Effects of Different Ligands on Fluorescent Properties of Nd3+ Organic Complexes

Rare Metal Materials and Engineering Volume 43, Issue 10, October 2014 Online English edition of the Chinese language journal Cite this article as: Rare Metal Materials and Engineering, 2014, 43(10): 2359-2364.

ARTICLE

Effects of Different Ligands on Fluorescent Properties of Nd3+ Organic Complexes Ye Yunxia,

Wei Lihua,

Sheng Weichen,

Hua Yinqun

Jiangsu University, Zhenjiang 212013, China

Abstract: Four complexes Nd(C2F5COO)3Tfpy, Nd(C3F7COO)3Tfpy, Nd(C2F5COO)2(C6F5COO)Tfpy, and Nd(C2F5COO)3Phen were synthesized to investigate the effects of different ligands on fluorescent properties of Nd3+. Their structures were characterized by UV absorption spectra, FT-IR spectra and 1H NMR spectra. On the basis of testing absorption spectra, photoluminescence spectra and emission decay curves of each solution of the complexes, Judd-Ofelt(JO) analysis was carried out for each fluorescent solution. Structure analysis indicates that all ligands have successfully coordinated with central rare earth ions. Relatively small Ω2 obtained through JO analysis demonstrates that the coordination bonds between Nd3+ and the vicinity ligand are ionic for four complexes. The benzene rings of ligands are favorable for improving the fluorescence quantum efficiency of Nd3+. Compared with Phen, Tfpy is an excellent neutral second ligand because it has more asymmetric chemical structure and less hydrogen atoms. The four Nd(III)complexes all have potential to become liquid laser material because of their large stimulated emission cross sections. Key words: Nd; coordination environment; nonradiative transition; quantum efficiency; Judd-Ofelt theory

Liquid rare earth-doped medium has great potential application in laser systems, especially in high power or high energy laser system for its cooling ability by circulation. Inorganic liquid media is not practical due to its high toxicity, corrosion and instability. So rare earth-doped organic system has been regarded as a promising candidate[1-6]. However, as is well known, high-energy vibrational chemical bonds in organic systems, such as O-H, C-H, easily make lanthanide ions suffering serious non-radiative fluorescent quenching[7]. As a result, the fluorescent quantum efficiency of rare earth-doped organic materials is very low, which is a major obstacle to the development of organic laser materials. So researchers have carried out extensive researches about design, preparation and luminescence properties of this kind of materials, and found several effective strategies to improve the emission properties of rare earth ions in organic system. To sum up, all strategies work from two major mechanisms, as shown in Fig.1 (taking a four-level system as an example):

improving the exciting light-absorbing ability of rare earth ions and suppressing nonradiative transitions from the excited level to enhance fluorescent quantum efficiency Φ. For rare earth ions, the forbidden f-f transition by the parity rule leads to very low light-absorbing ability. So one direct method to enhance absorption ability is constructing asymmetric coordination environments around rare earth ions and then making the forbidden f-f transition partially allowable to improve absorption[8,9]. Another important method is indirect, choosing a suitable ligand as an antenna to absorb the excitation light, and then transferring energy from ligand to lanthanide center through intra-molecular relaxation[10]. As for how to improve the fluorescent quantum efficiency, it can be realized through closely coordinating rare earth ions with low vibration ligands to suppress non radiative deactivation, specifically, through choosing deuterated or fluorinated organic complex ligand or solvent[11]. In addition, luminescent efficiency still has a close relationship with complex structure:

Received date: October 21, 2013 Foundation item: Natural Science Foundation of Jiangsu Higher Education Institutions of China (08KJD430009); Jiangsu University Senior Talent Starting Fund (08JDG025) Corresponding author: Ye Yunxia, Ph. D., Associate Professor, School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China, Tel: 0086-511-88780228, E-mail: [email protected] Copyright © 2014, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.

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Ye Yunxia et al. / Rare Metal Materials and Engineering, 2014, 43(10): 2359-2364 Radiative transition Non radiative transition Non steady state

Excited state (upper level) Direct excitation wr

Indirect excitation

war

Φ=

wr wr + war

Lower level

Ground state

Fig.1 Major transitions during photoluminescence

the larger the conjugate plane and the more rigid the structure of the complex, the higher the luminescent efficiency is[12]. According to the above effective strategies, in this paper, four perfluorocarbon neodymium complexes were designed and synthesized to investigate the effect of different ligands on fluorescent properties of Nd3+. Complexes with pentafluoropropionic acid (C2F5COOH), heptafluorobutyric acid (C3F7COOH) or pentafluorobenzoic acid (C6F5COOH) acting as the first ligand, and 2-amino-3-chloro-5-trifluorometly pyridine (C6H4ClF3N2, abbreviated as Tfpy) or 1, 10 phenanthroline (C12H8N2, abbreviated as Phen) as the second ligand were synthesized. FT-IR, UV and 1HNMR spectra were carried out to infer whether ligands have coordinated with central Nd3+ ions. Optical properties of the complexes have been characterized and discussed using the Judd-Ofelt theory.

1

Experiment

Pentafluoropropionic acid (98%), heptafluorobutyric acid (99%) and pentafluorobenzoic acid (99%) were purchased from Matrix Scientific Co., and 2-Amino-3-Chloro-5-Trifluor OmetlyPhyridine (98%) from Tokyo Kasei Industry Co Ltd. 1,10 phenanthroline (99%) and dimethyl sulfoxide-d6 were obtained from Acros Organics USA company. NaOH (99.99%) was from Alfa Aesar. NdCl3·6H2O (99.9%) was from Shanghai DiYang Chemical Co., Ltd. Anhydrous ethanol (Reagent grade) was obtained from Sinopharm Chemical Reagent Co., Ltd. All materials were used directly without further purification. The UV absorption spectra were obtained with UV-2450 using ethanol as solvent. FT-IR spectra were measured on Nexus 670 through KBr pellet pressing method. The 1H NMR spectra were recorded on AVANCEⅡ 400 MHz in DMSO-d6 solvent. The absorption spectra of Nd3+ complexes solutions were performed by UV2410. The fluorescence spectrum of the solution was obtained on QuantaMaster TM40 with excitation wavelength 365 nm. Fluorescence decay curves of solutions

were measured with Q-switched double frequency Nd3+:YAG laser as excitation source, DET210 photoelectric tube as detector and TDS3054B as oscilloscope. The synthesis procedure can be summarized as follows. 9 mmol carboxylic acids and 9 mmol NaOH were dissolved in anhydrous ethanol (15 mL), reacting at 60 °C for 4 h in water bath under stirring. Then the obtained solution was dried at 80 °C for 12 h in vacuum. Subsequently, the obtained solid powder and 3 mmol NdCl3·6H2O were added into 15 mL ethanol and then the reaction mixture was stirred at 60 °C for 6 h. After reaction, the obtained solution was distilled to filter off the white sodium chloride. Then 3 mmol Tfpy or Phen was added into the obtained solution and reacted at 60 °C for 6 h under stirring. After reaction, the solution was dried for 48 h under vacuum at 80 °C to get final product complexes: Nd(C2F5COO)3Tfpy, Nd(C3F7COO)3Tfpy or Nd(C2F5COO)3Phen. For Nd(C2F5COO)2(C6F5COO)Tfpy, there were two different carboxylic acids acting as the first ligand. So 9 mmol carboxylic acids consisted of 6 mmol C2F5COOH and 3 mmol C6F5COOH according to stoichiometric molar ratio. Other synthetic steps were completely similar to the above three complexes.

2

Results and Discussion

2.1 Structural analysis 2.1.1 FT-IR spectra The main FT-IR spectra data of all free ligands and their Nd(III) complexes are listed in Table 1. For carboxylic acids, there are three characteristic peaks, νC=O, νO-H and δO–H (out-plane). From Table 1, it can be found that these three characteristic peaks disappear after complexes forming. Meanwhile, two new peaks which are ascribed to asymmetric stretching vibrations (νasym(COO–)) and the symmetric stretching vibrations (νsym(COO–)) of the carboxylic group appear in the complexes. These phenomena indicate the successful coordination between carboxylic group and Nd3+ via Nd-O bond. Comparing the data of free second ligands with those of complexes, we also can infer that Tfpy and Phen both are successfully added to coordination. As shown in Table 1, for Tfpy, the stretching vibration peaks of C=N (νC=N, 1640 cm-1) have red-shifted to 1550 cm-1 or 1530 cm-1 in the complexes, and stretching vibrations νC-H (3100~3160 cm-1) and νN-H (3490, 3290 cm-1) also experience obvious blue shifts. For Phen, stretching vibration peaks of C=N for the free ligand, appearing at 1640 cm-1 have shifted to 1590 cm-1 after complex forming. 2.1.2 UV spectra The successful synthesis of the complexes can also be confirmed from the ultraviolet absorption spectrum. Fig.2 shows the UV spectra of ligands and complexes. The absorption bands of the ligands pentafluoropropionic acid and heptafluorobutyric acid are found at 218 and 235 nm, respectively which are attributed to n→π* transition.

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Table 1 Characteristic FT-IR bands (cm 1) of the ligand and complexes –

νC=O 1780 1770 1720

δO-H(out-plane ) 930 930 916

νasym(COO–)

νsym(COO–)

νC=N

νC-H

νN-H

Tfpy

1640

31003160

3290 3490

Phen

1640

C2F5COOH C3F7COOH C6F5COOH

νO-H 3300~2500 3300~2500 3300~2500

Nd(C2F5COO)3Tfpy

1670

1440

1550

3350

Nd(C3F7COO)3Tfpy

1670

1430

1530

3350

1650

1430

1530

3340

1670

1410

1590

Nd(C2F5COO)2 (C6F5COO)Tfpy Nd(C2F5COO)3Phen

Pentafluorobenzoic acid has two obvious absorption peaks at 212 and 267 nm, which are owed to n→π* and π→π* transition, respectively. For Tfpy and Phen, they both have three absorptions which are attributed to π→π* transition of conjugated system. When the complexes are formed, the n→π* transition of all acids have a blue shift. For Tfpy, the complexes have a red shift and move from 245 nm to 269 nm which indicates the formation of Nd-N. It is worth noting that coordination with Nd ion has a small effect on the π→π* transition (267 nm) in the pentafluorobenzoic acid and this peak overlaps with absorption band (245 nm) of Tfpy after forming the complexes. In Nd (C2F5COO)3Phen, the absorption band (260 nm) of free ligand Phen has a little red shift and moves to 264 nm, which also illustrates the

1.2

(2)

Absorption

1.0

(1)

a

coordination between Phen and Nd3+. 2.1.3 1H NMR spectra The 1H NMR spectra of free ligand and complexes were recorded with DMSO-d6 as the solvent. The chemical shifts are listed as follows. Tfpy: δ7.13(s, 2H), δ7.92(s,1H), δ8.24(s,1H). Nd(C2F5COO)3Tfpy: δ7.22(s,2H), δ8.22(s,1H), δ8.45(s,1H) Nd(C3F7COO)3Tfpy: δ7.20(s,2H), δ7.89(s,1H), δ8.44(s,1H). Nd(C2F5COO)2(C6F5COO)Tfpy: δ7.19(s, 2H), δ7.94(s,1H), δ8.49(s,1H). Phen: δ7.74(s, 2H), δ7.95(s, 2H), δ8.46(s, 2H), δ9.10(s, 2H). Nd(C2F5COO)3Phen: δ7.78(s, 2H), δ8.00(s, 2H), δ8.54(s, 2H), δ9.43(s, 2H).

3.0 (1) C3F7COOH (2) Tfpy (3) Nd(C3F7COO)3Tfpy

b

2.0

0.6

1.5

0.4

1.0

0.0

(1)

2.5

0.8

(3)

0.2 200

2.0

(2) (3)

0.5 300

(2)

1.5 Absorption

(1) C2F5COOH (2) Tfpy (3) Nd(C2F5COO)3Tfpy

3480 3670 3500 3670 3460 3650

400

(1) C2F5COOH c (2) C6F5COOH (3) Tfpy (1) (4) Nd(C2F5COO)2(C6F5COO)Tfpy (3)

0.0

200

300

2.5 (3)

2.0 1.5

(1) C2F5COOH (2) Phen (3) Nd(C2F5COO)3Phen

400 d

(2)

1.0 1.0 (4)

0.5 0.0

0.5

200

300 Wavelength/nm

400

0.0

(1)

200

300 Wavelength/nm

400

Fig.2 UV absorption spectra of ligands and complexes

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Absorption Coefficient/cm

-1

3

6 5 4

0.004 0.002 0.000 0.00000

3

0.00001 0.00002 0.00003 Time/s

2 1 0 800

900

1000 1100 1200 1300 1400

b

Nd(C3F7COO)3Tfpy Intensity/a.u.

6 5 4

0.004 0.002 0.000

3

0.00000

0.00001 0.00002 0.00003 Time/s

2 1 0 800

c

Nd(C2F5COO)2(C6F5COO)Tfpy Intensity/a.u.

4

900 1000 1100 1200 1300 1400

3 2

0.01

0.00 0.00000

0.00001 Time/s

0.00002

1 0 800

7 6

900

1000 1100 1200 1300 1400

d

Nd(C2F5COO)3Phen Intensity/a.u.

Fluorescence Intensity/a.u.

Fluorescence Intensity/a.u.

7

5 4

0.004 0.002 0.000

3

0.00000 0.00001 0.00002 0.00003 Time/s

2 1 0 800

Nd(C2F5COO)2(C6F5COO)Tfpy

a

Nd(C2F5COO)3Tfpy Intensity/a.u.

Fluorescence Intensity/a.u.

7

Fluorescence Intensity/a.u.

For Tfpy, the single signal at 7.13×10-6 is assigned to -NH2 and the other two signals at 7.92×10-6 and 8.24×10-6 are designated to the CH- in the benzene heterocyclic. Upon coordination, all signals have shifted to a certain degree. This is probably due to the inductive effect of Nd-N bonds and a change in the conformation of the ligand in the complexes. For Phen, all signals are assigned to -CH- in the ring. Compared with the free ligand, the complex has a shift to low field. So the above analysis results of 1H NMR spectra further confirm the successful coordination between second ligands and central Nd3+. 2.2 Analysis and evaluation of fluorescence property 2.2.1 Judd-Ofelt analysis Judd-Ofelt(JO) theory[13,14] is the most practical theory for evaluating the radiative properties of rare-earth-doped materials．In this analyzing method, three intensity parameters Ωt (t=2, 4, 6) and the lifetime of the excited state can be calculated based on the characteristic absorption bands of rare earth ions in the matrix. Then combining fluorescent spectrum with the actually measured fluorescent lifetime, the fluorescent quantum efficiency Φ and the stimulated emission cross-section σem can be determined. Fig.3 shows the absorption spectra of 0.1 mol/L solution of each complex in DMSO-d6. Fig.4 shows the fluorescence spectra and fluorescence decay curves of the fluorescent solutions. According to Fig.4, we evaluate that the actual fluorescent lifetimes of Nd3+ in the solutions are (a) 8.3 μs, (b) 5.3 μs, (c) 6.7 μs, and (d) 7.4 μs. Table 2 shows the calculated results of Judd-Ofelt analysis. Ω2 is an important parameter indicating the coordination environment around rare earth ions in the solution, and it is very sensitive to the symmetry of coordination field and the degree of covalency between Nd3+ ions and ligand. As shown in Table 2, the values of Ω2 of four complexes are all relatively small, demonstrating that the coordination bonds between Nd 3+ and the vicinity ligand are ionic. Compared with Nd(C 2 F 5 COO) 3 Tfpy, it is obvious that Ω 2 of complex Nd(C2F5COO)2(C6F5COO)Tfpy has a very smaller Ω2, which indicates a more symmetric crystal field around Nd3+ ions. For

900

1000 1100 1200 1300 1400 Wavelength/nm

Nd(C2F5COO)3Tfpy

2

Nd(C3F7COO)3Tfpy Nd(C2F5COO)3Phen

are fluorescence decay curves): (a) Nd(C2F5COO)3Tfpy, (b) Nd(C3F7COO)3Tfpy, (c) Nd(C2F5COO)2(C6F5COO)Tfpy,

1

0

Fig.4 Fluorescence spectra of four complexes in DMSO-d6 (insets

and (d) Nd(C2F5COO)3Phen

500

600

700 800 900 1000 1100 Wavelength/nm

Fig.3 Absorption spectra of four complexes in DMSO-d6

this result, we analyze the probable reason as follows. In Nd(C2F5COO)3Tfpy complex, there is only one benzene ring in Tfpy ligand. However, there are two benzene rings in Nd(C2 F5COO) 2(C6F5 COO)Tfpy, one in benzoic acid and another in Tfpy. So two benzene rings in this organic system

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Table 2 Results of Judd-Ofelt analysis Complexe

Ω2/×10-20 cm2

Ω4/×10-20cm2

Ω6/×10-20cm2

τr/μs

σem/×10-20cm2

Φ/%

Nd(C2F5COO)3Tfpy

1.083

5.086

3.680

541

2.696

1.54

Nd(C3F7COO)3Tfpy

1.493

7.072

4.730

400

3.560

1.33

Nd(C2F5COO)2(C6F5COO)Tfpy

0.22

9.466

3.593

356

3.188

1.88

Nd(C2F5COO)3Phen

0.592

4.621

3.424

590

2.488

1.25

Nd(C2F5COO)2(C6F5COO)Tfpy is the weakest although it has the highest quantum efficiency. This is mainly because the absorption ability of this complex is very weak due to its more symmetry coordination crystal field as indicated by its smallest Ω2 parameter. Compared with Nd(C2F5COO)3Tfpy, the fluorescence intensity of Nd(C2F5COO)3Phen is weaker, which is inevitable results of its weaker absorption ability (smaller Ω2) and lower quantum efficiency. The fluorescence lifetimes of Nd3+ in liquid solutions with DSMO-d6 as solvent with different concentrations of 0.2 mol/L, 0.1 mol/L and 0.05 mol/L were tested, as shown in Fig.6. The results indicate a remarkable concentration quenching. Therefore, Nd3+ concentrations have great effects on emission properties. Nd(C2F5COO)3Tfpy Nd(C3F7COO)3Tfpy Nd(C2F5COO)2(C6F5COO)Tfpy Nd(C2F5COO)3Phen

7 Intensity/a.u.

6 5 4 3 2 1 0 800

900 1000 1100 1200 1300 1400 Wavelength/nm

Fig.5 Fluorescent spectra of four complexes

Fluorescence Lifetime/μs

maybe together form a more symmetric crystal field, which generates an unexpectedly low value of Ω2 as a result. In addition, the Ω2 of Nd(C2F5COO)3Phen is smaller than that of Nd(C2F5COO)3Tfpy and the main reason is that Phen has a more symmetrical structure than Tfpy. From the view of quantum efficiency, some exciting results have been found. The first is that introducing pentafluorobenzoic acid as the ligand can enhance the quantum efficiency evidently. The most probable reason is that the rigid structure in pentafluorobenzoic acid reduces the probability of internal conversion caused by vibration which leads a small energy loss[15]. In addition, the second ligand Tfpy also can bring higher quantum efficiency than Phen. The main reason is that Phen contains eight H atoms , while Tfpy have only four H atoms. So non-radiative transition for Nd(C2F5COO)3Phen is more serious than Nd(C2F5COO)3Tfpy . The quantum efficiency of Nd(C3F7COO)3Tfpy has a little difference with that of Nd(C2F5COO)3Tfpy. This result is because the increase of the carbon chain length in C3F7COOH is not obviously relative to C2F5COOH, and these two complexes have completely similar composition and structure. Stimulated radiation cross-section is an important parameter for assessing laser material. Four complexes of this paper all have relatively large stimulated radiation cross-section, as shown in Table 2. 2.2.2 Fluorescence intensity and concentration quenching As can be seen from the Fig.5, four complexes have the static luminescent location with the change of the ligands. This is the advantage of rare earth complexes. Because 4f electronic is shielded by 5s5p electronic in space, the florescent is almost not affected by ligands[16]. Three characteristic emission bands centered around 893 nm (4F3/2→4I9/2), 1063 nm (4F3/2→4I11/2), and 1335 nm (4F3/2→4I13/2), were observed, which indicates that the complexes have good photoluminescence emission．Fig.1 clearly manifests that the final emission intensities are the comprehensive effect of absorption efficiency and fluorescent quantum efficiency. Fig.5 shows the fluorescences of four complexes in DMSO-d6, with the same concentration and excitation conditions. As shown in Fig.5, the fluorescence intensities of Nd(C2F5COO)3Tfpy and Nd(C3F7COO)3Tfpy are almost closer due to their similar coordination structures and similar quantum efficiencies. The luminescent intensity of

Nd(C2F5COO)3Tfpy Nd(C3F7COO)3Tfpy Nd(C2F5COO)2(C6F5COO)Tfpy Nd(C2F5COO)3Phen

10 9 8 7 6 5 4 0.00

0.05

0.10

0.15

0.20

0.25

-1

Concentration/mol·L

Fig.6 Fluorescence lifetimes of complexes in DMSO-d6 at different concentrations

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3

Conclusions

1) Ligand and its structure has a significant effect on quantum efficiency and coordination environment. Introduction pentafluorobenzoic acid as the first ligand can enhance the quantum efficiency evidently, for its rigid ring can effectively reduce vibration energy transfer. 2) The neutral second ligand Tfpy is more favorable for enhancing the fluorescent quantum efficiencies of Nd3+ ion than another commonly used ligand Phen, because Tfpy has less H atoms. 3) The excitation light absorption ability and the fluorescent quantum efficiency together determine the final emission intensity. So designing the coordination environment of rare earth ions in organic system must comprehensively consider the symmetry, covalency and non radiative transition in the system. 4) Judd-Ofelt analysis presents higher values of stimulated emission cross sections for all complexes prepared in this paper.

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