Novel fluorene-carzazole-based conjugated copolymers

containing pyrazoline and benzothiazole segments

for blue light-emitting materials

Hao Chen, Xing Xu, He Gang Yan, Xian Rong Cai, Ying Li *,Qing Jiang, Ming Gui Xie

College of Chemistry, Sichuan University, Chengdu 610064, China

Received 26 June 2007

Abstract

A series of novel fluorene-carbazole-based copolymers with pyrazoline and benzothiazole units were synthesized successfully

through Suzuki coupling reactions. The molecular structures and thermal properties of these polymers were characterized by FT-IR,1H NMR, DSC and TGA. GPC results indicated that the weight-average molecular weight (Mw) and polydispersity of these

polymers were in range (12,000–14,000) and (1.8–2.0), respectively. The two resulting polymers have high photoluminescence

quantum efficiency implying that they may be promising candidates for polymer light-emitting diodes (PLEDs).

# 2007 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

Keywords: Fluorene-carbazole copolymers; Pyrazoline; Bezothiazole

Since the demonstration of polymer light-emitting diodes by Cambridge group in 1990 [1], many progress have

been made in the field of PLEDs due to their potential applications in large-area flat-panel displays. Recently, a lot of

works have been focused on the blue color emitting materials because the red and green LEDs can be readily obtained

from blue LEDs via energy transfer [2]. But the performance of blue LEDs, such as low luminescent efficiency, high

turn-on voltage and short lifetime, still needs to be improved. Therefore, it is a major challenge to design novel blue

materials with improved fluorescence efficiencies.

Fluorene-carbazole-based copolymers, although proved to be good candidates blue EL materials owing to their

good blue purity and high PL efficiencies, show relative poor thermostability under high drive voltage [3,4].

Bezothiazolypyrazoline derivatives have also shown great prospect as blue emitters due to their excellent fluorescence

and easy accessibility. One limitation of the small molecular pyrazolines is their tendency of crystallization that lead to

device degradation [5–7]. Herein, we report the synthesis and characterization of fluorene-carbazole-based

copolymers containing pyrazoline and benzothiazole units. The 3D shaped pyrazoline unit was introduced into the

fluorene-carbazole-based copolymers, which will yield a much higher glass transition temperature (Tg), better thermal

stability and higher photoluminescence quantum efficiency.

www.elsevier.com/locate/cclet

Available online at www.sciencedirect.com

Chinese Chemical Letters 18 (2007) 1496–1500

* Corresponding author.

E-mail address: profliying@sina.com (Y. Li).

1001-8417/$ – see front matter # 2007 Ying Li. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.

doi:10.1016/j.cclet.2007.10.005

1. Experimental

All chemicals were purchased from Aldrich and Acros chemical companies and were used without further

purification. All the solvents were properly purified before use. All manipulation involving air-sensitive reagents were

performed in a dry argon atmosphere. 9,9-Dihexylflurorene-2,7-bis (trimethylene boronates) [8], 3-bromo-N(2-

ethyLhexyl)-carbazole (2), 6-bromo-N-(2-ethylhexyl-carbazole)-3-carboxaldhy [9] (3), 1-[N(2-ethylhexyl-6-bromo-

carbazolyl]-3-phenyl-2-propylene-1-one [10] (4), 1-[N-(2-ethylhexyl)-6-bromocar-bazolyl]-3-4-bromophenyl)-2-

propyLene-1-one [10] (6), 2-hydrzinybenzothiazole [11], 6-bromo-2-hydrziny-benzothiazole [11] were prepared

following the already published procedure.

1.1. Synthesis of monomers

1-(2-Benzothiazolyl)-3-(4-bromophenyl)-5-[N-(2-ethylhexyl)-6-bromocarbazolyl]-4,5-dihydro-1-H-pyrazole (5).

One gram (2 mmol) of 4, 6-bromo-2-hydrzinybenzothiazole (0.54 g, 2 mmol) and 5 mL of ethoxy-ethanol were mixed

and refluxed under nitrogen for 3 h, then the mixture was cooled to room temperature, and the precipitate was filtered,

and recrystallized from ethanol: THF (v:v = 1:1) to give the title product as pale powders. Yield: 80%, mp: 172–

174 8C, 1H NMR (400 MHz, CDCl3, dppm): 0.81–1.98 (m, 15H) 3.43, 4.01(m, 2H pyrazoline-CH2), 4.08 (d, J = 6.4 Hz

2H, N-CH2), 6.05 (m, 1H pyrazoline CH), 7.20–8.15 (m, 14H, Ar-H).

3-Phenyl-1-(6-bromo-2-benzothiazolyl)-5-[N-(2-ethylhexyl)-6-bromocarbazolyl]-4,5-dihydro-1-H-py-razo-le (7).

Compound 7 was obtained as pale powder with a yield of 75% from reaction of 2-hydrzinybenzothiazol with 6

according to the similar procedure describe for 5. mp: 116–118 8C. 1H NMR (400 MHz, CDCl3, dppm): 0.84–1.99 (m,

15H) 3.42, 3.95 (m, 2H pyrazoline-CH2), 4.07 (d, J = 6.2Hz 2H, N-CH2), 5.99 (m, 1H, pyrazoline-CH), 7.06–8.15 (m,

14H, Ar-H).

1.2. Synthesis of polymers

1.2.1. General procedure

To a mixture of 9, 9-dihexylflurorene-2,7-bis (trimethylene boronates) (1.0 mmol), dibromo-substituted

compounds 5 or 7 (1.0 mmol), and catalyst amount Pd(pph3)4 was added a mixture of toluene (5 mL) and aqueous

2 mol/L potassium carbonate (5 mL). The reactant was stirred reflux under nitrogen for about 48 h. After the mixture

was cooled to room temperature, it was poured into 200 mL methanol and deionized water (10:1). A fibrous solid was

obtained by filtration. Then the solid was washed with methanol and water several times following by washing with

acetone for 24 h in a Soxhlet appratus.

P1 was obtained as pale powder with yield of 80% after drying under a vacuum. P1: 1H NMR (400 MHz, CDCl3,

dppm): 0.66–2.2 (m, 41H), 3.4, 3.9 (m, 2H, pyrazoline-CH2), 4.1 (m, 2H N-CH2), 5.9 (m, 1H pyrazoline-CH), 7.28–

8.29 (m, 20H, Ar-H). IR (KBr, pellet) 1602.1 cm�1 (s, C N).

P2: Yield: 75%, 1H NMR (400 MHz, CDCl3, dppm): 0.64–2.1 (m, 41H) 3.3, 3.9 (m, 2H, pyrazoline-CH2), 4.1 (m,

2H, N-CH2), 6.0 (m, 1H, pyrazoline-CH), 7.36-8.4 (m, 20H, Ar-H). IR (KBr, pellet) 1601.1 cm�1 (s, C N).

2. Results and discussion

The general synthetic routes toward the monomers and polymers are outlined in Scheme 1. The chemical structures

of the polymers were confirmed by 1H NMR and FT-IR. All of these polymers are readily dissolved in common

solvents, such as CH2Cl2, CHCl3 and THF. Their molecular weights were determined by gel permeation

chromatography (GPC) using polystyrene as standard. These copolymers have weight-average molecular weights

(Mw) of 12,000–14,000 with polydispersity indices (Mw/Mn) of 1.8 and 2.0, respectively. The thermal stabilities of the

polymers were determined by TGA and DSC measurements. All of these polymers show good thermal stability with

the onset decomposition (Td) of 362–375 8C. The glass-transition temperatures (Tg) of the polymers rang from 137 to

164 8C, which is much higher than typical polyflorene (PF) (Tg = 55 8C), suggesting that the incorporation of

benzothiazol and pyrazoline unit into the fluorene-carbazole main chain can inhibit the aggregation and crystallization

problem of the polmers film successfully.The optical properties of the polymers are summarized in Table 1. The

absorption and emission spectra were shown in Figs. 1 and 2, respectively. The two copolymers show very similar

H. Chen et al. / Chinese Chemical Letters 18 (2007) 1496–1500 1497

photophysical properties, i.e. UV–vis absorption and photoluminescence; this can be assigned to the similar

conjugation length of the two polymers. P1, P2 have two absorption peaks: one located in 299–310 nm and another

located in 361–364 nm. The former subpeak is assigned to electron transition of the aromatic side chains. The latter

maximum absorption peaks may be assigned to the p–p* electron transitions the copolymers. The absorption onset

wavelengths of the polymer in solid states were 450–470 nm, which correspond to the band gaps of 2.6–2.7 eV. As for

the fluorescence emission spectra of the polymer in 10�5 mol/L CHCl3 and in the thin films, the emission peaks locate

at 450–478 nm. It is showed that the emissions have a little tail peak in the solid state compared with that of in the

solution. This effect, very common in photoluminescence polymers, may be caused by the interchain interactions and

H. Chen et al. / Chinese Chemical Letters 18 (2007) 1496–15001498

Scheme 1. Synthetic routes for the monomers and the polymers. Reagents and conditions: (a) 1-bromo-2-ethylhexane, DMSO, NaOH; (b) 1-bromo-

2-ethylhexane, POCl3, DMF, 80 8C; (c) acetophenone, 5% NaOH aqueous, room temperature (RT); (d) 6-bromo-2-hydrzinybenzothiazole, ethoxy-

ethanol, reflux; (e) 4-bromo-acetophone, 5% NaOH aqueous, RT; (f) 2-hydrzinybenzothiazole, reflux; (g) Pd (pph3)4, 90 8C, toluene/2 mol/L K2CO3

aqueous.

Table 1

Properties of the copolymers P1–P2

Wwa Mw/Mn Td

b (8C) Tgc (8C) lmax

d PLmaxe Egopt (eV) FPL

f

Sol (nm) Film (nm) Sol (nm) Film (nm)

P1 12,000 1.8 375 164 310, 363 309, 361 479 478 2.7 0.88

P2 14,000 2.0 362 137 305, 364 299, 362 450 476 2.6 0.83

a GPC (THF), polystyrene as standards.b TGA under N2.c DSC under N2.d Absorption spectra were recorded in the 10�5 mol/L CHCl3 solution and in solid states on quarts.e Emission spectra was recorded in the 10�5 mol/L CHCl3 solution and in solid states, excited at absorption maxima.f Relative PL efficiencies in 10�5 mol/L CHCl3 solution with quinine sulfate in H2SO4 (0.1 mol/L) as reference.

p–p* stacking interaction between the pannar conjugated segments in the solid state. The relative PL quantum

efficiencies of P1and P2 are 0.88 and 0.83, respectively, using a solution at 0.1 mol/L quinine sulfate H2SO4 as

reference [12,13]. The high PL efficiencies can be attributed to the incorporation of carbazole, pyrazoline and

benzothiazole pigments into the fluorene mainchain.

3. Conclusion

Novel fluorene-carbazole-based copolymers containing pyrazoline and benzothiazole units were prepared through

Suzuki coupling polymerization. These polymers show satisfied thermal stability and film-forming property. Theirs

strong blue PL efficiencies indicated that it could be used as light-emitting materials.

Acknowledgment

This work was supported by the key Foundation of Education Ministry of China (No. 105142).

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H. Chen et al. / Chinese Chemical Letters 18 (2007) 1496–1500 1499

Fig. 1. UV–vis absorption of the P1 and P2 in 10�5 mol/L CHCl3 and in the thin films.

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