Polymer Bulletin

, Volume 69, Issue 3, pp 273–289

Synthesis and characterization of new polyfluorene derivatives: using phenanthro[9,10-d]imidazole group as a building block for deep blue light-emitting polymer

Authors

  • Zhiming Wang
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
    • School of Petrochemical EngineeringShenyang University of Technology
  • Zhao Gao
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Shanfeng Xue
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Yulong Liu
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Wensi Zhang
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Cheng Gu
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Fangzhong Shen
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
  • Yuguang Ma
    • State Key Laboratory of Supramolecular Structure and MaterialsJilin University
Original Paper

DOI: 10.1007/s00289-012-0710-5

Cite this article as:
Wang, Z., Gao, Z., Xue, S. et al. Polym. Bull. (2012) 69: 273. doi:10.1007/s00289-012-0710-5

Abstract

A series of novel polyfluorene derivatives P1/4, P2/4, and P3/4, containing phenanthro[9,10-d]imidazole group on backbone are designed, synthesized, and well characterized. They all show high-molecular weights, good solubilities, and excellent thermal stabilities. The CV results of all three compounds show the lower LUMO levels and higher HOMO levels than PF. Among them, P3/4 exhibits deep blue emission both in solution and in solid state. The PLED based on P3/4 shows higher device performance and locates in the deep blue region with a CIE coordinate of (0.17, 0.08).

Keywords

Deep blue emissionPhenanthrenequinoneImidazoleMultifunctionalPolymer

Introduction

Conjugated polymer-based electroluminescent devices are of growing interest in display applications because they can be made by spin-coating or inkjet printing technologies [18]. Remarkable progress has been made in achieving high-quality polymers that emit various colors in the past decade [913]. One of the remaining challenges is to develop polymers for achieving deep blue emission with high efficiency since the performance of deep blue polymer light emitting diodes (PLEDs) is relatively low behind. Unbalanced charge injection and transport is one of the major problems. The high-energy gap essential for deep blue light emitting polymer often results in low electron affinities, which hampers the electron injection and the balance of the charge carriers [14, 15]. To solve the problem, multilayered structure which constitutes an electron transport layer, an emitting layer and a hole transport layer are widely applied to improve the performance of the devices [1621]. But the sequential deposition of these layers provides additional complexity and the cost of such devices will be increased. Thus, the pursuit for new and multifunctional deep blue emitter with good charge injection properties remains as one of the most active areas in this field [2225].

Most conjugated molecules are p-type due to their inherent richness of π electrons which causes the unbalanced injection and transport of carriers [26, 27]. Thus, n-type unit is often introduced to increase the electron injection and transport property [2831]. In recent years, imidazole derivatives are found to be an important functional group for adjusting the LUMO energy level [32]. For example, benzoimidazole-based compound 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi), which has a LUMO level of −2.40 eV, have been widely used in OLEDs as electron injection layer. Generally speaking, when the LUMO level of a compound is lowered to enhance the electron injection and transport ability, the HOMO level will be decreased at the same time, leading to higher hole-injection barrier. Thus, TPBi also can be used as hole blocking layer but not as the emitting layer [33, 34]. Our group have designed and synthesized a new multifunctional compound BPPI, in which benzoimidazole unit in TPBi is replaced by phenanthro[9,10-d]imidazole (PI) unit to enhance the holes injection and transport ability. BPPI exhibits excellent thermal stability, highly efficient fluorescence, and more balanced carrier injection [35]. The double-layered device based on BPPI shows higher maximum luminance and lower turn-on voltage than the multilayered one which has an independent electron injection layer. Because of the adjustment of the carriers injection property as well as deep blue emission by PI, we thought it might be a good building block for deep blue light emitting polymer. In this article, we report three polyfluorene derivatives containing PI unit on backbone. Fluorene was chosen as the building block because of its blue emission, good solubility, and high efficiency [36, 37]. These polymers show good blue or deep blue fluorescence emission. Compared with polyfluorene, the thermodynamic properties have significantly increased. And they all show lower LUMO levels and higher HOMO energy levels compared with PF, exhibiting more balanced carrier injection property. The performance of double-layered device of P3/4 shows higher efficiency than PF and deep blue emission with a CIE coordinate of (0.17, 0.08).

Experimental section

Materials and instrumentation

Tetrahydrofuran (THF) was distilled under normal pressure from sodium benzophenone ketyl under nitrogen immediately prior to use. Other solvents of high purities were used without further purification. Pd(PPh3)4 was purchased from Aldrich and used as received. 2,7-dibromo-9,9-dihexylfluorene, 4,4,4′,4′,5,5,5′,5′-hexyl-2,2′-bi(1,3,2-dioxaborolane), and 2,7-dibromophenanthrenequinone were synthesized according to our previous published procedures [3539]. IR spectra were recorded on a Perkin-Elmer 16 PC FT-IR spectrophotometer. 1H NMR spectra were measured on an AVANCZ 500 spectrometer using chloroform-d (CDCl3) or dimethylsulfoxide as solvents and tetramethylsilane (δ = 0 ppm) as internal standard. The MALDI-TOF MS were recorded using an AXIMA-CFR plus instrument. UV–Vis absorption spectra were measured on a UV-3100 spectrophotometer. Thermogravimetric analysis (TGA) of the polymers was evaluated on a TA TGA Q500 instrument under nitrogen at a heating rate of 20 °C/min. The photoluminescence (PL) spectra were recorded on a RF-5301PC spectrofluorometer. The PL quantum yields (Φf) were estimated using quinine sulfate (Φf = 54% in 0.1 M sulfuric acid) as reference in THF. Weight (Mw) and number average (Mn) molecular weights and polydispersity indices (Mw/Mn) of the polymers were estimated by a Waters Associates gel permeation chromatography system equipped with refractive index and UV detectors. THF was used as eluent at a flow rate of 1.0 mL/min. A set of monodisperse polystyrene standards covering molecular weight range of 103–107 was used for the molecular weight calibration. Cyclic voltammetry (CV) were performed with a BAS 100 W Bioanalytical Systems, using a glass carbon disk (Φ = 3 mm) as working electrode, platinum wire as auxiliary electrode, with porous ceramic wick, Ag/Ag+ as reference electrode, standardized for the redox couple ferricinium/ferrocene.

Monomer synthesis

Polufluorene’s monomer M4 is synthesized according to our previous published procedures [36], whereas compounds M1, M2, and M3, are prepared according to the synthetic routes shown in Scheme 1. Typical experimental procedures for their syntheses are shown below.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Sch1_HTML.gif
Scheme 1

Synthesis of P1/4, P2/4, and P3/4 by palladium-catalyzed Suzuki coupling reaction of monomer M4 with monomers M1, M2, and M3

Preparation of 3,4-bis(hexyloxy)benzaldehyde

To a 250 mL round-bottom flask equipped with a stirrer were added 2.80 g (20.0 mmol) of 3,4-dihydroxybenzaldehyde, 4.10 g (30.0 mmol) of K2CO3, 8.00 g (50 mmol) of bromohexane and CH3CN (100 mL). After refluxing for 48 h, the reaction was stopped by adding dilute hydrochloric acid (HCl). The mixture was then washed with water, extracted by CHCl3 and dried over MgSO4. The filtrates were collected. And after solvent evaporation under reduced pressure, the crude product was purified by silica gel column chromatography using hexane as eluent to give the targeted product as colorless oil in 95% yield (2.02 g). 1H-NMR (500 MHz, CDCl3, ppm): 9.83 (s, 1H), 7.40 (d, J = 8.2 Hz, 2H), 6.90 (d, J = 8.2 Hz, 1H), 4.06 (m, 4H), 1.81 (m, 4H), 1.48 (s, 4H), 1.34 (s, 8H), 0.90 (s, 4H). MALDI-TOF (m/z): [M+] Calcd. C19H30O3, 306.44; Found, 307.5.

Preparation of 5,10-dibromo-1,2-diphenyl-1H-phenanthro[9,10-d]imidazole (M1)

To a 250 mL round-bottom flask equipped with a stirrer were added 1.83 g (5.0 mmol) of 2,7-dibromophenanthrenequinone, 2.30 g (25.0 mmol) of aminobenzene, 0.53 g (5 mmol) of benzaldehyde, 1.54 g (20.0 mmol) of amino acetic acid and acetic acid (120 mL). After stirring for 2 h at 120 °C, the mixture was filtered and the precipitates were washed with dilute acetic acid. The filtrates were collected, and after solvent evaporation under reduced pressure, the crude product was purified by silica gel column chromatography using CH2Cl2 as eluent to give M1 as white solid in 70% yield. 1H NMR (500 MHz, CDCl3, ppm): 9.02 (d, J = 2.1 Hz, 1H), 8.53 (d, J = 8.8 Hz, 1H), 8.47 (d, J = 8.8 Hz, 1H), 8.69 (dd, J = 7.6 Hz, 2.1 Hz, 1H), 7.70–7.62 (m, 3H), 7.61–7.55 (m, 3H), 7.50 (m, 2H), 7.37–7.28 (m, 3H), 7.20 (d, J = 2.1 Hz, 1H). MALDI-TOF (m/z): [M+] Calcd. C27H16Br2N2, 525.97; Found, 527.3. Anal Calc. for C27H16Br2N2: C, 61.39; H, 3.05; N, 5.30; Br, 30.25. Found: C, 61.20; H, 3.02; N, 5.31.

Preparation of 2-(3,4-bis(hexyloxy)phenyl)-5,10-dibromo-1-phenyl-1H-phenanthro[9,10-d]imidazole (M2)

To a 250 mL round-bottom flask equipped with a stirrer were added 1.83 g (5.0 mmol) of 2,7-dibromophenanthrenequinone [35], 2.30 g (25.0 mmol) of aminobenzene, 1.65 g (5 mmol) of M1, 1.54 g (20.0 mmol) of amino acetic acid, and acetic acid (80 mL). After stirring for 12 h at 120 °C, the mixture was added 2 mL of water. The mixture was then filtered and the precipitates were washed with dilute acetic acid. The crude product was purified by silica gel column chromatography using CH2Cl2 as eluent to give M2 as white solid in 80% yield. 1H NMR (500 MHz, CDCl3, ppm): 8.98 (s, 1H), 8.45 (d, J = 8.80 Hz, 1H), 8.39 (d, J = 8.80 Hz, 1H), 7.65 (dd, J = 8.8 Hz, 2.10 Hz, 1H), 7.62–7.56 (m, 3H), 7.50 (dd, J = 8.80 Hz, 2.1 Hz, 1H), 7.48–7.42 (m, 2H), 7.14–7.09 (m, 2H), 7.03 (s, 1H), 6.72 (d, J = 8.50 Hz, 1H), 3.91 (t, J = 6.7 Hz, 7.00 Hz, 2H), 3.72 (t, J = 6.70 Hz, 7.0 Hz, 2H), 1.73–1.68 (m, 4H), 1.44–1.21 (m, 12H), 0.89–0.79 (m, 6H). MALDI-TOF (m/z): [M+] Calcd. C39H40Br2N2O2, 726.15; Found, 727.3. Anal Calc. for C39H40Br2N2O2: C, 64.29; H, 5.53; Br, 21.93; N, 3.85; O, 4.39. Found: C, 64.12; H, 5.39; N, 3.88, O, 4.36.

Preparation of 4-(2-(3,4-bis(hexyloxy)phenyl)-5,10-dibromo-1H-phenanthro[9,10-d]imidazole-1-yl)benzonitrile (M3)

To a 250 mL round-bottom flask equipped with a stirrer were added 1.83 g (5.0 mmol) of 2,7-dibromophenanthrenequinone, 3.00 g (25.0 mmol) of cyanoaminobenzene, 1.60 g (5 mmol) of 3,4-bis(hexyloxy)benzaldehyde, 1.54 g (20.0 mmol) of amino acetic acid and acetic acid (120 mL). After stirring for 12 h at 120 °C, the mixture was added 5 mL of water. The mixture was then filtered and the precipitates were washed with dilute acetic acid. The crude product was purified by silica gel column chromatography using CH2Cl2 as eluent to give M3 as white solid in 70% yield. 1H NMR (500 MHz, CDCl3, ppm): 8.99 (d, J = 2.10 Hz, 1H), 8.53 (d, J = 8.80 Hz, 1H), 8.45 (d, J = 8.80 Hz, 1H), 7.94 (d, J = 8.50 Hz, 2H), 7.73 (dd, J = 8.80 Hz, 2.10 Hz, 1H), 7.64 (d, J = 8.50 Hz, 1H), 7.60 (dd, J = 8.80 Hz, 2.1 Hz, 1H), 7.15 (d, J = 2.10 Hz, 1H), 7.09 (d, J = 2.10 Hz, 1H), 6.87 (dd, J = 8.20 Hz, 2.1 Hz, 1H), 6.75 (d, J = 8.20 Hz, 1H), 4.00 (t, J = 6.70 Hz, 7.0 Hz, 2H), 3.88 (t, J = 6.70 Hz, 7.0 Hz, 2H), 1.86–1.74 (m, 4H), 1.51-1.41 (m, 4H), 1.40–1.29 (m, 8H), 0.96–0.86 (m, 6H). MALDI-TOF (m/z): [M+] Calcd. C40H39Br2N3O2, 751.14; Found, 723.5. Anal Calc. for C40H39Br2N3O2: C, 63.75; H, 5.22; Br, 21.21; N, 5.58; O, 4.25. Found: C, 63.52; H, 5.18; N, 5.77, O, 4.33.

Polymerization

All the polymerization reactions and manipulations were carried out under nitrogen, except for the purification of the polymers, which was done in open air. A typical experimental procedure for the polymerization of M1 with M4 is given below as an example. Into a baked 50 mL flask was added M1 (180 mg, 0.34 mmol), M4 (200 mg, 0.34 mmol), and Pd(PPh3)4 (9.30 mg, 0.008 mmol). The flask was evacuated under vacuum for half an hour and then flushed with nitrogen. Freshly distilled toluene (1.2 mL) and 0.8 mL of 2 M K2CO3 aqueous solution were injected into the flask. After reflux under nitrogen for 72 h, the solution was cooled to room temperature, washed with 20 mL of water. The organic layer was extracted with chloroform and dried over MgSO4. The mixture was then filtered and the filtrates were collected. After solvent evaporation under reduced pressure, it was precipitated in 150 mL methanol. After filtration, the polymer was dried in a vacuum oven at 40 °C to a constant weight. Yield: 72%. 1H NMR (500 MHz, CDCl3, ppm): 9.33–9.14 (br, m, PIM Ar–H, 1H), 8.94–8.55 (br, m, PIM Ar–H, 2H), 8.19–7.85 (br, m, PIM Ar–H, 3H and fluorene Ar–H, 2H), 7.53–7.28 (br, m, PIM Ar–H, 5H and fluorene Ar–H, 4H), 7.53–7.28 (br, m, PIM Ar–H, 5H), 2.27–1.92 (br, m, CH2, 4H), 1.30–0.95 (br, m, CH2, 12H), 0.95–0.55 (br, m, CH2 and CH3, 10 H). Mn = 7.5 × 103, Mw = 15.8 × 103, PD = 2.1. Anal. Calcd. for (C52H48N2)n: C, 89.10; H, 6.90; N, 4.00. Found: C, 85.24; H, 5.08; N, 3.86.

Other polymers are prepared by the similar procedure and their characterization data are given below. PF was synthesized to compare the properties with these PI-containing polymers.

P2/4

1H NMR(500 MHz, CDCl3, ppm): 9.34–9.16 (br, m, PIM Ar–H, 1H), 8.98–8.59 (br, m, PIM Ar–H, 2H), 8.13–7.83 (br, m, PIM Ar–H, 3H and fluorene Ar–H, 2H), 7.82–7.53 (br, m, PIM Ar–H, 4H and fluorene Ar–H, 4H), 7.53–7.29 (br, m, PIM Ar–H, 2H), 7.17–7.03 (br, m, PIM Ar–H, 1H), 6.90–6.73 (br, m, PIM Ar–H, 1H), 4.10–3.90 (br, m, PIM Ar–OCH2, 2H), 3.90–3.63 (br, m, PIM Ar–OCH2, 2H), 2.31–1.92 (br, m, CH2, 4H), 1.89–1.68 (br, m, CH2, 4H), 1.59–1.22 (br, m, CH2, 12H),1.27–0.98 (br, m, CH2, 12H), 0.98–0.49 (br, m, CH2 and CH3, 16H). Mn = 21.0 × 103, Mw = 58.8 × 103, PD = 2.7. Anal. Calcd. for (C64H72N2O2)n: C, 85.29; H, 8.05; N, 3.11; O, 3.55. Found: C, 82.69; H, 6.98; N, 2.99; O, 3.35.

P3/4

1H NMR(500 MHz, CDCl3, ppm): 9.26–9.15 (br, m, PIM Ar–H, 1H), 8.97–8.62 (br, m, PIM Ar–H, 2H), 8.12–7.86 (br, m, PIM Ar–H, 4H and fluorene Ar–H, 2H), 7.86–7.57 (br, m, PIM Ar–H, 1H and fluorene Ar–H, 4H), 7.51–7.29 (br, m, PIM Ar–H, 2H), 7.17–7.07 (br, m, PIM Ar–H, 1H), 7.02–6.92 (br, m, PIM Ar–H, 1H), 6.85–6.72 (br, m, PIM Ar–H, 1H), 4.09–3.96 (br, m, PIM Ar–OCH2, 2H), 3.94–3.81 (br, m, PIM Ar–OCH2, 2H), 2.36–1.91 (br, m, CH2, 4H), 1.90–1.65 (br, m, CH2, 4H), 1.62–1.22 (br, m, CH2, 12H), 1.21–0.97 (br, m, CH2, 12H), 0.96–0.53 (br, m, CH2 and CH3, 16H). Mn = 21.8 × 103, Mw = 48.8 × 103, PD = 2.2. Anal. Calcd. for (C65H71N3O2)n: C, 84.28; H, 7.73; N, 4.54; O, 3.45. Found: C, 82.09; H, 7.06; N, 4.32; O, 3.07.

PF

1H NMR (500 MHz, CDCl3, ppm): 7.91–7.75 (br, m, fluorene Ar–H, 2H), 7.76–7.50 (br, m, fluorene Ar–H, 4H), 2.27–1.92 (br, m, CH2, 4H), 1.30–0.95 (br, m, CH2, 12H), 0.95–0.55 (br, m, CH2 and CH3, 10H). 13C NMR (125 MHz, CDCl3): 154.81, 151.82, 140.55, 140.04, 136.56, 133.47, 129.59, 126.16, 121.55, 120.62, 119.97, 69.87, 55.34, 40.37, 31.40, 31.47, 29.68, 29.49, 29.16, 25.73, 23.86, 22.63, 22.56, 14.09, 14.02. Mn = 10.8 × 103, Mw = 20.6 × 103, PD = 1.9. Anal. Calcd. for (C25H32)n: C, 89.76; H, 10.24. Found: C, 86.90; H, 10.81.

Results and discussion

Monomer synthesis

To explore the properties of PI-containing polymers, we design the molecular structures of a series of functional monomers (M1M3) as shown in Scheme 1. All three new monomers (M1M3) are synthesized in one-pot reaction [4043]. The mixture of aniline, phenanthrenequinone, ammonium acetate, and corresponding aromatic aldehyde are refluxed for 2 h, then cooled to room temperature and filtered. The crude product of light yellow powder is purified by chromatography and the yields of all target products are above 75%. All the monomers are characterized by standard spectroscopic methods, from which satisfactory data corresponding to their molecular structures are obtained.

Polymerization

After obtaining the monomers, we then try to polymerize them by Pd(PPh3)4, a commonly used catalyst for Suzuki coupling reaction [44, 45]. Reaction of M1 with M4 in the presence of Pd(PPh3)4 in refluxed toluene gives a high-molecular weight polymer with a reasonably high yield (Table 1). Under the same conditions, M4 reacted with M2 and M3, respectively, producing P2/4 and P3/4 with Mw’s from 58,800 to 48,800 in 60–72% yields. P2/4 and P3/4 show higher molecular weights than P1/4 because the solubility of M2 and M3 are better than M1 after being attached with the long alkoxyl side chain [46]. All these polymers were readily dissolved in common organic solvents, such as THF, CHCl3, toluene, and DMF.
Table 1

The molecular weight, thermal, absorption, and emission characteristics of PF, P1/4, P2/4, and P3/4 in solution and film states

 

Mn (×103)

Mw (×103)

PD

Tda (°C)

Tg (°C)

In THF (nm)

Φflb (%)

In solid (nm)

Abs

PL

Abs

PL

PF

10.8

20.6

1.9

331

96

375

418(440)

75

380

425(450)

P1/4

7.5

15.8

2.1

397

207

368(311)

440(418)

88

378(309)

450(425)

P2/4

21.0

58.8

2.7

415

189

378(315)

440(418)

90

385(310)

450(425)

P3/4

21.8

48.8

2.2

397

195

380(308)

418(440)

92

388(310)

425(451)

aThe temperature for 5% weight loss of the polymers

bThe fluorescence quantum yield in THF solution using 0.1 M H2SO4 solution of quinine as reference (0.54)

Structural characterization

The polymeric products were characterized spectroscopically. All the polymers give satisfactory analysis data corresponding to their expected molecular structures. An IR spectrum of P3/4 is shown in Fig. 1 as an example. For comparison, the spectra of its monomers M3 and M4 are also provided in the same figure. The band at 2,200/cm associated with CN stretching is clearly seen in the spectrum of the monomer M3, which is also observed in the spectrum of P3/4, indicating that the CN group has been introduced into the new formed polymer. The boron ester stretching vibration at 1,350/cm of M4 almost disappears after polymerization. Meanwhile, the absorption band of alkyl side chain and alkoxyl side chain at 3,100–2,800/cm are all observed in these three spectra. All these spectral data prove that monomers M3 and M4 have been successfully transformed to polymer P3/4.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig1_HTML.gif
Fig. 1

IR spectra of (A) polymer P3/4, (B) monomer M3, and (C) monomer M4

Similar results were obtained from the NMR analyses. Figure 2 shows the 1H NMR spectra of P3/4 and its monomer M3 and M4. In the spectrum of M3, the absorptions of the CH2 protons linked with oxygen atom on the side chain are observed at 3.60 and 4.00 ppm, and P3/4 shows the similar proton resonances in this area. Compared with our earlier study, these are assigned to the typical resonances of PI group, although they become weaker in P3/4. In the area of 6.50–7.26 ppm, four absorptions of protons are observed in M3, while in P3/4 the number is decreased to three peaks because the other one moves to low field region. All this proved that the M3 block is inserted to the P3/4 polymer. The evidence of M4 linked to the backbone in P3/4 originates from the response around 2.00 ppm as observed in the other polyfluorenes derivatives [36, 37]. From 7.26 to 8.50 ppm, the absorptions of the aromatic fluorene hydrogen and aromatic phenanthrene hydrogen of P3/4 are hard to distinguish, presumably due to their overlapping. In short, no unexpected peaks are found, and all the peaks can be readily assigned. Thus, the polymeric product is indeed P3/4 with a molecular structure as shown in Scheme 1. Similar observations are found in the spectra of P1/4 and P2/4, respectively.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig2_HTML.gif
Fig. 2

1H NMR spectra of CHCl3-d solutions of (A) polymer P3/4, (B) monomer M3, and (C) monomer M4

Thermal stability

The thermal stability of the polymers is evaluated by TGA analysis. As shown in Fig. 3, all the polymers are thermally stable, losing 5% of their weights (Td) at 397–415 °C (Table 1), which are much higher than that of polyfluorene (331 °C). The high Td values of the polymers are understandable because they are constructed from aromatic rings, which possess a high resistance to thermolysis. More importantly, these polymers could form amorphous films with high-glass transition temperature (Tg) of 207, 189, and 195 °C, respectively, which are distinctly higher than that of PF (96 °C) (Fig. 4). Such high-Tg values implicate that they could form morphologically stable amorphous films upon thermal evaporation, which is highly important for application in OLEDs. At the same, it is proved that the PI unit is a good block to increase the materials thermal stability from oligomer to polymer. P1/4 shows the highest Tg due to its more rigid structure.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig3_HTML.gif
Fig. 3

TGA thermograms of P1/4, P2/4, P3/4, and PF recorded under nitrogen at a heating rate of 20 °C/min

https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig4_HTML.gif
Fig. 4

DSC thermograms of P1/4, P2/4, P3/4, and PF measured at a scan rate 10 °C/min under nitrogen

To investigate the fluorescence stability of the polymers in the solid state, we performed annealing experiments. The solid films of these polymers were first baked at 60 °C for 30 min in air, then at 180 °C for another 30 min, followed by 240 °C. P3/4 shows very good spectra stability. Figure 5 showed the normalized PL emission spectra of the P3/4 after annealing in air. It was reported that thermal treatment of the film of poly(dihexylfluorene) at 100 °C would lead to a significant increase in the shoulder peak centered at 460 nm because of the aggregation effect or keto formation in air [47]. However, P3/4 was very stable even after baking at 180 °C for 0.5 h, and the PL spectra were almost the same as those tested before annealing. Accompanying the further increased temperature, there appeared a new shoulder peak at ∼520 nm, which was not apparent after annealing at 180 °C for another 0.5 h, suggesting that bulky structure resulted in better spectra-thermal stability, and the formation of aggregation excimer and keto defects were effectively suppressed.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig5_HTML.gif
Fig. 5

PL spectra of the films of P3/4 before and after annealing at different temperatures for 30 min in air

Absorption and emission

Figure 6 shows the UV spectra of the polymers in their dilute solutions in THF. All these polymers exhibit their maxima absorption peaks around 365–380 nm, which are due to the π–π* transitions of the polymer backbones. The new absorption peak at 310 nm, which is not observed in PF but in the three PI-containing polymers, is assigned to the new formed imidazole ring. Compared to their inherent monomers, their absorption peaks are all red-shifted to longer wavelengths. For example, P3/4 absorbs at 310 nm and 385 nm, which are red-shifted from those of M3 and M4 by 37 and 22 nm, respectively. This indicates a more extended electronic conjugation in the polymer system because of the electronic communication between the aromatic chromophoric units. The emission of PF shows main peaks at 418 nm and a shoulder peak at 440 nm. When the PI group are introduced into the PF backbone, P1/4 and P2/4 exhibit the same emission peaks which mean that there is no energy transfer between the fluorene and PI units and insures the high efficiencies of these polymers. While the intensity of emission peak at 440 nm is higher than the peak at 418 nm, which might be due to the change of the polymer configuration by the imidazole ring and the long alkoxyl side chains. P3/4 shows the same emission as that of PF because the electron-withdrawing effect and steric hindrance of cyano group which stabilizes the phenyl ring at 1-N position and suppresses the interactions between the long alkoxyl side chains. P1/4, P2/4, and P3/4 all show higher quantum efficiencies than PF in dilute solution as summarized in Table 1.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig6_HTML.gif
Fig. 6

Normalized UV and PL spectra of THF solutions of P1/4, P2/4, P3/4, and PF. Concentration: 10 μM. Excitation wavelength: 370 nm

The absorption and emission spectra of the polymers as thin film in solid state are shown in Fig. 7. All the polymers show very similar absorption spectra with main peak at 380 nm, which are a little bit red-shifted compared with their solutions. The absorption peaks of imidazole ring at 310 nm for P1/4, P2/4, and P3/4 are all observed. All their emission spectra in solid state peak at 425 and 450 nm, showing 10 nm red shift compared with their solutions, respectively. The emission spectra of P2/4 are broader than the other two both in solution and in solid state due to interactions between the long alkoxyl side chains. Because of the electron-withdrawing effect and steric hindrance of cyano group as mentioned above, P3/4 shows similar profiles as that of P1/4 both in solution and in solid state.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig7_HTML.gif
Fig. 7

Normalized UV and PL spectra of P1/4, P2/4, P3/4, and PF in film state. Concentration: 10 μM. Excitation wavelength: 370 nm

Electrochemistry properties

Electronic structures (HOMO and LUMO) are characterized by CV using a glass carbon disk (Φ = 3 mm) as working electrode, platinum wire as auxiliary electrode, and Ag/Ag+ as reference electrode. As shown in Fig. 8, CV analysis for P1/4 exhibits one quasi-reversible oxidation wave with an oxidative onset potential of 0.84 V, which gives a HOMO level of −5.55 eV by comparison to ferrocene (EHOMO = −(eEox + 4.71 eV)) [48, 49]. The HOMO levels of P2/4 and P3/4 are measured to be −5.56 and −5.59 eV, respectively (Table 2). The HOMO levels of these PI-based polymers are similar, which are elevated 0.16–0.20 eV compared to that of PF (HOMO level of −5.75 eV), indicating a decreased holes injection barrier. The CV analysis gives the LUMO levels of −2.55, −2.58, and −2.63 eV for P1/4, P2/4, and P3/4, respectively. P3/4 shows the lowest LUMO level because of the electron-withdrawing cyano group, which are 0.18 eV lower than that of PF (LUMO level of −2.45 eV), revealing that the electrons injection barrier is also decreased. This reflects the more balanced carrier injection properties in these PI-based polymers, especially for P3/4.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig8_HTML.gif
Fig. 8

Cyclic voltammogram of PF, P1/4, P2/4, and P3/4

Table 2

The electrochemistry and device data of PF, P1/4, P2/4, and P3/4

 

HOMOa (eV)

LUMO (eV)

Egapb (eV)

Voltagec (V)

Lmax (cd/m2)

LEmax (cd/A)

EQEmax (%)

PF

−5.75

−2.45

3.30

15.0

18

0.012

0.015

P1/4

−5.55

−2.55

3.00

12.7

420

0.077

0.096

P2/4

−5.56

−2.58

2.98

12.5

71

0.15

0.18

P3/4

−5.59

−2.63

2.96

7.2

1,200

0.13

0.20

aCalculated by comparing with ferrocene (Fc) (4.8 eV) and calibrated using E1/2 (Fc/Fc+) = 0.09 V

bCalculated from the electrochemistry data

cDevice structure: ITO/PEDOT:PSS (40 nm)/PVK (≈40 nm)/polymer (≈50 nm)/LiF (0.5 nm)/Al (100 nm); the turn-on voltage (L > 1 cd/m2)

Device performance

To evaluate the potential application of these compounds in blue PLEDs, we fabricate the device with a configuration of ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/polymer (50 nm)/LiF (0.5 nm)/Al (100 nm). P1/4, P2/4, and P3/4 all show higher maximum luminous efficiencies than PF, and the turn-on voltages (L > 1 cd m−2) all become lower than PF (Fig. 9). This is because the carriers injection barriers are decreased, which is consistent with the results revealed by CV. The device based on P3/4 exhibits better performance with a lower turn-on voltage of 7.2 V, a maximum luminance of 1,200 cd/m2, a maximum luminous efficiency of 0.13 cd/A and an external quantum efficiency of 0.2%. As can be seen in Fig. 10, the electroluminescence locates in the deep blue region with main peak at 425 nm with a CIE coordinate of (0.17, 0.08), which is corresponded to its emission in solid state without red shift, indicating a good deep blue emission material. It also can be applied as host material for phosphorescence device. Better device performance could be expected provided that they can be measured under a more advanced condition.
https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig9_HTML.gif
Fig. 9

The luminous efficiency of P1/4, P2/4, and P3/4 compared with PF. Device structure: ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/polymer (50 nm)/LiF (0.5 nm)/Al (100 nm)

https://static-content.springer.com/image/art%3A10.1007%2Fs00289-012-0710-5/MediaObjects/289_2012_710_Fig10_HTML.gif
Fig. 10

Electroluminescent spectrum of P3/4. Device structure: ITO/PEDOT:PSS (40 nm)/PVK (40 nm)/polymer (50 nm)/LiF (0.5 nm)/Al (100 nm)

Conclusion

In summary, we have developed the PI building block for efficient blue light emitting materials. By inserting the alkoxyl side chain in PI unit, the molecular weight was improved. Comparing with a model polymer PF, the new-generated three blue polymers show comprehensive enhancement in the basic characteristics involving thermal stability, fluorescent efficiency, charge injection property and the performance in device. When the electron-withdrawing cyano group is introduced, the energy level of P3/4 has been tuned to be easier for the carrier’s injection. The device based on P3/4 shows better performance than PF, P1/4, and P2/4. The electroluminescence locates in the deep blue region with a CIE coordinate of (0.17, 0.08) and the external efficiency of the device could reach to 0.2%. The work demonstrates that PI unit is a good block to construct deep blue emission materials, and it indicates the chemical structure modification could improve material’s properties.

Acknowledgments

We are grateful for support from the National Science Foundation of China (Grant Nos. 20704016, 21174050), the Ministry of Education of China (Grant No. 20070183202).

Copyright information

© Springer-Verlag 2012