Synthesis, characterization, luminescence properties and antioxidant activity of Ln(III) complexes with a new aryl amide bridging ligand

Journal of Luminescence 132 (2012) 1357–1363

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Synthesis, characterization, luminescence properties and antioxidant activity of Ln(III) complexes with a new aryl amide bridging ligand Abdulaziz M. Ajlouni a,n, Ziyad A. Taha a, Khader A. Al-Hassan b, Abdullah M. Abu Anzeh a a b

Department of Applied Chemical Sciences, Jordan University of Science & Technology Irbid, 22110, Jordan Department of Chemistry, Faculty of Science, Yarmouk University Irbid, Jordan

a r t i c l e i n f o

abstract

Article history: Received 15 August 2011 Received in revised form 23 November 2011 Accepted 4 January 2012 Available online 14 January 2012

A novel Aryl amide ligand H2L and its eight complexes, [LnH2L(NO3)2  H2O]NO3 [Ln¼Sm(III), Er(III), Tb(III), Dy(III), La(III), Gd(III), Nd(III), and Pr(III)], are presented. The ligand and complexes were synthesized and characterized based on elemental analyses, molar conductance, IR, 1H and 13C-NMR, UV–VIS., and TGA studies. The conductivity data show a 1:1 electrolytic nature with a general formula [LnH2L(NO3)2  2H2O]NO3 The IR spectra reveal coordination of the ligand through the azomethine nitrogen and the phenolic hydroxyl of the ligand to the lanthanide ion. The coordinated nitrate ions behave in a bidentate fashion. The thermal decomposition studies indicate the presence of two water molecules in the inner coordination sphere. Under the excitation at 319 nm, the luminescence emission properties for Sm, Tb, and Dy complexes are observed. These observations show that the ligand favors energy transfers to the emitting energy level of these lanthanide ions. Furthermore, the antioxidant activity of the ligand and its Ln(III) complexes was determined by DPPH radical scavenging method, which indicates that the Ln(III) complexes exhibit more effective antioxidant activity than the ligand alone. & 2012 Elsevier B.V. All rights reserved.

Keywords: Aryl amide ligand Lanthanide(III) complexes Luminescent properties Antioxidant activity.

1. Introduction Highly luminescent lanthanide complexes are attracting attention in awide variety of photonic applications such as planar waveguide amplifiers [1,2], light-emitting diodes [3] and bioinspired luminescent probes [1–3]. Ln(III) in general, are extensively studied of various luminescent materials due to their long-lived, millisecond lifetime, narrow-width emission bands and hypersensitivity to coordination environment. However, direct excitation of Ln(III) is not efficient because of its small absorption cross section. To overcome this problem, an organic chromophore, which serves as an antenna or sensitizer, absorbing the excitation light and transferring the energy from its lowest triplet state energy level (T) to the resonance level of Ln(III) ion, is desired [4,5]. Such energy transfer is one of the most important processes determining the fluorescence properties of Ln(III) complexes. The inherent nature of a highly conjugated open-chain schiff base, ligand [6], enables shielding the encapsulated ion effectively from interaction with the surroundings and has strong antenna effect onto Ln(III) ion. Aromatic schiff bases have been coordinated to many lanthanide systems, because of their excellent coordination ability to the rare earth ions and the ability of sensitizing the luminescence of rare earth ions [7]. In this paper

Schiff base H2L (Fig. 1) was chosen as organic ligand in lanthanide complexes in order to obtain new fluorescence materials which have strong luminescence, and high thermodynamic stability. Aryl amid Schiff base ligands receive a considerable attention, mainly due to their biological and physiological activities, such as antimyco bacterial, antifungal, anticonvulsant, antimicrobial, and antioxidant properties [8,9]. The potential value of antioxidants has already prompted investigators to search for the cooperative effects of metal complexes to improve the antioxidant activity [10]. In the present work, a new derivative of aryl amid Schiff base H2L has been synthesized as shown in Scheme 1 and its eight complexes, [Ln(H2L)(NO3)2  H2O]NO3 [Ln¼La(III) , Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III) and Er(III)).], have been prepared and characterized based on elemental analyses, molar conductance, TGA, and IR, 1H and 13C-NMR, UV–VIS. spectral studies. Photophysical properties of these complexes are studied by means of UV–VIS absorption and luminescence spectroscopy. This ligand and its complexes are also tested for their antioxidant abilities.

2. Materials and methods 2.1. Materials and physical measurements

n

Correspondence author. Tel.: þ962 27201000x23531, 23532; fax: þ 962 27201071. E-mail address: [email protected] (A.M. Ajlouni). 0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2012.01.013

All solvents used were of analytical grade purchased from Aldrich Chemical Company and were used without further purification unless

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mentioned. [Ln¼La(III) , Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III) and Er(III)).] [La(NO3)3  6H2O], [Pr(NO3)3  6H2O] [Nd(NO3)3  6H2O], [Sm(NO3)3  6H2O], [Eu(NO3)3  5H2O], [Gd(NO3)3  6H2O], [Tb(NO3)3  5H2O], [Dy(NO3)3  xH2O], and [Er(NO3)3  6H2O] were purchased from Sigma Aldrich Chemical company and used as received. Isoaticanhydride, 2-hydroxy-1-naphthaldehyde and 1,3-propyl diamine were purchased from Merck Schuchardt. The metal ions were determined by EDTA titration using xylenol oranges as an indicator [11]. Carbon, nitrogen and hydrogen analyses were performed using aVario EL elemental analyzer. Infrared spectra (4000 400 cm  1) were obtained with KBr discs on a JASCO FT-IR model 470 spectrophotometer. 13C NMR and 1H NMR spectra of H2L ligand and Ln(III)–H2L complexes were recorded on a Bruker AVANCE-400 MHz NMR Spectrometer. Spectra were taken in deuterated DMSO or CDCl3 using TMS as an internal reference. Fluorescence measurements were made on a Edinburgh instrument model FS900SDT spectrometer equipped with quartz cuvettes of 1 cm path length at room temperature. UV–VIS spectra were recorded in DMF solution, concentration of 10  6 M, at 25 1C, and wavelength was reported in (nm) using a UV-2401 UV–VIS spectrophotometer. The molar conductance measurements were carried out in DMF using WTW LF 318 model conductivity meter equipped with WTW Tetracon 325 conductivity cell. The thermal analysis were performed on a PCT-2A thermo balance analyzer operating at a heating rate of 10 1C/min in the range of ambient temperature up to 650 1C under N2.

O

O

A mixture of 1H-benzo[d] [1,3]oxazine-2,4-dione (0.5 g, 3 mmol) with 1,3-diaminopropane(0.11 g, 1.5 mmol) in distilled water was stirred at room temperature for about 24 h. The white solid precipitate was collected by filtration and washed with water. Recrystallization from methanol gave light brown crystals of 2-amino-N-[2-(2-amino-benzoylamino)-propyl]-Benzamide 3. 0.624 g (2 mmol) of compound 3 was refluxed with 0.688 g (4 mmol) of 2-hydroxy-1-naphthaldehyde in ethanol for 2 h followed by stirring at room temperature for about 10 h to give a yellow solid precipitate. The yellow precipitate was collected by filtration and washed twice with cold ethanol and dried in air at room temperature, recrystallized from anhydrous ethanol, yield 1.029 g (83%, based on 2-hydroxy-1-naphthaldehyde). 1 H NMR (400 MHz, CDCl3) d: 1.79 (quintet, J¼7.0 Hz, 2H), 3.26 (t, 4H), 6.6–8.5 (m, 20H) 8.44 (s, 2H), 9.45 (d, 2H), 15.03 (2, 2H). 13 C NMR (100 MHz, CDCl3) d: 28.9, 39.9, 108.5, 118.8, 120.5, 121.3, 122.4,123.9, 124.3 ,124.9, 125.1, 125.8, 126.5, 127.6, 128.9, 130.6, 132.4, 133,4, 151.1, 171.6. 2.3. Preparation of the complexes 1.0 mmol (626.2 mg) Schiff base ligand, H2L, was dissolved in 15 mL chloroform. To this solution a 15 mL ethyl acetate solution of 1.0 mmol (760.9 mg) Sm(NO3)3  6H2O was dropwise added. The reaction mixture was stirred for 1 h at room temperature. The precipitated yellow solid complex was separated from the solution by filtration, purified by washing several times with ethyl acetate and chloroform, and then dried for 24 h under vacuum at room temperature. Pr(III), Nd(III), Gd(III), La(III), Er(III), Tb(III), Eu(III) and Dy(III) complexes were prepared by the same way.

C

C N H

N H

N

OH

2.2. Synthesis of the ligand H2L

3. Results and discussion N

H2L

HO

Fig. 1. Structure of ligand H2L.

3.1. Synthesis of H2L ligand and its complexes 2-Amino-N-[2-(2-amino-benzoylamino)-propyl]-Benzamide 3 was synthesized according to the literature [12]. Compound 3 was condensed with 2-hydroxy-1-naphthaldehyde 4 in ethanol to afford the Schiff base H2L. The reaction of H2L with Ln(NO3)3 resulted in the formation of complexes of the composition [LnH2L

Scheme 1. The synthetic route for the ligand H2L.

A.M. Ajlouni et al. / Journal of Luminescence 132 (2012) 1357–1363

(NO3)2  2H2O]NO3 [Ln¼Sm(III), Er(III), Tb(III), Dy(III), La(III), Gd(III), Nd(III), and Pr(III)]. All the complexes are air stable, yellow powder, soluble in DMSO, DMF and THF, slightly soluble in methanol, ethyl acetate and chloroform, and, insoluble in benzene, water and diethyl ether. 3.2. Composition analysis and molar conductivity. The analytical results of the synthesized compounds are shown in Table 1. These results permitted to establish the stoichiometry of the compounds, which is in agreement with the general formula [Ln(H2L)(NO3)2  2H2O]NO3, (Ln¼La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er). The molar conductance is used to measure the ability of the electrolyte to conduct electricity and the SI unit is O  1 cm2 mol  1. The molar conductance of these complexes in DMF (10  3 M) solution at room temperature are in the range of 75–116 O  1 cm2 mol  1, indicating that all complexes are 1:1 ionic compounds [13]. 3.3. Infrared spectroscopy The IR band assignments are given in Table 2. The comparison of the IR spectra of the free ligand and La(NO3)3 L has been shown in Fig. 3. The bands near 1600 cm  1 and 1280 cm  1 are associated with the n(C¼N) and n(C–O), respectively. The stretching frequencies change in profile in the complexes as compared to those observed for the isolated ligands. All of the complexes showed very similar infrared spectra above 400 cm  1 and, therefore, far-infrared spectra were recorded (400 cm  l) in an attempt to assign n(Ln–O) and n(Ln–N) vibrational modes. There is a paucity of such assignments in the literature [14], though it appears that n(Ln–O) and n(Ln–N) might be expected in the 400–600 cm  1 region. In the H2L ligand, the phenolic n(O–H) vibration band appeared at 3295 cm  1 while the coordinated vibration mode are shifted towards higher frequency region 3370 cm  1 that was

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due to the intramolucularly hydrogen bonded in the free ligand. Staying the n(O–H) in the complex indicates that the phenolic O–H oxygen is coordinated to the metal ion without deprotonation. The chemical shift of n(C–O) from 1274 cm  1 to 1283 cm  1 also confirms the coordination through phenolic oxygens. The band at 1622 cm  1 for the free ligand is assigned to the n(C¼N) stretch, which shifts to 1601 cm  1 upon coordination. This shift and the appearance of the band at 411 cm  1 which assigned to n(La–N) confirm that the nitrogen of the azomethine group is coordinated to the lanthanide ions [15]. The absorption bands of the coordinated nitrates (C2v) were observed at about 1482 cm  1 (n1), 1031 cm 1 (n2), 814 cm  1 (n3), and 1288 cm 1 (n4). In addition, the separation of the two highest frequency bands 9n1–n49 is approximately 200 cm  1, and accordingly the coordinated NO3 ion in the complexes is a bidentate ligand [16]. The presence of free nitrate groups (D3h) in the coordinating sphere is indicated by the appearance of a band at 1384 cm  1 in the spectra of these complexes [17]. The bands at 3200–3400 cm  1 are affected by metal coordination. The interpretation of these bands due to the n(OH) of the phenolic groups. The broad band of medium intensity occurring in the region is due to the symmetric and the antisymmetric O–H stretching vibrations of inner-spher water molecules which was further supported by the appearance of a new band at 1615 cm  1 due to the vibration mode of O–H [18].

3.4. NMR spectroscopy The paramagnetic ions have their induced shifts, including complex formation shift, contact shift and pseudo-contact shift. In order to minimize the influence of these kind of shifts, the diamagnetic complexes such as La(III) complex was chosen for 1 H NMR investigation [19,20]. A broad signal at 15.03 ppm in the H2L is ascribed to  OH (D2O exchangeable) of the phenolic hydrogen which is intramolecularly hydrogen-bonded with the nitrogen of azomethine moiety.

Table 1 Yields, conductivity and elemental analysis data H2L ligand and it is lanthanide complexes. Compound

[La (H2L) (NO3)2  2H2O]NO3 [Nd (H2L) (NO3)2  2H2O]NO3 [Dy (H2L) (NO3)2  2H2O]NO3 [Sm (H2L) (NO3)2  2H2O]NO3 [Pr (H2L) (NO3)2  2H2O]NO3 [Gd (H2L) (NO3)2  2H2O]NO3 [Er (H2L) (NO3)2  2H2O]NO3 [Tb (H2L) (NO3)2  2H2O]NO3

Molecular wt

981.65 986.98 1005.24 993.10 983.65 999.99 1010 1001.67

Yield%

88 90 92 96 90 93 97 92

La

Ln%

75.58 115.3 93.15 82.11 97.54 92.46 101.0 77.21

Elemental analysis Cal. (Exp.)

14.15(13.36) 14.61(14.90) 16.17(16.64) 15.14(14.28) 14.32 (14.29) 15.73(15.37) 16.56(16.99) 15.78(15.93)

C%

H%

N%

47.72(47.15) 47.46(46.94) 46.60(46.92) 47.17(47.87) 47.62(47.23) 46.84(46.46) 46.38(46.29) 46.76(46.67)

3.50(3.35) 3.68(3.59) 3.61(3.67) 3.65(3.81) 3.69(3.66) 3.63(3.59) 3.59(3.42) 3.62(3.57)

9.74(9.59) 9.83(9.97) 9.75(9.92) 9.87(9.98) 9.97(9.91) 9.80(9.72) 9.71(9.75) 9.79(9.66)

Table 2 Major infrared spectral data for the Schiff base H2L, and [Ln(H2L)(NO3)2  2H2O]NO3 complexes (cm  1). Compound

H2 L [Tb(H2L)(NO3)2  2H2O]NO3 [Pr(H2L)(NO3)2  2H2O]NO3 [Gd(H2L)(NO3)2  2H2O]NO3 [Er(H2L)(NO3)2  2H2O]NO3 [Nd(H2L)(NO3)2  2H2O]NO3 [Sm(H2L)(NO3)2  2H2O]NO3 [La(H2L)(NO3)2  2H2O]NO3 [Dy(H2L)(NO3)2  2H2O]NO3

nOH

3295 3370 3370 3370 3370 3370 3370 3370 3370

nC–O

1274 1283 1283 1283 1283 1283 1283 1283 1283

nC ¼ N

1622 1601 1601 1601 1601 1601 1601 1601 1601

uNO3

n1

n2

n3

n4

no

Ln–O

Ln–N

– 1482 1481 1483 1483 1480 1481 1481 1483

– 1031 1031 1031 1031 1031 1031 1031 1031

– 814 814 814 814 814 814 814 814

– 1288 1287 1288 1288 1288 1288 1288 1288

– 1384 1384 1384 1384 1384 1384 1384 1384

– 570 570 570 570 570 570 570 570

– 411 411 411 411 411 411 411 411

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A peak appeared at 9.44 ppm with integration to two protons was assigned to azomethine proton (–HC¼N). The signal of the N¼CH hydrogen appeared as a doublet. This splitting could be explained on the bases of a coupling with the neighboring –NH hydrogen, which resonates as a doublet and the existence of keto-enol toutomerism Fig. 2. The singlet at 15.03 ppm is assigned to OHgroup and the high down field shift is du to the H-bonding. The proton chemical shift of (–CO–NH–) appears at 8.44 ppm as a broad singlet which corresponds to two protons. The 1H NMR spectra exhibit many multiplet peaks in the range of 6.60– 8.50 ppm which are assigned to the aromatic protons. Two peaks appeared at high field shift region at 3.26 and 1.79 ppm. The triplet peak appeared at 3.26 ppm, which is integrated to four protons, is assigned to four methylene hydrogens (CH2–NH¼O). The quintet peak at 1.79 ppm is assigned to other two methylene hydrogens. In the 1H NMR spectrum of La(III) complex, the –NH signals remain almost unperturbed at 8.44 ppm indicating that this grouping not involved in coordination. The chemical shifts of the aromatic protons remain almost the same upon complexation. The signal of the phenolic OH group is observed in the complex spectrum, confirming the IR result which shows that the OH is coordinated to the metal ion with out deprotonation. 3.5. TGA analysis Thermogravimetric (TGA) and differential Thermogravimetric analysis (DTGA) were carried out up to 650 1C under N2 flow at heating rate of 10 1C min  1. All the Ln(III) complexes show similar thermal decomposition behaviours. The TGA–DTGA curves of Sm(III) complex are depicted in Fig. 3. The TGA–DTGA curves of Ln(III) complexes are direct evidence of the fact that those compounds undergo five stages of decompositions The first mass loss occurred between 50 and 234 1C and mass loss percentage was 3.07% to 3.82%. This mass loss is due to the removal of two water molecules. The decomposition temperatures of these hydrates occur after 160 1C suggests that the water molecules present in these complexes in the inner coordination sphere. The second stage occurs between 234 and 289 1C due to loss of one nitrate ion with a weight loss of 7.2%–7.8%. The third stage, is

ascribed to the loss of another two nitrate ions (14.1%–15.2%) at 289–339 1C. The fourth step processes is corresponding to the loss of the ligand molecule which occurred between 339 and 550 1C with a weight loss of 25.60 to 27.07%. These processes were followed by the last step which oxidation of Ln(III) residue occurs to give the Ln(III) oxide. [21]. The residue reached constant mass at 550 1C. The thermal decomposition reaction of the complex Ln(H2L)(NO3)2  2H2O]NO3. 3.6. UV–VIS absorption studies. The UV–VIS absorption spectrum of the free ligand H2L and those of the corresponding Ln-complexes were carried out in DMF solution at room temperature. The numerical values of the maximum absorption wavelength (lmax) are listed in Table 3. The electronic spectrum of the ligand H2L shows three absorption bands with maxima at 322, 331 and 434 nm as shown in Fig. 4. The absorption bands at maxima of 331 nm and 434 nm, are assigned to n–p* transitions of conjugation between the lone pair of electrons of C ¼N and C ¼O groups and a conjugated p bond of the aromatic ring [22]. The third band at 322 arises from p–p* transition within the aromatic ring, C ¼N and C¼O groups. The major absorption bands of the ligand are shifted to higher energy levels (blue shift) upon complexation. This change is essentially variant for each of the lanthanide complexes (La(III), Pr(III), Table 3 UV–VIS Spectral data of Schiff base H2L and its complexes. Compound

lmax(nm)

Band assignments

H2L

322 331 434 321 409 326 409 319 407 321 404 325 405 321 411 319 416 327 411

p-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p* p-p* n-p*

[Sm (H2L) (NO3)2  2H2O]NO3 [Gd (H2L) (NO3)2  2H2O]NO3 [La (H2L) (NO3)2  2H2O]NO3 [Er (H2L) (NO3)2  2H2O]NO3 [Tb (H2L) (NO3)2  2H2O]NO3

O

O

H

[Nd (H2L) (NO3)2  2H2O]NO3

H

N

[Pr (H2L) (NO3)2  2H2O]NO3

N

Enolic form

ketonic form

[Dy (H2L) (NO3)2.2H2O]NO3

Fig. 2. Keto-enol toutomerism in Schiff base H2L.

TGA 3.5 3.0

0.40

2.5 -0.005

2.0 1.5

-0.01 1.0 0.50

-0.02

0.0 80

160

240

320

400

480

560

640

Temperature/°C Fig. 3. TGA–DrTGA curves of [Sm(H2L) (NO3)2  2H2O]NO3.

Absorbance

0

mg/minute

Weight loss/mg

0.50

DTGA

I II III IV

0.30 0.20

H2L

0.10 0.0 300

350

400 450 Wavelength (nm)

500

550

Fig. 4. UV–VIS. spectra for ligand H2L, [Pr(H2L)(NO3)2  2H2O]NO3(I), [Dy(H2L) (NO3)2  2H2O]NO3(II), [Sm(H2L)(NO3)2  2H2O]NO3 (III), [Tb(H2L)(NO3)2  2H2O] NO3(IV).

A.M. Ajlouni et al. / Journal of Luminescence 132 (2012) 1357–1363

Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III) and Er(III)) and is attributed to metal coordination by the ligand. The two higherenergy bands in the free ligand are overlapped upon complexation to a single band [23].

3.7. Luminescent properties of the complexes The emission spectra of H2L ligand and its Ln(III) complexes were measured in DMF at room temperature and are displayed in Fig. 5. A strong fluorescence emission band is observed around 415 nm in all

I

v II

the Ln(III) complexes. This emission wavelengths is attributed to p–p* electron transitions of the aromatic Schiff base ligand H2L . Under the excitation of 319 nm, the emission spectrum of the Sm(III) complex displays four characteristic Sm(III) emission peaks in the region 525–660 nm corresponding to 4G5/2-6H3/2 (546 nm), 4G5/2-6H5/2 (560 nm), 4G5/2-6H7/2 (595 nm), and 4 G5/2-6H9/2 (650 nm), transitions (Fig. 6). Fig. 7 shows also four narrow emission peaks in the range 540–670 nm, characteristic of Tb(III) transitions. These transitions are assigned to 5D4-7F5 (545 nm), 5D4-7F4 (585 nm), 5D4-7F3 (650 nm), and 5D4-7F1 (670 nm) transitions. In Fig. 8, two narrow emission peaks are observed in the range 586 nm and 675 nm characteristic of Dy(III) transitions. These transitions are attributed to 4F9/2- 6H13/2 , and 4 F9/2- 6H11/2 . Eu (III) complex shows three emission peaks at 580, 593 and 619 nm, attributed to the characteristic emission 5 D0- 7F0, 5D0- 7F1 and 5D0- 7F2 transitions of Eu(III) ion Fig. 9 [24,25]. H2L ligand is a good organic ligand that can absorb and transfer energy intramoleculary from the triplet state of the

III IV

4 x 104

350

400

450

500

Wavelength (nm) Fig. 5. Luminescence spectra ligand H2L, [Pr(H2L)(NO3)2  2H2O]NO3(I), [Dy(H2L) NO3)2  2H2O]NO3(II), [Eu(H2L)(NO3)2  2H2O]NO3(III). [Sm(H2L)(NO3)2  2H2O]NO3 (IV), [Tb(H2L)(NO3)2  2H2O]NO3(V).

Fluorescence Intensity

Relative intensity

H2L

3 x 104

6

G5/2

4

H11/2

6

F9/2

H13/2

1 x 104

480

H9/2

520 560 600 Wavelength (nm)

640

680

Fig. 8. The fluorescence spectrum of the Dy complex in DMF solution (1.0  10  6 M) at room temperature. Fluorescence spectrum is obtained with lexc ¼319 nm.

8 x 104 6 x 104 4

4 x 104

π*

π

4

4

450

G5/2

500

6

G5/2 H3/2

6

G5/2

6

H7/2

6 x 104 5

H5/2

550 Wavelength (nm)

600

650

Fig. 6. The fluorescence spectrum of the Sm complex in DMF solution (1.0  10  6 M) at room temperature. Fluorescence spectrum is obtained with lexc ¼319 nm.

Fluorescence Intensity

Fluorescence Intensity

6

π

440 4

0

5

D4

7

5

D4

7

D4

7

D4

7

480

D0

7

F0 5

2 x 104

D0

7

F1

0 450

F3

500 550 Wavelength (nm)

H

0 440

F2

600

650

F4 5

π

7

Fig. 9. The fluorescence spectrum of the Eu complex in DMF solution (1.0  10  6 M) at room temperature. Fluorescence spectrum is obtained with lexc ¼319 nm.

F5 5

π*

D0

π 5

F1

4 x 104

π*

4 x 104

6 x 104

Fluorescence Intensity

π*

F9/2

0

1 x 105

2 x 104

4

2 x 104

1.2 x 105

2 x 104

1361

520 560 600 Wavelength (nm)

640

680

Fig. 7. The fluorescence spectrum of the Tb complex in DMF solution (1.0  0  6 M) at room temperature. Fluorescence spectrum is obtained with lexc ¼319 nm.

O2N

O

N N

+

NO2 NO2

O La

La O2N

N NH

+ NO2

NO2

Scheme 2. . Proposed mechanism for DPPH scavenging activity for the [La (H2L) (NO3)2  2H2O]NO3.

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Table 4 The influence of the Ligand H2L and its Ln(III) complexes on the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH). Tested compounds

H2L [Dy(H2L)(NO3)2  2H2O]NO3 [Tb(H2L)(NO3)2  2H2O]NO3 [La(H2L)(NO3)2  2H2O]NO3 [Pr(H2L)(NO3)2  2H2O]NO3 [Er(H2L)(NO3)2  2H2O]NO3 [Sm(H2L)(NO3)2  2H2O]NO3 [Nd(H2L)(NO3)2  2H2O]NO3 [Gd(H2L)(NO3)2  2H2O]NO3

DPPH scavenging activity (%) concentration (mM) 62.5

125

187.5

250

312.5

10.61 7 0.2 29.95 7 0.9 15.64 7 0.7 13.41 7 0.4 21.12 7 0.3 23.02 7 0.6 15.89 7 1.3 19.93 7 0.3 10.007 0.2

12.60 7 0.5 36.27 7 0.4 21.31 7 0.6 14.46 7 0.2 24.86 7 0.4 26.03 7 0.1 23.20 7 0.5 28.70 7 0.2 16.50 7 1.0

12.94 7 0.9 40.217 0.6 25.057 0.6 19.107 0.2 29.12 7 0.2 30.667 0.5 29.507 0.2 30.757 0.9 20.637 0.6

13.27 70.2 43.64 70.2 27.93 70.6 21.16 70.6 32.27 70.5 36.76 70.4 34.90 70.3 34.20 70.6 22.88 70.4

14.59 7 0.4 45.92 7 1.5 30.307 0.9 22.80 7 0.3 32.96 7 0.9 37.93 7 0.2 37.82 7 1.0 37.82 7 0.8 25.64 7 0.6

ligand H2L to excited state of the Ln(III) ions. At the same time the energy gap between the triplet state of the ligand and the emitting level of the Ln(III) ions only favours the energy transfer process for Sm(III), Tb(III) and Dy(III), Eu(III) ions [25]. The only f–f transitions of these Ln(III) complexes have been seen in case of Sm(III), Tb(III), Eu(III) and Dy(III) complexes with different luminescence intensities. The luminescence intensities of Tb(III) and Sm(III) complexes show stronger luminescence than those of Eu(III) and Dy(III) systems, which indicates that the triplet state energy is more suitable for the luminescence of Tb(III) and Sm(III) ions than Eu(III) and Dy(III) ions. The broad bands at 440– 480 nm are attributed to the emission of the ligand H2L. The less intense is seen in the Tb(III) and Sm(III) complexes (Figs. 7 and 8), this indicates that ligand H2L, to the metal energy transfer (antenna effect) in Tb(III) complex is more efficient than the others [26]. Fig. 10. The scavenging effect of the Ligand H2L and its Ln(III) complexes on the 2,2- diphenyl-1-picrylhydrazyl radical (DPPH).

3.8. DPPH radical scavenging assay The anti-oxidant potential of Schiff base ligand and its lanthanide complexes was determined mainly by its scavenging ability on the stable 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical as described in the literature [27]. The scavenging ability determines the antiradical power of an antioxidant by measuring the decrease in the absorbance of DPPH at 517 nm. Resulting from a colour change, the absorbance decreased when the DPPH is scavenged by an antioxidant, through donation of hydrogen to form a stable DPPH molecule as shown in Scheme 2. All of these compounds exhibit free radical scavenging ability at the different concentrations. The radical scavenging activity was expressed as a percentage, and is calculated using the following formula:

4. Conclusions A novel organic ligand, H2L, and its corresponding lanthanide (III) complexes, [Ln(H2L)(NO3)2  2H2O]NO3, (Ln¼La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Er)) have been synthesized and characterized. According to the data and discussion previously presented, H2L could form stable complexes with lanthanide nitrates (L:Ln¼ 1:1). In the complexes, the lanthanide ions are coordinated through the azomethine nitrogen atoms, and phenolic hydroxyl groups, forming Ln(III) complexes with a coordination number of 10. The most The H2L Schiff base ligand had a suitable conjugated system to sensitize Sm(III), Tb(III), Eu(III) and Dy(III) ions fluorescence efficiently. Therefore, considering this factor, more fluorescent materials can be obtained by using this type of ligand. The lanthanide complexes show stronger antioxidant activities than the ligand alone.

½%Inhibition ¼ 100  ðA0 2As Þ=A0  where As is the absorbance of the test sample and A0 is the absorbance of the control. The data of the suppression ratio for DPPH are listed in Table 4. It is found that the inhibitory effect of the compounds tested is concentration dependent and the suppression ratio increases with increasing sample concentrations in the range tested, Fig. 10. The Schiff base ligand has the minimum value compared to its complexes. and the arrangement of the complexes due to their % Inhibition is (Dy4Er4Nd4Sm4Pr4Tb4Gd4La). As shown in Table 4, Ln(III) complexes are significantly more efficient in quenching DPPH radical than the free ligand H2L. Ligand H2L interacts with the positively charged Ln(III). The electron density is drawn from the oxygen, that makes the O–H bond more polarized; as a result, the H atom have a greater tendency to ionize than those in the free ligand H2L (Scheme 2) [28,29]. Dy(III) does have the highest positively charged density due to its size. It can quench DPPHd radical more efficiently than the other lanthanide ions.

Acknowledgements We are gratefully acknowledge the financial support from Jordan University of Science and Technology (project no 160/2010).

Appendix A. Supplementary materials Supplementary materials associated with this article can be found in the online version at doi:10.1016/j.jlumin.2012.01.013.

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