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New thiophene-based donor-acceptor conjugated polymers carrying fluorene or cyanovinylene units: synthesis, characterization, and electroluminescent properties.

INTRODUCTION

The use of conjugated polymers as emitting layers in polymer light-emitting diodes (PLEDs) has attracted considerable academic and commercial interest because of the potential of these materials as active components in flat-panel displays and lighting applications (1-3). The great advantages of PLEDs are tunable emission color through molecular design and ease of device fabrication. Solution-processable conjugated polymers are suitable for large-area display devices fabricated using spin coating or printing methods (4-6). Furthermore, considerable attention has been received on conjugated polymers because of their high heat-resistance, excellent dimensional stability, and good compatibility with other polymers (7. 8). However, the development of conjugated polymers that possess both high performance and easy processability has become the major challenge for advanced lightweight PLEDs. Due to this reason, extensive research work is going on to develop highly efficient light-emitting polymers with tunable emission, long lifetime, and color purity (9). Toward this end, a wide range of electro and photoactive conjugated polymers have been studied in the field of PLEDs, such as poly(p-phenylene)s (10), poly (p-phenylenev inylene)s (PPV) (11), poly(thiophene)s (PTs) (12), poly(pyrrole)s (13), poly((fluorene)s (14), and poly(carbazole)s (15). In order to improve the device properties such as low turn-on voltage, high color purity, high luminance, and high efficiency, it is necessary to understand the structure-property relationship between the molecular structure of the polymers and their electroluminescent properties. In most of the electroluminescent conjugated polymers such as PPV, hole injection, and transport are more favorable than electron injection and transport due to its high LUMO energy level, which results an imbalance in the rates of electron and hole injection and thus lowering the device efficiency (16), (17). The high device efficiency can be achieved by balancing the electron and hole injection rates. To improve the charge injection/transport, many approaches have been attempted. One strategy is to insert an additional electron injection/trans-port layer between the emitter and cathode and/or a hole transporting layer between the emitter and anode (18), (19). Fabrication of such multilayer PLEDs is usually a difficult task (20). Other approach is that, a low work function metal such as calcium or magnesium can be used as cathode to lower the energy barrier so as to enhance injection rate of electrons. The high chemical reactivity of these metals to oxygen and moisture limits their practicality. In this regard, donor--acceptor (D--A) type polymers, introduced by Havinga et al. (21) in macromolecular systems via alternating electron-rich and electrondeficient substituents along a polymer backbone is the well-known approach to obtain efficient polymers. In this system, the electron or hole affinity can be enhanced simultaneously or controlled independently (22), (23). In the categories of conjugated polymers, PT and poly(fluorene), and their derivatives occupy a significant position. In particular, thiophene is one of the widely used materials for modifying the electroluminescent properties of PLEDs. The most prominent features of PTs are their good thermal stability both in the neutral and doped states and their wide electronic and optical tenability (24-26). In addition, PT possesses lower delocalization energy and the addition of thiophene to the backbone of conjugated polymeric materials narrows the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) band gaps. These result in improved electroluminescent properties and a lower initial voltage of the light-emitting device (27). Furthermore, presence of alkoxy chains at 3- and 4-positions of thiophene ring improves solvent processability of the polymer. Poly(fluorene) derivatives are attractive candidates for light emitting diodes because of their good thermal and chemical stability and their exceptionally high solution and solid-state fluorescence quantum yields (28), (29). Moreover, facile substitution at 9-position of the fluorene monomer allows control of polymer properties such as solubility, processability, and morphology. Since 3,4-dialkoxythiophene and fluorene derivatives are both electron rich and hole transporting, it is necessary to introduce electron withdrawing units to the main chains or side chains to attain large electron affinities. The strong electron withdrawing 1,3,4-oxadiazole unit is the first and the most extensively investigated class of electron-transporting and hole-blocking material in PLEDs because of its high electron affinity, good thermal, and chemical stability (30), (31). Whereas, the presence of cyano group provides high electron affinity to the polymer backbone and hence polymers such as cyano-substituted PPV (CN-PPV) exhibits high internal efficiency in PLEDs (32). Also due to their high electron affinity, air stable metals with higher work functions, such as aluminum can be used as cathode in PLEDs. This would further improve the device stability.

Against this background, in the present study we report the synthesis, characterization, and electroluminescent properties of two new D--A conjugated polymers PDTOFV and PDTOCN. The core structure of the polymers consist of 3,4-didodecyloxythiophene moiety as electron-donor unit and 1,3,4-oxadiazole moiety as electron-acceptor unit. Furthermore, fluorenevinylene segments are incorporated in the backbone of PDTOFV whereas in PDTOCN, cyano-containing phenylenevinylene units are attached along the polymer chain. The incorporation of vinylene linkages in the polymer backbone facilitates to planarize the chain structure by overcoming torsional interactions between the rings and thus extending the effective conjugation length. The general properties of the polymers, including thermal, optical, and electrochemical properties are discussed. Furthermore, the potential applications of these polymers as active layers in PLEDs are investigated by fabricating devices with a configuration of ITO/PEDOT:PSS/polymer/Al.

EXPERIMENTAL

Instrumentation

Proton nuclear magnetic resonance [.sup.1]H NMR) spectra were recorded with a BRUKER 400 MHz NMR spectrometer using trimethylsilane (TMS) as internal reference. Elemental analyses were performed on a Flash EA 1112 CHNS analyzer (Thermo Electron Corporation). Infrared spectra of all the compounds were recorded on a NICOLET AVATAR 330 Fourier transform infrared (FTIR) spectrometer (Thermo Electron Corporation). UV-visible absorption spectra were measured using a CIN-TRA-101 (GBC scienti tic equipment) spectrophotometer. Fluorescence spectra were recorded using a JASCO FP6200 spectrofluorometer. The electrochemical studies of the polymer were carried out using art AUTOLAB PGSTAT 30 electrochemical analyzer. Cyclic voltammo-grams were recorded using a three-electrode cell system, with a glass carbon disk as working electrode, a Pt wire as counter electrode and an silver (Ag)/silver chloride (AgC1) electrode as the reference electrode. Gel-permeation chromatography (GPC) technique was used to obtain the average molecular weights of the polymers and measurements were made using a Waters make GPC instrument with reference to polystyrene standards with tetrahydrofuran (THF) as eluent. Polymer films are prepared using an ACE-1020 (Dong Ah Trade & Tech. Corp.) spin coating unit. Thermogravimetric analysis (TGA) was carried out using an EXSTAR TG/DTA 7000 (SII Nanotechnology Inc.) thermal analyzer. The electroluminescence (EL) spectrum was determined using HR 2000 Ocean Optics spectrometer, having a CCD array and fiber optic probe. The current--voltage characteristic was studied using a Keithley 2400 programmable digital source meter. The thicknesses of the deposited layer were measured by ellipsometry.

Materials

4-Bromobenzoyl chloride, 4-formylphenylboronic acid, tetrakis(triphenylphosphine) palladium(0) [Pd[(PP[h.sub.3]).sub.4] and poly(3,4-ethylenedioxy thiophene):pol y(styrenesulfonate) (PEDOT:PSS) were purchased From Sigma Aldrich chemicals. Tetrabutylammonium perchlorate (TBAPC) was purchased from Lancaster Company (UK). All solvents and other reagents were purchased commercially and were used without Further purification.

Synthesis of Monomers and Polymers

The detailed synthetic procedures for the synthesis of the monomers and the polymers are as follows.

Synthesis of [N.sup.2], [N.sup.5]-di(4-bromobenzoyl)-3,4-bis(dodecyloxy)thiophene-2, 5-diearbohydrazide (2). To a mixture of dihydrazide monomer 1 (1 g, 1.75 mmol) and pyridine (0.1 ml) in N-methylpyiToliciinone (NMP) (10 ml), 4-bro-mobenzoyl chloride (0.85 g, 3.87 mmol) was added slowly at room temperature. The reaction mixture was stirred at room temperature for 12 h. After completion of reaction (progress of the reaction was monitored by thin layer chromatography (TLC)), it was poured into excess of ice water to get a precipitate. The precipitate obtained was collected by filtration, washed with water and dried in oven. The crude product was recrystallized from ethanol/chloroform (CHC13) mixture to get the pure product as white solid in 80% yield. M.P: 192-193[degrees]C. [.sup.1]H NMR (400 MHz, CD[C1.sub.3]. [delta]): 10.22 (s, 2H), 9.39 (s, 2H), 7.77-7.74 (m, 4H), 7.667.63 (m, 4H), 4.33 (t, 4H), 2.03-1.27 (m, 40H), 0.9 (1, 6H). FTIR [square root of (([cm.sup.-1]))]: 3288 (>NH), 2918 and 2850 (C-H), 1641 (> C=O), 1595, 1461, 1412, 1283, 1068. Element. Anal. Calcd. For [C.sub.44][H.sub.62][Br.sub.2][N.sub.4][O.sub.6]S (%): C, 56.52; H, 6.69; N, 6.0; S. 3.42. Found (%): C, 56.50; H, 6.64; N, 6.04; S. 3.46.

Synthesis of 5,5'-(3,4-bis(dodecyloxy)thiophene-2, 5-diyl)bis(2-(4-bromopheny1)-1,3,4-oxadiazole) (3). A mixture of dicarbohydrazide 2 (1 g, 1.06 mmol) in 10 ml phosphorus oxychloride (PO[Cl.sub.3]) was refluxed under nitrogen atmosphere for 6 h. The reaction mixture was then cooled to room temperature and poured in to excess of ice cold water. The resulting precipitate was collected by filtration and was washed with water and dried in oven. The crude product was further purified by recrystallization from ethanol to get the pure product as light-yellow solid in 70% yield. M.P: 114-115[degrees]C. [.sup.1]H NMR (400 MHz, CD[Cl.sub.3], [delta]): 8.04-8.00 (m, 4H), 7.74-7.70 (m, 4H), 4.35 (t, 4H), 1.92-1.28 (m, 40H), 0.9 (t, GH). FTIR [square root of (([cm.sup.-1]))]: 2916 and 2850 (C-H), 1592 (C=N), 1516, 1469, 1388, 1275, 1064 (=C-O-C.). Element. Anal. Calcd. For [C.sub.44][H.sub.58][Br.sub.2][N.sub.4][O.sub.4]S (%): C, 58.78; H. 6.51; N, 6.24; S. 3.56. Found (%): C, 58.74; H, 6.54; N, 6.22; S. 3.28.

Synthesis of 4', 4'-(5,5'-(3,4-bis(dodecyloxy)thiophene-2,5-diyi)bis(1,3,4-oxadiazole-5,2-diy1)) dibipheny1-4-car-baldehyde (MI). Monomer MI was synthesized by Suzuki biaryl coupling reaction method. Under argon atmosphere, to a mixture of dibromo compound 3 (0.5 g, 0.556 mmol) and 4-formylphenylboronic acid (0.18 g, 1.22 mmol) in THF and ethanol 10 ml (1: 1 volume ratio), 2 M aqueous sodium carbonate was added. After 30 min of degassing with argon, 3 mol% (0.019 g, 0.0166 mmol) of Pd[(PP[h.sub.3]).sub.4] was added. The reaction mixture was stirred at 75[degrees]C for 10 h under argon atmosphere. After completion of reaction (progress of the reaction was monitored by TLC), it was poured into distilled water and extracted with chloroform. The organic layer was dried with magnesium sulfate (MgS[O.sub.4]) and concentrated. The crude product was purified by silica gel column using a mixture of hexane and ethyl acetate (8/2) as an eluent, giving a brown solid in 65% yield. M.P: 196-197[degrees]C. [.sup.1]H NMR (400 MHz, CD[Cl.sub.3], [delta]): 10.13 (s, 2H, -CHO), 8.29-8.26 (m, 4H), 8.05-8.01 (m, 4H), 7.87-7.84 (m, 8H), 4.38 (t, 4H), 1.98-1.26 (m, 40H), 0.89 (t, 6H). FTIR [square root of (([cm.sup.-1]))]: 2919 and 2850 (C-H), 1697 (C=O), 1591 (C=N), 1485, 1375, 1213, 1043. Element. Anal. Calcd. For [C.sub.58][H.sub.68][N.sub.4][O.sub.6]S (%): C, 73.38; H, 7.23; N, 5.91; S, 3.37. Found (%): C. 73.42; H, 7.26; N, 5.95; S. 3.33.

Synthesis of 9,9'Di-n-hexylfluorene (4). To a solution of fluorene (1 g, 6 mmol) in dimethylformamide (DMF) (10 ml), sodium hydride (0.43 g, 18 mmol) was added portion wise slowly under nitrogen at 0[degrees]C. To this solution, n-hexyl bromide (2.19g. 13.2 mmol) was added drop wise. The solution was warmed to room temperature and stirred for 12 h. After completion of reaction (as monitored by TLC), the resulting mixture was poured into ice cold water. The aqueous solution was extracted with dichloromethane. The organic layer was separated, dried with MgS[O.sub.4] and concentrated. The crude product obtained was purified by recrystallization from hexane. Yield: 70%. M.P: 36-39[degrees]C. [.sup.1]H NMR (400 MHz, CD[C1.sub.3], [delta]): 7.70 (d, 2H), 7.34-7.28 (m, 6H). 1.95 (t, 4H), 1.12-1.06 (m, 12H), 0.88-0.83 (m, 4H), 0.75 (t, 6H). FTIR [square root of (([cm.sup.-1]))]: 2914, 2854, 1451, 731. Element. Anal. Calcd. For [C.sub.25][H.sub.34] (%): C, 89.75; H, 10.25. Found (%): C, 89.71; H, 10.22.

Synthesis of 2,7-bis(bromomethy)-9,9'-di-n-hexylfluor-ene (5). A mixture of 9,9'-di-n-hexylfluorene 4 (2 g, 5.98 mmol), paraformaldehyde (1.85 g, 50.98 mmol) and 18 g of 33% hydrogen bromide (HBr) solution in acetic acid was stirred for 24 11 at 70[degrees]C. The reaction mixture was cooled to room temperature and poured into 100 ml of ice-cold water. The mixture was extracted with dichloromethane and the extracted solution was washed with water, saturated sodium bicarbonate (NaHC[O.sub.3]) and sodium chloride (NaCl). The organic layer was separated, dried with Mg[SO.sub.4] and concentrated to get a pale yellow liquid. The crude liquid was purified by silica gel column using a mixture of hexane and ethyl acetate (10/1) as an eluent, giving a colorless liquid in 78% yield. [.sup.1]H NMR (400 MHz, CD[Cl.sub.3], [delta]): 7.39-7.25 (m, 6H), 4.59 (s, 4H), 1.97-1.93 (m, 4H), 1.13-1.03 (m, 12H), 0.89-0.82 (m, 4H), 0.77-0.73 (m, 61-1). FTIR [square root of (([cm.sup.-1]))]: 2920, 2853, 1455, 1210, 818. Element. Anal. Calcd. For [C.sub.27][H.sub.36][Br.sub.2] (%): C, 62.30; H. 6.98. Found (%): C, 62.32; H, 6.95.

Synthesis of 2,7-bis[(p-triphenylphosphonio)methy)]-9,9'-di-n-hexyffluorene dibromide (M2). A mixture of 2,7-bis(bromomethy)-9,9'-di-n-hexyllluorene 5 (1 g, 2.02 mmol) and triphenylphosphine (1.32 g, 5.06 mmol) in DMF (10 ml) was heated for 12 h at 105-110[degrees]C under nitrogen atmosphere. The reaction mixture was cooled to room temperature and was added slowly into 100 ml of diethyl ether while stirring. The white solid obtained was filtered, washed with ether, and dried in a vacuum oven at 40[degrees]C in 90% yield. M.P: >200[degrees]C. [.sup.1]H NMR (400 MHz, CD[Cl.sub.3], [delta]): 8.02-7.82 (m, 30H), 7.58-7.17 (m, 6H), 5.6-5.56 (m, 4H), 1.54-1.50 (m, 4H), 1.16-0.78 (m, 16H), 0.2 (br, 6H). FTIR [square root of (([cm.sup.-1]))]: 3407, 3340, 2921, 2852. 1434, 1109, 744. Element. Anal. Calcd. For [C.sub.63][H.sub.66][Br.sub.2][P.sub.2] (%): C, 72.40; H, 6.37. Found (%): C, 72.36; H, 6.42.

Synthesis of Polymer PDTOFV. A solution of sodium (20 mg, 0.948 mmol) in 2 ml of anhydrous ethanol was added drop wise at ambient temperature under argon to a mixture of monomer M1 (0.3 g, 0.316 mmol) and iluorene Wittig salt M2 (0.45 g, 0.316 mmol) in 6 ml of dry chloroform. The mixture was stirred at room temperature for 12 h. The reaction mixture was slowly poured into 100 ml of methanol. The precipitated polymer was filtered off. The crude polymer was redissolved in chloroform and precipitated in methanol several times to remove oligomers. After filtration and drying under vacuum a light-yellow powder was obtained in 60% yield. [.sup.1]H NMR (400 MHz, CD[C1.sub.3], [delta]): 8.25-6.52 (m, 26H, Ar and -CH=CH-), 4.36 (t, 4H, -OCH2-), 1.97-0.80 (m, 66H), 0.77 (t, 6H). FTIR [square root of ([cm.sup.-1]]: 2919 and 2850 (C-H), 1585, 1475, 1368, 1279, 1045, 953, 817. Element. Anal. Calcd. For [C.sub.85][H.sub.102][N.sub.4][0.sub.4]S (%): C, 80.02; H, 8.06; N, 4.39; S, 2.51. Found (%): C. 80.10; H, 8.12; N. 4.34; S. 2.48.

Synthesis of Polymer PDTOCN. To a mixture of monomer MI (0.3 g, 0.316 mmol) and 1,4-phenylenedia-cetonitrile (50 mg, 0.316 mmol) in 5 ml of dry chloroform under argon atmosphere, a solution of sodium (20 mg, 0.948 mmol) in 2 ml of anhydrous ethanol was added drop wise at ambient temperature. The mixture was stirred at room temperature for 12 h and then was slowly poured into 100 ml of methanol. The precipitated polymer was filtered off. The crude polymer was redissolved in chloroform and precipitated in methanol several times to remove oligomers. The precipitate was then filtered and dried under vacuum dried to get the polymer as light-yellow powder in 65% yield. [.sup.1]H NMR (400 MHz, CD[C1.sub.3], [delta]): 8.29-7.63 (m, 22H, Ar and -CH=C-), 4.37 (t, 4H, -[OCH.sub.2]-). 1.94-1.26 (m, 40H, -[(C[H.sub.2]).sub.10]-, 0.89 (t, 6H). FTIR [square root of ([cm.sup.-1])]: 2918 and 2849 (C-H), 2215 (-CN), 1595, 1480, 1354, 1370,1280, 1173, 1044, 954, 820. Element. Anal. Calcd. For [C.sub.68][H.sub.72][N.sub.6][O.sub.4]S (%): C, 76.37; H, 6.79; N, 7.86; S. 2.99. Found (%): C. 76.42; H, 6.72; N, 7.84; S. 3.04.

Fabrication of the PLEDs

In the device fabrication, we used the simplest sandwich structure for the device configuration with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) coated indium tin oxide (ITO) glass as the anode, the spin coated polymer (PDTOFV or PDTOCN) as the emissive layer and aluminum as the cathode. To fabricate PLEDs of device configuration ITO/PEDOT:PS S/PDTOFV/A1 and ITO/PEDOT:PSS/PDTOCN/A1, first the ITO coated glass substrates with a sheet resistance of 20 [ohm]/square and ITO thickness of 120 nm were cleaned using deionized water, acetone, trichloroethylene and isopropyl alcohol sequentially for about 20 min each using an ultrasonic bath and dried in vacuum oven. Then ITO surface was treated with oxygen plasma for about 5 min to increase its work function. Then, a hole injection layer of PEDOT:PSS was spin coated on the cleaned and patterned ITO substrates at a speed of 4000 rpm with about 50-60 nm in thickness and was dried by baking at 120[degrees]C in vacuum for ~l h. Then, the emitting layer (PDTOFV or PDTOCN) was spin cast onto the PEDOT:PSS layer at a speed of 2000 rpm from chloroben-zene solution (10 mg [mL.sup.-1]) through a 0.45 [mciro]m Teflon filter, followed by vacuum annealing at 150[degrees]C for ~2 h in order to remove the organic fraction. Finally, the coated ITO was transferred to a deposition chamber, where a layer of aluminum (Al) electrode was vacuum deposited on the polymer layer with about 200 nm in thickness by thermal evaporation at a pressure of 1 x [10.sup.-6] Torr. Four pixels, each of active area 4 x 4 [mm.sup.2] defined per substrate were used to assess the reproducibility of the device performance. The complete fabricated devices were finally annealed at 100[degrees]C in vacuum for 5 min before being characterized. All the characterizations of the light emitting devices were carried out at room temperature. EL spectra of the devices at different bias voltage were investigated. The current as a function of applied voltage of the devices were also recorded.

RESULTS AND DISCUSSION

Synthesis and Characterization

The synthetic routes of the monomers and the polymers (PDTOFV and PDTOCN) are outlined in Scheme 1. The well-known Wittig reaction methodology was employed to synthesize D--A conjugated polymer PDTOFV containing 3,4-didodecyloxythiophene, fluorene and 1,3,4-oxadiazole units. Whereas, polymer PDTOCN carrying 3,4-didodecyloxythiophene, 1,3,4-oxadiazole and cyanophenylenevinylene units was synthesized using a Knoevenagel condensation method. The chemical structures of all intermediate compounds and the polymers were confirmed by [.sup.1]H NMR and FTIR spectroscopic methods and elemental analysis. Compound 3,4-diclodecy-loxythiophene-2,5-carbonyldihydrazide (1) was prepared according to the previously reported method (33). To prepare [N.sup.2], [N.sup.5]-di(4-bromobenzoy1)-3,4-bis(dodecyloxy)thio-phene-2, 5-dicarbohydrazide (2), the dihydrazide (1) was condensed with 4-bromobenzoyl chloride in NMP in the presence of pyridine at room temperature. The formation of compound 2 was confirmed by [.sup.1]H NMR and FTIR studies. The [.sup.1]H NMR spectrum of compound 2 exhibited two >NH protons as singlets at [delta] 10.22 and 9.39 ppm. The mutiplet peaks in the range [delta] 7.77-7.74 and [delta] 7.66-7.63 ppm are assigned to aromatic protons. Furthermore, its FTIR spectrum showed sharp peaks at 3288 and 1641 [cm.sup.-1] indicating the presence of >NH and >C=0 groups respectively. Cyclization of compound 2 to bisomidiazole (3) was achieved using PO[Cl.sub.3]. The [.sup.1]H NMR spectrum of bisoxadiazole (3) showed no singlet peaks due to >NH protons which confirms the completion of the cyclization reaction. In the FTIR spectrum of compound 3, absorption peaks corresponding to >NH and >C=O groups were absent, where as new peaks appeared at 1592 and 1064 [cm.sup.-1] for -C=N- and =C-O-C= groups, respectively, indicating the formation of a 1,3,4-oxadiazole ring. To prepare monomer MI, a Suzuki coupling reaction was performed between compound 3 and 4-formylphenylbor-onic acid in THF and ethanol (1:1 volume ratio) at 75[degrees]C for 10 h. The formation of required monomer (MI) was evidenced by [.sup.1]H NMR spectrum which showed a singlet peak at [delta] 10.13 ppm indicating the presence of aldehyde (-CHO) protons in the monomer. In addition, the FTIR spectrum exhibited a sharp peak at 1697 [cm.sup.-1] indicating the presence of >C=O groups, which further confirms the chemical structure of the required monomer (M1). The fluorene Wittig salt monomer (M2) was prepared starting from fluorene. The alkylation of fluorene was carried out using sodium hydride and n-hexyl bromide to afford compound 4. The compound 5 and monomer M2 were prepared according to the literature procedures (34). The elemental analysis data of all intermediate compounds and the monomers (Ml and M2) are in good agreement with their molecular formulae. To prepare PDTOFV, a Wittig reaction was performed in the presence or sodium ethoxide between monomer MI and fluorene Wittig salt (M2) in ethanol/CH[Cl.sub.3] solvent system under argon atmosphere at room temperature. Whereas PDTOCN was prepared under a similar reaction conditions by treating the monomer MI with 1,4-phenylenedia-cetonitrile in the presence of sodium ethoxide using a Knoevenagel condensation methodology. The [.sup.1]H NMR spectrum of PDTOFV displayed complex multiple peaks in the range [delta] 8.25-6.52 ppm corresponding to the aromatic and vinylic protons. A triplet peak at [delta] 4.36 ppm is due to -[OCH.sub.2]- protons of the alkoxy chains of the thiophene ring. The multiple peaks in the range [delta] 1.97-0.77 ppm are due to the protons of the alkyl chains attached to thiophene and fluorene rings. Similarly. [.sup.1]H NMR spectrum of PDTOCN exhibited characteristic peaks corresponding to aromatic, vinylic, alkoxy, and alkyl protons. Absence of absorption peak corresponding to aldehyde (-CHO) group in the FTIR spectra of polymers further evidenced the complete conversion of monomers to the required polymers. In addition, the elemental analyses results further evidenced the formation of new polymers; the percentage elements data of the polymers matched with their actual molecular formulae. The overall yields of the yellow-colored polymers were between 60% and 65%. The newly synthesized polymers showed good solubility in common organic solvents, such as CH[Cl.sub.3]. THF and chloro-benzene resulting from the alkoxy chains at 3- and 4-positions of the thiophene ring. The average molecular weights of the polymers were measured by GPC with reference to polystyrene standards. The number average molecular weight ([M.sub.n]) is 19,130 and 16,100 with polydispersity (PD) of 2.6 and 2.8, respectively, for PDTOFV and PDTOCN.

Optical Properties

UV-Vis absorption and fluorescence emission spectroscopic techniques were used to evaluate the optical properties of the polymers. The UV-Vis absorption spectra of PDTOFV and PDTOCN in chloroform solution (Ca. [10.sup.-4] mol) and in thin film state are as shown in Fig. 1. The absorption spectrum of PDTOFV in solution showed an absorption maximum at 388 nm which corresponds to the [pi]--[pi]* transition in the polymer. For PDTOCN in solution, the absorption maximum was observed at 360 nm. The observed difference in the absorption maxima of the polymers is due to the difference in the chemical structures of the polymer chains. However, the red shift of about 28 nm in the absorption maximum of PDTOFV in comparison to that of PDTOCN can be understood in terms of more planar backbone and the increased conjugation along the polymer backbone by the introduction of electron rich fluorenevinylene unit in PDTOFV. Smooth and optically clear polymer thin films on glass substrates were obtained by spin-coating the chloroform solution of the polymers (I mg [mL.sup.-1]) at a spin rate of 1500 rpm. The absorption maxima of polymers in the film state exhibited red shifts of about 10 and 26 nm, respectively, for PDTOFV and PDTOCN as compared to those of their chloroform solutions. This red shill in the film state could be attributed to the [pi]--[pi]* stacking effect. In general, redshill and broadening of the absorption spectra of polymer films results from the enhanced interchain interactions in the solid state and are also related to increased polarizability of the film (35). The onset absorption edge of the PDTOFV film is at 472 nm corresponding to an optical band gap of 2.63 eV. Whereas the onset absorption edge of PDTOCN film is at 502 nm and its band gap is 2.47 eV. The polymers showed low band gap when compared to those of poly(fluorenevinylenes) (PFV) and some fluorene-based conjugated polymers containing thiophene/oxadia-zole pendants (36-38). The obtained low band gap can be attributed to the strong intramolecular charge transfer (ICT) between electron donor segments like thiophene and/or flu-orene units and strong electron acceptor segments like 1,3,4-oxadiazole and/or cyano containing phenyleneviny-lene units in the polymers' backbone. Fluorescence emission spectra of the polymers in dilute chloroform solution (Ca. [10.sup.-4] mol) and in thin film state are as shown in Fig. 2. PDTOFV and PDTOCN in solution state emit intense blue light under the irradiation of UV light with emission maxima at 457 and 440 nm. respectively. Whereas polymers in the film state emit green light with a bathochromic shift of about 33 and 74 nm in the emission maxima, respectively for PDTOFV and PDTOCN as compared to those of their solution state. This shift can be attributed to the interchain or/and intrachain mobility of the excitons and excimers generated in the polymer solid state (39). The fluorescence quantum yields of PDTOFV and PDTOCN in chloroform solution are 38% and 42% respectively, which were calculated by comparing with the standard of quinine sulfate (ca. 1 x [10.sup.-5] M solution in 0.1 M sulfuric acid having a fluorescence quantum yield of 55%) (40). The optical properties of the polymers are summarized in Table 1.

TABLE 1. Comparison of photophysical data of PTDOFV and PDTOCN.

                              UV-Vis absorption

Polymer  [[lambda].sub.max]  [[lambda].sub.max]         [MATHEMATICAL
                   (a) (nm)            (b) (nm)        EXPRESSION NOT
                                                         REPRODUCIBLE
                                                       IN ASCII] (eV)

PTDOFV                  388                 398                  2.63

PTDOCN                  360                 386                  2.47

                                         Fluorescence
                                             emission

Polymer  [[lambda].sub.em] (a)  [[lambda].sub.em] (b)  [[PHI].sub.fl]
                          (nm)                   (nm)         (c) (%)

PTDOFV                     457                    490              38

PTDOCN                     440                    514              42

                      EL

Polymer   CIE (x, y) (d)

PTDOFV      (0.25, 0.39)

PTDOCN      (0.32, 0.35)

(a.) Measured in CH[Cl.sub.3].

(b.) Cast from CH[Cl.sub.3] solution.

(c.) Fluorescence quantum yield of polymers in CH
[Cl.sub.3] solution.

(d.) Estimaied from EL spectral data. [MATHEMATICAL
EXPRESSION NOT REPRODUCIBLE IN ASCII] Optical band
gap estimated from the edge of the absorption
spectra in the film state.


Electrochentical Properties

The electrochemical property of the polymers was investigated by cyclic voltammetric (CV) analysis. The CV analysis can be used to estimate the HOMO and LUMO energy levels of the conjugated polymers, because the onset oxidation and reduction potentials obtained from the cyclic voltammograms correspond to the HOMO and LUMO energy levels, respectively (41). The CV experiments of polymer thin film coated on glassy carbon (GC) disk electrode was performed using 0.1 M tetrabutylammonium perchlorate in acetonitrile as the supporting electrolyte at room temperature at a scan rate of 10 mV [s.sup.-1] platinum wire was used as the counter electrode and Ag/AgCl electrode was employed as the reference electrode. The performance of the CV instrument was calibrated using a ferrocene/ferrocenium (FOC) redox couple as an external standard before and after the analysis with [E.sub.FOC] = 0.53 V versus Ag/AgCl (42). Figure 3 shows the cyclic voltammograms of the polymer films. When swept cathodically, PDTOFV showed a reduction peak at -1.2 V with an onset reduction potential of -0.92V. In the anodic scan, an oxidation peak was observed at 1.9 V with an onset oxidation potential of 1.67 V. For PDTOCN, the oxidation peak was observed at 1.85 V with an onset oxidation potential of 1.59 V. While sweeping cathodically, PDTOCN showed a two-step reduction process with reduction peaks at -1.02 and -1.59 V. The two-step reduction process can be attributed to the n-doping of two different heteroaromatic rings. This is in contrast to PPV and PTs, where the two-step reduction process has rarely been observed [431. However, in conjugated heteroaromatic alternating polymers, such as poly(furanqunoxaline)s, and poly(phenylene-1,3,4-oxadiazole)s, a main peak with a shoulder in the cyclic voltammogram has been observed (44), (45). The obtained reduction potential values are lower than those of 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD), one of the most widely used electron-transporting materials, and are comparable with those of some good electron-transporting materials (46), (47). The onset potentials of oxidation and reduction processes were used to estimate the HOMO and the LUMO energy levels of the conjugated polymers according to the Eqs:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] are the onset potentials for the oxidation and reduction processes of a polymer respectively. Accordingly, the HOMO energy levels were estimated to be -5.94 and -5.86 eV and the LUMO energy levels were found to be -3.35 and -3.55 eV for PDTOFV and PDTOCN, respectively. The electrochemical band gaps were estimated to be 2.59 and 2.31 eV, respectively, for PDTOFV and PDTOCN. The electrochemical band gaps of the polymers are almost similar to their optical band gaps estimated from their onset absorption edge. The high electron affinity values could be attributed to the presence of electron withdrawing groups like oxadiazole and/or cyano containing phenylenevinylene units in the polymer backbone. Hence it can be expected that the polymers may show an increased electron injection ability when they are used as active materials in PLEDs. The presence of additional electron withdrawing cyano moieties in PDTOCN lowers its LUMO level as compared to that of PDTOFV. In addition, the cyano groups enhance the ICT along the polymer chain thus lowering the band gap energy of the polymer. Hence, the band gap of PDTOCN is lower than that of PDTOFV. The cyclic voltammetry data of the polymers are summarized in Table 2.

TABLE 2. Electrochemical data of PTDOFV and PDTOCN.

           [MATHEMATICAL    [MATHEMATICAL  [E.sub.HOMO]  [E.sub.LUMO]
          EXPRESSION NOT   EXPRESSION NOT
         REPRODUCIBLE IN  REPRODUCIBLE IN
                  ASCII]           ASCII]

Polymer              (V)              (V)          (eV)          (eV)

PTDOFV              1.67            -0.92         -5.94         -3.35

PRDOCN              1.59            -0.72         -5.86         -3.55

           [MATHEMATICAL
          EXPRESSION NOT
         REPRODUCIBLE IN
                  ASCII]

Polymer             (eV)

PTDOFV              2.59

PRDOCN              2.31

[E.sub.g.sup.Ec] Electrochemical band gap estimated
from the difference between [E.sub.HOMO] and
[E.sub.LUMO].


Themial Properties

The thermal property of the polymers was studied by TGA and was carried out under nitrogen atmosphere at a heating rate of 10[degrees]C [min.sup.-1]. The polymers showed good thermal stability under these conditions. As shown in Fig. 4, the onset decomposition temperatures ([T.sub.d]) of PDTOFV and PDTOCN are 330[degrees]C and 310[degrees]C, respectively. As the temperature was increased above [T.sub.d], the weight loss increased abruptly, indicating the decomposition of the polymer backbone.

Electroluminescent Properties

To evaluate the electroluminescent properties of the polymers. PLED devices were fabricated using the polymers as the emissive material, with a device configuration of ITO/PEDOT:PSS/polymer (PDTOFV or PDTOCN)/Al. The devices were fabricated under inert atmosphere. Figure 5 shows the EL spectra of PDTOFV at various driving voltages. The EL spectra show that with an increase in the applied voltage (10-1.6 V), the intensity of the emitted light also increases as a function of wavelength. The EL device based on PDTOFV emitted green light with EL maximum centered at 490 nm. The CIE coordinates under a driving voltage at 12 V was found to be (0.25, 0.39). The EL peak position and the CIE coordinates of the device were not changed significantly under different driving voltages. These results indicate the color stability of the device under different applied voltages. The EL spectra of PDTOCN under three different driving voltages (13-15 V) are as shown in Fig. 6. By applying different bias voltages, the intensity of the emitted light was found to be increasing as a function of wavelength. The device based on PDTOCN showed a broad EL spectrum. The EL maximum was observed at 514 nm under a driving voltage of 13 V. Furthermore, no change in the EL maximum was observed under the driving voltage of 14 V. but a slight red shill of about 6 nm was observed at 15 V. The observed red shift in the EL spectrum is likely due to more efficient [pi]-stacking and increased order at the internal operating temperatures of the LEDs. The PLEDs using PDTCN emitted white light with CIE coordinates of (0.32, 0.35) under a driving voltage of 14 V. These values are very close to the standard CIE coordinates for white color (0.33, 0.33). Figure 7 shows the CIE 1931 chromaticity coordinates (x, y) of the devices fabricated using PDTOFV and PDTOCN. The EL spectra of PDTOFV and PDTOCN devices are almost identical to the corresponding solid state photoluminescence (PL) spectra in terms of emission maxima values, indicating that the same excitation may be involved in both the processes. Figure 8 shows the current density-voltage characteristics of the PLED devices based on PDTOFV and PDTOCN. It was observed that, the current density of the polymer increases exponentially with the increasing forward bias voltage, which is typically a diode characteristic. Polymers PDTOFV and PDTOCN showed low threshold voltages of 7.3 and 3.9 V. respectively. The obtained lower threshold voltages can be attributed to the lower energy barrier for electron injection (due to low-lying LUMO level of the polymers) from the aluminum electrode. The lower threshold voltage for PDTOCN compared to that of PDTOFV could be understood by the results obtained in the electrochemical studies that PDTOCN possesses low-lying LUMO level than that of PDTCOFV. The low threshold voltage value can be comparable with some of the previously reported light-emitting polymers which showed good EL performance (48), (49). These preliminary EL results suggest that the present polymers are good candidates for light emitting diodes due to their good color stability under different bias voltages and also because of their low threshold voltage. However, for practical applicability of these materials, a detailed study on EL efficiency and luminance properties may be required. Furthermore, the performance of the device can be improved by optimizing the device Fabrication conditions and also by selecting a proper device structure such as multilayer device, protective encapsulation of the device etc.

CONCLUSIONS

We have synthesized two new thiophene and 1,3,4-oxadiazole based D--A conjugated polymers PDTOFV and PDTOCN. The polymer, PDTOFV, carrying fluorene units was synthesized using a Wittig reaction whereas a Knoe-venagel condensation reaction was used to synthesize polymer PDTOCN which contains cyanophenylenevinylene units. The resulting polymers showed excellent solubility in common organic solvents with good film forming property. They possessed good thermal stability up to ~300[degrees]C. The cyclic voltammetry studies revealed that the polymers possess low-lying HOMO and LUMO energy levels. The optical band gaps of the polymers were found to be 2.63 and 2.47 eV, respectively for PDTOFV and PDTOCN. The lower band gap of these polymers could be due to the D--A structure of the polymer backbone. Polymer light-emitting devices were fabricated using these polymers as emissive layer with a device configuration of ITO/PEDOT:PSS/polymer/Al. The emission maximum of the EL device based on PDTOFV centered at 490 nm which corresponds to green light, with CIE coordinate value of (0.25, 0.39) under a driving voltage of 12 V. Also, the device showed good color stability under different bias voltages. The EL spectra of the device based on PDTOCN showed white light emission with CIE coordinate value of (0.32. 0.35) at 14 V. which is very close to the standard CIE coordinates of (0.33, 0.33) for white light emission. The EL spectra of both the polymers are almost identical to their solid state PL spectra. In addition. PLEDs based on PDTOFV and PDTOCN showed lower threshold voltages due to low-lying LUMO levels. These preliminary studies on EL properties of PDTOFV and PDTOCN indeed demonstrate that the polymers are promising candidates as emissive materials that can be used to fabricate efficient organic light emitting diodes.

Correspondence to: U. Dalimba; e-mail: udayaravi80@gmail.com

DOI 10.1002/pen.23371

Published online in Wiley Online Libraty (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Maluvadi G Murali, (1) Udayakumar Dalimba, (1) Vandana Yadav, (2) Ritu Srivastava (2)

(1.) Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore-575025, India

(2.) Organic Light-Emitting Diode Lab, Polymeric and Soft Material Division, National Physical Laboratory, New Delhi-110012, India
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Author:Murali, Maluvadi G.; Dalimba, Udayakumar; Yadav, Vandana; Srivastava, Ritu
Publication:Polymer Engineering and Science
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Date:Jun 1, 2013
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