Synthesis, photophysical and electroluminescence properties of anthracene-based green-emitting conjugated polymers

Author's Accepted Manuscript

Synthesis, Photophysical and Electroluminescence Properties of Anthracene-Based GreenEmitting Conjugated Polymers Alpay Kimyonok, Emine Tekin, Gülçin Haykır, Figen Turksoy

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S0022-2313(13)00598-X http://dx.doi.org/10.1016/j.jlumin.2013.09.034 LUMIN12186

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Journal of Luminescence

Received date: 8 July 2013 Revised date: 2 September 2013 Accepted date: 17 September 2013 Cite this article as: Alpay Kimyonok, Emine Tekin, Gülçin Haykır, Figen Turksoy, Synthesis, Photophysical and Electroluminescence Properties of Anthracene-Based Green-Emitting Conjugated Polymers, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2013.09.034 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

 

Synthesis, Photophysical and Electroluminescence Anthracene-Based Green-Emitting Conjugated Polymers

Properties

of

Alpay Kimyonoka*, Emine Tekinb, Gülçin Haykıra, Figen Turksoya      a

Chemistry Institute, The Scientific and Technological Research Council of Turkey (TUBITAK), Marmara Research Center, Gebze, Kocaeli, 41470, TURKEY b National Metrology Institute, TUBITAK, Gebze, Kocaeli, 41470, TURKEY *

Corresponding author. Tel: +90 533 5473535 E-mail address: [email protected]

ABSTRACT Anthracene-based conjugated polymers, which include carbazole and fluorene groups as host materials, are synthesized via Suzuki coupling reaction. Monomer feed ratios are varied in order to determine the effect of anthracene concentration on polymer properties. It is found that

anthracene

content

plays

a

crucial

role

both

in

photoluminescence

and

electroluminescence properties of the polymers. All polymers exhibit efficient energy transfer from host groups to anthracene moieties in solid state. Excimer emission is observed for all polymers in devices, leading to green emission around 530-545 nm. A device structure of ITO/PEDOT:PSS/Polymer/Alq3/LiF/Al based on a polymer with 5% anthracene-containing monomer produces a luminance efficiency of 1.9 cd/A. Keywords: Conjugated polymer, Fluorescence, Anthracene, Excimer, Energy transfer, OLED

1. Introduction Organic light-emitting diodes (OLEDs) are focus of intense research for display and lighting applications [1-3]. Although high efficiencies have been obtained for small moleculebased OLEDs by using vacuum deposition technique [4], solution-processable polymeric materials offer easier fabrication that is especially desirable for large area displays [5,6]. There are two main types of polymers that are employed in OLEDs: Side-chain functionalized non-conjugated polymers and conjugated polymers. In the former type, electroactive groups are attached to the polymers as side chains, and the polymer backbone does not involve in the charge generation. Both fluorescent and phosphorescent materials have been prepared by this methodology [7]. On the other hand, charge is generated along the backbone for the conjugated polymers, eliminating the need for side chains [8,9]. Conjugated polymers with various functionalities along the backbone such as hole-transport, electron-transport, and emissive groups have been reported [7,10]. Anthracene-based compounds have been extensively studied in organic electronics, especially in OLEDs [11]. High fluorescence quantum yields and good electrochemical properties have been obtained for small molecule anthracene derivatives. Triplet-triplet quenching in these materials contributes to singlet production, leading to high device efficiencies [12-14]. External quantum efficiencies as high as 6% with luminous efficiencies of around 20 cd/A were obtained for green-emitting anthracene derivatives [14]. Although different emmision colors have been obtained for anthracene-based small molecules, most of the literature reports describe molecules with emission in blue region [15-18]. Luminous efficiencies around 10 cd/A were obtained for blue-emitting anthracene derivatives [19]. Aggregate-induced emission plays an important role to obtain emission colors other than blue. For

example,

commercially

available

9,10-bis(2-phenylethynyl)anthracene

with

photoluminescence (PL) maximum at 475 nm displays electroluminescence (EL) maximum

of 532 nm due to aggregate effect [20]. Red emission due to excimer formation was observed for 10-(4-dimethylamino-phenyl ethynyl)-anthracene-9-carbonitrile [21]. Devices based on this molecule gave efficiencies as high as 8.5 cd/A. Another strategy for color tuning of anthracene derivatives is physical processing of the material, thereby tuning the molecular packaging [22]. Anthracene-containing cojugated polymers offer promise for solution processable OLEDs, however, device efficiencies of these polymers are still inferior to small molecule-based counterparts. The vast majority of the polymers reported in the literature emit in the blue region of the visible spectrum. For example, Zheng et. al reported blue-emitting anthracenebased conjugated polymer with a device efficiency of 0.4 cd/A [23]. In another report, polymer with 9,10-naphthalene substituted anthracene units and 2,7-diphenyl substituted fluorene groups gave device efficiency of 1.18 cd/A and emission around 450 nm [24]. Similar emission spectrum was obtained for a polymer with anthracene group along with hole-transport carbazole and electron-transport oxadiazole units for improved charge transport, resulting in a device efficiency of 5.1 cd/A [25]. Blue-green emission was obtained for polymers with fluorene and 9,10 vinylphenyl substituted anthracene units. It was observed that the emission spectrum is shifted towards longer wavelengths with increasing molecular weight but pure green color could not be obtained [26]. Another blue-green emitting polymer with 9,10-ethylhexyloxy phenyl substituted anthracene units resulted in a device with luminous

efficiency

of

0.27

cd/A

[27].

Green

emission

was

obtained

for

poly(fluorenevinylene-alt-anthracenevinylene) type polymer that exhibited excimer emission with EL maximum at 528 nm and external quantum efficincy of around 0.3% [28]. Another excimer forming polymer, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene-alt9,10-anthrylene vinylene], exhibited red emission with EL maximum at 610 nm [29]. Finally,

attachment of electron withdrawing cyano groups on anthracene ring resulted in emission maximum of 665 nm [30]. In this report, we describe the synthesis and characterization of anthracene containing conjugated polymers with host groups such as fluorene and carbazole. We studied both intraand intermolecular energy transfer from host groups to anthracene units and investigated the effect of anthracene concentration on photophysical and electroluminescence properties of the polymers.

2.

Experimental 

2.1. General Methods All reagents were purchased from Aldrich, Alfa Aesar or Merck, and used without further purification. Compounds 1 [14,31], 2 [32], 3 [33], 4 [34], 5 [35] were synthesized according to published procedures. The 1H-NMR spectra were obtained using a Bruker Spectrospin Avance DPX 500 spectrometer operating at 500 MHz. Gel permeation chromatography (GPC) analyses were carried out on Agilent 1100. THF solutions for GPC analysis were eluted at 25 oC at a flow rate of 1.0 ml/min and analyzed using a multiple wave detector. Molecular weights and molecular weight distributions are relative to polystyrene. Thermogravimetric analysis was carried on a Perkin Elmer Thermogravimetric Analyzer Pyris 1 TGA under nitrogen at a heating rate of 10 photoluminescence

spectra

were

recorded

on

a

o

C/min. Absorption and

Shimadzu

UV-1650PC-

UV-vis

spectrophotometer and Horiba Jobinyvon Fluorolog TCSPC Fluorescence spectrophotometer. Atomic force microscopy (AFM, Park Systems XE-150) was used to inspect the morphology of the poymer films. A non-contact mode was employed for scanning the surfaces. Cyclic voltammetry (CV) was performed in CH2Cl2 containing 0.1 M tetrabutylammonium

perchlorate at room temperature using Gamry Instrument Reference 600. Platinum, platinum wire and Ag/AgCl were used as the working, counter and reference electrodes, respectively. Each oxidation potential was calibrated using ferrocene as a reference. The HOMO and LUMO levels were determined using the following equations: EHOMO= [Eonsetox - E11/2 (ferrocene) + 4.8], ELUMO= EHOMO- Egopt. 2.2. Device Fabrication Tris-(8-hydroxyquinoline) aluminum (Alq3, sublimed grade, 99.995%) was acquired from Sigma Aldrich. Poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS) was purchased from Heraeus Clevios GmbH. The ITO coated glass substrates (ITO thickness 150 nm, 15 ohms/sq) were determined from Visiontek Systems. The substrates were firstly etched in aqua-regia solution (3:1:3) (HCl:HNO3:H2O) for 2 minutes, and then cleaned consecutively in ultrasonic baths containing de-ionized water, acetone, isopropyl alcohol. Finally, the substrates were dried in N2 flow. The layout of the fabricated devices can be seen below: ITO/PEDOT:PSS/PFA or PCFA-15 or PCFA-5/Alq3/LiF/Al Here, the thicknesses of the hole transport layer (PEDOT:PSS) are about 50 nm, the active layers are approximately 80 nm and electron transport layer (ETL; Alq3) are 10 nm. PEDOT:PSS was spin-coated at 3000 rpm for 30 seconds; afterwards dried at 110 oC for 10 min. Solutions of PFA, PCFA-5 and PCFA-15 were prepared in 25:75 v/v dichlorobenzenetoluene mixture yielding 10 mg/ml concentration. Subsequently active layers were spincoated at 1500 rpm for 40 s and cured at 120 oC for 5 minutes. Then the electron transport layer Alq3 was coated by thermal evaporation at a base pressure of 10-6 mbar. In each device structure, LiF (1 nm) and Aluminum (Al, 120 nm) were coated as the electron injection layer and cathode electrode respectively, by thermal evaporation through shadow mask under vacuum (pressure: 10-6 mbar).

Electroluminescence characteristics of the OLEDs were detected utilizing HamamatsuBrightness Light Distribution Measurement System-C9920-11. The layer thicknesses were measured by a stylus profiler (KLA Tencor P-6). 2.3. Synthesis A general method for the synthesis of the polymers is described for PCF: To a mixture of 4 ml toluene, 2 ml tetrabutylammonium hydroxide (1.5M in H2O), and 2 ml H2O, were added 0.196 g of monomer 3 (0.397 mmol) and 0.20 g of monomer 4 (0.397 mmol).

After

the

mixture

is

purged

with

argon,

0.011

g

palladium

tetrakis(triphenylphosphine) catalyst (0.0098 mmol) was added, and the mixture was stirred at 85

o

C under argon atmosphere for 48 hours, after which phenyl boronic acid and

bromobenzene were added to end-cap the polymer chain. The resulting polymer was precipitated in a methanol-water mixture, filtered, dissolved in CH2Cl2, passed through Celite, and precipitated in methanol. The final polymer was dried at 70 oC under vacuum. Other polymers were synthesized by the same procedure using the corresponding monomers. PCF: 1H-NMR (CDCl3): δ= 8.50 (b), 7.83 (b), 7.72 (b), 7.52 (b), 7.34 (b), 4.38 (b), 2.27 (b), 2.14 (b), 1.43 (b), 1.33(b), 1.11 (b), 0.88 (b), 0.82 (b). Mn: 10x103 Da, Mw: 6x103 Da PFA: 1H-NMR (CDCl3): δ= 8.49 (b), 8.07 (b), 7.84(b), 7.70 (b), 7.50 (b), 7.39 (b), 7.06 (b), 2.06 (b), 1.52 (b), 1.12 (b), 0.77 (b). Mn: 18.1x103 Da, Mw: 48.3x103 Da PCFA-15: 1H-NMR (CDCl3): δ= 8.49 (b), 8.32 (b), 8.05 (b), 7.83 (b), 7.75 (b), 7.53 (b), 7.36 (b), 7.07 (b), 4.39 (b), 2.12 (b), 1.95 (b), 1.45 (b), 1.12 (b), 0.88 (b), 0.80 (b). Mn:6,6x103 Da, Mw:11,8x103 Da

PCFA-5: 1H-NMR (CDCl3): δ= 8.49 (b), 8.32 (b), 8.05 (b), 7.83 (b), 7.75 (b), 7.53 (b), 7.36 (b), 7.07 (b), 4.39 (b), 2.12 (b), 1.95 (b), 1.45 (b), 1.12 (b), 0.88 (b), 0.80 (b). Mn: 11.8x103 Da Mw: 33.2x103 Da

3. Results and Discussion 3.1. Synthesis Monomers 1 [14,31], 2 [32], 3 [33], 4 [34], 5[35] were synthesized according to literature procedures. Figure 1 shows the structures of the monomers. Monomer 1 represents the emissive anthracene units in the polymers. The feed ratio of monomer 1 in the polymers are varied in order to determine the effect of the anthracene group content on the photophysical and electroluminescence properties of the polymers. Monomer 1 is copolymerized with carbazole-containing monomer 2 and fluorene-containing monomer 5 to obtain the final polymers. Carbazole and fluorene are extensively used in OLEDs as host materials due to their wide band gaps and good hole-transport properties [36,37]. In order to have a detailed understanding of the effect of these host materials on the photophysical properties of the final materials, a polymer without anthracene group is synthesized by the copolymerization of monomers 3 and 4. Structures of the polymers are shown in Figure 2. The polymers were prepared by Suzuki coupling method with palladium tetrakis(triphenylphosphine) as the catalyst. The polymers are end-capped with phenyl groups by using phenyl boronic acid and bromobenzene. All polymers are soluble in common organic solvents such as THF and chloroform. Three different feed ratios are used for monomer 1 to obtain polymers with varying amounts of anthracene units. For PFA, the feed ratio is 50%, for PCFA-5, and PCFA-15, the feed ratios are 5%, and 15%, respectively. The polymer without anthracene groups, PCF, is used to

compare the UV-Vis and PL spectra of the polymers to determine the contribution of the carbazole and fluorene groups on the absorption and emission properties of the polymers. Table 1 lists the physical properties of the polymers. No correlation was found between the monomer type and degree of polymerization. Mn values of the polymers are in the range of 618 kDa, while the highest PDI values are observed for PFA and PCFA-5. The polymers are thermally stable up to 400 oC. For PCF, PFA, and PCFA-5, 5% weight lost was observed around 440 oC, whereas for PCFA-15, lower decomposition temperature around 400 oC was detected. The reason for 40 oC difference in thermal decomposition temperature for PCFA-15 is not clear at this point. Figure 3 shows the 1H-NMR spectra of the polymers in CDCl3. For PCF, PCFA-5,and PCFA-15, characteristic peak of the carbazole unit is detected at around 4.4 ppm (-N-CH2). The double bond protons of the anthracene unit is observed at 7.1 ppm. The peak intensity increases as the anthracene unit concentration in the polymer backbone increases. This peak is not detectable for PCFA-5 due to the low content of the anthracene group. The aromatic rings of the anthracene unit give signals in the same region as the aromatic rings of the carbazole and fluorene groups. The region between 7.2-8.2 ppm gets more crowded with increased content of the anthracene moiety. This is easily noticed for PFA, where the concentration of the anthracene group is the highest. 3.2. Photophysical Properties In order to determine the effect of the anhtracene units on the photophysical properties of the polymers, anthracene content of the polymers are varied. Furthermore, carbazole and fluorene containing polymer PCF served as a model compound to determine the interaction between the host groups and the anthracene moiety. Figure 4(a) shows the UV-vis spectra of the polymers. Comparison of the spectra of the polymers with and without anthracene groups indicates that the sharp peak around 270 nm, and the longer wavelength peak around 405 nm

are due to the anthracene units. Intensities of these peaks increase with increasing anthracene content, whereas these peaks are not observed for PCF, which does not contain anthracene unit. Figure 4(b) shows the emission spectra of the polymers in dichloromethane. Examination of PCF emission spectrum shows that the two peaks at around 400 and 420 nm are due to carbazole and fluorene groups. In PCFA-5, there is an additional broad peak at 507 nm, which is attributed to the anthracene unit. The presence of the low wavelength emission of the host groups indicates that the energy transfer from the host groups to the anthracene group is incomplete. For PCFA-15, the low wavelength emission is still detectable, whereas there is only one peak for PFA, located at 513 nm, indicating complete energy transfer. In solution, the predominant energy transfer method is expected to be intramolecular due to the low concentration of the polymer chains. As the polymer chains get closer to each other intermolecular energy transfer becomes dominant. Solid state emission spectra of the polymers (Figure 4c) show complete energy transfer for all polymers thanks to more efficient interaction between host groups and anthracene units in solid state. Therefore, just 5% anthracene containing monomer is sufficient to obtain a pure anthracene-based emission in solid state. All polymers exhibited hypsochromic shift in solid state, indicating solvent effect [14,38]. Table 2 lists the photophysical data for the polymers. Increase in anthracene content results in decreased HOMO and LUMO levels but the decrease in LUMO level is more pronounced. Anthracene serves as the acceptor moiety in the polymers, whereas carbazole and fluorene groups are donors, therefore, increasing the amount of electron accepting anthracene units in the conjugated system affects the energy level of LUMO. As a result of the decreased LUMO level, the polymer with the highest amount of anthracene unit, PFA, has the lowest bandgap. Anthracene content has also impact on fluorescence quantum yield. Fluorescence efficiency

decreases with increasing anthracene content, indicating concentration quenching. These results suggest that PCFA-5 is the most promising candidate as emissive layer in OLEDs. 3.3. Electroluminescence Polymer OLEDs using PFA, PCFA-15 and PCFA-5 as active layers were fabricated and characterized. Figure 5 shows the electroluminescence spectra of PFA, PCFA-15, and PCFA-5. EL spectrum is shifted to longer wavelengths as the anthracene group concentration in the polymers increased. EL maximum for PCFA-5 is observed at 530 nm, whereas for PCFA-15 and PFA, EL maxima are at 535 and 544 nm, respectively. Furthermore, compared to their respective solid state PL emission, all polymers exhibited red-shifted EL emission. The red-shift in EL spectra is believed to be due to excimer formation of the anthracene units, which are known to exhibit excimer-based emission [39,40]. Broadening of the EL spectra with increased concentration of the anthracene groups (Figure 5) is consistent with the excimer emission. Relative orientations of phenyl groups lead to different types of excimeric structures for anthracenes [41]. Partial π-π stacking of the phenyl groups results in emission in blue region, whereas complete overlap gives red emission [42,43]. T-shaped excimer (edge to face) has also been reported, giving blue-green emission [44,45]. Therefore, it is possible to tune the emission wavelength of anthracene derivatives by taking advantage of excimer formation. For conjugated polymers, this effect is less pronounced but still present for some systems [29]. For example, similar to our study, Adachi and coworkers reported excimer emission in green region with a maximum at 528 nm for an anthracene-containing conjugated polymer [28]. For solution processable OLEDs, morphology of the polymer film has direct influence on device performance. Figures 6 (a-c) show atomic force microscope (AFM) images of PFA, PCFA-15, and PCFA-5. Total roughness values are similar for PFA and PCFA-15, 25.5 and 25.2 nm, respectively, whereas, 18.3 nm is obtained for PCFA-5. Average roughness values

are 1.24, 1.20, and 1.13 nm for PFA, PCFA-15, and PCFA-5, respectively. These data show that PCFA-5 film has the smoothest surface, making it more suitable for OLED applications. Device performances can be seen in Figure 7 and the results are summarized in Table 3. As it is clear from Figure 7(a), high current values are obtained at lover voltages from device 3 compared to those of devices 1 and 2. According to the luminance-voltage curves, the turn on voltage of the device fabricated from PCFA-5 is 7.5 V. It is increased for devices of PFA and PCFA-15. Luminance value and luminous efficiency of PCFA-5 are 4050 cd/m2 and 1.90 cd/A, respectively. These values are decreased for devices 1 and 2. As a result, in terms of current-voltage characteristics, luminance and device efficiencies, PCFA-5 yielded the best device performance. For PFA and PCFA-15, it is believed that high amounts of anthracene units result in aggregates that lead to charge carrier traps, which decrease device efficiency.

4. Conclusions We have synthesized anthracene containing emissive conjugated polymers with host groups along the backbone. All polymers are soluble in common organic solvents, and are stable up to 400 oC. Fluorescence studies showed that in solution, energy transfer from host groups to anthracene units increased with increasing anthracene content, whereas, in solid state, complete energy transfer was observed for all polymers. LUMO levels of the polymers decreased as the concentration of the anthracene units increased on the backbone, resulting in a decrease in bandgap. Devices fabricated from the polymers exhibited green emission. Turn on voltage of 7.5 V, luminance value of 4050 cd/m2 and luminous efficiency of 1.90 cd/A were obtained for an unoptimized device based on PCFA-5, making it a candidate for optoelectronic applications.

Acknowledgements

We gratefully acknowledge the Turkish State Planning Organization (DPT) for financial support. References 

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Table 1. Physical properties of the polymers Tda)

Monomer feed ratio 

Mn x103 

Mw x103 

(mol %) 

(Da) 

(Da) 

PCF 

3:4 (50:50) 

12.0 

21.8 

1.82 

444 

PFA 

1:5 (50:50) 

18.1 

48.3 

2.67 

434 

PCFA‐5 

1:2:5 (5:45:50) 

11.8

33.1

2.80

444

PCFA‐15 

1:2:5 (15:35:50) 

6.6 

11.8 

1.79 

397 

Polymer 

PDI 

(oC) 

a)

 Temperature at 5% weight lost 

Table 2. Photophysical properties of the polymers. Eoxonset  

HOMO  

LUMO

Egopt  

λmax 

λmax 

 (V) 

(eV) 

(eV) 

(eV) 

(nm)a) 

(nm)b) 

0.96 

5.31 

2.64

2.67

513

506 

0.54

PCFA‐15 

0.92 

5.27 

2.56 

2.71 

400,419,514 

497 

0.73 

PCFA‐5 

0.90 

5.25 

2.46 

2.79 

401,422,507 

494 

0.88 

Polymer 

PFA 

 

Фc) 

a)

 in dilute CH2Cl2 solution (10‐3M); b)solid state; c)solution fluorescence quantum yield, relative to  9,10‐diphenyl anthracene 

Table 3. Summary of device characteristics of the synthesized polymers; onset voltage (VON),  luminance (L), luminous efficiency (LE), external quantum efficiency (EQE) and EL wavelength.  Device  No 

Active 

VON  

L 

LE 

EQE 

λEL  

Layer 

(V)a)  

(cd/m2)b) 

(cd/A)b) 

(%)b) 

(nm)c) 

1 

PFA 

10.7 

1600 

0.60 

0.20 

544 

2 

PCFA‐15 

8.7 

690

0.50

0.16

535 

3 

PCFA‐5 

7.5 

4050 

1.90 

0.60 

530 

a)

Turn‐on voltage at 1 cd/m2; b)Maximum values; c)Emission maximum.  

 

Figure 1. Structures of monomers 1‐5.  Figure 2. Structures of polymers. For PCFA‐5 and PCFA‐15, anthracene monomer molar feed ratio is  5% and 15%, respectively.  Figure 3. 1H‐NMR spectra of the polymers in CDCl3.  Figure 4. (a) UV‐Vis; (b) solution emission; (c) solid state emission spectra of the polymers.  Figure 5. EL spectra of the devices fabricated from PFA, PCFA‐15 and PCFA‐5.  Figure 6. AFM images of the polymers: (a) PFA; (b) PCFA‐15;(c) PCFA‐5.  Figure 7. Current density‐voltage, luminance‐voltage and luminous efficiency‐current density curves  of the electroluminescent devices of PFA, PCFA‐15 and PCFA‐5.      Highlight 

• • • •  

Anthracene containing conjugated polymers with host groups are synthesized. Emission characteristics depend on the extent of energy transfer. Polymers exhibit excimer emission, leading to green-emitting devices. Polymer with 5% anthracene produces a luminance efficiency of 1.9 cd/A.

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