A New Poly(fluorene-co-carbazole) with a Large Substituent Group at the 9-Position of the Carbazole Moiety: An Efficient Blue Emitter

A New Poly(fluorene-co-carbazole) with a Large

Substituent Group at the 9-Position of the

Carbazole Moiety: An Efficient Blue Emittera

Junping Du,1 Qiang Fang,*1 Dongsheng Bu,2 Shijie Ren,1 Amin Cao,1 Xiaoyao Chen1

1Laboratory for Polymer Materials, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road,Shanghai 200032, P. R. ChinaFax: 86-21-6416-6128; E-mail: [email protected]

2R&D Center for Flat Panel Display Technology, SVA Company, Shanghai, P. R. China

Received: July 3, 2005; Revised: August 3, 2005; Accepted: August 24, 2005; DOI: 10.1002/marc.200500460

Keywords: anthracene derivatives; electrochemical properties; fluorene-based polymers; poly(fluorene-co-carbazole)s; opticalproperties

Introduction

One of the important applications of fluorene-based

polymers is their use in organic light-emitting diodes

(OLEDs).[1] For this application, many investigations[1–10]

on fluorene-based polymers have been carried out in the

past few decades. This research has involved the synthesis

and property determination of homopolymers of 9,9-

dialkylfluorenes and the copolymers between 9,9-dialkyl-

fluorene and various comonomers.[2,5,7,11,12]

Rigid-rod polyfluorenes are usually prone to forming a

nematic-type of packing arrangement in the solid state as a

result of chain aggregation;[13] such chain aggregation

usually results in the decrease of luminescence quantum

yields of the polymers. To decrease such chain aggregation

and obtain polymers with good luminescent efficiency, the

general method is to introduce large aryl groups or an alkyl

chain with special structures into the 9-position of the

fluorenes.[14,15] Recently, Advincula’s group[16] reported

that the introduction of carbazole units into the polyfluorene

chains can also depress the chain aggregation by the

Summary:Anew poly(fluorene-co-carbazole) (PFC-1) witha large substituent group (ADN, a naphthalene-anthracenederivative moiety) at the 9-position of carbazole wassynthesized. Compared with poly(fluorene-co-carbazole)sthat have an alkyl substituent group at the 9-position of thecarbazole, the UV-vis absorption (or photoluminescentemission) peaks of PFC-1 are in almost the same positionboth in solution and in the solid state, whereas films of theformer give peaks at longer wavelengths than those insolution. The photoluminescent (PL) spectra of PFC-1indicate that the attachment of ADN to the poly(fluorene-co-carbazole)s gives rise to an efficient blue emissionfrom non-aggregated ADN. There is no difference evidentbetween PFC-1 and other reported poly(fluorene-co-carbazole)s in PL quantum yield, thermostability, andelectrochemical behavior, which suggests that PFC-1 is anefficient blue emitter.

UV-Vis spectra of the poly(fluorene-co-carbazole) (PFC-1),with a large substituent group (ADN, a naphthalene-anthracene derivative moiety) at the 9-position of carbazole,in toluene and in the film.

Macromol. Rapid Commun. 2005, 26, 1651–1656 DOI: 10.1002/marc.200500460 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 1651

a : Supporting information for this article (including DSC tracesand some optical data of the new polymer) is available at thebottom of the article’s abstract page, which can be accessedfrom the journal’s homepage at http://www.mrc-journal.de, orfrom the author.

formation of a kink linkage. Following this, Tao’s group[17]

and Wong’s group[18] reported similar results using differ-

ent copolymerization routes. While these efforts to depress

the chain aggregation have been successful, amore efficient

way is still required because the synthesized poly(fluorene-

co-carbazole)s show UV-vis absorption and photolumines-

cence (PL) emission peaks at longer wavelengths in the

solid state than in solution,[16–18] which suggests that

there is still chain aggregation in the polymers and that the

luminescence efficiency of the polymers still needs to be

improved.

In this contribution, we report a new way to obtain

poly(fluorene-co-carbazole)s with good luminescence effi-

ciency by introducing a large side chain into the 9-position

of the carbazole moiety. The chemical structure and the

synthetic procedure of the new polymer, PFC-1, are shown

in Scheme 1. As can be seen from Scheme 1, the large side

chain at the 9-position of carbazole is derived from 9,10-

di(2-naphthyl)-anthracene (ADN), which is a famous blue

emitter[19] and is widely used in OLEDs.[20,21] Our aim is to

attach the ADN group to the side chain of the carbazole unit

in the poly(fluorene-co-carbazole)s, to endow good optical

properties to the polymers, and to investigate the properties

of the ADN-containing polymers.

Experimental Part

Instrumentation

1H NMR spectra were determined on a Bruker DRX 400spectrometer. FT-IR spectra were recorded on a Nicoletspectrometer with NaCl pellets. Elemental analysis wasperformed with a Carlo Erba 1106 elemental analyzer. APerkin Elmer Series 200 GPC system was employed tomeasure the molecular weight using polystyrene as standards.UV-Visible absorption spectra were obtained with a HitachiUV2800 spectrophotometer. PL was measured with a HitachiF-4500 fluorescence spectrophotometer. Thermal stability wasdetermined with a TA 2000 thermogravimetric analyzer at aheating rate 10 8C �min�1 in nitrogen. The cyclic voltammetryof cast films of the polymers on Pt wires was performed in anacetonitrile solution of [Bu4N]BF4 (0.10 M, Bu¼ butyl)) underargon using (0.10 M AgNO3)/Ag and a platinum wire asreference and counter electrodes, respectively. A CHI 600Banalyzer was used for the cyclic voltammetry.

Preparation of the Monomer

2-Methyl-9,10-di(2-naphthyl)anthracene (1)

To a stirring mixture including magnesium (1.44 g, 60 mmol),I2, and tetrahydrofuran (THF, 40 mL) was added a solution of

O

O

NH

BrBr

C8H17C8H17

BBO

O O

O

PFC-1

N

BrBr

Br

i) ii)

1

2

iii)

2

3

3

iv)N

C8H17 C8H17

n

Scheme 1. Synthetic route of the new polymer. Regents and conditions: i) a) Mg,2-bromonaphthylene, THF; b) KI, NaH2PO2, HOAc, refluxing. ii) NBS, BPO, CCl4. iii)t-BuONa, THF. iv) aq. Na2CO3, Pd(PPh3)4, toluene, reflux for 84 h.

1652 J. Du, Q. Fang, D. Bu, S. Ren, A. Cao, X. Chen

Macromol. Rapid Commun. 2005, 26, 1651–1656 www.mrc-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2-bromonaphthalene (10.35 g, 50 mmol) in THF (40 mL)dropwise under argon at about 60 8C. Themixturewas refluxedfor 3 h and the obtained solution was added dropwise to astirring solution of 2-methylanthraquinone (5.37 g, 24 mmol)in THF (80 mL) under argon at �78 8C. After addition, thesolution was warmed to room temperature and stirred over-night. The resulting mixture was poured into 100 mL of waterand extracted twice with dichloromethane (200 mL). Theorganic layers were washed with water and dried overanhydrous Na2SO4. After evaporation of the solvent, a yellowpowder was obtained (11.26 g), which was added to a mixtureof potassium iodide (34.03 g, 205 mmol), sodium hypopho-sphite (40.51 g, 382.14 mmol), and glacial acetic acid(100 mL). The resulting solution was refluxed under an argonatmosphere overnight to give a homogenous solution, whichwas poured into 200 mL of water to give a yellow precipitate.Compound 1 was obtained as a yellow solid (7.15 g) afterwashing the obtained precipitate with water and ethanol anddrying.

2-Bromomethyl-9,10-di(2-naphthyl)anthracene (2)

Under an argon atmosphere, a solution of 1 (7.15 g,16.10 mmol), benzoyl peroxide (30 mg), and N-bromosucci-nimide (NBS, 2.87 g, 16.10 mmol) in carbon tetrachloride(150 mL) was heated to reflux overnight to obtain a turbidsolution. After being cooled to room temperature, the organiclayer was washed with water, dried over anhydrous Na2SO4,and concentrated under vacuum. The resulting residue waspurified by column chromatographywith silica using amixtureof petroleum ether and dichloromethane (6:1 to 4:1 v/v) asan eluent. Compound 2 was thus obtained as a yellowpowder (4.12 g, 32.6% of overall yield based on 2-methylanthraquinone).

3,6-Dibromo-9-N-[2-methyl-9,10-di(2-naphthyl)anthracene]carbazole (3)

A 200 mL three-necked flask was charged with 3,6-dibromocarbazole (195 mg, 0.6 mmol), sodium tert-butoxide(192 mg, 2 mmol), and THF (20 mL). The obtained mixturewas heated to 50 8C and maintained at this temperature for30 min under an argon atmosphere. A solution of 2 (262 mg,0.5 mmol) in 20 mL of THF was then added. The mixture wasrefluxed overnight, and the reaction was quenched by adding10 mL of water. The obtained reaction mixture was extractedtwice with dichloromethane (50 mL), and the organic extractswere washed with water, and dried over anhydrous Na2SO4.After evaporation of the solvent, the residue was purified bycolumn chromatography using a mixture of petroleum etherand dichloromethane (4:1, v/v) as an eluent to give 3 as ayellow powder (342 mg, 88.8% yield). MS (EI): m/z¼ 767.5(Mþ). 1H NMR (300 MHz, CDCl3): d¼ 6.70–8.01 (m, 27H),5.46 (s, 2H). C47H29Br2N � 0.5(C7H8): Calcd. C 74.53, H 4.06,N 1.72, Br 19.68; Found: C 74.58, H 3.98, N 1.70, Br 20.66%.

Synthesis of the Polymer (PFC-1)

A solution of degassed aqueous potassium carbonate (2 M,8 mL), containing one drop of Aliquat 336, was added to a

mixture of 3 (615 mg, 0.8 mmol), 9,9-dioctylfluorene-2,7-bis(trimethyleneborate) (447 mg, 0.8 mmol), Pd(PPh3)4(92 mg, 0.08 mmol), and toluene (100 mL) with stirring atroom temperature. The reaction mixture was allowed to warmto reflux and maintained at that temperature for 84 h. Themixture was then cooled to room temperature and filtered.The filtrate was concentrated, and the resulting residue wasdissolved in dichloromethane (10 mL). The solution wasadded dropwise to 200 mL of methanol to give the polymerprecipitate, which was filtered off, washed with methanol andpetroleum ether, and dried under vacuum to afford PFC-1 as agrey powder, yield 85%. Molecular weight (GPC, eluent¼chloroform, detector¼RI):Mn, 4 000,Mw=Mn (PDI), 1.56.

1HNMR (300 MHz, CDCl3): d¼ 8.24 (s, 2H), 7.12–8.06 (br m,31H) 5.61 (s, 2H), 2.10 (m, 4H), 0.64–1.27 (br m, 30H). IR(NaCl): 3 054, 2 925, 2 852, 1 629, 1 599, 1 501, 1 463, 802,817 cm�1. Br(C76H69N � 0.2H2O)4C29H40B(OH)2: Calcd. C88.65, H 7.09, N 1.24, Br 1.78; Found C 88.66, H 7.09, N 1.28,Br 0.78.

Results and Discussion

Synthesis and Characterization

According to the procedure shown in Scheme 1, the new

polymer, PFC-1,was preparedwith a yield of 85%and had a

number-average molecular weight, Mn, of 4 000, and a

weight-average molecular weight,Mw, of 6 500. Although

PFC-1 seems to have a low molecular weight, a chloroform

solution of the polymer cast on glass and platinum plates

gives a smooth film, which is suitable for optical and

electrochemical measurements. Interestingly, according to

the content of the terminal Br (0.78%, see Experimental

Part), theMn of PFC-1 is estimated to be approx. 9 000. The

difference in molecular weight between the GPC and

elemental analysis may be a result of the special structure of

PFC-1.

The chemical structure of PFC-1 is confirmed by IR,1H NMR, and elemental analysis. The 1H NMR spectra

of the polymer shows that the hydrogens in the aryl rings are

at d 7.12–8.4, and the H signals in the benzyl and in

alkyl side chains are observed at d 5.61 and 0.64–2.10,

respectively. All the detected peaks, and the ratios

between the peak areas, are consistent with the proposed

structure.

Optical and Thermal Properties of the Polymer

UV-Vis spectra of PFC-1 in toluene and in the solid state are

shown in Figure 1. To compare the optical properties of

PFC-1 with those of poly(fluorene-co-carbazole)s posses-

sing an alkyl (or benzyl) side chain at the 9-position of the

carbazole moiety, we have prepared polymers PFC-2 and

PFC-3 according to a synthetic procedure similar to that of

PFC-1 (see Scheme 1).

A New Poly(fluorene-co-carbazole) with a Large Substituent Group at the 9-Position of the Carbazole Moiety: . . . 1653


NC8H17 C8H17

PFC-3

NC8H17 C8H17

C10H21

PFC-2

nn

As shown in Figure 1, PFC-1 and PFC-3 showmaximum

UV-vis absorption peaks at 344 nm both in toluene and in

the solid state, whereas theUV-vis peak of PFC-2 in the film

is shifted by about 10 nm to a longer wavelength compared

to that of its solution, which suggests that there is still chain

aggregation in PFC-2.On the other hand, the onset positions

of theUV-vis absorption band of PFC-1 in toluene and in the

solid state appear at 424 and 430 nm, respectively. Thus,

Donset for PFC-1 is (430� 424)¼ 6 nm. However, such

Donset for PFC-2 and PFC-3 are (410� 396)¼ 14 nm and

(402� 392)¼ 10 nm, respectively. Hence, Donset follows

the order: PFC-2 (an alkyl side chain at the 9-position of

carbazole)> PFC-3 (a benzyl side chain at the 9-position

of carbazole)> PFC-1 (anADN side chain at the 9-position

of carbazole), hinting that a large side chain at the

9-position of carbazole can efficiently depress the chain

aggregation of poly(fluorene-co-carbazole)s.

PL spectra of PFC1, PFC-2, and PFC-3 are given in

Figure 2. PFC-1 shows maximum PL emission peaks at

451 nm both in toluene and in the film, whereas the peaks of

PFC-2 and PFC-3 in the solid state are red-shifted by about

25 nm from that of their solutions. These results imply that

PFC-1 is not able to easily form a p-stacking structure in thesolid state because of the large ADN moiety, which is

similar to the case of triphenylamine-type polymers.[22] In

contrast, the presence of certain p-stacking structures in

PFC-2 and PFC-3 result in the red-shifts of the polymers

in the solid state.

300 400 5000

0.5

1

1.5

300 400 5000

0.5

1

1.5

300 400 5000

0.5

1

1.5

PFC-2 in toluene, 344 nm

PFC-2 in the film, 352 nm



424 nm

430 nm



410 nm

396 nm

392 nm

402 nm

Wavelength/nm

oN

rosba dezila

mr

)u.a( ecnab

Figure 1. UV-Vis spectra of PFC-1 in toluene and in the film. Forcomparison, the UV-vis spectra of PFC-2 and PFC-3 are alsogiven. The onset positions of the UV-vis absorption band of thethree polymers are indicated by arrows.

300 400 500 600 7000

0.5

1

1.5

300 400 500 600 7000

0.5

1

1.5

300 400 500 600 7000

0.5

1

1.5







Wavelength/nm

oN

r)u.a( ytisnetni

LP dezilam

Figure 2. PL spectra of three polymers in toluene and in the film.



To further investigate the nature of the PL emission of

PFC-1, the PL spectra of ADN in toluene and in the solid

state has been measured. It is seen that the PL emission

of PFC-1 mainly derives from the ADN moiety, which

suggests that attachment of the ADN to the polymer chain

gives rise to efficient emission from non-aggregated ADN

chromophores.

The PL spectra of PFC-1 and PFC-2 in toluene at

different concentrations (1� 10�4 and 1� 10�7M) are also

measured and the results are shown in the Supporting

Information. As shown in the Supporting Information, the

maximum emission peak of PFC-2 is red-shifted by about

20 nm upon increasing the solution concentration from

1� 10�7 to 1� 10�4M, whereas the red-shift for PFC-1 is

only about 4 nm, which implies that PFC-1 has a weak

tendency to formap-stacking structure. Such results exhibitthat PFC-1 maintains the PL properties of non-aggregated

ADN.

The reported poly(fluorene-co-carbazole)s show PL

quantum yields of about 35–70%.[16–18] In our case, the

quantum yield of PFC-1 in toluene is measured to be about

50%, using a 0.5 M H2SO4 solution of quinine (10�5M) as

a reference; such a value is close to those reported for

other poly(fluorene-co-carbazole)s, which suggests that the

introduction of a large ADN group to poly(fluorene-co-

carbazole) not only gives rise to blue emission but also

maintains the quantum yield level.

DSC traces of PFC-1 are shown in the Supporting

Information. PFC-1 has a glass transition temperature, Tg,

of 123 8C, which is similar to those of PFC-2 and PFC-3,

which indicates that attachment of a large ADN group to

poly(fluorene-co-carbazole) does not decrease the thermo-

stability of the polymer. The TGA curve of PFC-1 also

suggests that the polymer has good thermal stability: its

5 wt.-% loss temperature is more than 400 8C.

Electrochemical Properties

The electrochemical behavior of PFC-1 is characterized by

cyclic voltammetry (CV) with its film on platinum wires.

Figure 3 shows theCVcurves of PFC-1. The polymer shows

electrochemical oxidation onset at 0.77 V and gives an

oxidation peak at 1.25 V vs Agþ/Ag; such electrochemical

oxidation data agree with those of the poly(fluorene-co-

carbazole)s with an alkyl side chain at the 9-position of

carbazole, which have an electrochemical oxidation onset

at 0.8–0.9 V and show oxidation peaks at about 1.3 V vs

Agþ/Ag.[16] According to the relationship[16] between

oxidation onset potential (Eoxonset) and HOMO energy, the

HOMO value of PFC-1 is estimated as �(Eoxonsetþ 4.66)¼

�(0.77þ 4.66)¼�5.43 eV. This HOMO value is in accord

with those of the reported poly(fluorene-co-carbazole)s.[16]

The electrochemical reduction onset of PFC-1 is at

�1.89VvsAgþ/Ag. From the reduction onset potential, the

LUMO value of PFC-1 is estimated to be �(�1.89þ

4.66)¼�2.77 eV; such a result is also close to those of the

reported poly(fluorene-co-carbazole)s.[16]

Hence, there is no evident difference between PFC-1 and

the reported poly(fluorene-co-carbazole)s in electrochem-

ical behavior, which implies that the introduction of a large

ADNgroup into the carbazolemoiety does not influence the

electrochemical properties of the polymers.

Conclusion

We have synthesized a new poly(fluorene-co-carbazole)

(PFC-1) with a large substituent group (ADN, an anthra-

cene derivative moiety) at the 9-position of carbazole.

PFC-1 shows almost the same optical properties as those of

ADN, which suggests that the attachment of ADN to

poly(fluorene-co-carbazole)s achieves an efficient blue

emission from non-aggregated ADN.No evident difference

between PFC-1 and the reported poly(fluorene-co-

carbazole)s in PL quantum yield, thermostability, and

electrochemical behavior is observed, which indicates that

PFC-1 is a good blue emitter.

Acknowledgements: Financial support from the ChineseAcademy of Sciences (‘‘Bai Ren’’ Project) is greatlyacknowledged.

-3 -2 -1 0

1.25 V

0.72 V

oxidation onset = 0.77 V

E/V vs Ag+/Ag

0

1

2

3

I/A

m

0

0.5

-0.5reduction onset = -1.89 V

0 0.5 1.0 1.5

Figure 3. CV charts of the film of PFC-1 on a Pt electrode (wire)in an acetonitrile solution of 0.10M [Bu4N]BF6 (Bu¼ butyl)with asweep rate of 100 mV � s�1.

A New Poly(fluorene-co-carbazole) with a Large Substituent Group at the 9-Position of the Carbazole Moiety: . . . 1655


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A New Poly(fluorene-co-carbazole) with a Large Substituent Group at the 9-Position of the Carbazole Moiety: An Efficient Blue Emitter