Novel blue light-emitting hyperbranched polyfluorenes incorporating carbazole kinked structure

European Polymer Journal 44 (2008) 3169–3176

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Novel blue light-emitting hyperbranched polyfluorenes incorporating carbazole kinked structure Qi-Yuan He a, Wen-Yong Lai a,b, Zhun Ma a, Dao-Yong Chen a, Wei Huang b,* a

Institute of Advanced Materials, Fudan University, Shanghai 200433, China Jiangsu Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications (NUPT), 9 Wenyuan Road, Nanjing 210046, China

b

a r t i c l e

i n f o

Article history: Received 9 April 2008 Received in revised form 25 May 2008 Accepted 9 June 2008 Available online 18 June 2008

Keywords: Hyperbranched polymer Polyfluorenes Blue light-emitting Carbazole

a b s t r a c t A novel series of soluble hyperbranched polyfluorenes P1P6 with various branching degrees and contents of kinked carbazole units were successfully synthesized with good yields and high molecular weight via a facile ‘‘A2 + B2 + C3 + D2” approach. The thermal, optical, and electrochemical properties as well as thermal spectral stability of the resulting hypberbranched polymers were investigated. All polymers exhibited good thermal stabilities and bright blue emission in both solutions and solid-states. Hyperbranched polyfluorenes (P3 and P6) exhibited improved spectral stability upon annealing at 200 °C in air, in sharp contrast to the linear poly(9,9-dihexylfluorene) (PDHF) that showed significant additional green emission at ca. 530 nm within minutes. In particular, outstanding spectral stability was observed with carbazole-incorporating hyperbranched polyfluorene P6. Electrochemical characterization indicated that the presence of carbazole also effectively raised the HOMO level with respect to that of polyfluorene homopolymer, suggesting better holeinjection properties. Hence, the incorporation of kinked carbazole unit into hyperbranched polyfluorenes could provide a new methodology for preparing blue light-emitting polymers with improved optoelectronic characteristics. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction

p-Conjugated polymers have been extensively explored and investigated as the active components for incorporation into polymer light-emitting diodes (PLEDs) [1–5], which have emerged as one of the most promising new technologies in the flat panel display industry. For full-color display applications, efficient and stable red-, green-, and blue light-emitting materials with good color purity are required [2–5]. Among the three primary color light-emitting materials, the biggest challenge until now has been to obtain efficient and spectra-stable blue light-emitting polymers. Polyfluorene and its derivatives [6] have been regarded as the most promising blue light-emitting polymers due to * Corresponding author. Tel./fax: +86 25 8349 2333. E-mail addresses: [email protected] (W.-Y. Lai), iamwhuang@ njupt.edu.cn (W. Huang). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.06.005

their pure-blue emission, high fluorescent efficiency, excellent thermal and chemical stability, and facile chemical functionalization [6–8], etc. A key issue hampering their widespread applications is the fast spectral degradation with a red-shifted emission during the device operation or thermal annealing [9–11]. This turns the desired pure-blue color into a blue-green or even a yellow emission. Although there is still much debate regarding the origin of the redshifted green emission [9–11], chemical approaches based on releasing the inter/intra-molecular interactions such as incorporating spiro-configuration [12], kinking the backbone structure [13,14], attaching bulky substitution [15], or constructing other sterically demanding structures [16], have resulted in encouraging improvements. Among them, the introduction of disorder into the conjugated backbone seemed to be a good route. For example, carbazole linkage has been introduced through the 3,6-positions into linear poly(2,7-9,9-dioctylfluorene) polymers [13,14]. The meta

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linkages allow the polymers to bend and twist and thus effectively decreases the packing order of the polymer chains. Most of the extensively studied blue light-emitting materials are linear rigid-rod molecules. In the quest for efficient materials for blue PLEDs, we have recently become interested in fluorene-based macromolecular materials with three-dimensional (3D) structure [17–21]. A novel series of monodisperse multi-branched oligofluorenes and a series of hyperbranched polyfluorenes based on oxadiazole or triazine units have been developed and investigated [17–21]. The highly branched and globular 3D structure of these materials has made them attractive in reducing or eliminating strong intermolecular interactions and aggregation. We find that these 3D materials are efficient blue light emitters. Compared with the regular monodisperse dendritic materials that are only accessible through laborious multistep syntheses, 3D hyperbranched polymers are attractive for their synthetic simplicity and facile mass-production availability [20–26]. As part of our continuous efforts in the quest of efficient blue light-emitting materials for blue PLEDs, we herein report the design, synthesis, and characterization of a new series of hyperbranched polyfluorenes incorporating carbazole kinked units. We intended to further introduce a ‘‘kink” disorder into the 3D hyperbranched polyfluorenes backbone to maximize the chain separation effect to depress the aggregation phenomena of the rigid-rod polyfluorenes chain and its effect on the excimer formation. The introduction of carbazole units may also influence the HOMO energy level, resulting in better hole-injection ability for polyfluorenes. Truxene [27–29] was selected as the hyperbranched core for its special intrinsic disordered 3D topology and facile functionalization on C5, C10, and C15 positions for improved solubility as well as its interesting overlapped three-fluorene-fragment structure. The effect of the molecular structures and chemical components on their photophysical and functional properties has also been investigated.

2. Experimental 2.1. General methods Anhydrous tetrahydrofuran (THF) was distilled over sodium. Other commercially available chemicals were used as received. 1H NMR and 13C NMR spectra were recorded on a Varian Mercury plus 400 at 295 K. The gel-permeation chromatography (GPC) measurements were performed on a Shimadzu Lc-VP system with THF as the eluent. The calibration was based on polystyrene standards with narrow molecular weight distribution. Elemental analysis was performed on a CHNS-O (Elementar Co.). Differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA) were done on Shimadzu DSC-60A and DTG-60A equipment, respectively. UV–vis absorption spectra were recorded on a Shimadzu UV-3150. Fluorescence spectra were measured on Shimadzu RF-5300PC with a xenon lamp as a light source. Fluorescence quantum yields (UPL) of the samples both in THF solutions and as films were

measured by using 9,10-diphenylanthracene (UPL = 0.91 in ethanol and UPL = 0.83 dispersed in PMMA films with a concentration lower than 1  103 M) as standard. Values are calculated according to the literature [30]. Cyclic voltammetry (CV) measurements were performed on an AUTOLAB PGSTAT30 potentiostat/galvanostat system (Ecochemie, The Netherlands) with a three-electrode cell in a solution of tetrabutylammonium hexylfluorophosphate (Bu4NPF6) in nonaqueous acetonitrile (0.1 M) under nitrogen with a scan rate of 50 mV s1. 2.2. Monomer synthesis The starting compounds 2,7-dibromo-9,9-dihexylfluorene (M1) [31], 2,20 -(9,9-dihexyl-9H-fluorene-2,7diyl)-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (M2) [8], 5,5,10,10,15,15-hexakis hexyl-2,7,12-tribromo-10,15dihydro-5H-diindeno[1,2-a;10 ,20 -c] fluorene (M3) [29], 3,6-dibromo-9-hexyl-9H-carbazole (M4) [14], were prepared according to the literature methods. 2.3. Polymerization A general procedure of polymerization is used for the syntheses of the target hyperbranched polymers. Monomers in THF were mixed with 2 M K2CO3 in water in a Schlenk tube. The mixture was carefully degassed by bubbling with N2 before and after adding Pd(PPh3)4 and then was stirred at 80 °C (oil-bath temperature) for 72 h under a nitrogen atmosphere. Upon completion, the mixture was cooled to room temperature. After it was poured into a stirred methanol, a fibrous solid was precipitated. The solid was collected by filtration, washed with water and methanol, dried, and was continuously extracted with acetone for 72 h using a Soxhlet apparatus to remove the catalyst residue, unreacted monomers, and low molecular weight oligomers. The remaining solid residue was then extracted with THF using the Soxhlet apparatus for 24 h. The insoluble solid was discarded and the THF solution was concentrated and dried in a vacuum oven at 100 °C for 2 days to give the soluble target polymer. P1: M1 (83.9 mg, 0.171 mmol), M2 (200.0 mg, 0.341 mmol), M3 (123.2 mg, 0.114 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give P1 as a white solid (150 mg, 56%). 1H NMR (400 MHz, CDCl3, ppm): d 8.54 (s), 7.86–7.68 (m), 7.52–7.38 (m), 3.11–2.99 (br), 2.36– 1.97 (m), 1.36–0.64 (m). 13C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.7, 140.2, 129.0, 127.4, 126.4, 126.3, 121.7, 120.2, 55.6, 40.6, 31.8, 31.7, 30.0, 29.8, 24.1, 23.0, 22.8, 22.6, 14.4, 14.3, 14.2. Anal. Calcd for C188H247: C, 90.07; H, 9.93. Found: C, 88.94; H, 9.72. P2: M1 (111.9 mg, 0.227 mmol), M2 (200.0 mg, 0.341 mmol), M3 (82.1 mg, 0.076 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give the soluble polymer P2 as a white solid (155 mg, 61%). 1H NMR (400 MHz, CDCl3, ppm): d 8.57 (s), 7.86–7.51 (m), 7.49–7.38 (m), 3.07–3.09 (br), 2.25–2.13 (m), 1.26–0.65 (m). 13C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.7, 140.3, 140.2, 129.0, 127.4, 126.4, 121.8, 121.7, 121.6, 120.2, 55.6, 40.7, 40.6, 31.8, 31.7, 30.1, 29.9,

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7.83–7.38 (m), 4.42–4.33 (m), 3.09–3.06 (m), 2.27–1.89 (m), 1.43–0.64 (m). 13C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.7, 140.5, 140.3, 140.2, 129.0, 127.4, 126.4, 126.3, 121.7, 120.2, 120.1, 55.6, 40.7, 40.6, 31.8, 31.7, 29.9, 24.1, 22.8, 22.6, 14.3, 14.2. Anal. Calcd for C217H272N3: C, 89.18; H, 9.38; N, 1.44. Found: C, 87.97; H, 9.16; N, 1.29. P6: M1 (67.2 mg, 0.136 mmol), M2 (200.0 mg, 0.341 mmol), M3 (38.5 mg, 0.045 mmol), M4 (55.8 mg, 0.136 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give the soluble polymer P6 as a white solid (136 mg, 59%). 1H NMR (400 MHz, CDCl3, ppm): d 8.51 (s), 7.87–7.38 (m), 4.41– 4.38 (m), 3.09–3.03 (m), 2.35–1.87 (m), 1.47–0.57 (m). 13 C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.8, 140.5, 140.3, 140.2, 129.0, 127.4, 126.4, 126.4, 121.8, 121.7, 120.2, 55.6, 40.6, 31.8, 31.7, 29.9, 24.1, 22.8, 14.3, 14.2. Anal. Calcd for C292H368N3: C, 89.47; H, 9.46; N, 1.07. Found: C, 88.95; H, 9.32; N, 0.96.

29.8, 24.2, 24.1, 22.8, 22.6, 14.3, 14.2. Anal. Calcd for C263H343: C, 90.13; H, 9.87. Found: C, 89.06; H, 9.68. P3: M1 (125.9 mg, 0.256 mmol), M2 (200.0 mg, 0.341 mmol), M3 (61.6 mg, 0.057 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give the soluble polymer P3 as a white solid (154 mg, 62%). 1H NMR (400 MHz, CDCl3, ppm): d 8.56 (s), 7.86–7.59 (m), 7.53–7.37 (m), 3.11–3.04 (br), 2.27–2.12 (m), 1.25–0.64 (m). 13C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.7, 140.2, 129.0, 127.4, 126.4, 121.7, 120.2, 55.6, 40.6, 31.8, 31.7, 29.9, 24.1, 22.8, 22.6, 14.3, 14.2. Anal. Calcd for C338H439: C, 90.17; H, 9.83. Found: C, 89.64; H, 9.71. P4: M2 (250.0 mg, 0.426 mmol), M3 (48.2 mg, 0.057 mmol), M4 (140.0 mg, 0.342 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give the soluble polymer P4 as a white solid (130 mg, 56%). 1H NMR (400 MHz, CDCl3, ppm): d 8.50 (s), 7.84–7.49 (m), 4.39–4.35 (m), 3.15–2.96 (m), 2.41–2.04 (m), 1.50–0.63 (m). 13C NMR (100 MHz, CDCl3, ppm): d 152.0, 140.8, 140.7, 140.2, 129.0, 127.4, 126.4, 126.3, 121.8, 121.7, 120.2, 55.6, 40.7, 40.6, 31.7, 29.9, 24.1, 22.8, 14.3, 14.2. Anal. Calcd for C346H425N6: C, 89.02; H, 9.18; N, 1.80. Found: C, 87.81; H, 8.99; N, 1.62. P5: M2 (200.0 mg, 0.341 mmol), M3 (64.2 mg, 0.076 mmol), M4 (57.1 mg, 0.227 mmol), THF (5 mL), 2 M K2CO3 (aq, 0.6 mL), and Pd(PPh3)4 (35 mg, 0.030 mmol) were used to give the soluble polymer P5 as a white solid (127 mg, 52%). 1H NMR (400 MHz, CDCl3, ppm): d 8.50 (s),

3.1 Synthesis and characterization Fig. 1 shows the chemical structures and the synthetic routes toward the designed polymers. The starting momomers were prepared according to the literature procedures. The target polymers were synthesized via Suzuki Polycondensation (SPC). The starting momomer ratios were ad-

T q

C6H13 C6H13 Br

3. Results and discussions

R

R p

Br

N R

M1 C6H13

C6H13 O B O

o

R=n-Hexyl O B O

R R

M2 Br C6H13 C6H13

T C6H13 C6H13

R

r

N

R

C6H13 C6H13 M3

M4

n N R

R R t

C6H13 N Br

Br

l

m s

Br

T

RR

R

T Br M1:M2:M3:M4

1.5 : 3.0 : 1.0 : 0 3.0 : 4.5 : 1.0 : 0 4.5 : 6.0 : 1.0 : 0 0 : 7.5 : 1.0 : 6 0 : 4.5 : 1.0 : 3 3.0 : 7.5 : 1.0 : 3

Fig. 1. Synthesis of polymers P1P6.

P1 P2 P3 P4 P5 P6

RR

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justed in order to investigate the effect of polymer composition on the physical and optical properties. Carbazole units were introduced through 3,6-positions into the hyperbranched polymer systems to further interrupt the polymer backbone and improve the hole-injection ability. For comparison, the hyperbranched polyfluorenes P1P3 without carbazole units but with various branched degrees were also fabricated. A novel ‘‘A2 + B2 + C3 + D2” approach was used to introduce carbazole into the hyperbranched polymer systems to provide P4P6 (see Fig. 1) in good yields (5262%). The resulting polymers P1P6 are readily soluble in common organic solvents, such as toluene, THF, CHCl3, etc, and show good film-forming ability by solution spincasting processing. The chemical structure of the polymers was verified by 1H and 13C NMR spectroscopy and elemental analysis. 1H NMR spectra of P1P6 are shown in Fig. 2. In the 1H NMR spectra of P1–P3, as the content of the fluorene units increase, the proton NMR spectra exhibit a corresponding enhancement in the intensity of the peaks associated with them, such as the peaks between d 7.90 and 7.60 ppm belonging to the aromatic protons of the fluorene units. The incorporation of the carbazole unit into the hyperbranched architectures was indicated by the observation of their characteristic signals and their clear assignments in the 1H NMR spectra. For example, peaks at around d 4.40 ppm in 1H NMR spectra were assigned to the aliphatic protons adjacent to the nitrogen atom on carbazole unit. The intensity varies with various amounts of carbazole units, indicating the successful incorporation of the carbazole units into the hyperbranched polyfluorene structures. The number-average molecular weight (Mn) of the resulting polymers ranged from 14,800 to 32,000 Da (Table 1) with a polydispersity index (PDI) from 1.65 to 2.35, as determined by gel-permeation chromatography with polystyrene as a calibrated standard. 3.2 Thermal analysis The thermal properties of the polymers were investigated under nitrogen atmosphere by TGA (thermal gravimetric analysis) and DSC (differential scanning calorimetry). The TGA indicates that all polymers exhibit good thermal stability with 5% weight loss temperatures

Table 1 Structural and thermal properties of the polymers Polymer

Mn

Mw

PDI

Td (°C)

P1 P2 P3 P4 P5 P6

24,000 19,800 14,800 32,000 28,800 25,600

51,700 40,600 24,400 75,200 62,700 48,300

2.15 2.05 1.65 2.35 2.18 1.89

416 417 413 402 405 410

ranging from 402 to 417 °C as listed in Table 1. DSC analysis revealed that all polymers are amorphous solids. No distinct thermal processes, such as glass transitions, crystallization or liquid-crystal transitions, were observed during the scanning cycles of P1P6 from 20 to 300 °C, suggesting good amorphous properties and high branching degrees of the polymers, which could significantly limit the polymer chain motions. High decomposition temperature and good amorphous morphological stability against crystallization are desirable properties for a luminescent polymer to be used as active material for practical applications in PLEDs. 3.3 Optical properties The absorption and photoluminescence (PL) spectra recorded from dilute THF solutions and thin films are shown in Fig. 3, and the results are summarized in Table 2. Uniform colorless films were prepared on quartz substrates by spin-coating with 1% toluene solution at a spin rate of 2000 rpm. In THF solution, the polymers P1P3 exhibited absorption maxima in the range 376.5381.5 nm and emission maxima in the range 415417 nm with vibrational shoulders at 441443 nm. The emission behaviors of these hyperbranched polymers are quite similar to the optical performance of the well-known linear poly(9,9-dihexylfluorene) (PDHF, Mw = 24 kg/mol, PDI = 1.5, was synthesized according to the previous procedure [8]) (415, 440sh). The film UV and PL spectra of P1P3 show similar spectral patterns to those determined in solution, with only 13 nm red shifts being observed for both their absorption and emission maxima. This indicated that con-

Fig. 2. 1H NMR spectra of: (a) P1P3 and (b) P4P6.

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1.0

a

P1-UV P1-PL P2-UV P2-PL P3-UV P3-PL

0.8

P1-UV P1-PL P2-UV P2-PL P3-UV P3-PL

1.0 0.8 0.6

0.4

0.4

0.2

0.2

0.0 1.0

0.0

c

P4-UV P4-PL P5-UV P5-PL P6-UV P6-PL

0.8 0.6

d

P4-UV P4-PL P5-UV P5-PL P6-UV P6-PL

1.0 0.8

Intensity (norm.)

Intensity (norm.)

0.6

b

0.6

0.4

0.4

0.2

0.2 0.0

0.0 300

400

500

600

300

400

500

600

Wavelength (nm) Fig. 3. UVvis absorption and PL spectra of: (a) P1P3 in dilute THF solution (10-6 M); (b) P1P3 in films; (c) P4P6 in THF solution; (d) P4P6 in films.

Table 2 Photophysical data of hyperbranched polymers P1P6 and PDHF in THFa and films

P1 P2 P3 PDHF P4 P5 P6

kabs [nm] (THF)

kem [nm] (THF)

kabs [nm] (film)

kPL [nm] (film)

Ufb (THF)

Ufb (film)

Egc [eV]

376.5 380.0 381.5 378.5 356.5 362.5 369.0

415,441sh 417,443sh 417,441sh 415,440sh 405,428sh 406,425sh 415,438sh

374.5 381 383.5 376 358 365.5 374

420,443sh 420,444sh 420,447sh 424,446sh 410sh,431 411,431sh 420,444sh

0.76 0.75 0.72 0.81 0.62 0.64 0.71

0.65 0.64 0.63 0.70 0.54 0.55 0.64

2.96 2.95 2.94 2.91 3.08 3.04 2.97

a

All spectra were measured for 1  106 M THF solution at 295 K. Estimated error in PL quantum efficiencies ±10%. 9,10-Diphenylanthracene (UPL = 0.91 in ethanol and UPL = 0.83 dispersed in PMMA films with a concentration lower than 1  103 M) was used as standard. c Band gap estimated from the red edge of the longest wavelength absorption in thin film. b

structing hyperbranched polymer structure is effective to suppress the aggregation in the solid-states. In light of the absorption spectra of P4P6 in dilute THF solutions as shown in Fig. 3c, these carbazole-containing hyperbranched polymers exhibited strong p–p* electron absorption bands, which peak at 356.5, 362.5, and 369.0 nm, respectively, and are generally structureless and significantly blue-shifted (ca. 1015 nm) relative to those of the corresponding hyperbranched polyfluorenes P1P3. This result suggests that incorporating carbazole kinked structure into the hyperbranched polymer backbone effectively further disrupts their conjugative interaction and then the effective conjugation length. This suggestion has been further supported by the direct observation that the absorption maxima of these hyperbranched polymers progressively red-shifted from 356.5 nm (P4) to 369 nm (P6) with decreasing the contents of the carbazole units from P4 to P6. Regarding the absorption spectra of P4P6 in films (Fig. 3d), the trends observed with decreasing the carbazole contents in terms of spectral shapes and peak positions are nearly the same compared to the spectra we obtained in dilute solutions, except slight red-shifts

(ca. 25 nm), suggesting that there is little aggregation of the chromophores in solid-states. As shown in Fig. 3d, the trends observed in the emission spectra of P4P6 in thin films parallel those seen in dilute solutions, except for small red-shifts (ca. 5 nm) in peak positions and slightly broader spectral shapes for P4 and P5, the shoulder peaks of which also increased slightly. Note that the spectral shapes and peak positions of P6 in the solid state are almost the same as those in solution. No additional bluegreen emission in the region of 500600 nm, which is typical for polyfluorenes, was observed in the solid-state PL spectra of all the hyperbranched polymers P1P6. All these results reveal that the inter-/intra-molecular interactions of these hyperbranched polymers are rather weak. This should be attributed to novel 3D hyperbranched polymer structure that could limit close packing of the rigid chains, and thus prevent the aggregation or ketonic defects effectively. The optical band gaps derived from the absorption edge of the spectra in thin films gave values of 2.962.94 eV for P1P3 and 3.082.97 eV for P4P6, respectively. A careful comparison shows that the hyperbranched polymers P4P6 with carbazole units have

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carbon electrode at room temperature. The scan rate was 50 mV s1 and the CV curves were referenced to an AgNO3/Ag reference electrode. The HOMO and LUMO energy levels as well as the electrochemical energy gaps (Eg) were calculated using the following equations: HOMO = Eox  4.71 eV, LUMO = Ered  4.71 eV, and Eg = Eox  Ered, respectively, where Eox and Ered are the onset potentials for oxidation and reduction relative to the Ag/Ag+ reference electrode [19]. The curves of CV scans of P1P6 and the reference polymer PDHF are shown in Fig. 4 and the results are summarized in Table 3. Accordingly, the HOMO and LUMO values of the reference polymer PDHF were estimated to be 5.71 and 2.53 eV, respectively, which are quite similar to the values reported in the literature [32]. As shown in Fig. 4, P1P3 exhibited reversible redox processes with onset oxidation (p-doping) and reduction (n-doping) potentials locating in the range of +0.92 to +1.00 V, and 2.17 to 2.18 V, respectively. Based on the onset potentials, the HOMO and LUMO energy levels of P1P3 are estimated to be in the range of 5.63 to 5.71 eV, and 2.53 to 2.54 eV, respectively, whereas those of P4P6 are in the range of 5.44 to 5.50 eV, and 2.51 to 2.44 eV. Electrochemical band gaps were therefore obtained in the range of 3.10 to 3.18 eV for P1P3 and 2.93 to 3.06 eV for P4P6, respectively. With incorporating carbazole unit into the hyperbranched polymer backbone, P4P6 exhib-

slightly higher band gaps than their hyperbranched polyfluorene counterparts P1P3 and the values of the optical band gaps increase progressively with increasing the carbazole contents from P6 (2.97 eV) to P4 (3.08 eV). These results further support that incorporating carbazole kinked structure into the hyperbranched polyfluorene systems effectively decrease the effective conjugation length and the efficiency of electron delocalization in the hyperbranched polyfluorene backbones and thus increase their optical band gaps. The dilute sample solutions emitted very strong blue light under UV light excitation. The quantum yields of the hyperbranched polymers in THF solution were measured using a dilute solution of 9,10-diphenylanthracene in ethanol (UPL = 0.91) and in PMMA films with a concentration lower than 1  103 M (UPL = 0.83) as the standard. All polymers exhibited good quantum efficiency in solution (0.620.76) and in the solid films (0.540.65), and the results are summarized in Table 2. 3.4 Electrochemical properties Cyclic voltammetry (CV) analysis was used to study the electrochemical behavior of the resulting polymers. The experiments were carried out in a 0.1 M tetrabutyl-ammonium hexafluorophosphate (n-Bu4NPF6) electrolyte in nonaqueous acetonitrile using a thin film coated on a glassy

a

200

150

P4 P5 P6

100

Current, µA

100 50 0 -50 -100 -150

50 0 -50 -100 -150

-200

-200

-250 -3.2 -2.8 -2.4 -2.0 0.0

0.4

0.8

1.2

1.6

-3.2

+

c

-2.8

-2.4

0.0

0.4

0.8

1.2

Potential, V vs. Ag/Ag+

Potential, V vs. Ag/Ag 800

PDHF

600

Current, µA

Current, µA

b

P1 P2 P3

150

400 200 0 -200 -400 -600 -3.2

-2.8

-2.4

-2.0

0.0

0.4

0.8

1.2

Potential, V vs. Ag/Ag+ Fig. 4. Cyclic voltammograms of the polymer films: (a) P1P3, (b) P4P6, and (c) PDHF. Scan rate: 50 mV s1.

1.6

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3.5 Thermal spectral stability

Table 3 Cyclic voltammetric data of reference polymer PDHF and hyperbranched polymers P1P6a Compound

Eoxb (V)

Eredb (V)

EHOMOc (eV)

ELUMOc (eV)

EgCVd (eV)

P1 P2 P3 PDHF P4 P5 P6

0.92 0.99 1.00 1.01 0.73 0.74 0.79

2.18 2.17 2.18 2.16 2.20 2.27 2.27

5.63 5.70 5.71 5.72 5.44 5.45 5.50

2.53 2.54 2.53 2.55 2.51 2.44 2.44

3.10 3.16 3.18 3.17 2.93 3.01 3.06

Spectral and luminescence stability are very important characters for polymers to be used as the active materials in PLEDs. It is well-known that polyfluorenes thin films are readily subject to spectral degradation upon thermal annealing with exposure to air [9–11]. An additional structureless green emission band emerged, which turns the pure-blue emission into an undesirable blue-green or even a yellow color. To examine the thermal spectral stability of our newly designed polymers, thermal annealing experiments were carried out on the spin-coated polymer films on quartz substrates in air with a harsh temperature condition (200 °C). The significance of constructing hyperbranched polyfluorenes and incorporation of carbazole kinked functionality into the hyperbranched polyfluorenes backbone on the luminescence properties is demonstrated by annealing the films. P3 and P6 were chosen as the representative examples with which to investigate the thermal spectral stability of these two types of hyperbranched polymers. The results are compared with the well-known linear long-chain-length polyfluorene PDHF. Fig. 5 shows the normalized PL spectra of the annealed films at 200 °C for various times. Thermal treatment of PDHF film for just 5 min at 200 °C in air, an additional broad shoulder peak at ca. 530 nm emerged as shown in Fig. 5a. This band increased steadily with prolonged time and became pronounced upon annealing within 45 min. In sharp contrast, the PL spectra of the hyperbranched polymers seemed much more stable upon annealing under the same annealing conditions. The PL spectra of P3 only showed a small shoulder under the same heating treatment conditions (Fig. 5b). The impressive color stability was observed from the carbazole-containing hyperbranched polymer P6. Its PL spectrum remained almost identical in comparison with the fresh films even after thermal annealing at 200 °C for 45 min as shown in Fig. 5c. The additional emission band can only be detectable when we continuously annealed the P6 film at 200 °C for 3 h, demonstrating its outstanding spectral stability for this material. Such distinct difference in spectral stability observed from PDHF and P3 or P6 suggests the significance of the hyperbranched molecular structure that could

a Cyclic voltammogram curves of the drop coated film measured in 0.1 M Bu4NPF6 acetonitrile solution at a scan rate of 50 mV s1 at room temperature (versus Ag/Ag+). b Onset oxidation (p-doping) and reduction (n-doping) potentials versus Ag/Ag+. c Estimated from the onset oxidation and reduction potential by using EHOMO = Eox  4.71 eV and ELUMO = Ered  4.71 eV [19]. d Electrochemical band gaps determined using Eg = (EHOMO  ELUMO).

ited much lower oxidation potentials with the onset values of +0.73 to +0.79 V, while the onset reduction potentials remain almost the same with slight shifts to 2.20 to 2.27 V. These result in higher HOMO levels and lower electrochemical band gaps of P4P6 relative to those of P1P3 (Table 3). The Eg values are very similar to those obtained by means of optical measurements in thin films, within experimental errors, but the trends were observed to be opposite. The improved electron-donating properties resulted from incorporating electron-rich carbazole units into the hyperbranched polyfluorenes are thus likely to play a key role in this case. This is further supported by the trends observed with increasing the carbazole contents from P6 to P4, where the onset oxidation potentials also decrease progressively although this effect is small. Clearly, polymers P4P6 incorporating carbazole units exhibited higher HOMO energy levels relative to that of PDHF (5.72 eV) and those of the hyperbranched polyfluorenes P1P3. These results indicate that incorporation of carbazole unit is effective to raise the HOMO level of the hyperbranched polyfluorenes. Such energy levels may provide a closer match to the work function of ITO/PEDOT:PSS (work function = 5.2 eV), which means that a better hole injection and transporting ability can be expected when such polymers are used as active materials in PLEDs.

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hamper the aggregation formation of the polymer backbones. In the case of P6, this effect has been enhanced by incorporating carbazole kinked structure into the hyperbranched backbones, which could further make the polymers to bend and twist and thus maximize the chain separation effect, resulting in suppressed inter-/intra-chain interactions and superior thermal spectral stability. 4. Conclusions A novel series of hyperbranched polyfluorenes P1P6 with various contents of carbazole kinked structure have been successfully developed. Pure-blue emission was obtained for these polymers in both solutions and solid-states. The 3D hyperbranched structure has proven to be useful in improving spectral stability. Especially, incorporating carbazole kinked structure into the hyperbranched polymer backbones led to superior results. Electrochemical studies have also shown that incorporation of carbazole is effective to raise the HOMO levels of the hyperbranched polyfluorenes, which are desirable as active materials for PLEDs. Our studies implied that the resulting hyperbranched polyfluorenes with carbazole kinked structure functionality are promising candidates for blue light-emitting applications in terms of the suppression of the detrimental green emission and the improved hole-injection properties. Electroluminescent properties of these materials are under investigation. Acknowledgements This work was funded by the National Natural Science Foundation of China (Grants 60325412, 90406021, and 50428303), the NUPT Foundation (Grant NY207040). References [1] Burroughes JH, Bradley DDC, Brown AR, Marks RN, Mackay K, Friend RH, et al. Light-emitting diodes based on conjugated polymers. Nature 1990;347:539–41. [2] Akcelrud L. Electroluminescent polymers. Prog Polym Sci 2003;28(6):875–962. [3] Gustafsson G, Cao Y, Treacy GM, Klavetter F, Colaneri N, Heeeger AJ. Flexible light-emitting diodes made from soluble conducting polymers. Nature 1992;357(6378):477–9. [4] Forrest SR. The path to ubiquitous and low-cost organic electronic appliances on plastic. Nature 2004;428:911–8. [5] Kim DY, Cho HN, Kim CY. Blue light emitting polymers. Prog Polym Sci 2000;25(8):1089–139. [6] Neher D. Polyfluorene homopolymers: conjugated liquid-crystalline polymers for bright blue emission and polarized electroluminescence. Macromol Rapid Commun 2001;22(17):1365–85. [7] Pei Q, Yang Y. Efficient photoluminescence and electroluminescence from a soluble polyfluorene. J Am Chem Soc 1996;118(31):7416–7. [8] Liu B, Yu WL, Lai YH, Huang W. Blue-light-emitting fluorene-based polymers with tunable electronic properties. Chem Mater 2001;13(6):1984–91. [9] Montilla F, Mallavia R. On the origin of green emission bands in fluorene-based conjugated polymers. Adv Funct Mater 2007;17(1): 71–8. [10] List EJW, Guuentner R, Scanducci de Freitas P, Scherf U. The effect of keto defect sites on the emission properties of polyfluorene-type materials. Adv Mater 2002;14(5):374–8.

[11] Sims M, Bradley DDC, Ariu M, Koeberg M, Asimakis A, Grell M, et al. Understanding the origin of the 535 nm emission band in oxidized poly(9,9-dioctylfluorene): the essential role of inter-chain/intersegment interactions. Adv Funct Mater 2004;14(8):765–81. [12] Yu W-L, Pei J, Huang W, Heeger AJ. Spiro-functionalized polyfluorene derivatives as blue light-emitting materials. Adv Mater 2000;12(11):828–31. [13] Xia C, Advincula RC. Decreased aggregation phenomena in polyfluorenes by introducing carbazole copolymer units. Macromolecules 2001;34(17):5854–9. [14] Lu JP, Tao Y, D’iorio M, Li YN, Ding JF, Day M. Pure deep blue lightemitting diodes from alternating fluorene/carbazole copolymers by using suitable hole-blocking materials. Macromolecules 2004;37(7):2442–9. [15] Marsitzky D, Vestberg R, Blainey P, Tang BT, Hawker CJ, Carter KR. Self-encapsulation of poly-2,7-fluorenes in a dendrimer matrix. J Am Chem Soc 2001;123(29):6965–72. [16] Mishra AK, Graf M, Grasse F, Jacob J, List EJW, Müllen K. Chem Mater 2006;18:2879. [17] Lai WY, Zhu R, Fan QL, Hou LT, Cao Y, Huang W. Monodisperse sixarmed triazatruxenes: microwave-enhanced synthesis and highly efficient pure-deep-blue electroluminescence. Macromolecules 2006;39(11):3707–9. [18] Lai WY, Chen QQ, He QY, Fan QL, Huang W. Microwave-enhanced multiple Suzuki couplings toward highly luminescent starburst monodisperse macromolecules. Chem Commun 2006;18:1959–61. [19] Lai WY, He QY, Zhu R, Chen QQ, Huang W. Kinked star-shaped fluorene/triazatruxene co-oligomer hybrids with enhanced functional properties for high-performance, solution-processed, blue organic light-emitting modes. Adv Funct Mater 2008;18(2):265–76. [20] Xin Y, Wen GA, Zeng WJ, Zhao L, Zhu XR, Fan QL, et al. Hyperbranched oxadiazole-containing polyfluorenes: toward stable blue light PLEDs. Macromolecules 2005;38(16):6755–8. [21] Wen GA, Xin Y, Zhu XR, Zeng WJ, Zhu R, Feng JC, et al. Hyperbranched triazine-containing polyfluorenes: efficient blue emitters for polymer light-emitting diodes (PLEDs). Polymer 2007;48(7):1824–9. [22] Li J, Bo Z. ‘‘AB(2)+AB” approach to hyperbranched polymers used as polymer blue light emitting materials. Macromolecules 2004;37(6):2013–5. [23] Cao XY, Zhou XH, Zi H, Pei J. Novel blue-light-emitting truxenecontaining hyperbranched and zigzag type copolymers: synthesis, optical properties, and investigation of thermal spectral stability. Macromolecules 2004;37(24):8874–82. [24] Liu XM, He C, Hao XT, Tan LW, Li Y, Ong KS. Hyperbranched bluelight-emitting alternating copolymers of tetrabromoarylmethane/ silane and 9,9-dihexylfluorene-2,7-diboronic acid. Macromolecules 2004;37(16):5965–70. [25] Li Z, Di C, Zhu Z, Yu G, Li Z, Zeng Q, et al. New light-emitting hyperbranched polymers prepared from tribromoaryls and 9,9dihexylfluorene-2,7-bis(trimethyleneborate). Polymer 2006;47(23):7889–99. [26] Tsai LR, Chen Y. Novel hyperbranched polyfluorenes containing electron-transporting aromatic triazole as branch unit. Macromolecules 2007;40(9):2984–92. [27] de Frutos Ó, Gómez-Lor B, Granier T, Monge MÁ, Gutiérrez-Puebla E, Echavarren AM. Syn-Trialkylated truxenes: building blocks that selfassociate by arene stacking. Angew Chem Int Ed 1999;38(12):204–7. [28] Cao XY, Zhang WB, Wang JL, Zhou XH, Lu H, Pei J. Extendedconjugated dendrimers based on truxene. J Am Chem Soc 2003;125(41):12430–1. [29] Kanibolotsky AL, Berridge R, Skabara PJ, Perepichka IF, Bradley DDC, Koeberg M. Synthesis and properties of monodisperse oligofluorenefunctionalized truxenes: highly fluorescent star-shaped architectures. J Am Chem Soc 2004;126(42):13695–702. [30] Hamai S, Hirayama F. Actinometric determination of absolute fluorescence quantum yields. J Phys Chem 1983;87(1):83–9. [31] Ranger M, Rondeau D, Leclerc M. New well-defined poly(2,7fluorene) derivatives: photoluminescence and base doping macromolecules. Macromolecules 1997;30(25):7686–91. [32] Mallavia R, Montilla F, Pastor I, Velásquez P, Arredondo B, Álvarez AL, et al. Characterization and side chain manipulation in violet-blue poly-[(9,9-dialkylfluoren-2,7-diyl)-alt-co-(benzen-1,4-diyl)] backbones. Macromolecules 2005;38(8):3185–92.