简介概要

Luminescent properties of a new Nd³⁺-doped complex with two different carboxylic acids and pyridine derivative

来源期刊：Rare Metals2013年第5期

论文作者：Yun-Xia Ye Li-Hua Wei Wei-Chen Mei Chen Yin-Qun Hua

文章页码：490 - 495

摘要：A new Nd3+-doped organic complex featuring two different perfluorinated carboxylic acids as the first ligand and pyridine derivative 2-amino-3-chloro-5-（trifluoromethyl）pyridine as the second ligand was designed and synthesized.Successful coordination between the ligands and central rare earth ions was confirmed by Fourier transform infrared spectroscopy（FT-IR）spectra,1H nuclear magnetic resonance（1H NMR）spectra,and UV spectra,and the synthesized complex is inferred to be eight-coordinate structure.Solution of the complex dissolved in DMSO-d6 was prepared and then its fluorescence spectrum,UV–Vis–NIR absorption spectrum,and fluorescence decay curve were tested.The fluorescent lifetime is about 7 ls.Based on the above experimental research,Judd–Ofelt analysis was carried out,and the results indicate that appropriate coordination environment around Nd3?in this solution results in a high fluorescent quantum efficiency 2%and a large stimulated emission cross-section about 3.2 9 10-20cm2at 1,064 nm.

稀有金属 (英文版) 2013,32(05),490-495

Luminescent properties of a new Nd³⁺-doped complex with two different carboxylic acids and pyridine derivative

School of Mechanical Engineering, Jiangsu University

Institute of Laser Technology, Jiangsu University

School of Material Engineering, Jiangsu University

摘要：

A new Nd3+-doped organic complex featuring two different perfluorinated carboxylic acids as the first ligand and pyridine derivative 2-amino-3-chloro-5- (trifluoromethyl) pyridine as the second ligand was designed and synthesized.Successful coordination between the ligands and central rare earth ions was confirmed by Fourier transform infrared spectroscopy (FT-IR) spectra, 1H nuclear magnetic resonance (1H NMR) spectra, and UV spectra, and the synthesized complex is inferred to be eight-coordinate structure.Solution of the complex dissolved in DMSO-d6 was prepared and then its fluorescence spectrum, UV–Vis–NIR absorption spectrum, and fluorescence decay curve were tested.The fluorescent lifetime is about 7 ls.Based on the above experimental research, Judd–Ofelt analysis was carried out, and the results indicate that appropriate coordination environment around Nd3?in this solution results in a high fluorescent quantum efficiency 2%and a large stimulated emission cross-section about 3.2 9 10-20cm2at 1, 064 nm.

收稿日期：25 September 2012

基金：supported by the Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 08KJD430009);Jiangsu University Senior Talent Starting Fund (No. 08JDG025);

Luminescent properties of a new Nd³⁺-doped complex with two different carboxylic acids and pyridine derivative

Yun-Xia Ye Li-Hua Wei Wei-Chen Mei Chen Yin-Qun Hua

School of Mechanical Engineering, Jiangsu University

Institute of Laser Technology, Jiangsu University

School of Material Engineering, Jiangsu University

Abstract：

A new Nd3+-doped organic complex featuring two different perfluorinated carboxylic acids as the first ligand and pyridine derivative 2-amino-3-chloro-5- (trifluoromethyl) pyridine as the second ligand was designed and synthesized. Successful coordination between the ligands and central rare earth ions was confirmed by Fourier transform infrared spectroscopy (FT-IR) spectra, 1H nuclear magnetic resonance (1H NMR) spectra, and UV spectra, and the synthesized complex is inferred to be eight-coordinate structure. Solution of the complex dissolved in DMSO-d6 was prepared and then its fluorescence spectrum, UV–Vis–NIR absorption spectrum, and fluorescence decay curve were tested. The fluorescent lifetime is about 7 ls. Based on the above experimental research, Judd–Ofelt analysis was carried out, and the results indicate that appropriate coordination environment around Nd3?in this solution results in a high fluorescent quantum efficiency 2 % and a large stimulated emission cross-section about 3.2 9 10-20cm2at 1, 064 nm.

Keyword：

Nd; Organic solution; Liquid laser; Quantum efficiency; Judd–Ofelt theory;

Author： Yun-Xia Ye e-mail: yeyunxia@ujs.edu.cn ;

Received： 25 September 2012

1 Introduction

Rare earth ions-doped fluorescent materials are widely researched and applied in laser systems, medical sensors, illumination, and so on.However, rare earth-containing organic luminescent materials were not developed well due to serious non-radiative deactivation by high energy vibration of surrounding coordination environment.So fluorescent quantum efficiency and lifetime of rare earth ions in organic system are both very low, especially for some near infrared ions, e.g., Nd³⁺and Er³+.For these ions, too small minimum energy gap between exciting states and lower levels can be very easily accepted by high vibration mode of O–H and C–H in organic system and cause non-radiative deactivation of excited states and fluorescence quench.So, researchers keep on working to find strategies for suppressing fluorescence quench and improving the fluorescent quantum efficiency of rare earth ions in organic systems.As far as I know, the influence of coordination environment on emission properties of rare earth ions in organic systems was thoroughly investigated by Hasegawa research group[1–5].Their work led to the most effective strategy to prevent luminescent ions from vibrational deactivation:detailedly designing the composition and structure of complex, and then realizing the coordination of rare earth ions with low-vibration ligands.In addition, the forbidden f–f transitions of rare earth ions in a symmetric environment cause low efficiency of absorbing exciting light, and also weaken the fluorescence.So, another effective approach to obtain stronger luminescence is forming an asymmetric surrounding around rare earth ions to increase transition probability of absorption and then enhance fluorescence intensity[4, 5].Under the guidance of these effective strategies, a number of lanthanide (III) -doped fluorescent complexes were synthesized and investigated[6–10].Until now, one of the most successful fluorescent complex structures is ternary complex, with some organic anions acting as the first ligands satisfying charge balance and neutral complex as second ligands to meet the requirements of high coordination number of rare earth ions.In this kind of structure, rare earth ions can be closely wrapped in a low-vibration coordination sphere so as to effectively reduce radiationless transitions.For this kind of complex, the carboxylic acid is commonly used as the first ligand through forming carboxylates[8, 11].And, some aromatic heterocyclic compounds, such as 2, 2-bipyridine and 1, 10-phenanthroline, can provide nitrogen atoms to coordinate with lanthanide ions and usually act as the second ligands[8, 12].

In the present study, we synthesized a new Nd³⁺-doped complex with two different perfluorinated carboxylic acids, pentafluoropropionic acid (C₂F₅COOH) , and 2, 3, 4, 5, 6-pentafluoro benzoic acid (C₆F₅COOH) , as the first ligand and2-amino-3-chloro-5- (trifluoromethyl) pyridine (C₆H₄Cl F₃N₂, abbreviation:tfpy) as the second ligand.Introducing two different acids into the most inner coordination sphere aims at: (1) forming a more asymmetric coordinating sphere around the luminescent ions to improve absorbing exciting light; (2) investigating the influence of the benzene ring in the pentafluoro benzoic acid on fluorescence.Choosing 2-amino-3-chloro-5- (trifluoromethyl) pyridine is because it has four hydrogen atoms, while there are eight hydrogen atoms in 2, 2-bipyridine (C₁₀H₈N₂) and 1, 10-phenanthroline (C₁₂H₈N₂) .The structure of the obtained complex was characterized by FT-IR spectra, ¹H NMR spectra, and UV spectra.Solution of the complex dissolving in deuterated dimethyl sulfoxide (C₂D₆SO, DMSO-d6) , the best solvent for fluorescent rare earth-doped organic system ever reported in[4, 7, 13], was prepared and then its fluorescence spectrum, UV–Vis–NIR spectrum, and fluorescence decay curve were tested.Through Judd–Ofelt analysis, the coordination symmetry and fluorescent properties were investigated in detail.

2 Experimental

2.1 Materials and instruments

Pentafluoropropionic acid (98%) and 2, 3, 4, 5, 6-pentafluoro benzoic acid (99%) were purchased from Matrix Scientific Co.and 2-amino-3-chloro-5- (trifluoromethyl) pyridine (98%) from Tokyo Kasei Industry Co.Ltd.Na OH (99.99%) was obtained from Alfa Aesar, Nd Cl₃.6H₂O (99.9%) from Shanghai Di Yang Chemical Co.Ltd., deuterated dimethyl sulfoxide (DMSO-d6) from Acros Orcanics, and anhydrous ethanol (reagent grade) from Sinopharm Chemical Reagent Co.Ltd.Fourier transform infrared spectroscopy (FT-IR) spectra and UV spectra for characterizing the synthesized complex were obtained with Nexus 670 and UV-2450, respectively.The UV–Vis–NIR spectrum of Nd³⁺-doped solution was performed by UV2410.The¹H NMR spectra were recorded on AVANCEII 400 MHz in DMSO-d6solvent.The fluorescence spectrum of the solution was obtained on Quanta Master TM40.For measuring fluorescent decay curves, the solution was excited with Q-switched double frequency Nd³⁺:YAG laser, DET210photoelectric tube as detector and TDS3054B as oscilloscope.

2.2 Preparation and characterization of complex

Pentafluoropropionic acid (0.63 ml, 6 mmol) , 2, 3, 4, 5, 6-pentafluoro benzoic acid (0.64 g, 3 mmol) , and Na OH (0.36 g, 9 mmol) were mixed in 15 ml anhydrous ethanol.The reaction mixture was stirred at 60°C for 4 h and then dried for 12 h under vacuum at 80°C.The obtained white solid powder and Nd Cl₃.6H₂O (1.08 g, 3 mmol) were added into 15 ml anhydrous ethanol again and reacted at60°C for 6 h under stirring.After this reaction, we obtained clarified solution through removing sodium chloride by filtration.Then 0.602 g (0.003 mol) 2-amino-3-chloro-5- (trifluoromethyl) pyridine was added into the solution and reacted at 60°C for 6 h under stirring.After reaction, the solution was dried for 48 h under vacuum at80°C to get final product complex Nd (C₂F₅COO) ₂ (C₆F₅COO) tfpy.

The obtained complex was characterized by FT-IR, ¹H NMR, and UV spectra.FT-IR spectra of free ligands and complex were obtained through KBr pellet pressing method to ascertain whether CF₃CF₂COOH, C₆F₅COOH, and tfpy were coordinated with central Nd³⁺as predicted.¹H NMR spectra were also used to verify the successful coordination between tfpy and Nd³⁺.The solution of complex dissolving in DMSO-d6 was prepared with the concentration of 0.1 mol.L^-1.

3 Results and discussion

3.1 Structure analysis

Fig.1 Infrared spectra of free ligands and complexes

Fig.2 UV spectra of free ligands and complex in ethanol

Fig.3 Chemical structure of complex

Table 1 Main absorption peaks in IR spectra of free ligands and complexes (cm^-1) 下载原图

Table 1 Main absorption peaks in IR spectra of free ligands and complexes (cm^-1)

Fig.4 UV–Vis–NIR absorption spectrum of Nd (C2F5COO) 2 (C6F5COO) tfpy in DMSO-d6 (0.1 mol.L-1)

Above all, we can infer that the synthesized complex is eight-coordinate structure with six Nd–O coordination bonds to carboxylic acids and two Nd–N coordinating bonds to tfpy.The chemical structure of the complex is shown in Fig.3.

3.2 Luminescence analysis

To evaluate the probability of non-radiative transition and quantum efficiency of Nd³⁺in solution, Judd–Ofelt[14, 15]theory (JO theory) was applied to analyze the radiative properties of rare earth ions-doped organic liquid material.The main results of JO theory are as follows:three intensity parameters giving information about coordination environment around rare earth ions, the radiative lifetime s_rfor the excited⁴F_3/2level determining the fluorescent quantum efficiency and the stimulated emission cross-section being an important parameter for laser application.

Fig.5 Emission spectrum of Nd (C2F5COO) 2 (C6F5COO) tfpy in DMSO-d6 (0.1 mol.L-1)

Fig.6 Fluorescence decay curves of Nd (C2F5COO) 2 (C6F5COO) tfpy in DMSO-d6 (0.1 mol.L-1)

Table 2 Emission quantum efficiencies, lifetimes, and stimulated emission cross-sections of Nd³⁺-doped organic media 下载原图

Table 2 Emission quantum efficiencies, lifetimes, and stimulated emission cross-sections of Nd³⁺-doped organic media

In the design and development of laser systems, the stimulated emission cross-section is a very important parameter, which determines the oscillator threshold, the laser medium gain, saturation power, etc.With the value of the radiative transition rate obtained by JO theory and the corresponding emission spectrum, of this Nd³⁺-doped organic system is evaluated about 3.2 9 10^-20cm².Table 2 also lists the stimulated cross-section of several typical organic system.From Table 2, we can see that obtained in this article is not only higher than other Nd³⁺-doped organic systems, but also is comparable with some Nd³⁺-doped solid laser material.

4 Conclusion

In conclusion, a new Nd³⁺-doped complex was synthesized and the solution of new complex in DMSO-d6 was prepared.We chose two different perfluorinated carboxylic acids, C₂F₅COOH and C₆F₅COOH, as the first ligand and electrically neutral 2-amino-3-chloro-5- (trifluoromethyl) pyridine as the second ligand.The successful coordination between ligands and central Nd³⁺was verified through FT-IR, ¹H NMR, and UV spectra.The complex is inferred to be eightcoordinate structure with six Nd–O and two Nd–N coordination bonds.Based on absorption spectrum, luminescence spectrum, and fluorescent decay curve, three intensity parameters , fluorescent quantum efficiency, and stimulated emission cross-section of the Nd (III) complex in the solution were calculated according to Judd–Ofelt theory.Small X₂value indicates that two benzene rings in this complex form a more symmetric crystal field than Nd (C₂F₅COO) ₃tfpy, which may reduce the ability of rare earth ions to absorb exciting light.This finding also reminds us that we should reasonably choose different ligands to construct asymmetric coordination surrounding around rare earth ions so as to enhance pumping efficiency.Despite this, the fluorescent quantum efficiency of the solution reaches a relatively high value 2%, because deuteration and perfluorinations effectively suppress the non-radiative transitions of Nd³⁺and a small energy loss due to rigid structure in the complex.Furthermore, the stimulated emission cross-section of Nd³⁺in the solution is also very high, which indicates that the Nd³⁺-doped liquid material is a promising candidate for the laser materials in further application.