Copolymers Containing Various 1,3,4-Oxadiazole
Pendants
HSIAO-HSIEN SUNG, HONG-CHEU LIN
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, ROC
Received 1 November 2004; accepted 13 January 2005 DOI: 10.1002/pola.20741
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A series of soluble alternating poly(fluorene)-based copolymers containing electron-transporting 1,3,4-oxadiazole (OXD) and hole-transporting carbazole pend-ants attached to the C-9 position of fluorene units by long alkyl spacers were synthe-sized. These copolymers possess mesogenic and nonmesogenic pendants attached to a rigid mesogenic poly(fluorene) (PF) backbone. All these polymers exhibit glass-form-ing liquid crystalline properties, includglass-form-ing the nematic and smectic A (SmA) phases, and reveal much wider mesophasic temperature ranges than that of PF. The thermal properties and mesomorphism of these conjugated polymers are mainly affected by the nature of these pendants, and thus the mesophasic temperature ranges and glass-forming properties are greatly enhanced by introducing the OXD pendants. In addition, the tendencies of crystallization and aggregation of PF are also suppressed by introducing the OXD pendants. A single layer device with P4 as an emitter shows a turn-on voltage of 5 V and a bright luminescence of 2694 cd/m2 at 11 V with a
power efficiency of 1.28 cd/A at 100 mA/cm2. VVC 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2700–2711, 2005
Keywords: carbazole; glass-forming property; mesomorphism; oxadiazole (OXD) pendant; poly(fluorene)
INTRODUCTION
Poly(fluorene) (PF) is a well-known high mobi-lity hole transporting blue-emitting polymer for practical applications in polymer light-emitting diodes (PLEDs).1,2 This material displays extremely high photoluminescence (PL) efficien-cies in both solution and solid states.3,4 How-ever, PF has two major deficiencies as a poten-tial candidate for blue PLEDs. First, PF tends to aggregate and form excimers on heating dur-ing device formation or operation under forward biases, therefore, leading to blue-green emission
and fluorescence quenching.5,6 Second, there is an unbalance in charge injection and transpor-tation due to large injection barriers and differ-ent charge carrier mobilities.7To overcome these drawbacks, the physicochemical properties of PF are improved via side-chain substitution at the C-9 position of the fluorene units. When bulky groups are incorporated into the C-9 position of the fluorene units, it can not only provide large steric hindrance and reduce interchain interac-tion but also enhance good thermal and morpho-logical stability without substantially changing the electronic properties of the polymer back-bone.8,9In addition, the introduction of electron-deficient OXD groups into the C-9 position of the fluorene units will increase the electron affinity of resulting polymers and lead to a more Correspondence to: H.-C. Lin (E-mail: [email protected].
edu.tw)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 43, 2700–2711 (2005)
V
VC2005 Wiley Periodicals, Inc.
balanced charge injection and transportation as well as recombination.10,11
Linearly polarized blue organic light-emitting diodes have been demonstrated by taking advant-age of the thermotropic mesomorphism to be align-ed on a rubbing polyimide (PI) substrate.12–14 These liquid crystalline materials possess a potential for spontaneous uniaxial alignment of nematic mesomorphism by spin-coating on a rub-bing substrate and, therefore, they are enabled to generate polarized light emission. PF and mono-disperse oligofluorenes are among the most prom-ising candidates for efficient polarized blue lumi-nescence.15–21One of the best devices with mono-disperse oligofluorenes as an emitter displays polarized electroluminescence (EL) and the dichroic ratio is as high as 31.22It also was found that the side-chain length, pendant structure, and molecular aspect ratio affect the solid mor-phology, thermotropic properties, and phase tran-sition temperatures of the resulting polymers. Moreover, the absorption and photoluminescence dichroic ratios are influenced as well.23,24
Herein, we synthesize the first series of PF with electron-transporting mesogenic pendants in the C-9 position of the fluorene units by long alkyl spacers. This design is based on the con-sideration that the mesogenic groups with elec-tron-transporting capacity are introduced as pendants to the hole-transporting PF backbones. We hope these functionalized polymers can take advantage of introducing bulky mesogenic pend-ants to avoid the tendency of spontaneous aggre-gation and crystallization normally encountered with PF, and to enhance the charge carrier bal-ance as well as the mesophasic temperature range. On the other hand, we also introduce nonmesogenic groups to the PF backbones as pendants for comparative purposes. These intrinsic features help us to investigate the effect of mesogenic pendants on electronic, pho-tonic, and thermotropic properties of the result-ing polymers.
EXPERIMENTAL
Measurements 1
H NMR spectra were recorded on a Varian unity 300M Hz spectrometer using CDCl3 sol-vent. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Transition temperatures were determined by dif-ferential scanning calorimetry (Perkin–Elmer
Diamond) with a heating and cooling rate of 10 8C/min. The mesophases were studied using a polarizing optical microscope (Leica DMLP) equipped with a hot stage. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyzer under a heating rate of 20 8C/min. Gel permeation chromatogra-phy (GPC) analysis was conducted on a Water 1515 separation module using polystyrene as a standard and THF as an eluant. UV-visible absorption spectra were recorded in dilute chloroform solutions (105 M) on a HP G1103A spectrophotometer, and fluorescence spectra were obtained on a Hitachi F-4500 spectropho-tometer. Polymer thin films were spin-coated on a quartz substrate from chloroform solutions with a concentration of 10 mg/mL (this condition consists with that of the electroluminescent device). Fluorescence quantum yields were determined by comparing the integrated PL density of a reference 9,10-diphenylanthracene in toluene with a known quantum yield (ca. 5 106 M, quantum yield ¼ 1.0). Cyclic voltam-metry (CV) was performed at a scanning rate of 100 mV/s on a BAS 100 B/W electrochemical analyzer, which was equipped with a three-trode cell. Pt wire was used as a counter trode, and Ag/AgCl was used as a reference elec-trode in the CV measurement. The polymer thin film was cast onto a Pt disc as a working elec-trode with ferrocene as a standard in acetoni-trile, and 0.1 M tetrabutylammonium hexafluoro-phosphate (TBAPF6) was used as a supporting electrolyte. A series of double-layer EL devices with the configuration of ITO/PEDOT:PSS/Poly-mer/Ca/Al were made by spin-coating the poly-mer solutions (with a concentration of 10 mg/mL) onto PEDOT coated glass substrates. The thick-nesses of these films were measured on a Sloan DektakIIA surface profilometer. The thickness of PEDOT was about 40 nm, and the thicknesses of these polymers for P1 and P2-P4 were about 60 nm and 80 nm, respectively. The luminance-current-voltage characteristics were recorded on a power source (Keithley 2400) and photometer (MINOLTA CS-100A). To align the LC state into a monodomain, polymers were spin-coated from a 1 wt % solution in chloroform at 2500 rpm onto a glass substrate coated with a polyimide alignment layer. After vacuum-drying, pristine films were thermally annealed at corresponding mesomophic temperature ranges for 30–60 min, then cooled to room temperature.
Materials
Chemicals and solvents were reagent grades and purchased from Aldrich, ARCROS, TCI, and Lancaster Chemical Co. Dichloromathane and THF were distilled to keep anhydrous before use. The other chemicals were used without fur-ther purification. Compounds 2-7 and fluorine derivative monomer were synthesized by modi-fied procedures (shown in Scheme 1) that have been described in refs 25–30.
M1
2.25 g (4.77 mmol) of 7a was added to a solution of 2,7-dibromofluorene (0.72 g, 2.22 mmol) in THF at 0 8C. The solution was heated to 60 8C and stirred for 3 h, then cooled to room tempera-ture. After evaporation of the solvent, the resi-due was purified by chromatograph. Yield: 76%. 1 H NMR (ppm, CDCl3): 0.58 (br, 4H), 1.07–1.45 (m, 24H), 1.73–1.80 (m, 4H), 1.83–1.94 (m, 4H), 2.44 (s, 6H), 4.00 (t, 4H), 6.98 (d, J ¼ 9 Hz, 4H), 7.31 (d, J ¼ 8.7 Hz, 4H), 7.44–7.46 (m, 4H), 7.50–7.54 (m, 2H), 8.00 (d, J ¼ 8.1 Hz, 4H), 8.03
(d, J ¼ 9 Hz, 4H). Anal. Calcd for
C63H68Br2N4O4: C, 68.47; H, 6.20; N, 5.07. Found: C, 68.38; H, 6.28; N, 4.80. M2 Yield: 82%. 1H NMR (ppm, CDCl3): 0.58 (br, 4H), 1.06–1.48 (m, 24H), 1.80–1.90 (m, 4H), 1.91–1.94 (m, 4H), 2.45 (s, 6H), 4.07 (t, 4H), 7.15 (s, 2H), 7.19 (d, J ¼ 8.7 Hz, 2H), 7.33 (d, J ¼ 8.4 Hz, 4H), 7.43–7.46 (m, 4H), 7.50–7.53 (m, 2H), 7.81 (d, J ¼ 8.4 Hz, 2H), 7.83 (d, J ¼ 8.7 Hz, 2H), 8.05 (d, J ¼ 8.4 Hz, 4H), 8.12 (d, J ¼ 8.7 Hz, 4H), 8.51 (s, 1H). Anal. Calcd for C71H72Br2N4O4: C, 70.76; H, 6.02; N, 4.65. Found: C, 70.70; H, 6.18; N, 4.60. M3 Yield: 73%. 1H NMR (ppm, CDCl3): 0.58 (br, 4H), 1.06–1.58 (m, 24H), 1.75–1.80 (m, 4H), 1.89–1.94 (m, 4H), 2.45 (s, 6H), 3.99 (t, 4H), 6.97 (d, J ¼ 8.4 Hz, 4H), 7.33 (d, J ¼ 8.4 Hz, 4H), 7.44-7.47 (m, 4H), 7.51–7.54 (m, 2H), 7.57 (d, J ¼ 8.4 Hz, 4H), 7.70 (d, J ¼ 8.4 Hz, 4H), 8.03 (d, J ¼ 8.1 Hz, 4H), 8.16 (d, J ¼ 8.1 Hz, 4H). Anal. Calcd for C75H76Br2N4O4: C, 71.65; H, 6.09; N, 4.46. Found: C, 71.31; H, 6.27; N, 4.49. M4 Yield: 89%. 1H NMR (ppm, CDCl3): 0.51 (br, 4H), 1.07–1.26 (m, 8H), 1.68–1.70 (m, 4H), 1.77– 1.82 (m, 4H), 4.17 (t, 4H), 7.19 (t, 4H), 7.28–7.35 (m, 6H), 7.40–7.48 (m, 8H), 8.05 (d, J ¼ 7.8 Hz, 4H). Anal. Calcd for C49H46Br2N2: C, 71.53; H, 5.64; N, 3.41. Found: C, 71.77; H, 5.98; N, 3.43.
Polymerization
The synthetic route of polymers is shown in Scheme 2. A general procedure of polymeriza-tion is proceeded through the Suzuki coupling reaction. For polymers P1–P3, a mixture of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexyllfluorene (1 equiv), dibromo com-pound (1 equiv), and tetrakis(triphenylphos-phine) palladium (1.0 mol %) were added in a degassed mixture of toluene ([monomer] ¼ 0.2M) and aqueous 2M potassium carbonate (3:2 in volume). The mixture was vigorously stirred at 87 8C for 72 h. After the mixture was cooled to room temperature, it was poured into 200 mL of methanol. A fibrous solid was obtained by filtration. The solid was washed sequentially with methanol, water, and metha-nol. A similar procedure is carried out for the synthesis of P4, and the feed ratio of 2,7- bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene/M1/M4 is 2/1/1. The actual m/n ratio of the resulting polymer (P4) is about 1:1, which is calculated from proton NMR.
P1
Yield: 53%. 1H NMR (ppm, CDCl3): 0.77–1.38 (m, 50H), 1.70–1.76 (m, 4H), 2.11 (br, 8H), 2.42 (s, 6H), 3.96 (t, 4H), 6.95 (d, J ¼ 8.7 Hz, 4H), 7.29 (d, J ¼ 8.1 Hz, 4H), 7.68–7.85 (m, 12H), 7.98–8.04 (m, 8H). Anal. Calcd for C88H100N4O4: C, 82.72; H, 7.89; N, 4.38. Found: C, 82.72; H, 7.89; N, 4.05. P2 Yield: 61%. 1H NMR (ppm, CDCl3): 0.77–1.41 (m, 50H), 1.75–1.80 (m, 4H), 2.12 (br, 8H), 2.42 (s, 6H), 4.00 (t, 4H), 7.09–7.21 (m, 4H), 7.30– 7.35 (m, 4H), 7.68–7.85 (m, 16H), 8.02–8.15 (m, 6H), 8.48 (s, 2H). Anal. Calcd for C96H104N4O4: C, 83.68; H, 7.61; N, 4.07. Found: C, 82.93; H, 7.60; N, 3.82.
P3 Yield: 79%. 1H NMR (ppm, CDCl3): 0.77–1.38 (m, 50H), 1.67–1.73 (m, 4H), 2.13 (br, 8H), 2.42 (s, 6H), 3.93 (t, 4H), 6.93 (d, J ¼ 8.4 Hz, 4H), 7.30 (d, J ¼ 7.8 Hz, 4H), 7.53–7.82 (m, 20H), 8.00 (d, J ¼ 7.8 Hz, 4H), 8.12 (d, J ¼ 8.4 Hz, 4H). Anal. Calcd for C100H108N4O4: C, 83.99; H, 7.61; N, 3.92. Found: C, 83.61; H, 7.61; N, 3.67. P4 Yield: 82%. 1H NMR (ppm, CDCl3): 0.65–1.40 (m, 84H), 1.78 (br, 8H), 2.11 (br, 16H), 3.98 (t, 4H), 4.18 (br, 4H), 6.94 (d, J ¼ 9 Hz, 4H), 7.15 (t, 4H), 7.25–7.36 (m, 12H), 7.60–7.88 (m, 24H), 7.96–8.05 (m, 12H).
RESULTS AND DISCUSSION
Synthesis and Characterization
Our previous study reveals that 2-(6-alkoxy-naphthalen-2-yl)-5-phenyl-1,3,4 oxadiazoles con-taining various substituents at the phenyl 4-position (n-NPO-X) exhibit stable mesogenic properties as shown in Structure 1,
where n ¼ 6, 8, and 10, and X ¼ Me, OMe, F, Cl, CN, and NO25
2 , including the nematic and
SmA phases.25 These LC materials incorporated with strong polar electron-withdrawing terminal groups (F, Cl, CN, and NO2) exhibit the SmA phase and tend to form a highly ordered smectic E (SmE) phase. Whereas, when electron-donat-ing methyl and methoxy groups were served as terminal moieties, the mesophase of these LC materials do not show any SmE phase. Although the methoxy-substituted material (n-NPO-OMe) exhibits wider mesophasic temperature range than that of methyl-substituted materials (n-NPO-Me), choosing the methoxy group as a ter-minal group of the mesogenic pendant in this research will encounter a synthetic problem by converting a methoxy group into a hydroxy group (synthetic step 5 of Scheme 1). Therefore, the methyl group becomes our best choice of the ter-minal group. Besides, 10-NPO-Me possesses monotropic nematic and SmA phases along with the widest mesophasic temperature range (I 107 N 93 SmA 85 K) among these n-NPO-Me. Con-cluding the aforementioned views, we can formu-late the optimal structure (Na-OXD, 7b) that pos-sesses decyloxy side-chain spacers and methyl terminal groups as electron-transporting meso-genic pendants, which are attached to the PF backbones. Comparatively, we also introduce non-mesogenic pendants (Be-OXD, 7a, and carbazole, 7d) and mesogenic pendants (Bi-OXD, 7c, with wider mesophasic temperature range, i.e. K 134 SmA 187 N 195 I) to the PF backbones. In
tion, polymers containing OXD pendants can pro-vide large steric hindrance and reduce aggrega-tion, which also have been reported.10,31 To our best knowledge, the first series of PF copolymers containing PF backbones and electron-transport-ing mesogenic pendants are reported in this research.
The average molecular weights of these PF copolymers obtained by GPC are given in Table 1. The number-average-molecular weights (Mn) of polymers are between 10,200 and 55,700 g/mol, and the polydispersity indices (PDI) are between 1.8 and 2.6. The thermal stability of the poly-mers in nitrogen evaluated by thermogravi-metric analysis (TGA) indicates that the degra-dation temperatures (Td) of 5% weight loss in nitrogen are larger than 400 8C for all polymers (shown in Table 1). The mesomorphism charac-terized by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) thermograms are displayed in Figure 1. To avoid thermal decomposition, these polymers were heated to 280 8C, then subsequently cooled to 40 8C at a heating and cooling rate of 10 8C/ min, whereas their melting temperatures were not observed even up to 2808C. The DSC results show the glass transition temperatures of these polymers are between 60 and 101 8C (demon-strated in Table 1). All these polymers are found to display the nematic and SmA phases, and reveal isotropization temperatures above 340 8C (except P1) where thermal decomposition occurred.
Poly(9,9-dioctylfluorene) (PFO) and chiral PF containing 2S-methylbutyl side groups have been reported to exhibit mesomorphism but with a strong tendency to crystallize on both heating and cooling processes.23 Herein, we introduce electron-transporting Be-OXD (7a) groups, which
do not exhibit mesogenic properties, to PF back-bones to serve as pendants of the resulting poly-mer (P1). As revealed by DSC diagram (shown in Fig. 1), P1 is an amorphous polymer with Tg ¼ 60 8C and shows a stable glass-forming nem-atic phase corroborated by polarizing optical microscopy. P2 contains Na-OXD (7b) pendants, which is a monotropic mesogenic material with nematic and SmA phases, shows Tg ¼ 80 8C, and only exhibits a nematic phase without any SmA phase. On the other hand, P3 possesses Bi-OXD (7c) groups as pendants, and Bi-OXD has wider mesophasic temperature range (K 134 SmA 187 N 195 I) than that of Na-OXD. The resulting polymer displays nematic and SmA phases similar to their pendants with Tg ¼ 86 8C. In comparison with P2, P3 shows two mesophases including the nematic and SmA phases. The divergence may result from the wide SmA temperature range (53 8C) of Bi-OXD pendants, which induces the resulting polymer to exhibit the SmA phase. In contrast to Bi-OXD, Na-OXD only has SmA temperature range of 8 8C and consequently cannot effec-tively induce P2 to generate the SmA phase. Comparing P1 and P2-P3, we can find that although Be-OXD (7a) does not exhibit a meso-genic property, the large steric groups can still suppress the tendency of crystallinity and enhance the mesophasic temperature range. On the hand, P4 (which is a random copolymer com-prising carbazole and Be-OXD groups) is also an amorphous polymer with Tg¼ 83 8C, and exhib-its a nematic phase without any propensity to crystallize on heating or cooling. From these results, we can draw two conclusions. First, the
Table 1. Molecular Weights and Thermal Properties of Polymers P1–P4 Polymer Yield (%) Mn Mw/Mn Tda (8C) (8C)Tg (8C)Tcb P1 53 10200 2.2 406 60 > 300 P2 61 27000 2.1 414 80 > 340 P3 79 55700 2.6 438 86 > 340 P4 82 45600 1.8 438 83 > 340 a
Temperature of 5% weight loss measured by TGA in nitrogen.
b
Temperature measured by polarizing optical microscopy (POM).
Figure 1. DSC thermograms of polymers during the second heating scan at 10 8C/min. Symbols: G, glass; N, nematic; K, crystalline.
mesomorphism of these conjugated polymers are greatly affected by the nature of the pendants. Second, the mesophasic temperature range will be hugely promoted, compared with the meso-phasic temperature range of PFO ( 100 8C), as well as the tendency of spontaneous crystalliza-tion of PF can be suppressed by the introduccrystalliza-tion of the OXD pendant groups.
Optical Properties
The photophysical properties of the pendants (7a–7c) and polymers are summarized in Tables 2 and 3 and Figures 2–5 serially. It is noticed that all polymers exhibit two absorption bands (Fig. 2). The shorter absorption range originates from the absorption of the pendants; the longer wavelength peak in the region of 360–400 nm is attributed to the electron transition of the conju-gated PF backbones. The optical band gaps of the polymers from the edge absorption of the polymer solutions are all equivalent (2.95 eV) regardless of various pendants; the value is exactly the same with the optical band gap of PFO reported by Janietz.32 The identical band
gaps illustrate that the different pendant groups do not appreciably change the excited state of the polymers, which implies similar distribution of effective conjugation lengths in the solution state of these polymers.
The PL emission spectra in dilute solutions as well as in thin films are shown in Figure 3, where these polymers exhibit stable and strong purple-blue emissions from the PF backbones. Emissions from the OXD pendants are not observed, even when polymers are excited at the absorption peaks of the pendants. This indicates that the existence of efficient energy transfer from the pendants to the polymer backbones has occurred. Thin films and dilute solutions show nearly identical absorption values and shapes, indicating an absence of ground-state aggrega-tion in the solid state. However, thin films show a redshift ca. 10 nm in PL peaks in contrast to dilute solutions, which suggests that light emis-sion from a more planar structure has occurred in the solid state. The PL quantum yields are
Table 2. Absorption and PL Emission Spectral Data of Polymers in Chloroform Solutions and Thin Solid Films
Polymer labs,sol (nm) labs,film (nm) Optical Band Gap (eV)
lPL,sol (nm) lPL,film (nm) FPLa FPLb P1 300, 388 302, 389 2.95 417 427 1.06 0.80c P2 325, 389 334, 394 2.95 417 428 1.07 0.89d P3 320, 391 321, 395 2.95 417 428 1.09 0.95d P4 296, 392 299, 395 2.95 417 428 1.04 —
aExcited at 370 nm (the absorption region of polymer backbones). b
Excited at absorption regions of pendant groups.
cExcited at the maximum absorbance of pendant groups.
dExcited at the absorption intersection of Na-OXD and Bi-OXD pendant groups.
Table 3. Absorption and PL Emission Spectral Data of Pendants in Chloroform Pendant Group labsa (nm) lPL,solb (nm) Rel. FPL Be-OXD (7a) 300 354 1c Na-OXD (7b) 323 374 1.59d Bi-OXD (7c) 319 386 1.75d
aWavelength of the maximum absorbance. bWavelength of the maximum emission.
cExcited at 300 nm and the PL quantum yield sets as 1. dExcited at the absorption intersection of Na-OXD and
almost identical (Table 2) when these polymers are excited at the absorption region of the poly-mer backbone (370 nm). However, when these polymers are excited at the absorption region of pendants, the PL quantum yields (Table 2) will increase in the order of P1< P2 < P3 according to the order of the quantum yield in each pend-ant (Table 3).
Owing to the glass-forming LC property of these polymers, they are suitable to generate polarized emissions. The anisotropic films are thermally annealed in the nematic temperature ranges on rubbing PI substrates for 30–60 min to align these polymers. As depicted in Figure 4, the shape of the annealed PL emissions is differ-ent from the thin film PL emissions, which indi-cates a more planar structure or aggregation formation in the annealed thin films. The PL dichroic ratios are 1.6 2.8 at 430 nm and 2.2 4.2 at 457 nm (Table 4), and these values
are relatively lower than that of PFO. It is noted that thermally treated films in LC state show remarkable anisotropic properties under polariz-ing optical microscope (POM). Unfortunately, the annealed films result in a weakly anisotropic light while the aligned films are cold from LC states. This result of fast relaxation in align-ment may be due to a complex alignalign-ment mor-phology originated from the competition of the polymer backbone and the pendants under ther-mal alignment. Conversely, while P3 is annealed in the temperature range of the SmA phase, the PL spectrum of annealed film shows a red-shifted emission peak at ca. 544 nm (Fig. 5a). To identify the red-shifted emission peak that results from the aggregation (excimer formation) or the crystal packing (crystalline state) with different degrees of intermolecular interactions, P3 is sealed in an aligned LC cell with a thick-ness of 9 mm. The PL emission dichroic ratio is measured at 210 8C (which corresponds to the
Figure 4. Polarized PL emission spectra of P3 annealed at the temperature range of the nematic phase. The spectra were measured as the polarized light parallel (dash line) and perpendicular (solid line) to the rubbing direction.
Table 4. PL Emission Dichroic Ratios of
Thermally Annealed Spin-Coated Films at Different Emission Peaks Dichroic Ratio P1 P2 P3 P4 R430a 2.7 1.6 2.8 1.9 R457 b 3.8 2.4 4.2 2.9
aPL emission dichroic ratio measured at emission peak
of 430 nm.
b
PL emission dichroic ratio measured at emission peak of 457 nm.
Figure 3. PL spectra of polymers: (a) in dilute chloroform solutions; (b) in films.
temperature range of the SmA phase) with dif-ferent annealing time. As revealed in Figure 5b, the red-shifted peak becomes a dominated emis-sion peak after 5 min thermal treatment and then becomes saturated after ca. 30 min treat-ment. From this result, we preferably assign the red-shifted emission band (ca. 544 nm) as an emission of the aggregation rather than the crystal packing. The dense packing of the smec-tic phase may be responsible for the formation of the aggregation. The data of PL dichroic ratios are summarized in Table 5; the emission dichroic ratios from polymer backbones and exci-mers are about 2.4 and 3.0, respectively, and these dichroic ratios are almost regardless of the annealing time.
Electrochemical Properties
The redox behavior of polymer thin films were investigated by CV, and the potentials were esti-mated by the reference energy level of ferrocene (4.8 eV below the vacuum level) according to the following equation:32
EHOMO
/ELUMO ¼ [(Eonset – 0.45) 4.8] eV. The onset potentials were determined from the intersection of two
tan-Figure 6. Cyclic voltammetry of polymers during (a) the oxidation process and (b) the reduction process. Figure 5. (a) Polarized PL emission spectra of P3
annealed at the temperature range of the SmA phase. The spectra were measured as the polarized light par-allel (dash line) and perpendicular (solid line) to the rubbing direction. (b) Polarized PL emission spectra of P3 measured at 210 8C (the temperature range of the SmA phase) with different annealing time.
Table 5. PL Emission Dichroic Ratios of P3 with Different Annealing Time at the Temperature Range of the SmA Phase
Dichroic Ratio 1 min 5 min 10 min 30 min R446
a
2.2 2.6 2.4 2.3
R544b 3.0 2.9 2.9 3.0
aPL emission dichroic ratio measured at emission peak of
446 nm.
bPL emission dichroic ratio measured at emission peak of
gents drawn at the rising and background cur-rents of the CV measurements. As shown in Fig-ure 6, these polymers possess two anodic peaks, which can be attributed to the oxidation of both pendants and polymer backbones. The OXD sub-stituted polymers (P1–P3) show the onset poten-tials of oxidation between 1.46 and 1.52 V (Table 6) in the anodic scans. The onset potentials are similar to that reported value of PFO (1.4 V),32 and thus the onset potentials are due to the oxi-dation of the PF backbones. On the contrary, P4 has lower onset potentials of oxidation (ca. 1.31 V), and the lower value is attributed to the intro-duction of cabazole groups.33 In the cathodic scans, the onset potentials of reduction are remarkably promoted from 2.28 V of PFO to ca 1.9 V of these polymers by introducing OXD groups. The electrochemical results indicate that the incorporation of the OXD and cabazole groups into PF as pendants will efficiently reduce the LUMO energy levels and promote the HOMO energy levels, thus reducing the energy barrier of charge injection from electrodes to emitters. Electroluminescent (EL) Properties of PLED Devices The EL data are summarized in Table 7. The current-voltage and luminescence-voltage char-acteristics are displayed in Figure 7. All these devices show similar turn-on voltages for
cur-rent and light at ca. 5 V. These similar turn-on voltages for current and light illustrate that a matched balance of injection and transportation in charges is achieved. The device with P1 as an emitter has the highest luminance of 2104 cd/m2 at 10 V (with a power efficiency of 1.13 cd/A at 100 mA/cm2) among these OXD substituted mers (P1–P3). The devices based on these poly-mers (P1–P3) demonstrate much higher maxi-mum brightnesses than the device made of poly(9,9-dihexylfluorene),34 which has a maxi-mum brightness of 717 cd/m2. In addition, the devices based on these polymers (P1–P3) also exhibit much better EL performance than the devices made of poly[9,9-bis(20 -ethylhexyl)fluor-ene-2,7-diyl] (PBEHF)35 and poly(9,9-dioctyl-fluorene) (POF).36 From these results, we can conclude that the incorporation of electron-transporting moieties (OXD) into hole-transport-ing PF backbones will improve the performance of the EL device. The device performance can be further improved by the incorporation of caba-zole pendants into OXD-substituted PF (such as P4); a bright luminance of 2694 cd/m2 at 11 V (with a power efficiency of 1.28 cd/A at 100 mA/ cm2) can be reached.
It was reported that EL emissions of PF suf-fer from the excimer formation on the current flow due to its liquid crystalline behavior,37,38
Table 6. HOMO and LUMO Energies, and Electrochemical Properties of Polymers (P1–P4)
Polymer Ered/onset (V) Ered/peak (V) Eox/onset (V) Eox/peak (V) EHOMO (eV) ELUMO (eV) Band Gap (eV) P1 1.91 2.40, 2.59 1.46 1.57, 1.65 5.81 2.44 3.37 P2 1.94 2.42, 2.63 1.52 1.69, 1.87 5.87 2.41 3.46 P3 1.86 2.38, 2.67 1.47 1.56, 1.87 5.82 2.49 3.33 P4 1.90 2.36, 2.50 1.31 1.47, 2.16 5.66 2.45 3.21
Table 7. PLED DevicesaPerformance Data
Polymer lmax (nm) Voltageb(V) Brightnessb (cd/m2) Power Effiencyb (cd/A) Luminance Efficiencyb (lm/W) Max. Brightness (cd/m2) (V) P1 424 7.8 1134 1.13 0.46 2104 (10) P2 422 7.2 802 0.80 0.35 1672 (11) P3 424 7.6 1070 1.07 0.44 2036 (11) P4 422 7.1 1276 1.28 0.56 2694 (11)
aDevice structure: ITO/PEDOT:PSS/Polymer/Ca/Al. bMeasured at 100 mA/cm2.
and result in an additional 100 nm bathochro-mic shifted emission at a lower energy band. As shown in Figure 8a, EL emission spectra of these polymers are almost the same at 8 V and exhibit stable EL emissions under forward biases. Figure 8b shows that the EL spectra of P3 are almost unchanged and without any low energy emission band as voltages increase from 6 V to 10 V. These EL spectra well match with the corresponding PL emission spectra of thin films, which indicates that the similar radia-tively excited states are involved in both EL and PL processes. Consequently, we can successfully suppress the excimer formation in the PF back-bones by introducing OXD groups with long flex-ible side chains, even though these polymers possess unfavorable liquid crystalline behavior for PLEDs devices.
CONCLUSIONS
The thermal properties and mesomorphism of these glass-forming polymers are greatly affected by the nature of the pendants; these polymers exhibit stable mesogenic properties including the nematic and SmA phases. The incorporation of electron-transporting OXD pendants into PF backbones via long alkoxy spacers will effectively promote the mesophasic temperature range and depress the tendency of spontaneous crystallization of the PF backbones. For device performance, the introduction of charge-transporting pendants can not only tune
the HOMO and LUMO energy levels but also suppress the formation of aggregation.
The authors gratefully acknowledge financial support from the National Science Council of Taiwan (ROC) through NSC 92-2113M-009-016.
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