Luminescent properties of europium and terbium complexes with 2′-hydroxy-4′,6′-dimethoxyacetophenone

Displays 31 (2010) 116–121

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Luminescent properties of europium and terbium complexes with 20 -hydroxy-40 ,60 -dimethoxyacetophenone V.B. Taxak, Rajesh Kumar, J.K. Makrandi, S.P. Khatkar * Department of Chemistry, Maharshi Dayanand University, Rohtak – 124001, India

a r t i c l e

i n f o

Article history: Received 16 June 2009 Received in revised form 13 February 2010 Accepted 25 February 2010 Available online 16 March 2010 Keywords: Luminescence Eu+3 and Tb+3 Infrared TGA/DTA Elemental analysis EDX

a b s t r a c t The novel complexes Eu(L)32H2O and Tb(L)32H2O (where L = 20 -hydroxy-40 ,60 -dimethoxyacetophenone) were synthesized and characterized by infrared spectroscopy, 1H NMR spectroscopy, TGA/DTA, scanning electron microscopy, elemental analysis and energy dispersive analysis (EDX). The Eu(L)32H2O on excitation at 354 nm emits bright red luminescent with main peak at 612 nm and Tb(L)32H2O on excitation at 354 nm emits bright green luminescence with main peak at 547 nm. The luminescence intensity of Eu(L)32H2O is higher than Tb(L)32H2O. These complexes emitting red and green luminescence might be used to make the OLEDs for display applications. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction The luminescence properties of rare earth metal complexes are based on their transition between the 4f energy levels. Among the rare earth ions, trivalent Eu+3 and Tb+3 ions have attracted more attention due to their high luminescence intensity as well as high color purity [1–10]. Fluorescence complexes can be classified into different categories such as inorganic complexes [11], organic complexes [12], inorganic–organic hybrid complexes [13] based on the rare earth ions and ligands. Among these complexes, the organic complexes show excellent luminescence properties. The excellent luminescence of organic–metal complexes is exhibited due to energy transfer from organic ligand to central metal ion by ‘‘antenna effect”, which can increase the luminescence efficiency [14,15]. The Eu+3 ion mainly provides five narrow emission lines corresponding to 5D0 ? 7FJ transitions, where J = 0, 1, 2, 3 and 4 at about 578, 591, 610, 651 and 702 nm, respectively out of which the 5D0 ? 7F2 transition being strongest with emission of red light of high color purity. The Tb+3 ion also mainly provides five narrow emission lines corresponding to 5D4 ? 7FJ transitions, where J = 2, 3, 4, 5 and 6 at about 655, 620 , 582, 543 and 492 nm, respectively out of which the 5D4 ? 7F5 transition being strongest with emission of green light of high color purity. The rare earth ions are known to form stable complexes with O and N donor ligands like ketopyridyls, phenanthrolines, b-diketones, b-aminoketones and bipyridyls, * Corresponding author. Tel.: +91 9813805666. E-mail address: [email protected] (S.P. Khatkar). 0141-9382/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.displa.2010.02.007

etc. These complexes exhibit efficient energy transfer from absorbing chelated ligand to chelated rare earth ions therefore chelated rare earth ions show high luminescence efficiency than unchelated rare earth ions. The rare earth ions complexes with organic ligands have been doped in polymers for optical amplification [16–18]. However, in recent years organic light emitting devices (OLEDs) have been prepared representing a low cost fabrication route and low voltage operation for large area light display technology and large flat panel display [19–22]. The luminescent organic–metal complexes have attracted a great attention in the electronic industry because of their applications in organic light emitting devices (OLEDs). These materials find applications in electronic technological field especially in flat panel display technology. The organic light emitting devices (OLEDs) display offers the thinnest profile of any color flat panel technology. The whole assembly can be very small as little as 2 mm in depth that can replace the heavy and bulky cathode-ray tube of the display systems. Now days, organic luminescent materials are used commercially for examples in lap-top computers, mobile phones, car dashboards, advertisement panels, decorating lighting, etc. Some organic ligands efficiently sensitize Eu+3 luminescence but not Tb+3 at room temperature [17,23,24]. We proposed the new organic ligand 20 -hydroxy-40 ,60 -dimethoxyacetophenone keeping in view that this is found to sensitize Eu+3 as well as Tb+3 luminescence excellently. In present work complexes of Eu+3 and Tb+3 with 20 -hydroxy0 0 4 ,6 -dimethoxyacetophenone have been synthesized and characterized by infrared spectroscopy, 1H NMR spectroscopy, TGA/DTA,

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scanning electron microscopy and elemental analysis. The qualitative analysis of the complexes to detect the rare earth (Eu, Tb) has been performed by energy dispersive analysis (EDX). The luminescence properties of these complexes have been studied in details and are found to be as excellent as expected. 2. Experimental 2.1. Materials Phloroglucinol dihydrate (97%), acetonitrile (99.5+%), Eu (NO3)3xH2O (99.99%), Tb(NO3)35H2O (99.99%), sodium hydroxide (99.998%), potassium carbonate (99.99%), dimethyl sulphate (99+%) were purchased from Aldrich. All chemicals are used without further purification. 2.2. Measurements The complexes formed were identified on the basis of elemental analysis and infrared, 1H NMR spectra measurements. The elemental analysis was performed on the Perkin Elmer 2400 elemental analyzer, infrared spectra (4000–400 cm1) were recorded with Perkin Elmer spectrum RX-I FT infrared spectrophotometer, 1 H NMR spectra were recorded on Bruker Advance 300 spectrometer (300 MHz) using chloroform (CDCl3) as solvent, thermal analysis was carried out by using simultaneous thermal analyzer(STA; Scinco, STA S-1500) with heating rate of 5 °C/min and photoluminescence spectra were recorded with spectroradiometer CS-1000. The energy dispersive analysis of the complexes was performed by EDX, PV 99.

sparingly soluble in methanol, ethanol and ethyl acetate but insoluble in benzene, hexane and dichloromethane. 3.2. Elemental analysis Elemental analytical data for the complexes has been presented in Table 1. The results of elemental analysis indicated that the composition of the complexes confirm to 1:3:2, i.e. metal to ligand to water stoichiometry. The presence of Eu in Eu(L)32H2O and Tb in Tb(L)32H2O has been confirmed by the EDX technique (Fig. 1a and b). 3.3. Infrared studies The infrared spectrum of complex in KBr pellets was recorded with Perkin Elmer RXI FT IR spectrophotometer. The Table 2 gives the characteristic bands of ligand and complexes. The shift of characteristic stretching peak at 1615 cm1 in Eu(L)32H2O and 1617 cm1 in Tb(L)32H2O from 1640 cm1 due to C@O group of the 20 -hydroxy-40 ,60 -dimethoxyacetophenone, indicated that the C@O group participated in coordination with M+3 ion (Eu+3, Tb+3). The peak for PhAO vibration (1258 cm1) of 20 -hydroxy-40 ,60 dimethoxyacetophenone shifted 38 cm1 in Eu(L)32H2O and Tb(L)32H2O which indicates that phenolic group included in coordination with M+3 ion (Eu+3, Tb+3). The absorption peak at about 428 cm1 in Eu(L)32H2O can be assigned to the EuAO vibration absorption band and 425 cm1 in Tb(L)32H2O can be assigned to the TbAO vibration absorption band [26]. Overall, the results indicated that the 20 -hydroxy-40 ,60 -dimethoxyacetophenone coordinated with M+3 ion (Eu+3, Tb+3) through carbonyl group and the phenolic group as shown in Fig. 2.

2.3. Synthesis of ligand 20 -Hydroxy-40 ,60 -dimethoxyacetophenone was synthesized according to the method sited in literature [25] and recrystalized three times in ethanol. 2.4. Synthesis of complexes 2.4.1. Synthesis of Eu (L)32H2O Eu(L)32H2O was prepared as follows, 0.2 mmol of Eu(NO3)3xH2O was dissolved into 30 ml water and 0.6 mmol of 20 -hydroxy40 ,60 -dimethoxyacetophenone was dissolved into 50 ml ethanol. The aqueous solution of Eu(NO3)3xH2O was slowly added to ethanolic solution of 20 -hydroxy-40 ,60 -dimethoxyacetophenone with constant stirring on magnetic stirrer and the pH of mixture was adjusted to 6.5–7 with N/20 aqueous NaOH. This resulted into formation of white precipitates. These precipitates were stirred for 3 h at 40–45 °C and then allowed to stand for 1 h. The precipitates were filtered, washed with water and ethanol, dried in air and then in vacuum desiccator. 2.4.2. Synthesis of Tb(L)32H2O The Tb(L)32H2O was prepared by same way as Eu(L)32H2O except ligand dissolved in methanol instead of ethanol and precipitates were stirred for 3.5 h at 40 °C.

3.4. 1H NMR studies 1

H NMR spectrum was recorded on Bruker Avance 300 spectrometer (300 MHz) using chloroform (CDCl3) as solvent. Abbreviations used to describe the spectrum are: s = singlet, d = doublet, t = triplet, m = multiplet and b = broad. All chemical shifts are given in ppm with respect to tetramethylsilane (TMS). Values for various 1 H NMR chemical shifts for Eu(L)32H2O are: 2.72(s, 9H), 3.90– 4.15(bs, 18H), 5.80(s, 3H), 6.15(s, 3H) and Tb(L)32H2O are: 2.75(s, 9H), 3.8–4.1(bs, 18H), 6.05–6.32(bs, 6H). 3.5. Thermal stability The thermal analysis of the complexes were carried out using simultaneous thermal analyzer (STA; Scinco, STA S1500) at heating rate of 5 °C/minute. The TGA/DTA curves of the Eu(L)32H2O show that the weight loss at about 100 °C is due to loss of coordinated water molecules and the weight loss starts at about 320 °C is due to elimination/or decomposition of ligand 20 -hydroxy-40 ,60 dimethoxyacetophenone. However, above 480 °C the complex Eu (L)32H2O completely get decomposed (Fig. 3a). The TGA/DTA curves of complex Tb(L)32H2O show that the weight loss at about 110 °C is due to loss of coordinated water molecules and the weight loss at about 480 °C is due to elimination or decomposition of 20 -hydroxy-40 ,60 -dimethoxyacetophenone. However, above

3. Results and discussion 3.1. Solubility Solubility of the synthesized complexes was checked in various organic solvents. Both the complexes were found to be soluble in dimethylsulfoxide, dimethylformamide, chloroform and acetone,

Table 1 Elemental analytical data of the complexes. Complex

C (%) found (calc.)

H (%) found (calc.)

Eu(L)32H2O Tb(L)32H2O

46.42 (46.57) 46.03 (46.15)

4.88 (4.79) 4.83 (4.74)

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Fig. 1. EDX spectra of Eu(L)32H2O(a) and Tb(L)32H2O (b).

Fig. 3. TGA/DTA curves of Eu(L)32H2O (a) and Tb(L)32H2O (b).

Table 2 The relevant characteristic IR bands (cm1). Compound

m (CAH)

m (C@O)

m (PhAO)

L Eu(L)32H2O Tb(L)32H2O

2935 2937 2 2937

1640 1615 1617

1258 1220 1220

grain size distribution is highly desirable for efficient OLED production and applications. 3.7. Photoluminescence studies

H3CO O H3CO

Ln .2H2O

O H 3C

3 Where Ln = Eu,Tb

Fig. 2. General structure of complexes.

480 °C the complex Tb(L)32H2O completely get decomposed (Fig. 3b). 3.6. Scanning electron micrographs (SEM) The surface morphological features (shape and particle size) of the powder complexes were studied by Philips XL-30, scanning electron microscope (SEM). The instrumental parameters, accelerating voltage, spot size, magnification and working distance are indicated on the SEM images (Fig. 4a and b). The scanning electron micrographs for the complexes demonstrate that homogeneous luminescence materials were obtained having regular shaped particles with size less than 1 lm and no phase separations were observed. This kind of morphology with small particles with narrow

The emission spectra of Eu(L)32H2O and Tb(L)32H2O are shown in Fig. 5a and b, respectively and the luminescence characteristics of complexes in solid state are listed in Table 3. The schematic diagram for excitation of 20 -hydroxy-40 ,60 -dimethoxyacetophenone by UV light and energy transfer from 20 -hydroxy-40 ,60 -dimethoxyacetophenone to Eu+3, Tb+3 ions has been shown in Fig. 6. Emission spectrum of Eu(L)32H2O has been shown in Fig. 5a. The luminescence spectrum is sharp near 612 nm. The peak is the spectrum of red luminescence of europium. The energy difference of various transitions (DE) is calculated from transition (5D0 ? 7F1) at 591 nm having energy 16,920 cm1 [27]. The emission spectrum of Eu(L)32H2O consists of four peaks at 591 nm (5D0 ? 7F1), 612 nm (5D0 ? 7F2), 651 nm (5D0 ? 7F3) and 705 nm (5D0 ? 7F4). The highest intensity observed for the peak at 612 nm, indicating strong emission due to 5D0 ? 7F2 transition, this peak is sharp and has high color purity, while the transition (5D0 ? 7F0) not observed in Eu(L)32H2O. The emission color was analyzed and confirmed with help of the Commission Internationale de Eclairage (CIE) chromaticity coordinate diagram. The color coordinates for the complex shown in Fig. 7. It is clear from the figure that color coordinates of the complex fall in red region (x = 0.6029 and y = 0.3073). The complex having bright red emission and good PL intensity (64.23 cd/m2) might be promisingly applicable for various display applications. The present europium complex has better photoluminescent intensity (64.23 cd/m2) than earlier reported complexes Eu(L1)32H2O (61.17 cd/m2), where L1 = 20 -hydroxy-40 -

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Fig. 4. SEM micrographs of Eu(L)32H2O (a) and Tb(L)32H2O (b).

(b)

λex =354

D4

D0

λex =354nm

5

5

(a)

F5

F2

7

7

5

D4 5

D4 5 5

7

F2

7

F3

F4

F5

F3

F1

F4

D4

D4

F4

7

7

7

F6

7

5

D3

7

D3

D0

7

5

5

D0

7

5

D0

Intensity(a. u.)

5

Intensity (a.u.)

Wavelength (nm)

Wavelength (nm)

Fig. 5. Emission spectra of Eu(L)32H2O (a) and Tb(L)32H2O.

Table 3 Photoluminescence data of the complexes. Complex

kex (nm)

E (cm1)

kem (nm)

DE (cm1)

Assignment

Eu(L)32H2O

354

16,920 16,340 15,361 14,184

591 612 651 705

0 580 1559 2736

5

24,691 22,989 20,325 18,282 17,212 16,077 15,267

405 435 492 547 581 622 655

0 1702 4366 6409 7479 8614 9424

5

Tb(L)32H2O

354

D0 ? 7F1 D0 ? 7F2 5 D0 ? 7F3 5 D0 ? 7F4 5

D3 ? 7F5 D3 ? 7F4 5 D4 ? 7F6 5 D4 ? 7F5 5 D4 ? 7F4 5 D4 ? 7F3 5 D4 ? 7F2 5

methoxy-2-phenylacetophenone, Eu(L3)32H2O (61.83 cd/m2), where L3 = 20 -hydroxy-40 ,60 -dimethoxy-2-phenylacetophenone, Eu(L4)32H2O (62.06 cd/m2), where L4 = 20 -hydroxy-40 ,60 -dimethoxy-2-(p-methoxyphenyl) acetophenone [2], Eu(HMAP)32H2O (62.13 cd/m2) where HMAP = 20 -hydroxy-40 -methoxyacetophenone [28] and Eu(TNB)3.phen, where TNB = 4,4,4-trifluoro-1-(2naphthyl)-1,3-butanedione and phen = 1,10-phenanthroline [6]. Emission spectrum of Tb(L)32H2O has been shown in Fig. 5b. The luminescence spectrum is sharp near 492 nm, 547 nm and

581 nm. The peaks are the spectrum of blue1 and green luminescence of terbium. The energy difference of various transitions (DE) is calculated from transition (5D3 ? 7F5) at 405 nm having energy 24,691 cm1 [27]. The emission spectrum of complex in solid state consists of seven peaks at 405 nm (5D3 ? 7F5), 435 nm (5D3 ? 7F4), 492 nm (5D4 ? 7F6), 547 nm (5D4 ? 7F5), 581 nm (5D4 ? 7F4), 622 nm (5D4 ? 7F3) and 655 nm (5D4 ? 7F2). The highest intensity observed for the peak at 547 nm, indicating strong emission due to 5D4 ? 7F5 transition. This peak is sharp and has high green color purity. The intensity of blue emission peaks (405 nm and 435 nm) is much weaker than that of the green emission peaks (492 nm and 547 nm) as a consequence of the cross relaxation from the 5D3 to the 5D4 energy levels. The emission color was analyzed and confirmed with the help of Commission Internationale de Eclairage (CIE) chromaticity coordinate diagram. The color coordinates for the complex shown in Fig. 7. It is clear from the figure that color coordinates of the complex fall in green region (x = 0.2876 and y = 0.5234). The complex having bright green luminescence and good PL intensity (29.12 cd/m2) might be promisingly applicable for various display applications. The present terbium complex has better photoluminescent intensity (29.12 cd/m2) than earlier reported complexes [1,28].

1 For interpretation of color in Fig. 5, the reader is referred to the web version of this article.

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S1(L) 30 5

D3

5

DJ, J= T1(L)

2

D4

655 nm

622 nm

581 nm

547 nm

492 nm

435 nm

405 nm

705 nm

651 nm

612 nm

10

5

1 0

591 nm

E (103cm-1)

20

T1(L)

7

Fj, j=

7

F J, J=

4 3

0

2

2

3

S0(L)

4

5

6

1

Eu +3

Ligand

Tb +3

Fig. 6. Schematic diagram of the 20 -hydroxy-40 ,60 -dimethoxyacetophenone to Eu+3 and Tb+3 energy transfer mechanism.

intensity of Eu(L)32H2O is (Lv = 64.23 cd/m2) higher than Tb(L)32H2O (Lv = 29.12 cd/m2). Both complexes are good fluorescence materials and might be used to make the OLEDs. The possible use of them may be in lap-top computers, mobile phones, car dashboards, advertisement panels, decorative lighting, etc. Acknowledgements Authors express their profound thanks to the University Grant Commission, New Delhi for providing financial assistance in the form of a major research project, No. F.12-26/2004(SR). Instrumental help provided by the Photo- & Electro-Materials Research Center, Korea Institute of Energy Research (KIER), Daejeon, South Korea is thankfully acknowledged. References

Fig. 7. CIE coordinates of Eu(L)32H2O (a) and Tb(L)32H2O.

4. Conclusion A novel organic ligand, 20 -hydroxy-40 ,60 -dimethoxyacetophenone, has been synthesized and corresponding lanthanide (III) complexes, Ln(L)32H2O (Ln@Eu, Tb) have been synthesized. The synthesized complexes are electroneutral molecules, in which the each central lanthanide ion is wrapped with three 20 -hydroxy-40 ,60 -dimethoxyacetophenone molecules and two water molecules. 20 -hydroxy-40 ,60 -dimethoxyacetophenone not only excite Eu+3 luminescence but also excite Tb+3 luminescence. The europium complex emits very strong red characteristic emission of Eu+3 ions and terbium complex emits very strong green characteristics emission of Tb+3 in solid states at room temperature. Hence the 20 -hydroxy-40 ,60 -dimethoxyacetophenone sensitizes europium (III) emission as well as terbium (III) emission. The luminescence

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