Elsevier

Polymer

Volume 52, Issue 4, 17 February 2011, Pages 976-986
Polymer

Carbazole/fluorene copolymers with dimesitylboron pendants for blue light-emitting diodes

https://doi.org/10.1016/j.polymer.2010.12.060Get rights and content

Abstract

A series of random and alternating carbazole/fluorene copolymers with various dimesitylboron-containing carbazole derivative contents were synthesized by Suzuki polymerization for use as a light-emitting layer in blue light-emitting diodes. Two carbazole derivatives, CzPhB and CzPhThB consisted of a carbazoyl group as the donor and a dimesitylboron group as the acceptor group, separated by phenyl and phenyl-thiophene groups π-conjugated systems, respectively. The copolymers exhibited good thermal stability and blue emission in both solution and the solid state. Moreover, the CzPhB/fluorene and CzPhThB/fluorene copolymers exhibited a higher PL quantum efficiency than the fluorene-based homopolymer (POF). Higher brightness and larger current efficiency were observed for the CzPhB/fluorene and CzPhThB/fluorene copolymer-based devices compared to the POF-based device. Additionally, the CzPhThB/fluorene copolymer-based devices had better EL performances than the CzPhB/fluorene copolymer-based devices. The turn-on voltage, maximal brightness, and highest luminescence efficiency of the carbazole/fluorene copolymer-based devices were found to be 4.5–8.5 V, 436 cd/m2, and 0.51 cd/A, respectively.

Introduction

In the past decades, polymer light-emitting diodes (PLEDs) have received attention because of their applications in full-color flat panel displays and solid state lighting sources [1], [2]. PLEDs have a number of advantages over inorganic or organic small molecule-based light-emitting diodes, including easy processing, low operating voltages, low cost fabrication, and high flexibility [3]. For a full-color display, the need to develop more stable and highly efficient three primary color (red, green, and blue) emitters is important for allowing PLEDs to become commercial products [4]. Of particular interest are blue light-emitting materials, which can serve as either blue light sources in a full-color display or as host materials for lower energy fluorescent or phosphorescent dyes [5], [6], [7], [8]. Therefore, developing stable blue EL materials with high efficiency and excellent Commission Internationale de L’Enclairage (CIE) coordinates (y-coordinate value < 0.15) is essential to realizing such applications.

Conjugated polyfluorenes (PFs) have evolved to be the most promising candidates for blue-emitting materials for PLEDs because of their highly efficient blue emission in photoluminescence (PL) and electroluminescence (EL), excellent thermal and chemical stability, and good solubility in common organic solvents [9], [10], [11], [12], [13]. However, an undesired emission appearing in the long-wavelength division (from 500 to 600 nm) of the emission spectra of PF homopolymers not only hampers the EL efficiency, but impairs the color purity. Either the formation of aggregates/excimers or degradation of the polymers during operation of the PF-based PLEDs has been proposed as the source of this problem [14], [15], [16]. The long-wavelength emission could be curtailed by introducing bulky substituents or long alkyl chains at the C-9 position of the fluorene unit through copolymerization with appropriate co-monomers or the attachment of bulky end-capping groups, among others [17], [18], [19], [20], [21], [22], [23], [24]. Additionally, most PF-type polymers with low highest occupied molecular orbital (HOMO) levels have a high energy barrier to hole-injection from the anode, resulting in imbalanced charge mobility and subsequent low quantum efficiency [25], [26]. Chemical structure modification of PFs by incorporating electron-donor moieties, such as triarylamine, carbazole, and thiophene groups, seems to improve the deficiency in hole-injection properties [20], [27], [28]. It is well-known that attaching a carbazole moiety to the molecular scaffold can significantly enhance the thermal stability and HOMO energy level of light-emitting polymers (LEPs). Moreover, the 3-, 6- or 9-position of the carbazole moiety can be easily functionalized, and thus the photo-physical properties of the polymers can be tuned [29], [30], [31], [32]. N-arylated carbazoles, in which a phenyl or napthyl group is attached at the 9-position of the carbazole, have shown excellent thermal stability and good electro-optical properties in small-molecule OLEDs [33]. As far as the conjugated polymer is concerned, the imbalanced transport properties between the holes and electrons are another crucial factor deciding the efficiency of PLEDs. Due the poor electron-mobility of LEPs, the attachment of electron-deficient groups (such as pyridine, benzothiadiazole, quinoxalines, and oxadiazole, etc.) onto a polymer chain has proven to be an effective methodology to promote its electron transport capabilities [34], [35], [36], [37]. Several studies have also demonstrated that borane-derivatives are potential luminescent and electron-transporting materials [38], [39], [40]. The air and moisture stabilities of electron-deficient arylboranes can be improved by incorporating the non-coplanar dimesityl group into the molecules [41], [42], [43]. Moreover, the non-coplanar structure of dimesitylborane could hinder the molecular close packing in the solid state. A stable amorphous film was formed for a dimesitylborane moiety containing carbazole derivative [40]. Although arylborane-containing fluorophores have been successfully developed as light emitters, only a few arylborane moieties containing LEPs have been proposed for PLED applications [44], [45], [46], [47]. Chujo et al. synthesized a series of alternating copolymers by the hydroboration polymerization of aromatic diynes and mesitylborane [44]. Blue emission was observed for the copolymers in dilute solution state. The copolymers containing boron atoms in the main-chains are expected to act as electron-deficient π-conjugated systems, where π-conjugation length is extended via the vacant p-orbital of the boron atom [44]. However, the EL properties of the main chain organoboron polymer-based emitting layers have not been investigated. In addition, Yamaguchi and coworkers reported a series of highly emissive diborylphenylene containing poly(arylenethyynylene)s [45], [46]. The presence of bulky diarylboryl substituents not only acts as a good electron-accepting unit, but also prevents the interaction and aggregation between the polymer backbones. However, these poly(arylenethyynylene)s exhibited sky-blue to green emission as thin films due to the strongly intramolecular charge transfer (ICT) transition from the π-conjugated backbone to the diborylphenylene unit. EL properties of the poly(arylenethyynylene)s were not discussed even through the fact that highly absolute quantum yields were observed for these polymer-based thin films [46]. More recently, two N-p-(diarylboryl)phenyl-substituted polycarbazoles were reported by Lambert et al. [47]. Although the authors demonstrated a light-emitting device based on N-p-(diarylboryl)phenyl-substituted 3,6-linked polycarbazoles, but only the EL spectrum was shown in the literature.

Base on the above, highly blue emissive conjugated polymers could be obtained by the incorporation of the electron-deficient arylborane unit into the conjugated polymers [44], [45], [46], [47]. It is important to note that EL properties have never been discussed for these arylborane unit containing conjugated polymers. In addition to the bipolar structure in the polymer, the conjugation length in both side-chain moiety and polymer backbone should be taken into account for pursuing a polymer-based device with excellent EL performances. In this study, a series of random and alternating carbazole/fluorene copolymers with various contents of dimesitylboron-containing carbazole derivatives were designed and synthesized for use as blue emitters in PLEDs because the bipolar carbazole-π-dimesitylboron unit exhibits a high PL quantum efficiency in both solution and the solid state [40]. Two carbazole derivatives, CzPhB and CzPhThB consisted of a carbazoyl group as the donor and a dimesitylboron group as the acceptor group, separated by phenyl and phenyl-thiophene groups π-conjugated systems, respectively. Incorporating the bipolar carbazole derivative as the pendant of the conjugated polymer was favorable for improvement of the charge-injection/transporting characteristics of the PF. Excellent EL properties were expected for these carbazole/fluorene copolymer-based PLEDs. The thermal stability, electrochemical properties, photo-physical behavior, and EL performances of the carbazole/fluorene copolymer-based devices are discussed in detail as the chemical structures and carbazole derivative content of the copolymers are taken into account.

Section snippets

Materials

All reactions and manipulations were performed in a nitrogen atmosphere using standard Schlenk techniques. All chromatographic separations were carried out on silica gel. 9H-carbazole, 1,4-dibromobenzene, N-bromosuccinimide (NBS), copper powder and 18-crown-6 were purchased from Acros Co. Potassium carbonate (K2CO3) was bought from Fisher Scientific. Tetrakistriphenylphosphine palladium(0) [Pd(PPh3)4] was purchased from Strem Chemicals. 2.7-Dibromo-9,9-dioctylfluorene (compound 3), n-butyl

Synthesis and Characterization

Scheme 1 illustrates the synthetic routes of the monomer and carbazole/fluorene copolymers. Compound 1 was synthesized from carbazole and 1,4-dibromobenzene through an Ullmann coupling reaction [48] and then further reacted with thiophen-2-ylboronic acid via Suzuki coupling to obtain compound 2. Compound 1 and compound 2 were then further reacted with n-BuLi in THF at −78 °C, followed by the addition of dimesitylboron fluoride to give compounds 3 and 4, respectively. Compounds 5 (CzPhB) and 6

Conclusion

A series of novel carbazole/fluorene copolymers with dimesitylboron pendants were synthesized by Suzuki coupling. Excellent thermal stability was observed for the copolymers due to incorporation of the rigid carbazole-π-boron (CzPhB and CzPhThB) pendants in the fluorene-based backbone. The CzPhB/fluorene and CzPhThB/fluorene copolymers showed a higher PL quantum efficiency than the POF. Moreover, higher brightness and larger current efficiency were observed for the CzPhB/fluorene and

Acknowledgements

Financial support from National Science Council (NSC) and Ministry of Education, Taiwan under ATU plan are gratefully acknowledged.

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