Synthesis and Characterization Red Emitting Iridium (III) Complex with 2-(4-cynophenyl)-4 Phenyl quinoline for PhOLEDs

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ScienceDirect Materials Today: Proceedings 5 (2018) 22163–22170

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ICASE_2017

Synthesis and Characterization Red Emitting Iridium (III) Complex with 2-(4-cynophenyl)-4 Phenyl quinoline for PhOLEDs Neha B. Khotelea, H. K. Dahulea, S. J. Dhobleb* a

Department of Physics, Shivaji Science College, Congress Nagar, Nagpur-440012, India. b Department of Physics, R. T. M. N. U. Nagpur University, Nagpur - 440033,India.

Abstract A new red light emitting highly efficient Ir(III) complex Ir(CN-DPQ)2(acac), was synthesized for phosphorescent organic lightemitting diodes (PhOLEDs), and their photo physical, structural properties were investigated. The Ir(III) complex, including acetyl acetone (acac) as ancillary ligand, are comprised with 2-(4-cynophenyl)-4 Phenyl quinoline (CN-DPQ),main ligands. The synthesized iridium metal complex, Ir(CN-DPQ)2(acac), and ligand (CN-DPQ) were characterized by different techniques, e.g., Fourier transform infra-red spectroscopy (FTIR), the thermo gravimetric analysis (TGA) and the differential thermal analysis (DTA), UV-visible absorption and photoluminescence spectra. FTIR spectra confirms the presence of chemical groups such as C=O, NH, or OH in the synthesized complex and ligand and TGA/DTA curve revels that the synthesized complex is thermally stable up to 350 0 C . The synthesized 2-(4-cynophenyl)-4 Phenyl quinoline (CN-DPQ) phosphor when excited at 388 nm, demonstrate emission in blue region at 442 nm in formic acid with CIE co-ordinate X=0.1592 & Y=0.0786. A red emitting PL peaks at 626 nm in powder, 628 nm inTHF,631 nm in DCM and 638 nm in acetic acid solution was observed. It is proposed that the synthesized complex Ir(CN-DPQ)2(acac) may be efficiently used as the red emitter materials and CN-DPQ organic phosphor may be efficiently used as the blue emitter materials in Organic Light-Emitting Diodes (OLEDs). It is promising candidates for potential applications in organic light-emitting diodes (OLEDs), light-emitting electrochemical cells and solid-state organic lighting applications. The synthesized quinoline-based iridium complex are promising candidates for efficient red emitters in PhOLEDs. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Science & Engineering ICASE - 2017. Keywords: Phosphorescent; cyclometalation; PhOLEDs; CN-DPQ; Ir(CN-DPQ)2(acac).

* Corresponding author. Tel.: +91-7721001752. E-mail address:[email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Science & Engineering ICASE - 2017.

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1. Introduction Organic light-emitting diodes (OLEDs) have been actively investigated as an important technology for display and lighting applications can be attributed to the development of highly efficient phosphorescent emitters. OLEDs are highly promising candidates for next-generation solid state lighting sources to replace conventional incandescent bulbs and fluorescent lamps .The best known class of phosphorescent emitters includes the cyclometallated complexes with iridium (III) as the central atom [1–2].With specifically designed chelating ligands for color tuning, RGB (red, green and blue) emitters with nearly 100% (internal) quantum efficiency have been demonstrated. Phosphorescent OLEDs (PhOLEDs) fabricated by thermal high vacuum evaporation technology can deposit multilayer device architectures with excellent efficiencies. However, the use of thermal evaporation deposition, renders the fabrication process with low utilization of expensive materials. Recently, solution-processed fabrication of PhOLEDs has become another attractive technique for inexpensive device manufacture, because it facilitates the co-doping of several dopants and is compatible with large area manufacture such as spin coating, screen and inkjet printing technologies [3,4].For the development of full color display, a set of red, green, and blue emitters are needed with color purity and high device performance. Therefore, the interest has been focused on developing red PhOLEDs with high device performance [5,6]. Many researchers have studied the modification of bis 2-(4,6-difluorophenyl)pyridinato-C2,N] (picolinate)iridium(III) (FIrpic) to obtain high color purity of blue PhOLEDs [7–9] by introducing various electron withdrawing and/ or donating substituents to the C^N and LX ligands [6,10]. Among the Ir(III) complexes for blue PhOLEDs, the (dfppy)2Ir(acac) and (dfppy)2Ir(pic) have been well characterized [11–13].We reported highly efficient phenylquinoline based Ir(III) complexes with acetyl acetone (acac)as an ancillary ligand to improve the performance of red PhOLEDs, and found that the increment in performance is mainly due to the synergistic effect of both phenyl pyridine (ppy) and phenylquinoline (PhQ) based main ligands as well as ancillary ligand [14,15]. Recently, much attention in the area of solution-processed PhOLEDs has been given to the design and preparation of suitable small molecules and to the optimization of device architecture. For this purpose, the design and development of solution-processable phosphorescent emitters gained much attention. The majority of solution-processed PhOLEDs based on Ir(III) complexes are composed of layers that contain oligomers or conjugated polymers [19,20], and more recently dendrimers [21,22]. However, the complexities such as multi-step reaction sequences, difficulty to get high purity and molecular weight dependent property changes lessen their utility. Consequently, the design of novel small molecule based Ir(III) complexes are highly preferred alternative for solution-processed PhOLEDs. Therefore, we herein report the synthesis, photophysical, and electroluminescent (EL) properties of a new red Ir(III) complexes containing 2-(4-cynophenyl)-4 Phenyl quinoline (CN-DPQ) main ligand and acetyl acetone (acac) as ancillary ligand. 2. Experimental details 2.1. Measurements Structural properties of CN-DPQ are evaluated by Fourier Transform Infrared spectra recorded on Bruker-Alpha at room temperature. Thermal properties of the synthesized complexes are explored by SHIMADZU thermo gravimetric analyzer. Optical properties of CN-DPQ and Ir(CN-DPQ)2(acac) are investigated by UV–Vis absorption and emission spectroscopy. It was carried out on SHIMADZU-1800 Spectrophotometer and SHIMADZU RF 5301spectrofluorometer at room temperature. The XRD pattern was obtained by X-ray Analytical diffractometer with Cu Kά radiation (λ=1.5418 Å) operated at 40 kV and 20 mA. All reagents and solvents were used as received without further purification. All reactions were performed under an argon atmosphere. 2.2. Synthesis of 2-(4-cynophenyl)-4-Diphenyl-quinoline (CN – DPQ) The cyno substituted quinoline-derived ligand 2-(4-Cynophenyl)-4 Phenyl quinoline (CN-DPQ) were synthesized conveniently according to Scheme-1 from the condensation of 2-Aminobenzophenone with 4-cynoacetophenone using the acid-catalyzed Friedlander reaction [23].

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2-Amino-benzophenone (1gm) C13H11NO, and 4-cynoacetophenone (1gm),C22H14N2 were added along with Di-phenyl Phosphate(DPP)(1gm) C12H11O4P, and m-Cresol (3 ml) in a glass reactor fitted with mechanical stirrer, two glass inlets and one side arm. The reaction mixture in a glass reactor heated at 90o C for 1 hour and then finally to 140o C for 4 hours. After cooling, dichloromethane [CH2Cl2], (50 ml) and 5 % NaOH (50 ml) were added to the reaction mixture. The organic layer was separated and collects this layer on watch glass to evaporate remaining dichloromethane, then washed with distilled water (20 ml x 5) until it was neutral. It dried in oven and washed with hexane (5 ml x 3) to obtain off-white crystalline solid of Cyno–Diphenyl quinoline abbreviated as CNDPQ, relative molar mass = 306.36gm

O C

N

CH3

DPP/ m-crysol

O

1400C

NH2

CN

2-amino-benzophenon

CN

4-Cynoacetophenone

IrCl3.nH2O 2-ethoxyethanol

O

N

CH3

Ir 2

Ir

N

C5H8O2

O CN

Na2CO3

CH3

Ir(CN-DPQ)2(acac) complex

CN

2

Cl Cl

Ir

N CN

2

m-chloro-bridged dimer

Scheme1:Synthesis of (CN-DPQ) and Ir(CN-DPQ)2(acac) Complex 2.3 Synthesis of Ir(CN-DPQ)2(acac) Complex Cyclometallated Ir(III)-chloro-bridged dimers were synthesized according to the Nonoyama method [24]. Ir(CNDPQ)2(acac) was synthesized according to scheme 1. (CN-DPQ) (2.2mmol) was dissolved in Ethoxyethanol (10ml) in a 50ml round bottom flask. Iridium trichloride hydrate [IrCl3.3H2O] (1 mmol ) and 3ml of water were then added to the flask. The mixture was stirred under argon at 1200C for 24hrs.The mixture was cooled to room temperature and the precipitate was collected and then dried to give a cyclometallated chloride bridged dimmer as red powder. In a 50 ml flask, the chloride bridged dimmer, acetyl acetone (1.0 m mol) and Na2CO3 (2.5 m mol) were dissolved in 2ethoxyetenol (15 ml) and the mixture was the refluxed under a argon atmosphere for 12 h. After cooling to room temperature, the precipitate was filtered off and washed with water ,ethanol and ether give to desire pure Ir(CN-DPQ)2(acac) complex. 3. Results and discussion 3.1 Fourier Transform Infrared (FTIR) Spectroscopy By using FTIR spectra we can confirm that the presence of different molecules of polymeric compounds. Fig.1 (a) shows FTIR spectra of CN-DPQ. Strong band at 1400 cm-1 due to the (C=N) group and characteristic of

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the quinoline ring were observed. Above spectra shows the presence of the aromatic CH vibration peak appears at 3100-2800 cm-1. The peak at 865 cm-1, is characteristic peak of the benzene ring. Also some peaks observed at 916 and 766 cm-1 are due to CH phenyl ring substituted bands. This is usually an excellent confirmation of the completion of cyclization reaction forming quinoline rings. The peak at 1682cm-1 is due to the conjugated C=O stretching vibrations and the peaks at 1590 and 1557cm-1 may be assigned to the conjugated C=C stretching vibrations. Peaks at 1104, 1070, and 1024 cm-1 are assigned to the single C–N bond stretching vibrations (Amines groups).The absorption corresponding to 2200 cm-1, indicates the presence of cyno-group (C≡N).

Fig. 1: FT-IR spectra of (a) CN-DPQ (b) Ir(CN-DPQ)2 (acac) complex FTIR Spectra of Ir(CN-DPQ)2(acac) is shown in Fig.1 (b).The peaks at 3444 and 3020 cm-1 are due to the C-H stretching frequency of aromatic (C-H) ring. The peaks at 3100-3000 cm-1 are due to the CH vibration stretch. The peaks between1615–1700 cm-1 due to the C-N stretching frequency are noticed. The peak at 1670 cm-1 due to C-C bond. The peak 1750 cm-1 due to C=O stretching frequency and at 1242 cm-1 due to the C-N stretching frequency are noticed. The peak at 1450 cm-1 aromatic C= C bond. The peak between 1220–1260 cm−1 are due to aromatic C─O bond. The peak at 1040-1060 cm-1 is related to the C-O stretching frequency, while between 790- 870 cm-1 are due to the >C=CH bending. 3.2. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) 3.2.1 TGA and DTA of CN-DPQ The thermo gravimetric analysis (TGA) and the differential thermal analysis (DTA) evaluated the thermal stability, chemical reactivity and phase transitions properties of synthesized CN-DPQ material. Thermal stability was evaluated by TGA and was carried out in the temperature range of 30–3500C. TGA curve of CN-DPQ as shown in Fig.2 (a). From graph it is conclude that the thermal stability of the synthesized compound is around 238.80C and then after continues heating there is a gradual loss of weight material. At Lower temperature, the material is stable and no weight losses are observed at lower temperatures. DTA curve of CN-DPQ as shown in Fig.2 (b). The study DTA curve allows the detection of every physical or chemical change in synthesized material, accompanied by a change in weight. DTA result of CN- DPQ shows the melting temperature at about 101 0 C and glass transition temperature is at 173 0C. 3.2.2 Thermo gravimetric Analysis (TGA) of Ir(CN-DPQ)2(acac) Thermo gravimetric analysis of the Ir(CN-DPQ)2(acac) is shown in Fig.3. From Fig.3 it is clear that the complex is thermally stable up to 3500C. A weight loss (2.0444 %) at 3750C might be attributed to the loss of some absorbed water molecules. In the second step weight loss of about 8.444% were taken place up to 4000C. The weight

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loss start at 3500C and completed at 4500C. This huge weight loss suggests that the decomposition of complex start in this range, hence complex was stable up to this temperature. b.

a.

Fig.2: (a) TGA and (b) DTA curve for CN-DPQ

10 9

Sample weight(%)

8

T G A o f Ir(C N -D P Q ) 2 (a c a c )

7 6 5 4 3 2 10

100

200

300

T e m p e ra tu re (

400 0

500

C)

Fig.3. Thermo gravimetric analysis curve of Ir(CN-DPQ)2(acac) complex 3.3. X-ray structural analysis 3.3.1. Analysis of X-ray Diffraction spectra of CN-DPQ and Ir(CN-DPQ)2(acac) The X-ray diffraction pattern Fig.4 (a) of CN–DPQ and (b) Ir(CN-DPQ)2(acac) powder has many strong, sharp diffraction peaks at lower values of 2θ and diffused background. Several sharp XRD peaks appear in the pattern of pure CN-DPQ corresponding to 2θ of 12.92°, 16.55°, 17.13°, 18.96°, 19.75°, 20.26°, 21.70°, 23.46°, 25.62° etc., indicating poly crystals. This indicates the crystalline nature of the polymeric material. The crystallinity parts give sharp narrow diffraction peaks and the amorphous component gives a very broad peak. CN-DPQ showed a much weaker diffraction peak at higher 2θ values, indicating lower crystallinity or orientation. In addition, its higher volume fraction of insulating side chains may also contribute to its low field-effect mobility. By using XRD data we have calculated d-spacing value, hkl-plane, lattice parameter, unit cell, crystalline size etc. The knowledge of these reveals the crystalline structure of CN-DPQ and Ir(CN-DPQ)2(acac). According to analyzing XRD-data the average d-spacing value of CN-DPQ is found to be 4.6792 A0 and lattice parameter a= 3.684 A0. There are all integers and all quotients are integers that indicate a primitive unit cell. The average crystalline size by using Scherer formula (2) of CN-DPQ is found to be 84.54 A 0. We have calculated (hkl) planes by using formula where C.F. common factor for all values, are (100) ,(110),(111), (200) for CN-DPQ organic phosphor. The d-spacing value and the crystalline size of CN-DPQ and Ir(CN-DPQ)2(acac) is calculated by equation (1) and (2) respectively.

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2d Sin θ = n λ………….(1) . = ……………...(2) Where, λ=Wavelength of radiation of Cu-K (λ =1.54 A 0) β=Full width half maxima, θ=Angle of reflection. b 3000

20.26

XRD pattern of CN-DPQ

0

0

16.71

21.78

0

Counts

1000

0

1000

1500 13.14 0 15.65

0 0

17.13

21.70

18.96

1500

0

XRD of Ir(CN-DPQ)2(acac) 2000

0

2000

2500

12.92 0 15.55

Intensity [counts]

2500

25.57

0

19.67

0

25.62

19.75

3000

0

0

a

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500

0

0 10

20

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Fig.4 XRD pattern of (a) CN-DPQ (b) Ir(CN-DPQ)2(acac) complex 3.4. Photo physical properties In order to realize the functioning of organic materials in device applications, study of photo physical properties is important. Using UV–Vis absorption and photoluminescence (PL) spectroscopy, basic photo physical characteristics such as the absorption maxima (vAbs), optical band gaps (Eg), maximum intensity and emission wavelength of the molecules were determined [14]. 3.4.1 UV-Vis absorption UV–vis absorption spectra of 10-4 M solutions of CN-DPQ organic phosphor in acetic acid and formic acid, at room temperature are shown in Fig.5 (a). Pure acetic and formic acid solutions show absorption shoulders at 263 and 265 nm, respectively, and no absorption above 320 nm (results not reproduced here). Result indicates the strong absorption peak of CN-DPQ at 305 nm and the shoulder at 361 nm in formic acid (curve a). Whereas, the strong peak at 304 nm and the shoulder at 318 nm are observed when CN-DPQ is dissolved in acetic acid (curve b).The values of absorption maxima in different acids solution are nearly equals, this shows that the different solvent not affect the absorption wavelength. The intensity of various peaks is slightly higher in CN-DPQ dissolved in formic acid solutions .It is found that the absorption maxima for substituted electron-donating cyno-groups at the 2-para position of 2-4 Diphenyl quinoline, has been blue shifted. In both cases, the intensity of main absorption peak approximately reduces to zero at 400 nm. The maximum absorption peaks in visible region is attributed to the π- π* transition of the conjugated polymer main chains. The PL spectra of Cyno-DPQ in powder form Fig.6 (a) displayed blue phosphorescence, which may be ascribed to the charge transfer between the electron-donating substituent at the 4-position and the electron accepting carbonyl groups. The excitation band in formic acid is centered at 388 nm and the polymer emits blue emission of wavelength 442 nm, due to the presence of electron-withdrawing Cyno–substituent in 2-4 Diphenyl quinoline. 2-4 Diphenyl quinoline phosphor emits intense blue light of wavelength 436 nm [13]. It is found that the electron-donating Cyno-group substituent at the 4-position of 2-4 Diphenyl quinoline, has been red shifted. The absorption and photoluminescence emission spectra measured for 10-4M solutions of Ir(CN-DPQ)2(acac) in DCM, THF and formic acid at room temperature are displayed in Fig.5 (b). The strong absorption bands in the UV region are assigned to the spin-allowed 1π–π*transition of the cyclometallated quinoline ligands. The lowest energy absorption peak in the region 440–460 nm could be ascribed to a typical spin allowed metal-to-ligand charge transfer (1MLCT) transition. We believe the weak bands at the long wavelength side are associated to both spin–

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orbit coupling enhanced 3π–π* and 3MLCT transitions. It is noted that the formally spin forbidden 3MLCT transition gains intensity by mixing with the higher lying 1MLCT transition through the strong spin–orbit coupling of Ir, which is comparable to the allowed 1MLCT transition. b a 243 nm

Abs.Intensity (Arb.Unit)

4

278 nm 295 nm (a)Ir(CN-DPQ)2(acac) 2 in dcm (b)Ir(CN-DPQ)2(acac) 2 inthf (c)Ir(CN-DPQ)2(acac) 2in formic acid

3 262 nm 363 nm

2

1

287 nm

351 nm 354 nm

442 nm

0

Fig.5 Absorption spectra of (a) CN-DPQ and (b) Ir(CN-DPQ)2(acac) complex Furthermore, we have observed absorption peaks at 243, 276, 295, 363, and 442 nm in THF. The highly intense PL emission peak of Ir(CN-DPQ)2(acac) complex in THF was located in the red region with the λmax value at 628 nm in THF with CIE coordinates of (0.703, 0.295). However the emission peaks of the complex Ir(CNDPQ)2(acac) in different solvents show clear differences. This emission of complex Ir(CN--DPQ)2(acac) peaks at around 626 with CIE coordinates of (0.700, 0.297) nm in solid state ,628 with CIE coordinates of (0.703, 0.295) in THF (polar solvent) and 631 nm with CIE coordinates of (0.706, 0.291). in CH2Cl2 (medium polarity) and 634 nm with CIE coordinates of (0.709, 0.286) in acetic acid. The peak shift can be attributed to the stronger interaction are shown in Fig.6 (b) between the solvents and the excited molecules. The broad, structure less spectral features lead us to conclude that the phosphorescence originates primarily from the 3MLCT state. b a 300

PL Intensity (Arb.Unit)

250

626 nm

628 nm

631 nm 634 nm

B Dichloromethane(DCM) CTetrahydrofuran (THF) D Solid emission spectra E Acetic Acid

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Wavelength in nm

Fig.6. .PL spectra of (a)CN-DPQ and (b) Ir(CN-DPQ)2(acac) complex Conclusion: We have successfully synthesized a new red emitting Ir(III) phosphorescent complex Ir(CN-DPQ)2(acac) bearing a 2-(4-cyno phenyl)-4 Phenyl quinoline (CN-DPQ) Ligand. Structural, Optical and thermal studies of the Ir(CN-DPQ)2(acac) were investigated using FTIR, UV–Vis absorption spectrometry, PL spectrometry and TGA/DTA respectively. The synthesized complex is thermally very stable over a wide range of temperature and suitable for the use as a red emissive material in OLEDs.

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