Materials Science and Engineering B85 (2001) 232–235

Synthesis and characterization of novel fluorene–thiophene-basedconjugated copolymers

Bin Liu a, Wang-Lin Yu a, Jian Pei a, Yee-Hing Lai a, Wei Huang a,*, Yu-Hua Niu b,Yong Cao b

a Institute of Materials Research and Engineering (IMRE) and Department of Chemistry, National Uni�ersity of Singapore, 3 Research Link,Singapore 117602, Singapore

b College of Materials Science, South China Uni�ersity of Technology, Wushan Road, Guangzhou 510640, People’s Republic of China

Abstract

A novel series of soluble alternating conjugated copolymers comprised of 9,9-dihexylfluorene and substituted bithiophene orthiophene moieties have been synthesized by palladium-catalyzed Suzuki coupling reaction. The polymer structures werecharacterized and confirmed by NMR, FT-IR, and elemental analysis. The defined structures of the polymers provide us thepossibility to study the structure–property relationships of the polymers based on the hybrid backbones, especially by controllingthe substitution on thiophene rings and the coupling configurations between two adjacent thiophene rings. All of the fourpolymers have demonstrated efficient blue-to-green light emission, good solubility in common organic solvents, good thermalstability, relatively high glass transition temperatures and good electrochemical properties. © 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Synthesis; Conjugated copolymers

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1. Introduction

In the last decade, great efforts have been devoted tothe design and synthesis of light-emitting polymers(LEPs) for the fabrication of polymer light-emittingdiodes (PLEDs) [1,2]. A number of conjugated back-bone structures, including poly(para-phenylenevinylene) (PPV) [3–6], poly(para-phenylene) (PPP) [7–9], polythiophene (PT) [10–12], polyfluorene (PF) [13–15], have been demonstrated to be of great value inrealizing different emissive colors. Recently,polyfluorene and its derivatives (PFs) have been of greatinterest due to their blue emission and high efficienciesboth in photoluminescence (PL) and in electrolumines-cence (EL) [16–18]. However, the drawbacks of PFs,such as aggregation and/or excimer formation in solidstates, insufficient stability and high energy barriers forhole-injection, have limited their application in PLEDs[13,17,19,20]. It was found that copolymerization offluorene with various aryl partners allows for tunability

of electronic properties and enhancement of stability[19,21]. This co-polymerization approach is particularlyinteresting for PFs, whose electronic properties are noteasily tuned by conventional chemical modifications.

Recently, Tsuie et al. reported the synthesis of somefluorene-heterocycle hybrid polymers, including the co-polymers of fluorene and thiophene or bithiophene,through the Stille coupling reaction [22]. In their synthe-sis, however, thiophene and bithiophene were un-substi-tuted, and the molecular weights of the polymers wererelatively low (Mn�9000 g mol−1). For thiophene-based polymers, one of the most striking features is theeasy and wide electronic tunability by side chain modifi-cation [11]. Moreover, different structural regularitiesarising from side chain substitution in PTs offer addi-tional opportunities for tuning electronic properties bycontrolling the coupling configurations [23–25]. In thispaper, we have incorporated several different thiophenederivatives, specifically chosen for their regioregularstructures, along with the fluorene moiety, in order toexplore some new materials and study the relationshipbetween the polymer structures and their optical andelectrical properties.

* Corresponding author. Tel.: +65-8748592; fax: +65-8720785.E-mail address: wei-huang@imre.org.sg (W. Huang).

0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0921 -5107 (01 )00593 -1

B. Liu et al. / Materials Science and Engineering B85 (2001) 232–235 233

2. Results and discussion

The polymers were synthesized through the Suzukicoupling reaction in good yields. After purification and

drying, the polymers P1, P2, P3, and P4 were obtainedas yellow, light yellow, bright yellow and greenishyellow powders, respectively. All these polymers (withstructures depicted in Fig. 1) readily dissolve in com-mon organic solvents, such as chloroform, THF, tolu-ene, and xylene. The number average molecular weights(Mn) of the polymers were determined by gel perme-ation chromatography (GPC) against polystyrene stan-dards to be �15 000 to �23 000 with thepolydispersity index of 1.4–1.9 (Table 1). The chemicalstructures of the polymers were verified by 1H NMR,13C NMR, FT-IR, and elemental analyses.

The thermal stability of the polymers in nitrogen wasevaluated by thermogravimetric analysis (TGA). Thepolymers showed weight loss starting at 394°C for P1,390°C for P2, 381°C for P3, and 374°C for P4, respec-tively, an indication of good thermal stability. Ther-mally induced phase transition behavior of thepolymers was investigated with differential scanningcalorimetry (DSC) in a nitrogen atmosphere. P1 ex-hibits a clear glass transition starting at �63°C, withP2 having a Tg slightly higher (by �14°C) than that ofP1. When the bithiophene moieties are replaced by�-substituted thiophene (P3), a phase transition wasobserved at 203°C, which may be attributed to the meltof the polymer. When the thiophene moiety is disubsti-tuted (P4), a clear glass transition was observed atabout 78°C. The determined glass transition tempera-tures of P2 and P4 are comparable to those of 9,9-di-alkyl polyfluorenes [26]. The relatively high glasstransition temperatures are essential for many applica-tions, such as in light-emitting diodes as emissivematerials.

2.1. Optical properties

The spectroscopic properties of P1–P4 were mea-sured both in solution (CHCl3) and as thin films. TheUV–visible absorption and photoluminescence (PL)spectra of P1–P4 in chloroform (ca. 1×10−5 M) areshown in Fig. 2. P1 exhibits an absorption maximum at401 nm. Its PL spectrum peaks at 482 nm with ashoulder around 515 nm. Interestingly, P2 gives almostidentical absorption and PL spectra with P1. The UV–visible absorption of P3 in chloroform solution bears aresemblance to P2 and P1, with the maximum at 403nm, but with its PL spectrum blue-shifted by about 20nm compared to those of P1 and P2. The PL differencemay be attributed to the different radiative decay pro-cess of excitons in the polymers. In comparison withP3, P4 shows an obvious spectral blue shift both inabsorption (��=36 nm) and in emission (��=14 nm).

Transparent and uniform films of the polymers wereprepared on quartz plates by spin-casting their solu-

Fig. 1. The structures of P1–P4.

Table 1Number average (Mn) and weight average (Mw) molecular weight ofP1–P4

Mn Mw Mw/MnPolymer

1.9P1 15 200 29 10039 40022 600P2 1.7

18 700P3 26 600 1.418 600 1.629 300P4

Fig. 2. The UV–visible absorption spectra and photoluminescencespectra of P1–P4 measured from the solutions (�1×10−5 M) inchloroform at room temperature.

B. Liu et al. / Materials Science and Engineering B85 (2001) 232–235234

Fig. 3. The UV–visible absorption spectra and photoluminescencespectra of P1–P4 measured from the spin-coated films on quartzplates at room temperature.

Fig. 4. Cyclic voltammograms of P1–P4 films coated on platinumplate electrodes in acetonitrile containing 0.1 M Bu4NClO4. Counterelectrode: platinum wire. Reference electrode: Ag/AgNO3 (0.10 M inacetonitrile). Scan rate: 50 mV s−1.

tions in toluene at room temperature. The films of P1and P2 emit intensive green light by the excitation ofUV light. P3 film emits bluish green light, while theemissive color of P4 film is blue. The UV–visible andPL spectra of P1–P4 as films are displayed in Fig. 3.Both the absorption spectra and the PL spectra of P1and P2 are only very slightly red shifted with respect totheir corresponding spectra in solution, an indicationthat there is no noticeable molecular conformationchange from solution to solid film states. The spectro-scopic parameters of the four polymers in film statesare summarized in Table 2.

2.2. Electrochemical properties

The electrochemical behavior of the polymers wasinvestigated by cyclic voltammetry (CV). A platinumelectrode (�0.08 cm2) was coated with a thin polymerfilm and was used as the working electrode. A Pt wirewas used as the counter electrode and an Ag/AgNO3

electrode was used as the reference electrode.

As shown by the cyclic voltammograms in Fig. 4, allthe polymers (P1–P4) exhibit partial reversibility inboth n-doping and p-doping processes. The electro-chemical data for the four polymers are also summa-rized in Table 2. The electrochemical properties of P1and P2 are very similar, which demonstrates again thatthe conformational difference in P1 and P2 does notcause a noticeable electronic difference between the twopolymers. Comparing the electrochemical data of P3with those of P1 and P2, we found a small decrease inthe reduction potential and a slight increase in theoxidation potential for P3. This reveals that due to theaddition of one more thiophene ring, a strong �-exces-sive group, into the repeat unit, P1 and P2 are moreelectropositive than P3, although P1 and P3 have thesame band gap. In comparison with P3, P4 exhibitsboth higher oxidation potential (by 0.19 V) and higherreduction potential (by 0.16 V). This is evidently due tothe decrease in effective conjugation of the polymermain chain caused by the increased steric hindrance ofthe second decyl chain on the thiophene ring.

Table 2Optical and electrochemical data for P1–P4

Polymer �max (solution)a (nm) �max (films)a (nm) Egb (eV) p-dopingc (V) n-dopingc (V)

Em. Abs. Em. EpaAbs. Epc E1/2 Epc Epa E1/2

– −2.11 –482 (515) 403 490 (520) 2.57 1.30 0.92P1 1.11401−2.50 −2.07 −2.28P2 483 (520)398 401 493 (520) 2.60 1.28 0.93 1.10−2.31 −2.09 −2.20461 (490) 412 492 (477) 2.63P3 1.27403 1.02 1.15

458 (475)378447 −2.36−2.23−2.481.341.18367 1.49P4 2.70

a The data in parentheses are the wavelengths of shoulders and sub-peaks.b Stands for the band gap energy estimated from the onset wavelength of optical absorption.c Epa and Epc: anodic peak potential and cathodic peak potential, respectively.

B. Liu et al. / Materials Science and Engineering B85 (2001) 232–235 235

3. Conclusions

Four fluorene-based conjugated copolymers com-posed of alternating 9,9-dialkylfluorene and substitutedthiophene or bithiophene were synthesized through pal-ladium-catalyzed Suzuki coupling reaction. The optical,electrochemical, and thermal properties are all sensitiveto the changes of the thiophene ring number in therepeat unit and the substitution on the thiophene ring,while for the polymers comprised of bithiophene moi-eties, their optical and electrochemical properties areindependent of the coupling configurations of the sub-stituted bithiophene moieties. Good solubility in com-mon organic solvents, good thermal stability, andrelatively high glass transition temperatures weredemonstrated with these backbone structures. The pre-liminary characterization results have shown potentialapplications of this series of polymers in polymer lightemitting diodes (PLEDs).

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