Synthesis and Optical Property of a New Zinc Complex Based on the Derivative of 2-(2?-Hydroxyphenyl)-1H-benzimidazole and Phenanthroline①

YIN Guo-Jie ZHANG Ho-Yu TIAN Wen-Jie ZHAO Dn ZHANG Bin CHEN Dong-Mei

Synthesis and Optical Property of a New Zinc Complex Based on the Derivative of 2-(2?-Hydroxyphenyl)-1H-benzimidazole and Phenanthroline①

YIN Guo-Jiea②ZHANG Hao-YuaTIAN Wen-JieaZHAO DanaZHANG Binb②CHEN Dong-Meia

a(471023)b(450000)

A novel zinc complex (ZnE) has been designed and synthesized based on the derivative of 2-(2?-hydroxyphenyl)-1-H-benzimidazole (HBI) and the neutral nitrogen-containing ligand (phen).The crystal of the title complex crystallizes in the monoclinic system, space group21/with= 10.2631(2),= 34.2166(6),= 11.4103(3) ?,= 96.771(2)°,M= 844.24,= 3978.99(14) ?3,= 4, the final= 0.0400 and= 0.1001 for8107 observed reflections (2()).In the title complex, the free protonated phenoxide moiety (4-OH) is successfully retained to realize pseudo-intramolecular hydrogen bonds with the coordinated O atom from the other ligand.

synthesis, proton transfer, intramolecular hydrogen bond;

1 INTRODUCTION

Recently, fluorophore based on excited-state intramole- cular proton transfer (ESIPT) in particular has attracted considerable attention from both experimental and theoretical viewpoints, because it show uniquely optical properties[1-5].A dual emission originating from both the initial excited form and the proton-transferred tautomer is usually found by the ESIPT chromophores, which are of special interest in white light-emitting materials[6-8], and a large panel of optical applications based on ESIPT lumophores has been made[9-13].Despite these encouraging results, syntheses of hydroxybenzazole derivatives for efficient ESIPT emission with different color are challenged until now[14-19].It is well established that ESIPT is a fast photoisomerization process, which mainly relies on the keto-enol tautomerism.Upon photoexcitation, the enol conformer (–O–H···N= or –N–H···N=) of these compounds undergoes ultrafast intramolecular proton transfer to afford the excited keto tautomer (=O···H–N– or =N···H–N=)[20-26].

It is well known that the metal ion in the complexes provide the best combination of versatility, stability and efficiency, which provide opportunities for harnessing satisfactory photo-properties[27].Thus, achieving ESIPT fluorophore based on the pseudo-intramolecular hydrogen bond in complexes is one of our recent targets of fundamental research.The zinc complexes based on 2-(2?-hydroxyphenyl) benzimidazole (HBI) are excellent emitting materials with desired physical and optical properties[28].Meanwhile, hydroxybenzazole derivatives are the most extensively studied ESIPT fluorophores due to its relative good performance in photoluminescence[29, 30].In this report, the HBI was functionalized with additional hydroxy moiety in the 4 position and methoxy moiety in the 4? position to form the 2-(2?-hydroxy-4?-methoxyphenyl)-1-ethyl-4-hydroxyben-zimidazole (EtL).Moreover, in order to achieve better proton transfer luminescence, a neutral nitrogen-containing ligand was introduced on the basis of the original complex structure[31].Upon complexation with Zn(II) ions and the ligands, as expected, the free protonated phenoxide moiety (4-OH) is successfully retained to realize pseudo-intramole- cular hydrogen bonds with the coordinated O atom from the other ligand (Scheme 1).It is looking forward to obtain excited state proton transfer luminescent material with better luminescenceproperty.

Scheme 1.Structure of the zinc complex ZnE

2 EXPERIMENTAL

All the purchased solvents, except for C2H5OH and CH3OH, were distilled from appropriate drying agents prior to use.The commercially available reagents were used as received, unless otherwise stated.1H NMR spectra of the compounds were recorded on a Bruker DPX-400MHz spectrometer in DMSO-6.Electrospray ionization (ESI) mass spectra of the compounds were conducted in positive ion mode using a Bruker Esquire 3000 instrument in CH3OH.Elemental analysis for C, H, and N were acquired by employing Carlo-Erba1106 Elemental Analyzer.The absorption spectra were recorded by using Perkin-Elmer Lambda 900 spectrometer.Steady state excitation and emission spectra of the compounds were carried out on a Jobin-Yvon-Horiba FluoroMax-4P spectrometer.

According to the classic benzimidazoles synthesis[32, 33], the ligand 2-(2?-hydroxy-4?-methoxyphenyl)-1-ethyl-4- hydroxylbenzimidazole (EtL) was synthesized in good yield by the reaction with 2-amino-3-nitrophenol and the corresponding salicylaldehyde (Scheme 2).Subsequent reaction of the ligand (EtL) with zinc(II) acetate in methanol afforded zinc complexes (ZnE).Note that easy and specific synthesis of the complex is essential for real applications.

Scheme 2.Synthesis of the ligand EtL

2.1 Preparation of HL

2-Amino-3-nitrophenol (15 mmol, 2.31 g) and 2-hydroxy- 4-methory-benzaldehyde (17 mmol, 3.06 g) were dissolved in 300 mL of a 5:1 EtOH/H2O mixture.Then Na2S2O4(45 mmol, 7.83 g) was added to it.After heating at 70 ℃ for 5 h, the reaction mixture was cooled to room-temperature and poured into water under stirring.The precipitate was filtered and purified by column chromatography on silica gel using a mixture of ethyl acetate and-hexane as eluent to afford a white powder.Yield: 87.5% (3.36 g).1H NMR (400 MHz, DMSO):13.44 (s, 1H), 12.92 (d,= 31.8 Hz, 1H), 10.02 (d,= 92.1 Hz, 1H), 8.20～7.91 (m, 1H), 7.14～6.98 (m, 2H), 6.70～6.60 (m, 3H), 3.81 (s, 3H); ESI-MS (m/z): 256.9 [M+l]+.Elemental analysis calcd.(%) for C14H12N2O3: C, 65.62; H, 4.72; N, 10.93.Found (%): C, 65.65; H, 4.69; N, 10.92.

2.2 Preparation ofHTsL

HL (7 mmol, 1.79 g) and triethylamine (3.5 mL, 25 mmol) were dissolved in CH2Cl2(20 mL).The mixture was cooled to 0 ℃, and then 4-methylbenzenesulfonyl chloride (1.37 g, 7.2 mmol) in CH2Cl2(10 mL) was added dropwise under N2.After worming the reaction mixture to room temperature for 2 h, it was quenched by water.The mixture was extracted by dichloromethane.The organic layer was dried with Na2SO4and concentrated under reduced pressure.The crude residue was purified by silica gel column chromatography with ethyl acetate/-hexane as eluent to afford a white powder.Yield: 85% (3.36 g) yield.1H NMR (400 MHz, DMSO)12.42 (s, 1H), 7.86 (d,= 8.3 Hz, 2H), 7.60 (d,= 8.7 Hz, 1H), 7.47～7.28 (m, 5H), 7.20～7.08 (m, 2H), 6.95～6.80 (m, 4H), 3.85 (s, 3H), 2.36 (s, 3H), 2.12 (s, 3H); ESI-MS (m/z): 565.3 [M+l]+.Anal.Calcd.(%) for C28H24N2O7S2: C, 59.56; H, 4.28; N, 4.96.Found (%): C, 59.59; H, 4.29; N, 4.96.

2.3 Preparationof EtL

Under N2atmosphere, a mixture of HTsL (2 mmol, 1.13 g) and tetrabutylammonium bromide (TBAB, 64.5 mg, 0.2 mmol) were dissolved in DMSO (10 mL).Aqueous K2CO3(2 M, 3.3 mL) was added dropwise to the solution and stirred at room temperature for 1 h.Then ethyl bromide (0.16 mL, 2.2 mmol) was added slowly.The reaction mixture was refluxed for 12 h.After cooling to room temperature, the mixture was poured into water and extracted by dichloromethane.The organic phase was then dried (Na2SO4), evaporated, and concentrated to dryness under reduced pressure.The resulting crude product (EtTsL) was dissolved in methanol (20 mL) and refluxed for 2 h after the addition of aqueous NaOH (2.0 mol/L, 5 mL).The mixture was cooled and neutralized by dilute aqueous HCl.The mixture was then extracted by ethyl acetate and the organic layers were dried with Na2SO4.The solvent was removed under reduced pressure, and the crude product was further purified by silica gel column chromatography using ethyl acetate/-hexane as eluent to afford a white powder.Yield: 74% (0.42 g).1H NMR (400 MHz, DMSO)12.11 (s, 1H), 9.92 (s, 1H), 7.55 (d,= 8.5 Hz, 1H), 7.11～6.97 (m, 2H), 6.69～6.56 (m, 3H), 4.29 (q,= 7.1 Hz, 2H), 3.80 (s, 3H), 1.34 (t,= 7.2 Hz, 3H); ESI-MS (m/z): 285.2 [M+l]+.Anal.Calcd.(%) for C16H16N2O3: C, 67.59; H, 5.67; N, 9.85.Found (%): C, 67.57; H, 5.66; N, 9.86.

2.4 Synthesis of ZnE

Zn(Ac)2·2H2O (0.22 g, 1 mmol) in methanol (5 mL) was added in drops to a solution of EtL (2 mmol, 0.57 g) in methanol (30 mL) and 1,10-phenanthroline (Phen) (0.18 g, 1 mmol).The resulting mixture was stirred and refluxed for 5 h.The precipitate was then filtered and recrystallized in methanol as white powder.Colorless single crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of their methanol solutions in air.Yield: 61% (0.51 g).ESI-MS (m/z): 811.4 [M+l]+.Anal.Calcd.(%): C, 65.07; H, 4.72; N, 10.35.Found (%): C, 65.02; H, 4.73; N, 10.33.

2.3 X-ray crystallography studies of ZnE

Crystal diffraction data for complexes ZnE were collected on an Oxford Diffraction Xcalibur CCD diffractometer using graphite-monochromatized Moradiation (= 0.7107 ?) at 293 K by an-scan technique.Program CrysAlisPro was employed for data collection and reduction[34].The structure was solved by direct methods, and refined anisotropically by full-matrix least-squares method on2using the OLEX2-1.2 and SHELX[35, 36].All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were included in the structure factor calculation at idealized positions using a riding model and refined isotropically.Hydroxyl protons were located from the difference-Fourier map and refined isotropically, and no restrictions were put on the O–H distances.The coordinated water molecule of ZnE was refined to be disordered over two possible sites with site occupancy factors of 0.50:0.50.Details of selected bond lengths (?) and bond angles (°) for the crystals of the title compound are shown in Tables 1 and 2.

Table 1.Selected Bond Lengths (?) and Bond Angles (°)

Table 2.Hydrogen Bond Lengths (?) and Bond Angles (°)

3 RESULTS AND DISCUSSION

In order to elucidate the structure features of the zinc complex (ZnE), the crystal was grown by evaporation of their methanol solutions in air.The complex crystallizes in monoclinic with space group121/1.As shown in Fig.1, the zinc ionis six-coordinated by N(1)^O(1) and N(3)^O(4) atoms in the two deprotonated ligands with a bidentate bonding mode and N(5)^N(6) atoms of thephen, forming a deformed octahedral coordination environment.Interestingly, the N(1) and N(3) atoms in the two ligands (EtL) intogether with the N(5)^N(6) of the phen ligand constitute the octahedral equatorial plane.The bond lengths of Zn(1)–N(1) and Zn(1)–N(3) between the znic ion and the nitrogen atoms of the two deprotonated ligands are 2.1680(18) and 2.2003(17) ?, respectively.The bond lengths of Zn(1)–N(5) and Zn(1)–N(6) between the zinc ion and the nitrogen atoms of phen are 2.2294(18) and 2.2609(17) ?, respectively.The two oxygen atoms O(1) and O(4) in the ligand EtL are in trans and occupy two vertices of the octahedron, with the bond lengths of Zn(1)–O(1) and Zn(1)–O(4) being 2.0533(15) and 2.0582(14) ?, respectively.At the same time, the distortion of the two EtL ligands caused by the steric hindrance effect is also slightly different.The dihedral angle of the 2-phenyl group containing O(4) atom and the benzimidazole ring is 48.7°, while the other one containing O(1) atom is 44.2°, so the conjugation degree of benzimidazole itself is further reduced.Furthermore, there are strong intramolecular hydrogen bonds in this complex: O(2)–H(2)···O(4) (O/O 2.557(4)?, H···O 1.76 ?,DNHO, 165°) and O(5)–H(5)···O(1) (O/O 2.656(4)?, H···O 1.85 ?,DOHO, 166°).

Fig.1.Molecular structure of the complex ZnE

The UV-Vis absorption spectra of the free ligands (EtL and phen) and the corresponding zinc complex (ZnE) were recorded in methanol solvent at room temperature.As shown in Fig.2, the main absorption region for the ligand EtL is at 210～220 nm (Table 3), the high-energy absorption bands mainly correspond to the-* transition on the 4-hydroxy- benzimidazole moiety, and the lowest-energy absorption bands at ca.316 nm can be attributed to the coupling between the benzimidazole and the substituted phenyl ring[37, 38].Upon complexation with Zn(II) ion, the absorption profile is still dominated by the ligands.Meanwhile, the main absorption wavelength of the ligand phen is in the short-wave region, which enhances the absorption intensity of the complex in the short-wave region.Consequently, due to the dihedral angles between the phenolate and benzimidazole moieties in the complex, the intensity of the long wavelength absorption maxima is reduced relative to that of the maximum absorption wavelengths.It should be noted that the absorption intensities of the complex in the long wavelength region decrease significantly more slowly than that of the corresponding free ligands.The absorption with small extinction coefficient values can be reasonable due to the intramolecular charge-transfer (ICT) between the two coordinated ligands (EtL), as evidenced by the structure, red-shifted absorption tail in addition to the main absorption peak.

Fig.2.UV/Vis absorption spectra of the ligand and the zinc complex in methanol solution (1.0 × 10-5mol×L-1)

Table 3.UV/Vis Absorption Data of the Ligand and the Corresponded Zinc Complex in Methanol Solution (1.0 × 10-5 mol×L-1)

Generally speaking, the HOMO (H) is mainly distributed on electron-rich groups, while LUMO (L) on the electron- deficient group.For the energy of HOMO, the size of the conjugated substituent group has a significant effect.With the increase in delocalized electrons, the energy of HOMO shows an increasing tendency[39].Furthermore, the HOMO-LUMO gap is an important parameter to characterize the electronic structure of molecule.It is well known that the energy gap retains close connection to some molecular properties[40].In order to obtain the electronic density distribution of complex, the energy type of the initial spatial configuration was investigated at the B3LYP[41]/3-21G level with density functional theory[42]based on the ground state by using the Gaussian 09 program package[43].As shown in Fig.3, the HOMO energy level of –4.78 ev is distributed on the group of ligand EtL and the contribution rate of this moiety in the highest occupied orbital reached 97.6%, while the LUMO energy level of –2.02 ev is distributed on the group of ligand phen and the contribution rate of this moiety in thelowest-unoccupied orbital is up to 98.7% with its energy gap to be 2.68 eV.Therefore, the H→L transition is a predominantly ligand-to-ligand charge transfer transition (LLCT).Normally, the intensity of LLCT is too weak, and it is difficult to observe in the UV/Vis absorption spectrum, which eventually leads to fluorescence quenching.

Fig.3.HOMO-LUMO (H-L) energy levels and the energy gap of the complex

In order to reveal the stability of the crystal structure, thermal gravimetric analysis (TGA) was performed for the complex under nitrogen atmosphere in the range of room temperature to 700 ℃.As depicted in Fig.4, the initial weight loss of 4.02% from room temperature to 67.6 ℃corresponded to the release of CH3OH guest molecules (calcd.3.79%).After that, there is a plateau region up to 248.2 ℃, and then the weight loss is attributable to the decomposition of the coordination framework.The final residue of 9.81% for compound 2 is in agreement with the percentage of ZnO in all cases (calcd.9.59%).

Fig.4.TGA curve of the complex

4 CONCLUSION

In conclusion, a novel zinc complex has been designed and synthesized based on 2-(2?-hydroxy-4?-methoxy-phenyl)-1-ethyl-4-hydroxybenzimidazole (EtL) and neutral nitrogen-containing ligand (phen).As expected, the free protonated phenoxide moiety (4-OH) is successfully retained to realize pseudo-intramolecular hydrogen bonds with the coordinated O atom from the other ligand.However, unfortunately, the H → L transition in the complex is a predominantly ligand-to-ligand charge transfer transition (LLCT) from the group of hydroxybenzimidazole to the nitrogen-containing ligand, which leads to the fluorescence quenching of the complex.Moreover, the introduction of neutral ligands increased the distance of intermolecular hydrogen bonds within the ligands.The luminescence phenomenon of the complex was not observed under solid conditions at the same time.Next, we will revise the strategy and synthesize a new complex on the basis of the zinc salt and the unique ligand (EtL) to eliminate the phenomenon of fluorescence quenching and the influence of introducing nitrogen-containing neutral ligands on the formation of intramolecular hydrogen bonds so as to achieve better luminescence properties.

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20 July 2020;

13 October 2020 (CCDC 2017298)

① The project was supported by the Training Plan for Young Key Teachers in Colleges and Universities of Henan Province (No.2020GGJS243) and Scientific and Technological Project of Henan Province (Nos.182102210102 and 202102210058)

E-mail: 591941522@qq.com and 13523612522@163.com

10.14102/j.cnki.0254–5861.2011–2941