Structure and luminescence of benzidine 5-methylfurfural lanthanide complexes

WANG Xin-Yue, KANG Li, YU Ze-Qi, LI Qiu-Lei, LUAN Fang

(Department of chemistry and chemical engineering, college of pharmacy, Jiamusi university, Jiamusi 154007, China)

Abstract：Heterocyclic Schiff base ligands were synthesized by benzidine and 5-methylfurfural with conventional synthesis method, and then three kinds of lanthanide complexes were synthesized with the synthesized Schiff base ligands and Ln(NO3)3·6H2O(Ln=Eu3+,Tb3+,Sm3+). The heterocyclic Schiff base ligands were characterized by ultraviolet spectroscopy, infrared spectroscopy and nuclear magnetic resonance spectroscopy. The fluorescence spectroscopy luminescent properties of the lanthanide complexes were determined. The characterization results showed that benzidine 5-methylfurfural and three lanthanide complexes were successfully synthesized. The intensity of the fluorescence luminescence properties of europium and samarium lanthanide complexes exhibits wonderful. Semi-rigid benzidine 5-methylfurfural heterocyclic Schiff base organic ligands can effectively sensitize the luminescence of Eu3+ and Sm3+.

Key words：Benzidine; 5-methylfurfural; heterocyclic Schiff base; lanthanide complexes; synthesis; fluorescence

0 Introduction

According to the f-f transition barrier, the direct excitation of lanthanide ions always present weak luminescence intensity[1]. Therefore, the appropriate organic ligands are used commonly to sensitize and develop the luminescence properties of the lanthanide ions. Constructed by the heterocyclic Schiff base, the organic ligands often own various structures, which are easily to prepare, and present excellent performance. Up to now, the lanthanide complexes synthesized by them have broad development prospects in luminescent materials[2-4], NIR materials[5-6], magnetic materials[7-8], catalysis[9], anti-cancer and bacteriostasis and other biological activities[10].

Due to the single bond between two benzene rings, the long conjugated system of benzidine does not present a coplanar structure, so less literatures can be found in the field of luminescence. In this paper, heterocyclic Schiff base organic ligands were synthesized from benzidine and 5-methylfurfural, and then three lanthanide complexes were synthesized by benzidine 5-methylfurfural with Ln(NO3)3. Ultraviolet spectroscopy, infrared spectroscopy and nuclear magnetic resonance spectroscopy were used to characterize the structures of the ligands and complexes, and molecular fluorescence spectroscopy was used to test the solid fluorescent emission properties of lanthanide complexes, hoping to provide a reference for their further application in the field of luminescent materials.

1 Material and experimental methods

1.1 Instruments and reagents

Nuclear magnetic resonance spectrometer (Bruke, Avance 300 MHz); Antaris II near-infrared spectrometer (Thermo); Fourier transform infrared spectrometer (Bruke); 2550 ultraviolet visible spectrophotometer (Shimadzu).

Benzidine; 5-methylfurfural; Eu(NO3)3·6H2O; TbCl3·6H2O;Sm(NO3)3·6H2O;Methanol;Petroleum ether.

1.2 Synthesis of Schiff base ligand benzidine 5-methylfurfural (L)

5-methylfurfural (5.1 mmol, 0.592 4 g) was dissolved in 5 mL methanol, then put them into a 100 mL flask. With the ultrasound assistance, benzidine (1.7 mmol, 0.326 5 g) was dissolved in methanol completely, then slowly added the methanol solution of benzidine into the methanol solution of 5-methylfurfural. The mixed solution was stirred at 30 ℃ until the yellow precipitate no longer increased, cool down to room temperature, filtered, then washed them with methanol for three times, at last we obtained yellow crystals.

1.3 Synthesis of complexes 1-3

5-methylfurfural benzidine (0.1 mmol, 0.037 0 g) and 5 mL dichloromethane were added together into a 50 mL round bottom flask, stirred when they were completely dissolved, then added 3 mL acetonitrile solution of Eu(NO3)3·6H2O(0.1 mmol, 0.047 3 g);Tb(NO3)3·6H2O(0.1 mmol, 0.047 3 g) and Sm(NO3)3·6H2O(0.1 mmol, 0.045 0 g) slowly into the dichloromethane solution of 5-methylfurfural benzidine at room temperature. Then the mixed solution was stirred for a while, its color turned from light yellow to brick red, and some dark red sediment was precipitated. Stopped stirring until no precipitation operated (>5 h), filtered at a medium speed, then dried at room temperature. At last, we obtained some brick red yellow and brick red powder which are complexes1-3.

1.4 Characterization of Schiff base ligands and lanthanide complexes 1-3

Using methanol as a blank sample in the wavelength range of 200～400 nm, the UV spectra of the Schiff base ligand benzidine 5-methylfurfural and its lanthanide complexes were determined. Also, the infrared spectra of the ligand benzidine 5-methylfurfural and lanthanide complexes were determined by KBr compression in the wavenumber range of 400～4 000 cm-1. Using deuterated DMSO as solvent, the1H NMR spectra of the ligand benzidine 5-methylfurfural was determined to further confirm its molecule structure.

1.5 Luminescent property test of lanthanide complexes

With the excitation wavelength and emission wavelength optimized, the solid-state molecular fluorescence spectrum of 5-methylfurfural lanthanide complexes1-3were measured at room temperature.

2 Results and discussions

2.1 1H NMR spectrum analysis of Schiff base ligand

The1H NMR spectrum of the heterocyclic Schiff base ligand benzidine 5-methylfurfural is shown in Fig.1.

1H NMR spectra results of benzidine 5-methylfurfural showed that the δ (9～10)×10-6characteristic peak of -CHO in furfural disappeared, and the peak of C=N in ligand present a distinct signal at δ 8.27×10-6. δ 2.47×10-6due to the methylhydrogen of furfural, and the peak of δ 6.23×10-6is contributed by hydrogen on the furfural ring, the peak of δ 6.94×10-6belongs to the hydrogen on the benzidine ring. Which mentioned above shows that the ligand benzidine 5-methylfurfural was successfully synthesized.

2.2 Infrared spectrum analysis of ligand and complexes 1-3

The infrared spectra of heterocyclic Schiff base ligand benzidine 5-methylfurfural and its complexes1-3are shown in Fig.2.

Fig.2 shows that the absorption peaks of complex1at 1 040 cm-1, 1 447 cm-1and 3 396 cm-1are contributed to benzene ring C-H deformation vibration, C=C skeleton stretching vibration and benzene ring C-H stretching vibration respectively, and 1 660 cm-1is generated by the C=N characteristic absorption peak in Schiff base ligand benzidine 5-methylfurfural (35 cm-1blue shift compared with the ligand). Similarly, complex2has strong infrared absorption at 1 627 cm-1, which is a strong absorption of Schiff base ligand C=N stretching vibration feature. Complex3has strong IR absorption peaks at 1 040 cm-1, 1 295 cm-1, 1 657 cm-1, 3 423 cm-1, of which 1 657 cm-1is the C=N characteristic absorption of Schiff base ligand, and the C=N characteristic absorption peak of the ligand is strong (32 cm-1blue shift compared with the ligand). These infrared spectra indicates that complexes1-3were successfully synthesized.

Fig.1 1H NMR of heterocyclic Schiff base ligand Benzidine 5-methylfurfural

Fig.2 IR of ligand and complexes 1-3

2.3 UV spectral analysis of ligand and complexes 1-3

The UV spectra of the heterocyclic Schiff base ligand benzidine 5-methylfurfural and its lanthanide complexes1-3are shown in Fig.3.

Fig.3 UV-vis of ligand and complexes 1-3

Fig.3 shows that the UV absorption peak of the complexes has shifted apparently compared with the ligand. There are three strong absorption peaks of the ligand at 206 nm and 288 nm. Complex1has strong absorption peaks at 209 nm and 364 nm, and complex2has strong absorption peaks at 361 nm. The absorption peaks of complex3are at 207 nm and 365 nm. The above absorption peaks come from the absorption of benzene ring E2 band and the n-π*transition of Schiff base C=N characteristic group. Compared with the UV absorption data of the ligand, the big red shift occurred with the UV spectra of complexes1-3. The reason may be attributed to the interference of the ligand energy level after coordination with lanthanide metal ions, which weakens the conjugated system and causes the red shift of the peak. In addition, the molar absorptivity of organic ligands is far greater than that of lanthanide metal elements, and through coordination, the space distance between lanthanide and ligands is drawn to a large extent. Therefore, the energy absorbed by the ligand can be transferred to the central ion in the form of energy level matching, so that the complexes exhibit the characteristic emission of the lanthanide ions.

2.4 Solid fluorescence spectrum analysis results of complexes

Fig.4 Fluorescence spectra of complex 1

The solid fluorescent spectrum of complex1are shown in Fig.4.Fig.4 shows that complex1is excited at 404 nm, and presents strong emission at 573 nm and 618 nm, which indicates that the ligand center can transfer energy to the lanthanide ion Eu3+for its luminescence, corresponding to the transitions of5D0-7F0and5D0-7F2respectively. Significantly, the strongest emission peak of complex1appears at 618 nm, which is the main characteristic absorption peak of Eu3+and shows the characteristic red light of Eu3+. The solid fluorescent spectrum of complex2are shown in Fig.5.Fig.5 shows that complex2has 547 nm and 609 nm emission peaks with 400 nm excitation wavelength, which correspond to the5D4-7F5and5D4-7F3transitions of the lanthanide ion Tb3+respectively. Complex2shows that the strongest orange red light emission exits at 609 nm, and the characteristic green light emission intensity of Tb3+at 547 nm is weak. The relative fluorescence intensity ratio at5D4-7F5and5D4-7F3is about three times, which indicates that Tb3+is not in the symmetric center of the complex, and the ligand cannot effectively sensitize the luminescence of Tb3+ions.

Fig.5 Fluorescence spectrum of complex 2

Fig.6 Fluorescence spectra of complex 3

The solid fluorescent spectrum of complex3are shown in Fig.6.Fig.6 shows that the characteristic emission peaks of complex3at 576 nm and 647 nm are obtained at 400 nm excitation wavelength, which indicates that the ligand center can transfer energy to lanthanide ion Sm3+for its luminescence, corresponding to the4G5/2→6H5/2and4G5/2→6H9/2transitions of Sm3+respectively. Also, it can be observed that the fluorescence intensity at 647 nm is higher than the other peak, because 647 nm is the main characteristic absorption peak of Sm3+, where it presents orange red light.

The luminescence mechanism and their energy levels of lanthanide complexes1-3can be calculated with the lowest triplet energy level of the organic ligand and the lowese excited state level of lanthanide irons[11-12].

As mentioned above, all of three synthesized benzidine 5-methylfurfural lanthanide complexes own better fluorescence properties. Compared with the other Schiff base lanthanide complexes, the ligand benzidine 5-methylfurfural has a longer π-conjugation system and greatly enhances the fluorescence of the complexes[13]. The principle may be that the triplet state of the ligand longer conjugated system matches the resonance energy level of Eu3+ion and Sm3+, while it can not match the resonance energy level of the Tb3+ion. Therefore, the heterocyclic Schiff base ligand sensitizes the fluorescence emission strength of the different lanthanide ions respectively.

3 Conclusions

In conclusion, the heterocyclic Schiff base ligand benzidine 5-methylfurfural was successfully synthesized from benzidine and 5-methylfurfural in this experiment, and a new Schiff base lanthanide complex was successfully synthesized from the ligand and lanthanide salt. Through the characterization of ligands and complexes and the solid fluorescence test of the complexes, complexes1-3present better fluorescence performance. This indicates that the organic heterocyclic Schiff base ligand benzidine 5-methylfurfural can effectively transfer energy with lanthanide ions, which lays a foundation for further development of heterocyclic Schiff base lanthanide fluorescence sensing materials.