Synthesis and Characterization of Kinked and Hyperbranched
Carbazole/Fluorene-Based Copolymers
Chung-Wen Wu and Hong-Cheu Lin*
Department of Materials Science and Engineering, National Chiao Tung UniVersity, Hsinchu, Taiwan (ROC)
ReceiVed August 4, 2006; ReVised Manuscript ReceiVed August 25, 2006
ABSTRACT: To decrease the aggregation phenomena and improve the luminescent properties of polyfluorenes (PFs), a series of novel kinked and hyperbranched carbazole units were copolymerized by the Suzuki coupling polycondensation reaction to introduce disorder packing into the copolymer backbones. The thermal, photophysical, electrochemical, and electroluminescent properties of the copolymers were investigated. All of these polymers possess excellent thermal stability with onset decomposition temperatures at 407-466°C and glass transition temperatures at 109-140°C. Photoluminescent (PL) studies showed that these polymers were promising blue light-emitting materials, which exhibited high quantum efficiencies in solution and solid states. Long wavelength emissions at 500-600 nm, which were typical for PFs due to their self-aggregation in the solid state, were suppressed in these polymers. Annealing studies under air showed both improved thermal and photoluminescence stability of the polymers. It was proven that HOMO energy levels of the copolymers can be enlarged by increasing the carbazole content in the electrochemical measurements; hence, the hole injection was greatly enhanced. Pure-blue electroluminescence (EL) spectra with narrow fwhm (full width at the half-maximum) values (39-42 nm) and negligible low-energy excimer emission bands were successfully achieved, indicating that these copolymers could be good candidates for blue light-emitting materials.
Introduction
Over the past few years, polyfluorene (PF) derivatives have emerged as a promising class of blue-light-emitting conjugated polymers for use in polymer-based emissive displays because of high photoluminescent (PL) and electroluminescent (EL) yields as well as easy tunability of physical properties through chemical structure modification and copolymerization.1Some thorny problems associated with PFs are their tendencies of forming aggregates, excimers, and ketone defects during either annealing or passage of current processes, which lead to red-shifted and less-efficient emissions and reduced color purity.2 Many studies have been carried out to solve the aggregation-related problems by increasing the structural hindrances of PFs and thus to reduce their self-aggregation tendency in the solid state. For example, studies of PFs containing long (or bulky) and branched side chains,3copolymerization techniques,4 den-drimer attachment,5 end-capping of PFs with bulky groups,6 cross-linking of PFs,7 and oligomer approaches4,8 have been reported.
On the other hand, another problem of PFs is their large band gaps between the LUMO and HOMO energy levels.9The high energy barrier between the emissive layer and the electrodes as well as the imbalance between the hole- and electron-transport-ing properties in the emissive layer are also possible causes of poor performance of a light-emitting diode (LED) device. In this regard, it is well recognized that the charge injection and transport can be facilitated by sandwiching the emissive layer between a hole-injecting/-transporting layer (HTL) above the anode and an electron-injecting/-transporting layer (ETL) under the cathode. An alternative approach addressing these issues is to vary the chemical structures of PFs. For example, the incorporation of electron-withdrawing or electron-donating groups into the PF main chains or side chains can influence the
electron- or hole-injecting/-transporting capabilities of the polymers.10It is well-known that carbazole-based molecules and polymers are good hole-transporting materials due to the electron-donating capabilities associated with the nitrogen in the carbazole.11 Several studies of random and alternating fluorene/carbazole copolymers have been designed and synthe-sized to be used as a light-emitting layer in blue light-emitting diodes.12 It is demonstrated that the incorporation of 3,6-carbazole units into the fluorene polymer chains will effectively raise the HOMO energy levels and improve the luminescent stability by interfering with the packing order to suppress the aggregation and excimer formation. However, the maxima of UV-vis absorption and PL emission were significantly blue-shifted compared with PF homopolymers, indicating that by copolymerization of PFs with 3,6-linked carbazole units, the effective conjugation lengths of the copolymers have been reduced to some extent. In general, these copolymers showed decreased PL efficiencies compared with PF homopolymers in previous research.
In this study, a series of carbazole/fluorene-based copolymers (P1-P6) were prepared by the Suzuki polycondensation reac-tion, and the influence of the functionalized carbazole units on thermal, photophysical, electrochemical, and electroluminescent properties of the resulting polymers were investigated. The noticeable difference between our investigation and those of previous research is that the properties of polyfluorenes were improved by introducing minimal carbazole derivatives (2-4) into poly(2,7-9,9-dihexylfluorene)s to avoid breaking the delocalization of π-electrons seriously along the polymer backbones. To suppress the aggregation phenomena and reduce the excimer formation, “kinked” and “hyperbranched” disor-dered structures were introduced into the conjugated polyfluo-rene backbones. Scheme 1 shows the chemical structures and the synthetic procedures of multifunctional carbazole derivatives (2-4). The introduction of hole-transporting dibromocarbazole (2) groups into the PF chains can suppress the chain aggregation * Corresponding author. E-mail: [email protected]. Telephone:
8863-5712121 ext. 55305. Fax: 8863-5724727.
10.1021/ma061770j CCC: $33.50 © 2006 American Chemical Society
by the formation of kinked linkages and improve the mismatch of energy levels between the anode and PF. In addition, the hyperbranched approach has proven to be an effective method for the design of amorphous nonaggregation in optoelectronic materials.13,14It has been well established that the solubility was poor in common organic solvents for hyperbranched polymers with higher contents of branched units by step-growth polymerization. Therefore, a small amount of tribromocarbazole (3) or tetrabromocarbazole (4) units are suitable as branched units to make the PF polymers with hyperbranched architecture. Thus, PF copolymers with kinked and hyperbranched structures derived from a series of carbazole-based compounds are expected to improve the material properties of polyfluorenes, which can be eventually useful for PLED device applications. Experimental Section
Materials. Compounds 9-(4-bromophenyl)-9H-carbazole (1),15
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylflu-orene (5),16and 2,7-dibromo-9,9-dihexylfluorene (6)16were
syn-thesized according to the literature procedures. Chemicals and solvents were reagent grade and purchased from Aldrich, ACROS, TCI, and Lancaster Chemical Co. Dichloromethane and THF were distilled to keep anhydrous before use. The other chemicals were used without further purification.
Measurements and Characterization.1H NMR spectra were
recorded on a Varian Unity 300 MHz spectrometer using CDCl3
or 1,4-dioxane solvent. Elemental analyses were performed on a HERAEUS CHN-OS RAPID elemental analyzer. Phase transition temperatures were determined by differential scanning calorimetry (Perkin-Elmer Diamond) with a heating and cooling rate of 10°C/ min. Thermogravimetric analysis (TGA) was conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravi-metric analyzer under a heating rate of 20°C/min. Gel permeation chromatography (GPC) analysis was conducted on a Waters 1515 separation module using polystyrene as a standard and THF as an eluent. UV-visible absorption spectra were recorded in dilute chloroform solutions (10-6M) on a HP G1103A spectrophotometer, and photoluminescence (PL) spectra were obtained on a Hitachi F-4500 spectrophotometer. Thin films of UV-vis and PL measure-ments were spin-coated on a quartz substrate from chloroform solutions with a concentration of 1 wt %. The relative photolumi-nescent quantum yields (ΦPL) of the compounds in chloroform were
determined using a solution of 9,10-diphenylanthracene as a standard (cyclohexane,ΦPL) 0.90). Dilute sample solutions were
used for the determinations (absorbance < 0.1). Values are
calculated according to the equation,Φunk ) Φstd(Iunk/Aunk)(Astd/ Istd)(Πunk/Πstd)2, whereΦunkis the fluorescence quantum yield of
the sample,Φstdis the fluorescence quantum yield of the standard, Iunk and Istd are the integrated emission intensities of the sample
and the standard, respectively, Aunkand Astdare the absorbances of
the sample and the standard at the excitation wavelength, respec-tively, and Πunk and Πstd are the refractive indexes of the
corresponding solutions (pure solvents were assumed). Electro-chemistry measurements were performed using an Autolab PG-STAT30 potentiostat/galvanostat with a standard three-electrode electrochemical cell in a 0.1 M tetrabutylammonium hexafluoro-phosphate (TBAPF6) solution (in acetonitrile) at room temperature
with a scanning rate of 100 mV/s. A platinum working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode were used. The onset potentials were determined from the intersection of two tangents drawn at the rising current and background current of the cyclic voltammogram (CV).
EL Device Fabrication. The devices were fabricated on ITO substrates that had been ultrasonicated and sequentially washed in detergent, methanol, 2-propanol, and acetone, and further treated with O3plasma for 10 min before use. Thin layers of PEDOT (40
nm) on the anode and polymers (from 1 wt % of polymers in chloroform solutions) were consecutively spin-coated on the ITO surface, after which a thin layer of LiF(0.5 nm)/Al(200 nm) was deposited on the polymer films by thermal evaporation under a vacuum of 10-6Torr. All measurements of the EL devices were carried out in air at room temperature. The luminance-current-voltage characteristics were recorded on a power source meter (Keithley 2400) and a photometer (Minolta CS-100A).
Synthesis. The synthetic routes of monomers 2-4 and polymers are shown in Schemes1 and 2, and their synthetic procedures are shown as follows:
9,9′-Bis-(4-bromophenyl)-9H,9′H-[3,3′]bicarbazolyl (2). To a stirred solution containing 2.93 g (9.1 mmol) of 9-(4-bromophenyl)-9H-carbazole (1) in 70 mL of chloroform under nitrogen, 3.0 g (18.5 mmol) of iron(III) chloride was added. Water (75 mL) was added after stirring for 40 h at room temperature. The mixture was extracted with CH2Cl2, and the organic layer was dried over MgSO4,
filtered, and concentrated. The mixture was purified by column chromatography with EA/hexane (1:10) to get a white solid. Yield: 70%.1H NMR (ppm, CDCl
3): δ 7.30-7.39 (m, 4H),
7.41-7.52 (m, 8H), 7.73-7.78 (m, 6H), 8.23 (d, 2H, J ) 7.5 Hz), 8.43 (d, 2H, J ) 1.2 Hz). MS (FAB): m/z [M+] 642, calcd m/z [M+] 642.38. Anal. Calcd for C36H22Br2N2: C 67.31, H 3.45, N 4.36.
Found: C 67.28, H 3.68, N 4.42.
3,6-Dibromo-9-(4-bromophenyl)-9H-carbazole (3). In a flask covered with aluminum foil, a stirred solution containing 2 g (6.2 Scheme 1. Synthetic Routes of Monomers 2-4
mmol) of 9-(4-bromophenyl)-9H-carbazole (1) in 200 mL of THF was cooled to 0°C. N-bromosuccinimide (NBS, 2.2 g, 12.4 mmol) was added in small portions. The mixture was allowed to warm to react at room temperature overnight. Then, the solvent was evaporated, and the crude product was purified by extraction with CH2Cl2and water. The combined organic phases were dried over
MgSO4. After removal of the solvent, the crude product was further
purified by column chromatography on silica gel eluted with CH2
-Cl2/hexane (1:10) and crystallization (acetone), respectively, to
afford compound 3 as a white crystal. Yield: 65%.1H NMR (ppm,
CDCl3): δ 7.22 (d, 2H, J ) 8.7 Hz), 7.38 (d, 2H, J ) 8.4 Hz),
7.51 (dd, 2H, J ) 1.5 Hz, J ) 8.7 Hz), 7.74 (d, 2H, J ) 9.0 Hz), 8.19 (d, 2H, J ) 1.8 Hz). MS (EI): m/z [M+] 480, calcd m/z [M+] 479.99. Anal. Calcd for C18H10Br3N: C 45.04, H 2.10, N 2.92.
Found: C 45.49, H 2.40, N 3.19.
6,6′-Dibromo-9,9′-bis-(4-bromophenyl)-9H,9′H-[3,3′ ]bicarba-zolyl (4). The synthesis of compound 4 was also followed by the similar procedure of compound 3. Compound 2 (2 g, 3.1 mmol), 1.1 g (6.2 mmol) of NBS, and 100 mL of THF were used. Compound 4 was purified by column chromatography with CH2
-Cl2/hexane (1:10) to get a white solid. Yield: 73%.1H NMR (ppm,
1,4-dioxane): δ 7.34 (d, 2H, J ) 8.7 Hz), 7.48 (d, 2H, J ) 8.4
Hz), 7.55 (dd, 2H, J ) 2.1 Hz, J ) 8.7 Hz), 7.60 (d, 4H, J ) 8.4 Hz), 7.83-7.87 (m, 6H), 8.48 (d, 2H, J ) 2.1 Hz), 8.55 (d, 2H, J ) 1.2 Hz). MS (FAB): m/z [M+
] 800, calcd m/z [M+] 800.17. Anal. Calcd for C36H20Br4N2: C 54.04, H 2.52, N 3.50. Found: C
53.92, H 2.87, N 3.73.
General Procedure for the Synthesis of Polymers. The synthetic route of polymers is shown in Scheme 2. A general procedure of polymerization is proceeded through the Suzuki coupling reaction. The monomers, alq336 and [Pd(PPh3)4], were
dissolved in toluene and 2 M K2CO3(3:2 in volume). The mixture
was degassed and stirred at 85°C for 2 days. After cooling to room temperature, the reaction mixture was poured into 200 mL of methanol. A fibrous solid was obtained by filtration. The obtained solid was followed by the Soxhlet extraction with acetone for 24 h to remove the catalyst residues and oligomers. Because polymers P3-P6 tend to produce insoluble cross-linking portions in the step-growth polymerization, the solid residues of P3-P6 were further extracted with THF using a Soxhlet apparatus for 24 h, and the insoluble solid (cross-linking networks) was discarded. After removal of THF, the soluble polymer products were dried in a vacuum oven for 24 h.
PFDiCAZ. Monomer 2 (219 mg, 0.34 mmol), monomer 5 (200 mg, 0.34 mmol), K2CO3 (1.1 g), [Pd(PPh3)4] (8 mg), toluene (6
mL), and H2O (4 mL) were used in the reaction mixture. PFDiCAZ
was obtained as a gray solid. Yield: 85%. 1H NMR (ppm,
CDCl3): δ 0.78-0.83 (m, 10H), 1.14 (m, 12H), 2.16 (broad, 4H),
7.37 (t, 2H, J ) 6.6 Hz), 7.50 (t, 2H, J ) 8.1 Hz), 7.57 (d, 2H, J ) 8.4 Hz), 7.64 (d, 2H, J ) 8.1 Hz), 7.74 (m, 8H), 7.88 (t, 4H, J ) 9.0 Hz), 7.97 (d, 4H, J ) 7.8 Hz), 8.30 (d, 2H, J ) 7.8 Hz), 8.53 (s, 2H,).
P1. Monomer 2 (27.4 mg, 0.043 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (104.9 mg, 0.21 mmol), K2CO3 (1.1 g),
[Pd(PPh3)4] (8 mg), toluene (6 mL), and H2O (4 mL) were used in
the reaction mixture. P1 was obtained as a light-yellow solid. Yield: 89%.1H NMR (ppm, CDCl
3): δ 0.78 (broad), 1.23 (broad),
2.12 (broad), 7.32-7.35 (broad), 7.47-7.49 (broad), 7.54-7.94 (broad), 8.27 (d, J ) 8.1 Hz), 8.50 (s).
P2. Monomer 2 (14.9 mg, 0.023 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (114.5 mg, 0.23 mmol), K2CO3 (1.1 g),
[Pd(PPh3)4] (8 mg), toluene (6 mL), and H2O (4 mL) were used in
the reaction mixture. P2 was obtained as a light-yellow solid. Yield: 80%.1H NMR (ppm, CDCl
3): δ 0.79 (broad), 1.14 (broad),
2.13 (broad), 7.30-7.36 (broad), 7.68 (broad), 7.83 (broad), 8.28 (d), 8.52 (s).
P3. Monomer 3 (18.9 mg, 0.039 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (96.9 mg, 0.20 mmol), K2CO3 (3.3 g),
[Pd(PPh3)4] (8 mg), toluene (18 mL), and H2O (12 mL) were used
in the reaction mixture. P3 was obtained as a light-yellow solid. Yield: 62%.1H NMR (ppm, CDCl
3): δ 0.78 (broad), 1.12 (broad),
2.09 (broad), 7.22-7.28 (broad), 7.67 (broad), 7.82 (broad), 8.39 (s), 8.54 (s).
P4. Monomer 3 (10.7 mg, 0.022 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (109.5 mg, 0.22 mmol), K2CO3 (3.3 g),
[Pd(PPh3)4] (8 mg), toluene (18 mL), and H2O (12 mL) were used
in the reaction mixture. P4 was obtained as a light-yellow solid. Yield: 74%.1H NMR (ppm, CDCl
3): δ 0.78 (broad), 1.13 (broad),
2.11 (broad), 7.27-7.30 (broad), 7.57 (broad), 7.82-7.85 (broad), 8.45 (s), 8.56 (s).
P5. Monomer 4 (29.2 mg, 0.037 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (90 mg, 0.18 mmol), K2CO3(3.3 g),
[Pd-(PPh3)4] (8 mg), toluene (18 mL), and H2O (12 mL) were used in
the reaction mixture. P5 was obtained as a light-yellow solid. Yield: 43%.1H NMR (ppm, CDCl
3): δ 0.80 (broad), 1.22 (broad),
2.12 (broad), 7.26-7.36 (broad), 7.51-7.58 (broad), 7.68 (broad), 7.83-7.86 (broad), 8.59 (broad).
P6. Monomer 4 (17.1 mg, 0.021 mmol), monomer 5 (150 mg, 0.26 mmol), monomer 6 (104.9 mg, 0.21 mmol), K2CO3 (3.3 g),
[Pd(PPh3)4] (8 mg), toluene (18 mL), and H2O (12 mL) were used
in the reaction mixture. P6 was obtained as a light-yellow solid. Yield: 55%.1H NMR (ppm, CDCl
3): δ 0.79 (broad), 1.21 (broad),
2.11 (broad), 7.30 (broad), 7.35 (broad), 7.67 (broad), 7.82 (broad), 8.58 (broad).
Results and Discussion
Synthesis and Characterization. Scheme 1 shows the synthetic routes of monomers 2-4. 9-(4-Bromophenyl)-9H-carbazole (1) was synthesized from 9-(4-Bromophenyl)-9H-carbazole as the starting material by a modified Ullmann reaction.15bTwo of compound 1 were linked via the 3- or 6-position of carbazoles to form dibromocarbazole monomer 2. NBS was used to brominate compound 1 or 2 to produce tribromocarbazole monomer 3 or tetrabromocarbazole monomer 4, respectively, with high yields. Monomers 2-4 were satisfactorily characterized by1H NMR, MS spectroscopy, and elemental analyses. The synthetic routes of PFDiCAZ and P1-P6 are shown in Scheme 2. PFDiCAZ, P1, and P2 were obtained by reaction of 2, 5, and 6 with different molar ratios in toluene containing Pd(PPh3)4(1 mol %). The obtained polymers were further purified by washing with acetone in a Soxhlet apparatus for 24 h to remove oligomers and catalyst residues and were dried under reduced pressure at
room temperature. After purification and drying, PFDiCAZ, P1, and P2 were obtained as yellow fibrous solids in overall good yields. P3 and P4 were synthesized by reacting monomers 3, 5, and 6 with different molar ratios. The molar ratios of monomer 3 in copolymers were found to be key factors of the solubilities in hyperbranched P3 and P4. The solubilities were poor in common organic solvents for hyperbranched polymers with higher contents of monomer 3. The polymers with the molar ratios (monomer 3/5/6) of 1:6.5:5 (P3) and 1:11.5:10 (P4) were approximately soluble in THF. P3 and P4 were purified by two-step extractions: first with acetone to remove catalyst residues and unreacted monomers (soluble in acetone), and then with THF to extract the soluble portions of hyperbranched polymers (insoluble cross-linking parts were discarded). The synthesis of P5 and P6 were also followed according to the synthetic procedures of P3 and P4 by reacting monomers 4, 5, and 6 with different molar ratios. The obtained PFDiCAZ and P1-P6 exhibited good solubilities in common organic solvents, e.g., toluene, THF, and chloroform.
Figure 1 shows1H NMR spectra of monomer 2, PFDiCAZ, P1, and P2 in CDCl3.1H NMR spectra of PFDiCAZ, P1, and P2 showed doublet signals atδ ) 8.3 and singlet signals at δ ) 8.5, respectively, which were assigned to the proton signals of 9,9′-diphenyl-bicarbazolyl linkages. Comparing 1H NMR spectra of PFDiCAZ, P1, and P2, it was found that the integrated signals of the kinked units increased with more amounts of monomer 2 added. The number-average molecular weights and the weight-average molecular weights of the polymers listed in Table 1 were 15100-24300, and 34500-66300, respectively, which were determined by means of gel permeation chromatography (GPC) using THF as eluent against polystyrene standards.
Thermal Properties. The thermal stability of the polymers was determined by thermogravimetric analysis (TGA) under nitrogen. As shown in Table 1, all of these polymers possess excellent thermal stability with 5% weight loss temperatures (Tds) ranging from 407 to 466 °C. These data reveal that all polymers have excellent thermal stability.
Thermally induced phase transition properties of the polymers were also investigated by differential scanning calorimetry Figure 1. 1H NMR spectra of monomer 2, PFDiCAZ, P1, and P2.
(DSC) under nitrogen. PF homopolymer had a crystallization temperature of 113°C and a melting point of 159°C, indicating the crystalline nature of this polymer as reported previously.17 The incorporation of a series of carbazole units into the copolymers resulted in DSC traces that show no crystallization and melting peaks but only glass transition temperatures (Tgs). This clearly indicates that the presence of the kinked or hyperbranched structures in these copolymers effectively sup-presses the crystallizability (or chain aggregation) of the polymer chains. The glass transition temperatures (Tgs) of P1-P4 were ranged from 109 to 140 °C. Compared with P2 possessing analogous kinked molecular architectures, P1 with a higher kinked carbazole content has lower Tg, which is conceivable by reason of a higher kinked density of carbazole units in P1. Compared with P4 possessing similar hyperbranched molecular architectures, P3 with a higher hyperbranched carbazole content has higher Tg, which is plausible because of a higher hyper-branched density of carbazole units in P3. Hence, polymers with higher kinked density or lower hyperbranched density of carbazole units possess lower Tg relatively. No distinct glass transitions were observed for PFDiCAZ, P5, and P6 in their DSC curves of the second heating. In addition, the measured values of Tds and Tgs were higher than that of the linear polyfluorene.18
Optical Properties. UV-vis absorption and PL emission data of the polymers are shown in Table 2 and Figures2-5. PFDiCAZ (alternative copolymers) displayed an absorption maximum (λAbs,sol) at 351 nm in CHCl3solution, resulting in a substantially blue shift compared with that of PF homopolymer. This behavior is similar to the analogous alternating fluorene/ carbazole copolymers,19which is due to the interruption of the delocalization ofπ-electrons along the polymer backbones by 9,9′-diphenyl-bicarbazolyl linkages. This effect also manifests
itself in the solution PL spectrum, where the emission peak is blue-shifted to 402 nm as compared with that of PF. With considerable decreases in the amounts of 9,9′ -diphenyl-bicar-bazolyl moieties, P1 and P2 show their absorption peaks with the values ofλAbs,sol) 380 nm for P1 and λAbs,sol) 388 nm for P2 (which are close to those of PF). In the case of hyperbranched polymers, the absorption peaks in solutions appeared at 380 nm for P3 and P5, and 388 nm for P4 and P6, were similar to those of copolymers P1 and P2. The result indicated that the absorption maxima are blue-shifted in analogous systems with the increasing contents of kinked or branching units. Compared with the absorption spectra of monodisperse oligofluorenes reported in the literature,20the average conjugated lengths of the polymers can be estimated. According to this assumption, P1, P3, and P5 should have comparable persistent conjugation lengths to that of an oligomer with eight fluorene units. However, the average conjugated lengths of P2, P4, and P6 are not conceivable but similar to those of PF homopolymers with high molecular weights. It has been reported that, as the number of fluorene units is larger than five, the emission wavelengths of the oligofluorenes approach those of polyfluo-renes with high molecular weights.20As shown in Figure 3, the PL spectra of P1-P6 in CHCl3 solutions are strikingly similar to each other, which show the maximum band around 417 nm along with a vibronic band at 440 nm. In general, the
Table 1. Molecular Weights and Thermal Properties of Polymers
polymer Mna Mwa PDI Tg(°C)b Td(°C)c PFDiCAZ 23400 43600 1.87 n.d.d 466 P1 24300 39900 1.64 109 424 P2 21400 40600 1.90 115 420 P3 15100 34500 2.28 140 433 P4 21800 66300 3.04 120 449 P5 15500 49500 3.19 n.d.d 407 P6 17800 49500 2.78 n.d.d 407
aMolecular weight determined by GPC in THF, based on polystyrene
standards.bGlass transition temperature (°C) determined by DSC at a
heating rate of 10°C min-1under nitrogen.cTemperature (°C) at 5% weight
loss measured by TGA at a heating rate of 20°C min-1under nitrogen.
dNo noticeable T
gwas observed.
Table 2. Absorption and PL Emission Spectral Data of Polymers in Solution and Solid States
polymer λAbs,sol (nm)a λPL,sol (nm)a λAbs,film (nm) λPL,film (nm) band gap (eV)b Φ PL,solc ΦPL,reld PFDiCAZ 351 402 355 414 3.1 0.47 0.44 P1 380 417, 440 379 425, 444 2.9 0.99 0.94 P2 388 417, 440 380 424, 447 2.9 0.94 0.88 P3 380 417, 440 388 423, 445 2.9 0.96 0.90 P4 388 417, 440 380 424, 444 2.9 0.81 0.76 P5 380 417, 441 388 422, 446 2.9 0.90 0.85 P6 388 417, 440 388 423, 446 2.9 0.88 0.83
aChloroform as solvent with a concentration of 10-6M.bBand gaps
were calculated from the onsets of UV-visible absorption spectra of
P1-P3 in solid films.cSolution fluorescence quantum efficiency measured in
chloroform, relative to 9,10-diphenylanthracene in cyclohexane (ΦPL) 0.9).
dRelative fluorescence quantum efficiencies in solutions were estimated
relative to the synthesized PF homopolymer, i.e., poly-2,7-(9,9-dihexy-lfluorene) as a reference, with Mn) 26700, PDI ) 1.94, and ΦPL,rel) 1.
Figure 2. Normalized UV-vis absorption spectra of polymers in chloroform (10-6M).
Figure 3. Normalized PL spectra of polymers excited at the maximum absorption of the polymer backbones in chloroform (10-6M).
presence of well-defined vibronic structures in the emission spectra indicates that the polymers have rigid and well-defined backbone structures.21The similar emissions observed for these copolymers in comparison with that of PF homopolymer might suggest that the carbazole derivatives in the polymer backbones do not prevent efficient energy transfer from the short conjugated emissions to the long ones in these copolymers. Therefore, as discussed above, the carbazole segments of the copolymers efficiently interrupt the conjugation of the polymer backbones, but most of the excitons migrate to the fluorene segments to emit light with lower energy. UV-vis and PL spectra of thin solid films were measured by spin-coating of solutions on quartz substrates. The optical band gaps (Eg) of P1-P6 were deter-mined to be around 2.9 eV from the UV-vis absorption spectra (solid films). As shown in Figure 4, the PL spectra of all polymers in the solid state showed 5-8 nm of red-shifts and some broadening of the emission bands in comparison with their solution PL spectra. Unlike the PL spectrum of polyfluorene possessing an aggregation peak around 520 nm besides two excitonic emission peaks, the PL spectra of P1-P6 only show peaks at 424 and 446 nm. The lack of the excimer emission in the solid films of P1-P6 proves that even introducing a small amount of carbazole derivatives can influence the packing of the polymer chains in the solid state and thus to enhance the quantum efficiency.
To investigate luminescent stability of the polymers in the solid state, annealing experiments were performed on polymers P1-P6. Because the incorporation of the carbazole derivatives into PFs in this work suppressed the crystallizability of the polymers by lack of crystallization and melting peaks in the DSC measurements, it could be expected that these copolymers would be prone to have less chain aggregation and thus to possess improved fluorescent stabilities. After being spin-coated on a quartz substrate, the solid film was annealed at 200°C for 1 h under nitrogen. Annealing at such a temperature, which is much higher than the glass transition temperature, can cause the movement of chain segments and increase the packing order. In fact, annealing of PFDiCAZ and P1-P6 at 200°C for 1 h under nitrogen resulted in no obvious changes in the PL spectra as compared with those of pristine (as-prepared) films. There was no additional band that emerged to change the pure-blue emission into another undesirable blue-green color. This clearly suggests that the formation of excimer aggregation in these copolymers is effectively suppressed even at temperatures much higher than their Tgs. However, different results can be observed after annealing at 200°C under air because traces of defects
formed via oxidation could also cause drastic spectral changes.22 As shown in Figure 5, PL spectra of kinked polymers P1 and P2 show a little change at the peak of 445 nm by annealing at 200°C for 0.5 h under air. In comparison with kinked polymers P1 and P2, all hyperbranched polymers P3-P6 have better thermal stability by annealing under air (but no difference under nitrogen). Besides, PL spectra of P1-P4 showed a weak long wavelength emission tail around 520 nm after annealing under air. These changed features of PL spectra were attributed to the interchain excimer formation and/or keto defects, which are commonly observed in 9,9-disubstituted PFs after annealing.23 Nevertheless, the excimer peak increases very slowly compared with previously reported PF homopolymers.24It is noteworthy that no green-blue band emission was observed in the thermal treatment of P5 and P6. Apparently, the introduction of kinked or hyperbranched disorder in the conjugated polymer chains shows more stable PL spectra of these copolymers upon annealing, where the aggregation and excimer formation of polymer chains can be avoided efficiently. Similar phenomenon regarding the effect of disorder on chromic behavior for other polyfluorene copolymers have also been reported by Leclerc,25 which was attributed to the increased conformational flexibility of the copolymers to interchain interaction, resulting in the incorporation of nonfluorene comonomer units. The PL quantum yields (ΦPL) of PFDiCAZ and P1-P6 excited at the maximum absorption of the polymer backbones in solutions were measured with 9,10-diphenylanthracene as a reference standard (cyclo-hexane, ΦPL ) 0.90).26 Except PFDiCAZ, the quantum ef-ficiencies of P1-P6 reach 0.81-0.99, which are much higher than the hyperbranched polyfluorene in the previous report.27 The high PL quantum yields (ΦPL) of these polymers were attributed to the introduction of minimal carbazole derivatives into poly(2,7-9,9-dihexylfluorene) to avoid reducing the effective conjugation lengths of the copolymers seriously. Besides, The PL quantum yields (ΦPL) of polymers P1-P6 were also compared with that of PF homopolymer, i.e., poly-2,7-(9,9-dihexylfluorene). The values of ΦPL for P1-P6 are slightly lower than that of PF homopolymer. The measured data of PL quantum yields are listed in Table 2.
Electrochemical Characterization. The electrochemical properties of the polymers were investigated by using cyclic voltammetry (CV), and their anodically scanned cyclic volta-mmograms are shown in Figure 6. PFDiCAZ (50% of the fluorene units was replaced by diphenylcarbazole units) exhib-ited reversible processes in the oxidation scan and had an Figure 4. Normalized PL spectra of polymers excited at the maximum
absorption of the polymer backbones in solid films.
Figure 5. Normalized PL spectra of polymers excited at the maximum absorption of the polymer backbones in solid films: before annealing (pristine film) and annealing at 200°C for 0.5 h under air.
oxidative onset potential at 0.96 V that was much lower than the reported value (1.4 V) of poly(9,9-dioctylfluorene) (POF).28 The decrease of the oxidative onset potential indicated the introduction of the 9,9′-diphenyl-bicarbazolyl linkages facilitates better hole injection from the ITO anode. When the 9,9′ -diphenyl-bicarbazolyl contents decreased substantially, the co-polymers display a slightly higher onset potential of oxidation with values of 1.20 V for P1 and 1.28 V for P2, but still lower than the value of POF. The similar effect was also observed in the previous report where only a small amount of carbazole units was incorporated into the polymers.19It is noticeable that the onset potential of oxidation in P1 is slightly lower than that in P2 due to the higher carbazole content in P1. In the meanwhile, similar trends were also observed in the oxidation processes of hyperbranched polymers P3-P6. The HOMO energy values of the polymers were estimated according to that (-4.8 eV) of ferrocene (Fc) with respect to zero vacuum level. Furthermore, the optical absorption edge was utilized to derive the band gap and calculate the LUMO energy values for the polymers.29The measured oxidation potentials along with HOMO and LUMO energy values of the polymers are summarized in Table 3. As a result, all of the polymers have higher HOMO energy levels than that of the POF, which means that better hole-transporting capabilities can be expected in polymers P1-P6.
Electroluminescence (EL) Properties. Owing to the short conjugation length and low PL efficiency of PFDiCAZ, the applications of polymers P1, P3, and P5 in PLED devices were surveyed in this study. The representative polymers P1, P3, and P5 were used as emitting layers in double-layered PLED devices
with a configuration of ITO/PEDOT:PSS/polymer/LiF/Al, and their EL data of PLED devices are shown in Table 4. As can be seen in Figure 7(a), the pure-blue EL emissions with negligible emissions in the region of 500-600 nm (which commonly appears for PF-based PLEDs) can be achieved from P1, P3, and P5. In Figure 7b, the EL spectra of P3 are almost unchanged and without any low-energy emission bands as the Figure 6. Cyclic voltammograms of the polymers: (a) PFDiCAZ,
P1, and P2; (b) P3, P4, P5, and P6.
Table 3. Oxidation Potentials and HOMO and LUMO Energy Values of Polymers
polymer Eox/onset(V) HOMO (eV) LUMO (eV)a
PFDiCAZ 0.96 -5.36 -2.26 P1 1.20 -5.60 -2.70 P2 1.32 -5.72 -2.82 P3 1.33 -5.73 -2.83 P4 1.36 -5.76 -2.86 P5 1.27 -5.67 -2.77 P6 1.36 -5.76 -2.86
aLUMO energy values were deduced from HOMO energy values and
optical band gaps.
Table 4. EL Data of PLED Devicesa
polymer λEL (nm) fwhm (nm) Vonb (V) power efficiency (cd/A)c luminance efficiency (cd/W)c max brightness (cd/m2) P1 425 42 7 0.11 0.05 133 (14 V) P3 426 42 6 0.35 0.08 213 (13 V) P5 421 39 8 0.07 0.03 87 (13 V)
aDevice structure: ITO/PEDOT/Polymer/LiF/Al.bV
on: the turn-on
voltage of light.cMeasured at 100 mA/cm2.
Figure 7. (a) Normalized EL spectra of P1, P3, and P5 at 10 V. (b) Normalized EL spectra of ITO/PEDOT/P3/LiF/Al device at different voltages.
voltage was increased from 8 to 14 V. The fwhm values of the EL spectra in P1, P3, and P5 are 42, 42, and 39 nm, respectively. The narrow EL spectra of these polymers are almost identical to the PL spectra of the corresponding polymer films. Figure 8 shows the current and luminance of PLED device containing P3 as a function of the applied voltage. The device emitted bright-blue light starting at about 6.0 V and reached a brightness of 213 cd/m2at a bias voltage of 13 V. The results suggest that the improvement in EL device performance with these copolymers is due to the introduction of kinked or hyperbranched disorder in the conjugated polymer chains. These preliminary data show the applicability of these novel polymers as PLED blue-emitters.
Conclusion
In summary, a series of novel kinked and hyperbranched carbazole/fluorene-based copolymers by one-pot Suzuki condensation (SPC) were successfully synthesized. The poly-mers exhibited high thermal stability with their decomposition temperatures (Tds) in the range of 407-466°C. The PL quantum efficiencies of P1-P6 in chloroform solutions were in the range of 0.81-0.99. The PL spectra of P1-P6 in solid films exhibited similar spectral patterns (only with 5-8 nm of red-shift in their emission maxima) in contrast to those measured in solutions. No long wavelength excimer-like emissions at 500-600 nm attributed to self-aggregation of polyfluorenes in the solid state were observed in these polymers. Thus, the polymers were less easy to self-aggregate in the solid state due to their kinked and hyperbranched structures. The polymer films exhibited very stable blue light emission even annealing at 200°C for 1 h under nitrogen. Besides, the tunability of hole-transporting properties and the stability of excellent luminescent properties were all demonstrated by these kinked and hyperbranched structures, which render them promising candidates for use as light-emitting materials in stable blue PLED devices. Thus, the possibility of improved luminescence properties in polyfluorene materials through carbazole copolymerization has been demonstrated.
Acknowledgment. We thank the financial support from the National Science Council of Taiwan (ROC) through NSC 93-2113-M-009-011. Prof. Kung-Hwa Wei (CV and GPC measure-ments) at the Department of Materials Science and Engineering, National Chiao Tung University (in Taiwan), and Mr. Menq-Dan Jiang (vacuum deposition systems) at the Chung-Shan Institute of Science and Technology (in Taiwan) are also acknowledged for their instrumental support.
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