Macromolecules 2011, 44, 17–19
17
DOI: 10.1021/ma102446b
*Corresponding authors. E-mail:
[email protected] (Y.M.); hbwu@ scut.edu.cn (H.W.);
[email protected] (X.C.).
In this Communication, we report the synthesis a soluble poly(9,9-dialkoxyphenyl-2,7-silafluorene) (PSF) by the Yamamoto reaction. The thermal, photophysical, and EL properties of the obtained polymer were investigated. Experimental Section. The synthesis of PSF is shown in Scheme 1. 9,9-Dichloro-2,7-dibromosilafluorene17 is the key intermediate for the synthesis of 9,9-di(4-(3 0 ,70 -dimethyloctyloxy))-2,7-dibromosilafluorene (1). We tried another intermediate;9-phenyl-9-chloro-2,7-dibromosilafluorene;but it is difficult to get the objective product (1). In order to get high yield of 9,9-dichloro-2,7-dibromosilafluorene, excess SiCl4 was added to suppress the formation of spirosilafluorene18 and was stripped eventually before the addition of alkoxyphenyllithium. PSF was obtained via the Yamamoto reaction. After the solution of 1 and nickel catalyst was stirred at 85 C for 2 days, bromobenzene was added to diminish the bromo end groups which may cause formation of green emission aggregates.19After purification, PSF with Mn of 23 000 and Mw of 92 000 was obtained. PSF was readily soluble in common organic solvents, including toluene, chlorobenzene, chloroform, and THF. Results and Discussion. The UV-vis absorption spectra and photoluminescent spectra (PL) of PSF in CH2Cl2 and in solid state are shown in Figure 1. It can be seen that the absorption spectrum of PSF in solution are almost identical to that in film, with the absorption maximum at 391-392 nm, implying that the conjugation length does not change upon the film was formed after spin-coating. The absorption onset of PSF is around 438 nm; thus, the optical bandgap (Eg) of PF is estimated at ca. 2.83 eV, slightly smaller than that of poly(9, 9-dialkysilafluorene) (2.93 eV).5 As the onset potential of the oxidation process (p-doping) of PSF occurs at about 1.38 V against Hg/Hg2Cl2 (as shown in Figure S5), the HOMO level of PSF was calculated to be -5.78 eV according to the empirical formula EHOMO = -(Eox þ 4.4) (eV),20 which is quite close to that of poly(9,9-dialkysilafluorene) (-5.77 eV).5 The LUMO level was calculated to be about -2.95 eV from the HOMO level and the optical bandgap (Eg). As can be seen from the PL spectra of PSF in CH2Cl2 (ca. 5 ppm) and in the solid state, which were obtained by a Fluorolog JY luminescence spectrometer under excitation of 340 nm, the PL spectrum of PSF in film shows a 14 nm of red shift as compared with that in CH2Cl2 while both PL spectra show strong vibronic structure. The PL maximum of PSF in film is located at 437 nm, which shows about 12 nm of red shift compared with poly(9,9-dialkysilafluorene) film,5 with two vibronic sidebands at 461 and 495 nm, respectively. The PL quantum yields of PSF in dilute 1,2-dichloroethane and in film were estimated to be ca. 95% and 75% as measured in an integrated sphere, respectively. To investigate the electroluminescence properties of the PSF, single-active-layer devices with the configuration ITO/ PEDOT:PSS/PSF/Ba/Al and ITO/PEDOT:PSS/PVK /PSF/ Ba/Al have been fabricated, where poly(N-vinylcarbazole) (PVK) was used as hole transporting layer. The EL spectra of the devices and the PL spectra of the PSF are shown in Figure 2. We note that the EL spectra of the devices in both configurations are nearly identical to that of the PL spectrum, with an emission peak at ca. 435 nm and a shoulder at 458 nm, implying that EL and PL have the same origin. Unlike poly(dioctylfluorene), in which pronounced emission in the longer wavelength
r 2010 American Chemical Society
Published on Web 12/15/2010
Highly Efficient and Stable Deep Blue Light Emitting Poly(9,9-dialkoxyphenyl- 2,7-silafluorene): Synthesis and Electroluminescent Properties Jun Wang,† Chang-qing Zhang,† Cheng-mei Zhong,‡ Su-jun Hu,‡ Xue-yi Chang,§ Yue-qi Mo,*,† Xiwen Chen,*,^ and Hong-bin Wu*,‡ † Key Laboratory of Special Functional Materials, South China University of Technology, Guangzhou 510640, P. R. China, ‡ Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China, §Canton Oledking Optoelectronic Materials Co., ltd, Guangzhou 510640, P. R. China, and ^CSIRO Materials Science and Engineering, Bayview Avenue, Clayton, VIC 3168, Australia
Received October 27, 2010 Revised Manuscript Received December 5, 2010 Introduction. Highly efficient blue emitting polymer with good color purity is one of the key issues of the commercialization of the polymer light emitting diodes (PLEDs)1 for the next-generation flat-panel displays. Although many efforts have been focused on poly( p-phenylene)s, polycarbazoles, polythiophenes, etc., 2,7-fluorene-based polymers2 are regarded as the most successful blue light emitters. Very recently, Yang et al.3 and Chen et al.4 reported highly efficient blue emitting PLEDs based on alkoxyphenyl-substituted poly(2,7-fluorene) derivatives. On the other hand, poly(9,9-dialkylsilafluorene)s are another promising candidate for blue light emitting PLEDs, which was first reported by Holmes’ group5 and us nearly the same time in 2005.6 In general, polysilafluorenes exhibit even more stable blue emission due to the higher oxidative stability of Si at the 9-position of polysilafluorenes as compared to that of the C-9 carbon of polyfluorenes, which usually result in the formation of the keto defects.7 Moreover, Si-containing polymers are expected to have higher electron affinity owing to the σ*-π* conjugation.8 As a result, photovoltaic cells based on 9,9-dialkylsilafluorene-based copolymers have been demonstrated to have higher open voltage circuit as compared to that based on its polyfluorene analogue,9 and the related PLEDs exhibit very good EL performance,10,11 which received intense attention.12 However, we note that the device based on the homopolymer of 9,9-dialkylsilafluorene shows a moderate external quantum efficiency (EQE) of 0.66% and a maximal luminous efficiency of 0.53 cd/A.10 Given the fact that polymers with aryl side chains usually have higher thermal stability due to higher glass transition temperatures than those with alkyl side chains,13,14 bulky aryl substitutes were thought to result in more effective suppression of aggregation than alkyl substitutes. As it can lead to the suppression of excimer formation and longwavelength emission,15 it is very interesting to develop homopolymers of 9,9-diarylsilafluorene for blue light emitting devices. Although some 9,9-diarylsilafluorene-containing compounds had been reported previously,16 to the best of our knowledge, processable homopolymers of 9,9-diarylsilafluorene have not appeared in the literature.
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Communication Scheme 1. Synthesis of the PSF
Table 1. Device Performance of the PLEDs Based on PSFa entry 1 2 3 4
treatment 120 C, 30 min 120 C, 30 min
LEmax (cd/A)
EQEmax (%)
Von (V)
Lmax (cd/m2)
CIE
0.32 0.53
0.31 0.51
3.2 2.8
1540 2362
(0.16, 0.12) (0.16, 0.12)
2.1 2.3
2.02 2.21
3.2 3.4
6278 6107
(0.17, 0.13) (0.17, 0.13)
a
Configurations of 1 and 2: ITO/PEDOT:PSS/PSF/Ba/Al. Configurations of 3 and 4: ITO/PEDOT:PSS/PVK/PSF/Ba/Al. LE: luminous efficiency; EQE: external quantum efficiency; L: luminance; Von: turn-on voltage.
Figure 1. Absorption and PL spectra of PSF in CH2Cl2 and in solid state.
Figure 2. PL and EL spectra of the devices fabricated from PSF in device configuration of ITO/PEDOT:PSS/PSF/Ba/Al and ITO/PEDOT:PSS/PVK/PSF/Ba/Al.
region typically exists and unstable blue emission upon increase of temperature and operation stress,15 the EL spectra of the PSF devices remain nearly unchanged even the devices were annealed at 120 C for 30 min. The excellent spectral stabilities of PSF can be attributed to the bulky 9,9-diphenyl substitutes, which could result in the suppression of aggregation and/or excimer formation. The CIE coordinates of the blue emission were found to be around from (0.17, 0.12) to (0.17,0.13), representing deep blue emission in the 1931 CIE diagram. Table 1 summarizes the performance of the PSF devices fabricated from two device configurations. With a device configuration of ITO/PEDOT: PSS/PSF/Ba/Al, the maximal LE of 0.53 cd/A (corresponding to an EQE of 0.51%.) was obtained after annealing at 120 C for 0.5 h. Upon the incorporation of a thin layer of PVK (∼30 nm), despite the turn-on voltage (defined as the voltage at which a luminance of 1 cd/m2 was measured) slightly increased, the LE dramatically increased to 2.3 cd/A, while the EQE of the device reached 2.2%. We noticed that the best performance of a
polyfluorene analogue;poly[9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene-2,7-diyl];was reported to a luminous efficiency of 1.9 cd/A (corresponding to an EQE of 1.8%) with CIE coordinates of (0.16, 0.12).3 The current density-luminancevoltage (J-L-V ) characteristics of device A and the LEcurrent density (LE-J ) characteristics of both devices are plotted in the Figures S6 and S7. The thermal properties of PSF were investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. PSF exhibits excellent thermal stability from the TGA and DSC measurements (shown in Figures S8 and S9). In the DSC trace of heating (10 C/min), PSF exhibited a glass transition at about 120 C, which is slightly higher than that of poly(9,9-dialkoxyphenyl2,7-fluorene) (108 C)14 and much higher than that of poly(9, 9-dihexyl-3,6-fluorene) (83 C).6 TGA curve shows that decomposition temperature of PSF starts at 426 C (5% weight loss), which is close to that of poly(9,9-dialkysilafluorene).5 We attribute the excellent thermal stabilities of PSF to the 9,9-diphenyl substitutes. Conclusion. In conclusion, the soluble poly(9,9-dialkoxyphenyl-2,7-silafluorene) was prepared via Yamamoto reactions. This polymer with high glass transition temperature of 120 C exhibits a high external quantum efficiency of 75% and a luminous efficiency of 2.1-2.3 cd/A. Acknowledgment. The authors are grateful for financial support from Fundamental Research Funds for the Central Universities of China (No. 2009ZM0307), Ministry of Science and Technology Project (No. 2009CB623601), and the National Nature Science Foundation of China (No. U0634003). We thank Prof. Hongzhi Wang for help with the PL measurements. Supporting Information Available: NMR spectra of monomers and PSF, cyclic voltammograms of PSF, DSC heating curves of PSF, TGA of PSF, the current density-luminance-voltage (J-L-V ) characteristics and the LE-current density (LE-J ) characteristics of the devices from PSF. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, 539–541.
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