102(5)Å. The two benzimidazole groups of ligand L₁ are contorted giving rise to a screwed structure. Right-handed (R) and left-handed (M) helicates of compound 1 whose screw-type is depended on the R and M helical L₁ ligands, severally (Fig. 1b). In complex 1, hydrogen bonding indwells between O atom of methanol molecule and Br atom in the complex unit. Similarity, hydrogen bond also occurs between N atom and O atom in the contiguous complex unit, through which the adjacent mononuclear complexes are connected to build a 1D chain shape. Two adjacent 1D chains were concatenated into a 2D network patterning by hydrogen bonding through another methanol solvent. A compound most similar to 1 is compound 1’. Compounds 1’ and 1 are 1D chain structure. Complex 1’ contains one Cd(II) atom, one L₁, two I anions and two CH₃OH molecules in the Fig. 1’(g). The crystal system of compound 1’ is triclinic and P-1 space group [13]. Compound 1’ has the same coordination environment as compound 1, but different coordination metals and they can also form 2D networks through hydrogen bonds. Comparison of some important parameters of compounds 1 and 1’ in the Table 3. (Fig. 1c O₂-H_2B…Br₂ is 2.368 Å, N₂-H_2A…O₂ is 1.192 Å, O₁-H_1A…Br₁ is 2.486 Å, N₄-H_4A…O₁ is 1.953 Å). In Fig. 1(d), complex 1 displays its void structure. Figure 1 (e-f) Stacked drawing of compound 1 under different views. (g) Structural unit of compound 1’ (h) Stacked drawing of compound 1’.

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Fig.1

(a) Asymmetric unit of compound 1. (b) Space-filling view of R-helicate and M-helicate of ligand. (c) Hydrogen bonding in complex 1. (d) Insert sphere in compound 1. (e) Stacked drawing of compound 1 in the b-axis direction. (f) Stacked drawing of compound 1 in the c-axis direction. Unnecessary H atoms were omitted for clarity. (g) The structure unit diagram of 1’, which is similar to compound 1. (h) Stacked drawing of compound 1’.

{[L₁Co(CH₃COO)CH₃O]·CH₃OH} (2). Its asymmetric unit in Fig. 2 (a) involves one Co atom, one acetate ion, one methanol molecular and one ligand L₁. Co employs a four-coordination mode to form a twisted tetrahedron. The bond lengths among Co₁-O₁, Co₁-O₂, Co₁-N₁, Co₁-N₂ are 1.953, 1.983, 2.063, and 2.039 Å, respectively. Figure 2 (b) reveals the weak hydrogen bonding which the bond length of N₂-H₂…O₁ is 1.672 Å, N₄-H₄…O₄ is 1.922 Å, N₄-H_4B…O₃ is 1.881 Å. As we can see, a double helix cylindrical insertion drawing is formed in the presence of hydrogen bonds in Fig. 2 (c). Figure 2 (d) demonstrates a stacked view of compound 2 to construct a cylindrical insertion structure. In Fig. 2 (e), it exhibitions many porous shapes in some aspects. Figure 2 (f) shows a another stacked view of compound 2.

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Fig.2

(a) Asymmetric unit of compound 2. (b) Hydrogen bonding picture in compound 2. (c) double helix cylindrical insertion drawing of compound 2. (d) construct a cylindrical insertion structure of compound 2. (e) Stacked diagram of compound 2 with hole structure.

{[L₁(NO₃)₂]} (3). As is shown in Fig. 3a, its asymmetric unit includes one ligand L₁ and two nitrate ion. Despite its simple structure, it has abundant hydrogen bonds. The length of the N-H…N bond is ranging from 2.57 to 2.75 Å, the length of the N-H…O bond is within the scope of 1.90 to 2.77 Å. Figure 3b exerts a stacked view of compound 3 to form a flower clumps-like structure.

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Fig.3

(a) Asymmetric unit of compound 3. (b) Hydrogen bond graph of compound 3. (c) Stacked diagram of compound 3.

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Fig.4

Power X-ray Diffraction of compounds 1-3.

The purity of the bulk microcrystalline materials obtained from the syntheses was checked by Powder X-ray diffraction analysis. Powder X-ray diffraction (PXRD) patterns were recorded using Cu-Kα radiation on a PAN analytical X’ Pert PRO diffractometer. The purity of compounds 1-3 are confirmed by powder XRD analyses, in which the main peaks of the experimental spectra of compounds 1-3 are almost consistent with its simulated spectra.

3.3 UV-vis diffuse reflectance property and calculation of optical band gap

Figure 5. shows the UV-vis absorption spectra of compounds 1-3. For these compounds, the higher energy absorption peaks are 272 nm, which should be initially rooted in the absorption peak of intraligand. It also manifests that the peaks are generated by φ-φ* transition of the ligand. Complex 2 shows the higher energy absorption peaks, confining to 400–800 nm, which attributes to the Co²⁺ d-d transition.

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Fig.5

The UV absorption spectrum of compounds 1-3.

For the sake of study the conductivity of the three compounds, their band gap (Eg) are acquired by the measurements of diffuse reflectance. The band gap energies were evaluated to be 1.672, 1.683, 1.672 eV for 1-3, respectively. (Fig. 6) They are display that 1-3 have feaible semiconductor peculiarity and are serve as underlying photocatalysts [25]. In addition, they have the latent capacity for elecient sunlight harvesting for the maximum photon flux of sunlight is similar and small [26, 27].

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Fig.6

Band gap diagram of compounds 1-3.

Figure 7 The emission spectrum of compound 1 demonstrates a main peak at 378 nm on excitation at 347 nm; The emission spectrum of compound 2 manifests two main peak at 405 and 426 nm on excitation at 307 nm; compound 3 exhibits a main maximum at 435 nm in excitation at 309 nm. These emisions are likely to put down to π*-π transition of ligand [13, 15]. Whereas, the intensity of compounds 2 3 are lower than that compound 1, the probable causes is that long-distance hydrogen bonding in the molecular infortify the distance of the electron transition, causing the energy to dissipate and resulting in a decrease in fluorescence degree [28, 29]. Generally speaking, we compared the fluorescence spectra of compounds 1-3, we can find that a fluorescence distinct blue shift occurred for compound 1, this fluorescence emission phenomenon also indicates that the conformations of ligand undergo certain changes that lead to a change.

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Fig.7

Emission spectra of compounds 1-3.

Figure 8, It exhibits various metal ions have significant effect on luminescence performance. Amongst the tested metal ions, only Fe³⁺ can induce decrease of fluorescence emission drastically in compounds 1-3. What’s more, the luminescence emission of compounds 1-2 can be quench quickly by Cr³⁺, Al³⁺ ions. This may be due to electrostatic interactions between compounds 1-3. In the liquid state, the different pH on the fluorescence changes of compounds 1-3 was recorded via preparing a series of B-R buffer solutions with a series of pH 2-12 (Fig. 9). The fluorescence intensity (Ex = 325 nm) was gradually increased with decreasing pH = 2–7, then decreases gradually in compound 1; In compound 2, the fluorescence intensity achieved maximum at pH = 8 (Ex = 251 nm, 254 nm). At pH = 10, the fluorescence intensity reached its maximum in compound 3 (Ex = 273 nm).

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Fig.8

(a)(c)(e) The fluorescence spectra of detecting metal ions excited at 255 nm, 254 nm, 256 nm, 300 nm and 327 nm respectively. (b)(d)(f) Photoluminescence intensity change of the compound 1-3 treated with 0.01 mol/L different metal ions.

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Fig.9

(a)(c)(e) The fluorescence spectra of different pH excited at 325 nm, 254 nm, 251 nm, 254 nm and 273 nm respectively. (b)(d)(f) photoluminescence intensity change of compounds 1-3.

The adsorption spectra of compounds 1-3 are showed in Fig. 10–12. We can see from that: In three dyes, compound 1 can adsorb RHB well. MB can be good adsorption by compound 2; compound 3 could adsorb MO well. In a word, in three compounds, there is good adsorption capacity of compound 1 to organic dye RHB. What matters most is that these compounds have pore structure and reinforce their adsorption nature.

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Fig.10

UV absorption spectrum curves of compound 1 and blank control test in organic dyes solution for various adsorption times. (a-b) adsorption of MB solution, (c-d) MO solution, (e-f) RhB solution.

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Fig.11

Absorption spectrum curves of compound 2 and blank control test in organic dyes solution for various adsorption times. (a-b) adsorption of MB solution, (c-d) MO solution, (e-f) RhB solution.

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Fig.12

UV absorption spectrum curves of compound 3 and blank control test in organic dyes solution for various adsorption times. (a-b) adsorption of MB solution, (c-d) MO solution, (e-f) RhB solution.

We researched the photocatalytic degradation property of compounds 1-3 on used the same organic dyes (MB MO RHB). It can be seen from the figure that compound 1 has better catalytic effect on MB, compound 3 have better catalytic performance on MO (Fig. 13–15).

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Fig.13

The insets are photos of the corresponding solutions at various times. (a-c) photocatalysis of MB solution (a), MO solution (b) and RhB solution (c) with the use of compound 1 and the control experiment without any catalys.

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Fig.14

The insets are photos of the corresponding solutions at various times. (a-c) thotocatalysis of MB solution (a), MO solution (b) and RhB solution (c) with the use of compound 2 and the control experiment without any catalyst.

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Fig.15

The insets are photos of the corresponding solutions at various times. (a-c) photocatalysis of MB solution (a), MO solution (b) and RhB solution (c) with the use of compound 3 and the control experiment without any catalyst.