Synthetic Metals 156 (2006) 809–814
N-Aryl carbazole derivatives for non-doped red OLEDs Hui-ying Fu a , Huan-rong Wu b , Xiao-yuan Hou b , Fei Xiao a,∗ , Bing-xian Shao a b
a Department of Materials Science, Fudan University, Shanghai 200433, China State Key Laboratory of Applied Surface Physics, Fudan University, Shanghai 200433, China
Received 13 January 2006; received in revised form 21 April 2006; accepted 24 April 2006 Available online 16 June 2006
Abstract Two N-aryl carbazole derivatives: 3-2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl-N-naphthyl-carbazole (NCz-2CN) and 3,6-bis(2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl-N-phenyl-carbazole (PCz-4CN), with the molecular structure of donor-acceptor, have been synthesized and characterized. They are red emitters in the solid films with a peak wavelength at 630 nm of NCz-2CN and 666 nm of PCz-4CN. Non-doped orange-red electroluminescent devices with the structure of ITO/NPB/NCz-2CN/BCP/Alq3 /LiF/Al were fabricated. The device showed orange-red emission at λmax = 628 nm and a maximum luminance of 4110 cd/m2 obtained at 15 V. The maximum luminous efficiency was 0.49 lm/W and the current efficiency was 2.09 cd/A. © 2006 Elsevier B.V. All rights reserved. PACS: 78.60.Fi; 78.55.Kz Keywords: N-Aryl carbazole; Organic light-emitting diodes; Red emission; Non-doped device
1. Introduction Great progress has been made on organic light-emitting diodes (OLEDs) since the initial works by Kodak’s team  and Cambridge’s group  on small-molecule-based and polymerbased diodes. Displays based on OLEDs can be found in the fields of digital cameras, mobile phones, personal stereos, etc. However, molecular materials for non-doped red OLEDs still remain among the urgent demanded for the easier fabrication of devices in the mass production process compared with the dopants for doped devices [3,4]. Molecules for nondoped pure red OLEDs with high brightness and efficiency are extremely scarce [5–10]. The first reported red emitter for non-doped red OLED is [2-(p-dimethyl-aminophenyl)ethenyl]phenyl-methylene-propane-dinitrile (DPP) . A series of benzo-[␣]aceanthrylenes (ACENs) were synthesized for nondoped red OLEDs by Lin co-workers, however, the nondoped red OLEDs with ACENs as the emitting layer or as the dual functional layer (emitting and hole-transporting) showed unsatisfactory luminance and efficiency . 4-
Corresponding author. Tel.: +86 21 65643267; fax: +86 21 65103056. E-mail address: [email protected]
0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.04.013
Amino-1,8-naphthalic anhydride (ANA) derivatives were also been used to fabricate EL devices functioning as emitting layer or both emitting and electron-transporting layer . Most of the recently reported non-doping red molecules for OLEDs are fluorophores with donor--acceptor structure [5–8,13–16] and some of them showed promising EL properties. Red OLED with 2,3-bis-N,N-1-naphthylphenylamino)-Nmethylmaleimide (NPAMI) as the red-emitting layer showed a maximum luminance of 8000 cd/m2 at 15 V and a CIE coordinates of (0.66, 0.32) . Device fabricated with bis(4-(N(1-naphthyl)phenylamino)phenyl)fumaronitrile (NPAFN) as the red emitter and hole-transporting material exhibited a maximum luminance of 9359 cd/m2 at 14.5 V, and the CIE coordinates of (0.64, 0.36) . Non-doped red EL device based on fluorene-containing triarylamine derivative showed the remarkable brightness of 12410 cd/m2 , the EL spectrum peaks at 636 nm and the CIE coordinates of (0.65, 0.35) . Carbazole molecules exhibit inherent electron-donating nature, excellent photoconductivity and relatively intense luminescence. The thermal stability or the glass-state durability of organic compounds can be greatly enhanced by the introduction of a carbazole group in the core structure. Unlike diphenylamine, carbazole has a planar structure, and it can be imagined as the bonded diphenylamine. The carbazole group
H.-y. Fu et al. / Synthetic Metals 156 (2006) 809–814
Scheme 1. Molecular structures of the N-aryl carbazole derivatives.
can be easily functionalized at the 3-, 6-, or 9-position and covalently linked to other molecular groups. The N-aryl carbazoles, in which a phenyl or a naphthyl group is attached on the 9-position of carbazole, have shown excellent thermal stability and good electro-optical properties [17–19]. In this paper, we designed and synthesized novel red emitters based on N-aryl carbazole for non-doped OLEDs. The molecular structures of the fluorophores: 3-2-(3,3-dicyanomethylene-5,5-dimethyl-1cyclohexylidene)vinyl-N-naphthyl-carbazole (NCz-2CN) and 3,6-bis(2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl-N-phenyl-carbazole (PCz-4CN), are shown in Scheme 1. Their thermal, photophysical, electrochemical and electroluminescent properties were investigated. A non-doped red OLED was fabricated using NCz-2CN as the emitter, with the maximum brightness of 4110 cd/m2 obtained at 15 V and the emission peak at 628 nm. 2. Experimental 2.1. Measurements All solvents involved in the experiments were reagent grade and were purified by the usual methods before use. 1 H NMR spectra were recorded on a DMX-500 spectrometer operating at 500.134 MHz. Mass spectra (MS) were measured on an Agilent 5973N MSD instrument using standard conditions. Elemental analyses were performed on a Carlo Erba 1106 analyzer. Infrared spectra were taken in KBr pellets using a Nicolet
Magna-IR 550 ◦ C Spectrometer. Absorption spectra in solutions were recorded on a TU-1800PC UV spectrometer. Fluorescence spectra in the solid-state films were obtained using a Hitachi F-4500 fluorescence spectrophotometer. Differential scanning calorimetry (DSC) was performed using a PerkinElmer DSC Pyris1 with a scan rate of 20 ◦ C/min. The melting points of the compounds and the thermal stability were analyzed on a Shimadzu DTG-60H simultaneous DTA-TGA apparatus. The surface morphology of the NCz-2CN thin film on ITO was observed by scanning tunneling microscopy (AJ Nanoview I) and atomic force microscopy (PSI AutoProbe CP). Cyclic voltammetric experiments were performed using a CHI660 electrochemical analyzer. All measurements were carried out at room temperature with a conventional three-electrode configuration consisting of a working electrode, an auxiliary platinum electrode, and a saturated Hg/Hg2 Cl2 reference electrode. The working electrode was made by depositing thin solid film of sample onto ITO coated glass. The solvent in all experiments was distilled water, and the supporting electrolyte was 0.5 M Na2 SO4 . All potentials reported are referenced to saturated Hg/Hg2 Cl2 electrode at 4.75 eV. 2.2. Materials Alq3 , BCP and NPB were purchased from Sigma–Aldrich Company. The synthetic route of the N-aryl carbazole derivatives are shown in Scheme 2. The N-aryl carbazoles were synthesized
Scheme 2. Synthetic route of the N-aryl carbazole derivatives.
H.-y. Fu et al. / Synthetic Metals 156 (2006) 809–814
from carbazole by Ullmann reaction. The red fluorescent materials, NCz-2CN and PCz-4CN, were synthesized by a one-pot condensation of intermediate (M), which was obtained from the reaction of isophorone and malononitrile without separation, and the corresponding aldehyde under the normal Knovenagel reaction condition . Selected data of the aldehydes, NCz-2CN and PCz-4CN are as follows. 2.3. 3-Formyl-N-naphthyl-carbazole (FNCz) Phosphorus oxychloride (POCl3 , 2.3 ml, 24.5 mmol) was slowly added into the mixture of N-naphthyl-carbazole (2.93 g, 10.0 mmol) in 1,2-dichloroethane (20 ml) and dimethyl formamide (DMF, 1.9 ml, 24.5 mmol) cooled in ice-water bath while stirring. After 20 min, the reaction mixture was heated at 95 ◦ C for 21 h. The mixture was cooled to 50 ◦ C, poured into ice-water (150 ml), and neutralized with sodium hydroxide. The mixture was extracted with ether. The extract was washed with water, dried over Mg2 SO4 , and concentrated. The pale-yellow solid FNCz (1.02 g, 32%) was obtained after column chromatography with toluene as an eluent; 1 H NMR (500 MHz, CDCl3 ): δ 10.13 (s, 1H), 8.74 (s, 1H), 8.27 (d, J = 6.7 Hz, 1H), 8.09 (d, J = 8.1 Hz, 1H), 8.04 (d, J = 8.2 Hz, 1H), 7.88 (d, J = 8.5 Hz, 1H), 7.69 (t, J = 7.7 Hz, 1H), 7.64 (d, J = 7.1 Hz, 1H), 7.56 (t, J = 7.5 Hz, 1H), 7.40–7.38 (m, 2H), 7.34 (t, J = 7.5 Hz, 1H), 7.19 (d, J = 8.6 Hz, 1H), 7.05 (m, 2H); IR (KBr): 2820, 2720, 1686, 875, 840, 805, 769, 700 cm−1 . 2.4. 3,6-Diformyl-N-phenyl-carbazole (DFPCz) DFPCz was prepared in 16.5% yield after column chromatography with petroleum ether–ether (3:1) as an eluent from N-phenyl-carbazole by analogy to FNCz; H NMR (500 MHz, CDCl3 ): δ 10.16 (s, 2H), 8.74 (s, 2H), 8.03 (d, J = 8.5Hz, 2H), 7.69 (t, J = 7.7 Hz, 2H), 7.61 (m, 1H), 7.55 (d, J = 7.4 Hz, 2H), 7.47 (d, J = 8.5 Hz, 2H); IR (KBr): 2820, 2720, 1686, 900, 813, 761, 698 cm−1 . 2.5. 3-2-(3,3-Dicyanomethylene-5,5-dimethyl-1cyclohexylidene)vinyl-N-naphthyl-carbazole (NCz-2CN) Malononitrile (0.80 g, 12.1 mmol) and isophorone (1.47 g, 12.7 mmol) were added to a solution of acetic acid (14 l), acetic anhydride (11 l), piperidine (159 l) and DMF (3.6 ml). The mixture was stirred at room temperature for 1 h and then at 80 ◦ C
for 1 h. Then FNCz (1.00 g, 3.1 mmol) was added, and the reaction mixture was stirred at 80 ◦ C for 1.5 h. The mixture was poured into 150 ml of hot water containing 5 ml concentrated HCl and the precipitate was collected and stirred in a solution of iso-propanol (50 ml) and water (7 ml). The solid was collected by filtration under reduced pressure and recrystallized from ether to afford red crystals with a yield of 53%, m.p.: 225 ◦ C; 1 H NMR (500MHz, CDCl3 ): δ 8.36 (s, 1H), 8.23 (t, J = 6.5 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H), 8.04 (d, J = 8.3 Hz, 1H), 7.69 (t, J = 7.9 Hz, 1H), 7.64 (d, J = 7.1 Hz, 1H), 7.55 (m, 2H), 7.38–7.33 (m, 3H), 7.28 (d, J = 16.0 Hz, 1H), 7.25 (d, J = 8.6 Hz, 1H), 7.08 (d, J = 16.0 Hz, 1H), 7.01 (t, J = 7.5 Hz, 2H), 6.87 (s, 1H), 2.62 (s, 2H), 2.54 (s, 2H), 1.11 (s, 6H); MS (EI): m/z (%): 491 (M + 2, 100.00), 492 (M + 3, 38.43), 435 (32.70), 44 (22.39), 490 (M+ , 15.68), 304 (15.31), 128 (14.13), 291 (12.10); IR (KBr): 2958, 2213, 1390, 893, 801, 763, 750, 700 cm−1 ; Anal. Calcd. for C35 H27 N3 : C 85.85; H 5.57; N 8.58; Found: C 85.94; H 5.39; N 8.46. 2.6. 3,6-Bis(2-(3,3-dicyanomethylene-5,5-dimethyl-1cyclohexylidene)vinyl-N-phenyl-carbazole (PCz-4CN) PCz-4CN was prepared in 56% yield from DFPCz by analogy to NCz-2CN and recrystallized from acetonitrile. m.p.: 316 ◦ C; 1 H NMR (500 MHz, CDCl ): δ 8.32 (s, 2H), 7.68–7.62 (m, 4H), 3 7.58–7.55 (m, 3H), 7.40 (d, J = 8.6 Hz, 2H), 7.33 (d, J = 16.0 Hz, 2H), 7.09 (d, J = 16.0 Hz, 2H), 6.89 (s, 2H), 2.63 (s, 4H), 2.54 (s, 4H), 1.12 (s, 12H). MS (MALDI): m/z: 635 (M); IR (KBr): 2953, 2222, 1388, 898, 816, 758, 698 cm−1 ; Anal. Calcd. for C44 H37 N5 : C 83.15; H 5.83; N 11.02; Found: C 83.83; H 5.58; N 10.72. 2.7. LEDs fabrication and characterization NPB was used as the hole-transporting layer, BCP was used as the hole-blocking layer, and Alq3 was used as the electrontransporting layer. LiF (1 nm) was used between electrontransporting layer and cathode to enhance electron injection and reduce the turn-on voltage of EL device. ITO coated glass with a sheet resistance of 30/square were used as the substrate and anode. EL spectra and brightness–current–voltage characteristics of the devices without packaging were measured using Keithley 236 source measure unit and Keithley 2000–20 multimeter with ST-86LA luminance meter at room temperature under dry nitrogen atmosphere.
Table 1 Physical data for NCz-2CN and PCz-4CN Compound
Tg /Tm /Td (◦ C)
λabs (nm) film
λem (nm) film
CIE (x, y)
EHOMO a (eV)
ELUMO a (eV)
Eg b (eV)
nac /225/332 na/315/365
(0.64, 0.36) (0.62, 0.34)
a b c
Obtained from cyclic voltammetric methods at a scan rate of 50 Mv s−1 . ELUMO (or EHOMO ) = 4.75 + e Vreduction (or Voxidation ) Electrochemical band gap EHOMO − ELUMO . na, not observed.
H.-y. Fu et al. / Synthetic Metals 156 (2006) 809–814
Fig. 1. STM topography (A, B) and AFM image (C, D) of NCz-2CN/ITO thin film.
3. Results and discussion
3.2. Optical and electrochemical properties
3.1. Synthesis and characterization
The absorption spectra and photoluminescence (PL) spectra measured in a thin film on quartz plate are shown in Fig. 2. A similar broad absorption band of the derivatives covers the range from 400 to 600 nm, and fluorescence emission with the peak at 630 nm for NCz-2CN and 666 nm for PCz-4CN. The absorption data, emission data and the CIE color coordinates of the new compounds are presented in Table 1. There is hardly an overlap between absorption and emission spectra of emitters, thus self-absorption can be avoided. In addition, these molecules
As illustrated in Scheme 2, the N-aryl carbazole derivatives can be easily synthesized in relatively good yields. The thermal properties of the compounds were determined by DSC and DTA-TGA measurements (Table 1). The thermal decomposition temperature (Td , 1% weight loss) were 332 ◦ C for NCz-2CN and 365 ◦ C for PCz-4CN, obtained by thermogravimetric analysis. NCz-2CN can be purified by sublimation without decomposition. However, slight decomposition of PCz-2CN was observed if the sublimation temperature was higher than 280 ◦ C. Although DSC measurement did not show Tg transition of NCz-2CN and PCz-4CN, the smooth surface of NCz-2CN can be observed by scanning tunneling microscopy (STM) image. Fig. 1(A and B) shows the 1000 nm × 1000 nm and 300 nm × 300 nm topography of NCz-2CN thin film on ITO, fabricated via vacuum vapor deposition with the deposition rate of 1.0 A/s, about 20 nm in thickness, taken by STM. The measured root mean square average (RMS) is 2.3 and 1.3 nm for the 1000 nm × 1000 nm and 300 nm × 300 nm images, respectively. The boxed region of 300 nm × 300 nm image has the topography with the RMS of 1.0 nm. As seen from the images, the film surface is uniform and nano-flat. The AFM surface morphology of NCz-2CN film deposited on ITO substrate is also shown in Fig. 1(C and D). The average deviation of roughness is 0.6 and 0.4 nm for film 5000 nm × 5000 nm and 2000 nm × 2000 nm, respectively. These all indicates that NCz-2CN film is smooth.
Fig. 2. Normalized absorption and emission spectra of the N-aryl carbazole derivatives.
H.-y. Fu et al. / Synthetic Metals 156 (2006) 809–814
Fig. 4. The energy level diagram of ITO/NPB/NCz-2CN/BCP/Alq3 /LiF/Al devices. Fig. 3. Solvatochromism property of NCz-2CN in different solvents.
show strong excited-state solvatochromism . Fig. 3 exhibits this behavior of compound NCz-2CN of solvents with different polarity (toluene, methanol, dichloromethane). The value of λem increases as the solvent polarity increases (λem : toluene, 569 nm; dichloromethane, 595 nm; methanol, 599 nm). This indicates NCz-2CN possess strong intramolecular charge transfer (ICT) from the donor (N-naphthyl-carbazole) to the acceptor (dicyanovinyl) unit. Compound PCz-4CN shows greater solvatochromism effect than NCz-2CN because of an additional donor-acceptor system in the molecular structure (λem : toluene, 552 nm; dichloromethane, 599 nm; methanol, 611 nm). The electrochemical properties of the red-emitting compounds were studied by cyclic voltammetric methods. The electrochemical data and the energy band structures of the compounds are shown in Table 1. The ionization potentials (IP, HOMO levels) are 5.33 for NCz-2CN and 5.48 for PCz-4CN. NCz-2CN and PCz-4CN have donor--acceptor structure, with N-aryl carbazole as the donor and dicyanovinyl as the acceptor. The electrochemical oxidation site of the compounds is probably on the electron-rich N-aryl carbazole group. 3.3. Electroluminescence Two devices with the following structure were fabricated: (A) ITO/NPB (60 nm)/NCz-2CN (10 nm)/BCP (10 nm)/Alq3 (60 nm)/LiF (1 nm)/Al and (B) ITO/NPB (60 nm)/NCz-2CN (10 nm)/BCP (10 nm)/Alq3 (90 nm)/LiF (1 nm)/Al. Fig. 4 shows the energy level diagram of the devices. The HOMO energy level difference between Cz-2CN and NPB is only 0.1 eV, there is less energy barrier for hole to transport from NPB to NCz-2CN. As a triarylamine derivative, NCz-2CN is likely a hole-transporting material and HOMO energy difference between NCz-2CN and Alq3 is not large enough to block hole injection into Alq3 layer, therefore a hole-blocking layer is required between NCz-2CN and Alq3 . Otherwise the recombination of electrons and holes will occur in Alq3 layer as the reported results on naphthylaminebased red emitters . BCP has a much lower HOMO energy level (6.7 eV) than NCz-2CN (5.3 eV) and is a good holeblocking material to confine the recombination of electrons and holes in NCz-2CN layer.
The EL spectra and the CIE color coordinates of device A at different voltage are shown in Fig. 5. The EL device shows an orange-red emission with a peak at 628 nm and there is very slight change in spectrum and CIE color coordinate with increase of the driven voltage. The EL emission of device A comes from the NCz-2CN layer, however, minor contribution from Alq3 was detected and the CIE color coordinates fall into the orange-red region, imperfect red emission compared with the PL CIE color coordinate. And the emission intensity of Alq3 increases slightly with the increasing voltage. The results imply that NCz-2CN has good hole-transporting property and the BCP layer cannot block the hole transporting to Alq3 thoroughly. The current density–voltage–luminance characteristics of device A is presented in Fig. 6. The turn-on voltage, which is defined as the voltage when the luminance is 1 cd/m2 , was 6.5 V, and a luminance of 2530 cd/m2 was obtained at 15 V. Device B was fabricated with the same configuration with device A except the thickness of the electron-transporting layer Alq3 was increased. Fig. 5 exhibits the EL spectra of device B with the red emission λmax at 628 nm. No clearly change of the CIE color coordinates of the device with increasing of the voltage was found. There is also Alq3 emission trace detected, the emission intensity of Alq3 increased with the increasing of the voltage. Compared with device A, the emission intensity of Alq3 is weakened. The current density–voltage–luminance character-
Fig. 5. EL spectra and CIE color coordinates of devices A and B at different voltage.
H.-y. Fu et al. / Synthetic Metals 156 (2006) 809–814
as the light-emitting layer in non-doped orange-red OLEDs. The compounds show good thermal stability and thin film with amorphous surface morphology can be obtained by vacuum deposition. OLEDs fabricated with NCz-2CN as the emitter showed bright orange-red color centered at 628 nm. The device with the configuration of ITO/NPB (60 nm)/NCz-2CN (10 nm)/BCP (10 nm)/Alq3 (90 nm)/LiF (1 nm)/Al exhibited a maximum luminance of 4110 cd/m2 and a maximum luminous efficiency of 0.49 lm/W. These results demonstrate that NCz2CN is a promising red-emitting host material. Acknowledgements
Fig. 6. Luminance–voltage–current density characteristics of devices A and B.
This work was supported by Science and Technology Fund of Shanghai (015261049) and Foundation for University Key Teacher by the Ministry of Education. References
Fig. 7. Current density–efficiency characteristics of device B.
istics of device B is presented in Fig. 6. A turn-on voltage of 6.0 V and a brightness of 4110 cd/m2 at 15 V were observed. Fig. 7 shows the current density–efficiency and a maximum luminous efficiency of 0.49 lm/W at 13.5 V with the current efficiency of 2.09 cd/A was obtained. The better performance of device B is probably owing to the more balanced carrier injection and transport in the EL device compared to device A. However, thicker Alq3 layer may also retard the electron transport to the emitting layer. Alq3 emission from devices indicates the hole injection to the electron transport layer, so the thicker blocking layer (BCP) may be needed. Both devices A and B show worse red color purity compared with the PL of NCz-2CN film. To obtain a device with pure NCz-2CN red color, optimization of the device configuration to eliminate the emission of Alq3 is required and the experiments are in progress. 4. Conclusion In summary, we have developed the donor--acceptor molecules with N-aryl carbazole as the donor, which can be used
 C.W. Tang, S.A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913.  J.H. Buroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns, A.B. Holmes, Nature 347 (1990) 539.  L.S. Hung, C.H. Chen, Mater. Sci. Eng. R 39 (2002) 143.  C.T. Chen, Chem. Mater. 16 (2004) 4389.  W.C. Wu, H.C. Yeh, L.H. Chan, C.T. Chen, Adv. Mater. 14 (2002) 1072.  H.C. Yeh, S.J. Yeh, C.T. Chen, Chem. Comm. 2632 (2003).  K.R.J. Thomas, J.T. Lin, M. Velusamy, Y.T. Tao, C.T. Chuen, Adv. Funct. Mater. 14 (2004) 83.  C.L. Chiang, M.F. Wu, D.C. Dai, Y.S. Wen, J.K. Wang, C.T. Chen, Adv. Funct. Mater. 15 (2005) 231.  A. Islam, C.C. Cheng, S.H. Chi, S.J. Lee, P.G. Hela, I.C. Chen, C.H. Cheng, J. Phys. Chem. B 109 (2005) 5509.  S. Toguchi, Y. Morioka, H. Ishikawa, A. Oda, E. Hasegawa, Synth. Met. 111/112 (2000) 57.  W.Y. Lai, Z.Q. Gao, T.C. Wong, L.F. Cheng, C.L. Lee, S.T. Lee, X.H. Zhang, S.K. Wu, Phys. Status Solidi A 173 (1999) 491.  T.H. Huang, J.T. Lin, Y.T. Tao, C.H. Chuen, Chem. Mater. 15 (2003) 4854.  S.G. Kim, S.H. Yoon, S.H. Kim, E.M. Han, Dyes Pigments 64 (2005) 45.  K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.H. Chuan, Adv. Mater. 14 (2002) 822.  T.H. Huang, J.T. Lin, Chem. Mater. 16 (2004) 5387.  S. Chen, X. Xu, Y. Liu, G. Yu, X. Sun, W. Qiu, Y. Ma, D. Zhu, Adv. Funct. Mater. 15 (2005) 1541.  K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.W. Ko, Adv. Mater. 12 (2000) 1949.  K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.W. Ko, J. Am. Chem. Soc. 123 (2001) 9404.  H. Li, Y. Zhang, Y. Hu, D. Ma, L. Wang, X. Jing, F. Wang, Macromol. Chem. Phys. 205 (2004) 247.  R. Lemke, Synthesis (1974) 359.  J. Li, D.L.Z. Hong, S. Tong, P. Wang, C. Ma, O. Lengyel, C.S. Lee, H.L. Kwong, S. Lee, Chem. Mater. 15 (2003) 1486.  H.Y. Fu, Y.Q. Zhan, J.X. Xu, X.Y. Hou, F. Xiao, Opt. Mater., doi:10.1016/j.optmat.2005.09.074.