Bipolar π-conjugation interrupted host polymers by metal-free superacid-catalyzed polymerization for single-layer electrophosphorescent diodes

Abstract

Novel aromatic bipolar host polymers (P1 and P2) containing pyridine as an electron transporting unit and carbazole and fluorene as hole transporting units in the π-conjugation interrupted polymer backbone have been synthesized by a metal-free superacid-catalyzed polyhydroxyalkylation. The present polymers show good thermal stability, with high glass transition temperatures and decomposition temperatures. The conjugation lengths of the polymers are effectively confined into the repeating units due to the δ-C bond interrupted polymer backbone, giving rise to quite high triplet energy (2.79 eV for P1) and a wide band gap of around 3.33 eV, which make them promising hosts for phosphorescent OLEDs. The results suggest that the strategy of incorporating bipolar blocks into the π-conjugation interrupted polymer backbone can be a promising approach to obtain host polymers with high triplet level for green and even blue phosphorescent polymer light-emitting diodes on a simple device structure and using a solution-processed technique.

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1 Introduction

Organic light-emitting diodes (OLEDs) based on polymer materials have attracted considerable attention in fabricating flexible and large area flat-panel displays and lighting devices, due to their low-cost spin-coating or use in ink-jet printing technology.^1,2 To effectively utilize the radiative recombination of both singlet and triplet excitons, OLEDs usually dope a phosphorescent emitter into a polymer matrix to form a host–guest system and achieve nearly quantitative internal quantum efficiency (IQE).^3,4

Typical host polymers, such as polyfluorenes, have a relatively low triplet energy (E_T) (<2.4 eV), due to the π-electron delocalizations along the polymer backbone, and thus can only host green and red phosphorescent emitters.^5–9 For hosting blue phosphorescent emitters, the object host polymer should possess an appropriate E_T (>2.65 eV), i.e., higher than that of the phosphorescent emitter to prevent energy transfer back.^10–13 One of the representative host polymers suitable for phosphorescent blue emitters is poly(N-vinylcarbazole) (PVK), which possesses a comparatively high E_T level of 3.0 eV, which is nearly identical to that of the single carbazole unit, due to non-conjugated polymer backbone.¹⁴ However, PVK-based devices usually suffer from a high driving voltage, because of the imbalanced carrier injection/transportation, since the hole mobility of PVK is about three orders higher than that of the electron.^15–17

Blending an electron transported small molecule with PVK is a good approach to balance the carrier transportation. For example, the electrophilic small molecule PBD (2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole) is usually blended into PVK to improve electron transportation.^17–20 The added small molecule has quite a lower glass transition temperature (T_g) than PVK, possibly leading to phase separation under operation and hence a decreased device performance. The covalent attachment of the electron-transport small molecule into the host polymer could be a good solution to suppress this phase separation.

Regarding the E_T level and charge transport properties, non-conjugated bipolar polymer hosts simultaneously incorporating electron-rich and electron-deficient moieties have been studied recently.^21,22 Yan et al. reported a polysiloxane-type host PCzMSi containing a silicon–oxygen linkage in the backbone and carbazole moieties in the side chain with a high E_T of 3.0 eV.²³ Ding et al. reported a bipolar poly(aryl ether) host containing phosphine oxide in the main chain and carbazole in the side chain with an E_T of 2.96 eV.²⁴ Ogino et al. reported a series of random and block copolymers containing triphenylamine and oxadiazole moieties as charge transportation layer via nitroxide-mediated radical polymerization.²⁵ Scherf et al. reported a host copolymer containing conjugated fluorenetetrafluorobenzene building blocks in a non-conjugated backbone with an E_T of 2.40 eV in the solid state.²⁶

We previously reported an aromatic host polymer PICzFB containing carbazole–tetrafluorinebeneze–carbazole repeating units in the π-conjugation interrupted backbone with a considerably high E_T level of 2.73 eV, and successfully applied this in green and even in blue PLEDs.²⁷ However, the π-electron delocalization in the repeating units of PICzFB was not interrupted and it could be extended along the elongated molecular axis (carbazole–tetrafluorinebeneze–carbazole) due to the para linkages of tetrafluorinebeneze with carbazole, which leads to a lowering of the E_T level.²⁸ To further interrupt the π-electron delocalization, in this work, we use meta linkage carbazole–pyridine–carbazole and fluorene–pyridine–fluorene as repeating units, respectively, in a δ-C connection polymer backbone to prepare two new bipolar host polymers (P1 and P2) via superacid-catalyzed metal-free polyhydroxyalkylation. Such metal-free copolymerization is essentially quite favorable for avoiding the exciton quenching resulting from the heavy metal catalyst residue in traditional cross-coupling copolymerization reactions.²⁹ The presented polymer P1 shows quite a high triplet energy of 2.79 eV, due to the further all meta linkages in the repeating units (carbazole–pyridine–carbazole) allowing the conjugation between the carbazole and pyridyl units to be interrupted. Furthermore, the simultaneous introduction of both electron-deficient pyridine and electron-rich carbazole in each repeating unit should exhibit bipolar carrier transportation characteristics.

2 Experimental part

2.1 Materials and measurements

Tetrakis(triphenylphosphine)palladium(0), trifluoromethanesulfonic acid (TFSA), and N-methylisatin were purchased from Sigma-Aldrich Chemical Industry. Toluene and dichloromethane were distilled and dried overnight over 4A molecular sieves prior to use. Other commercially available reagents were used without further purification.

¹H NMR and ¹³C NMR spectra were recorded on a Bruker 300 MHz spectrometer in deuterated chloroform solution, with tetramethylsilane as the internal reference. UV-visible absorption spectra were recorded on a HP 8453 spectrophotometer. PL spectra were measured with a cooled charge coupled device (CCD) coupled to a monochromator, using the 325 nm line of the He–Cd laser as the excitation. Elemental analyses were performed on a Vario EL elemental analysis instrument (Elementar Co.). Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 at a heating rate of 20 °C min⁻¹. DSC measurements were performed on a Netzsch DSC 204 under N₂ flow, at a heating rate of 10 °C min⁻¹. All the samples were typically subjected to a heating scan to about 250 °C, a cooling scan to room temperature (RT), and a re-heating scan. Liquid chromatograph-mass spectroscopy (LC-MS) was recorded on a LCQ DECA XP spectrometer. The molecular weights of the polymers were estimated by Waters GPC 2410, with tetrahydrofuran (THF) as the eluent. Cyclic voltammetry (CV) data were measured on a CHI660A electrochemical workstation, using tetrabutylammonium hexafluorophosphate (Bu₄NPF₆, 0.1 M in acetonitrile) as electrolyte at a scan rate of 50 mV s⁻¹ at room temperature under the protection of argon. A platinum electrode coated with a thin polymer film was used as the working electrode, Pt wire as the counter electrode, and Ag wire as the reference electrode. At the end of the measurement, the ferrocene/ferrocium potential was measured and used as the reference.

2.2 Fabrication of PLEDs

The devices with the structure of ITO/PEDOT:PSS/LEL/CsF/Al (ITO: iridium tin oxide, PEDOT:PSS: poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid), LEL: light-emitting layer) were fabricated following a well-established process: PEDOT:PSS was spin-coated onto the cleaned ITO-coated glass substrate from aqueous solution and then heated at 120 °C for 20 min to remove the residual water solvent. Polymer doped with phosphorescent emitters was used as the light-emitting layer, which was spin-coated in chlorobenzene to form a uniform 80 nm thickness film on top of the PEDOT:PSS layer. All the films were then annealed at 100 °C on a hotplate for 20 min. A Tencor Alpha-step 500 Surface Profilometer was used to evaluate the thicknesses of the polymer films. Finally, 1.5 nm of CsF, followed by 120 nm aluminum, were thermally evaporated through a shadow mask at a base pressure of 3.0 × 10⁻⁴ Pa to form the cathode. The current density–luminance–voltage (J–L–V) characteristics were measured in the nitrogen dry-box using a keithley 236 source-measurement unit with a calibrated silicon photodiode. The electroluminescence spectra and CIE coordinates were recorded on a PR-705 SpectraScan spectrophotometer (Photo Research).

2.3 Syntheses of monomers and polymers

9-Decyl-3-bromo-9H-carbazole (2). 40 mL dried THF in a 100 mL three-neck flask was purged with nitrogen and stirred, then 1.2 g (0.05 mol) NaH was added carefully. 9.88 g (0.04 mol) 3-bromocarbazole (1) was added slowly into the mixture. 11.05 g (0.05 mol) 1-bromodecane diluted by 10 mL THF was added dropwise into the flask and refluxed for 12 h. The reactant was cooled to room temperature and then the solvent was removed by evaporation. The residue was mixed with 60 mL H₂O and extracted with methylene chloride. The extracted solution was dried with anhydrous MgSO₄ and then filtrated. Compound (2) was isolated by silica gel column chromatography with petroleum ether and ethyl acetic (20/1, v/v) as the eluent (yield 80%). ¹H NMR (300 MHz, DMSO, δ, ppm): 8.39 (s, 1H, ArH), 8.20 (d, 1H, J = 6.0 Hz, ArH), 7.56–7.62 (m, 3H, ArH), 7.48 (d, 1H, J = 7.20 Hz, ArH), 7.21 (t, 1H, J = 7.92 Hz, ArH), 4.38 (t, 2H, J = 7.02 Hz), 1.72–1.78 (m, 2H), 1.17–1.25 (m, 14H), 0.81 (t, 3H, J = 7.02 Hz). ¹³C NMR (75 MHz, CDCl₃, δ, ppm): 14.09, 22.64, 26.84, 27.31, 29.07, 29.24, 29.42, 29.49, 29.53, 31.83, 108.58, 109.11, 118.17, 118.78, 120.36, 122.87, 122.99, 125.61, 127.50, 133.71, 139.79, 149.68; anal. calcd (%) for C₂₂H₂₈BrN: C, 68.39; H, 7.30; N, 3.63. Found: C, 68.82; H, 7.41; N, 3.69; EIMS: m/z 386 (M + 1)⁺.

9-Decyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9H-carbazole (3). Compound (2) was dissolved in dry tetrahydrofuran (THF, 150 mL) and cooled to −78 °C, and then 1.6 M n-BuLi solution in hexane (1.2 equiv.) was slowly dropped into it under an argon atmosphere. The resulting solution was allowed to stir for 2 hours at −78 °C and 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3 equiv.) was then added. The mixture was warmed to RT and stirred for 24 h, then it was poured into water and extracted with ethyl ether. The organic layer was dried by anhydrous magnesium sulfate. Silica gel column chromatography was performed with petroleum ether and ethyl acetic (15/1, v/v) as the eluent to isolate compound 3 as a colorless oil (yield 63%).¹H NMR (300 MHz, CDCl₃, δ, ppm): 8.62 (s, 1H, ArH), 8.15 (d, 1H, J = 7.74 Hz, ArH), 7.93 (d, 1H, J = 8.25 Hz, ArH), 7.40–7.48 (m, 3H, ArH), 7.24 (s, 1H, ArH), 4.32 (t, 2H, J = 7.08 Hz), 1.85–1.90 (m, 2H), 1.25–1.42 (m, 26H), 0.87 (t, 3H, J = 6.27 Hz).

2,6-Bis(9-decylcarbazole-3-yl)pyridine (M1). 2,6-Dibromopyridine (2.7 g, 6.23 mmol), 9-decyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-9H-carbazole (3) (0.68 g, 2.83 mmol), 2 M Na₂CO₃ (6 mL), toluene (20 mL), and ethanol (8 mL) were added in to a 100 mL three-neck flask, and degassed for 10 minutes, then tetrakis(triphenylphosphino)Pd(0) (0.23 g, 0.1 mmol) was added to the mixture, stirred, and refluxed for 24 h under an argon atmosphere. The reaction was quenched by adding 10 mL water, and then extracted by chloroform (100 mL). The organic layer was subsequently washed with brine and water, and dried over anhydrous Na₂SO₄. The solvents were removed by rotary evaporation, and the residue was purified by column chromatography over silica gel using a mixture of petroleum ether and ethyl acetic (10/1, v/v) to give a white solid 1.17 g (yield, 70%). ¹H NMR (300 MHz, CDCl₃, δ, ppm): 8.94 (s, 2H, ArH), 8.41 (d, 2H, J = 7.08 Hz, ArH), 8.26 (d, 2H, J = 7.77 Hz, ArH), 7.80–7.88 (m, 3H, ArH), 7.44–7.57 (m, 6H, ArH), 7.30–7.32 (m, 2H, ArH), 4.38 (t, 4H, J = 7.02 Hz), 1.89–1.96 (m, 4H), 1.27–1.43 (m, 28H), 0.89 (t, 6H, J = 6.18 Hz). ¹³C NMR (75 MHz, CDCl₃, δ, ppm): 14.08, 22.64, 27.32, 29.01, 29.07, 29.25, 29.42, 29.50, 29.52, 31.84, 108.77, 108.86, 117.47, 119.02, 119.18, 120.60, 123.24, 123.34, 125.17, 125.72, 130.89, 137.30, 141.01, 141.17, 157.72; anal. calcd (%) for C₄₉H₅₉N₃: C, 85.34; H, 8.56; N, 6.09. Found: C, 85.30; H, 8.34; N, 6.08; EIMS: m/z 690 (M + 1)⁺.

2-Bromo-9,9-dioctylfluorene (5). The synthesis of compound 5 was similar to that of compound 2, except that the 3-bromocarbazole (1) was replaced with 2-bromofluorene (4). ¹H NMR (300 MHz, CDCl₃, δ, ppm): 7.66–7.70 (m, 1H, ArH), 7.58 (d, 1H, J = 6.0 Hz, ArH), 7.47 (s, 1H, ArH), 7.45 (s, 1H, ArH), 7.33–7.36 (m, 3H, ArH), 1.98 (t, 4H, J = 9.18 Hz), 1.06–1.30 (m, 20H), 0.88 (t, 6H, J = 6.99 Hz), 0.59–0.63 (m, 4H).

9,9-Dioctylfluorene-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (6). The synthesis of compound 6 was similar to that of compound 3 except that the 9-decyl-3-bromo-9H-carbazole (2) was replaced with 2-bromo-9,9-dioctylfluorene (5).

2,6-Bis(9,9-dioctylfluorene-2-yl)pyridine (M2). The synthesis of M2 was similar to that of M1, except that the 9-decylcarbazole boronic ester (3) was replaced with 9,9-dioctylfluoren boronic ester (6) to give a white solid 0.5 g (yield, 70%). ¹H NMR (300 MHz, CDCl₃, δ, ppm): 8.20 (d, 4H, J = 9.48 Hz, ArH), 7.78–7.91 (m, 7H, ArH), 7.36–7.41 (m, 6H, ArH), 2.06 (d, 8H, J = 8.76 Hz), 1.08–1.40 (m, 48H), 0.73–0.87 (m, 12H). ¹³C NMR (75 MHz, CDCl₃, δ, ppm): 14.00, 22.56, 23.83, 29.23, 30.10, 31.77, 40.45, 55.18, 118.38, 119.80, 119.96, 121.40, 122.93, 125.93, 126.78, 127.23, 137.35, 138.38, 140.73, 142.12, 151.22, 151.35, 151.17; anal. calcd (%) for C₆₃H₈₅N: C, 88.36; H, 10.00; N, 1.64. Found: C, 87.30; H, 11.10; N, 1.56; EIMS: m/z 857 (M + 1)⁺.

Syntheses of polymers. The polymers were prepared according to the literature method²⁶ as follows: trifluoromethanesulfonic acid (0.5 mL) was added to a mixture of dichloromethane (3 mL), N-methylisatin (0.5 mmol), and M1/M2 (0.5 mmol). The reaction mixture was stirred for 12 h at room temperature under an argon atmosphere, before being poured slowly into methanol. The resulting mixture was neutralized with 2 M NaOH aqueous solution and extracted with chloroform. The organic layer was subsequently washed with brine and water, and dried over anhydrous Na₂SO₄. The solution was concentrated by rotary evaporation, and poured slowly into methanol. The white polymers were filtered off, and then washed with hot methanol and acetone, and subsequently washed using Soxhlet extraction with methanol and acetone to remove the oligomers and catalyst residues, before being finally dried under vacuum to give white polymer fibers.

Poly{[2,6-bis(9-decylcarbazole-3-yl)pyridine-6,6′-diyl]-alt-[N-methylisatin-2-one-3,3-diyl]} (P1). 260 mg, yield: 90%. ¹H NMR (300 MHz, CDCl₃, δ, ppm): 8.64 (s, 2H, ArH), 8.46 (d, 2H, J = 9 Hz, ArH), 8.13 (s, 2H, ArH), 7.73 (s, 3H, ArH), 7.47–7.54 (m, 5H, ArH), 7.34–7.38 (m, 3H, ArH), 7.17 (t, 1H, J = 7.32 Hz, ArH), 7.02 (d, 1H J = 7.62 Hz, ArH), 4.3 (s, 4H), 3.41 (s, 3H), 1.87 (s, 4H), 1.23–1.34 (m, 28H), 0.85 (t, 6H, J = 6.06 Hz). ¹³C NMR (75 MHz, CDCl₃, δ, ppm): 14.10, 22.64, 26.84, 27.31, 29.07, 29.24, 29.42, 29.49, 29.53, 31.83, 62.69, 108.91, 120.17, 123.04, 126.87, 134.34, 140.11, 141.47, 157.56, 178.86.

Poly{[2,6-bis(9,9-dioctylfluoren-2-yl)pyridine-7,7′-diyl]-alt-[N-methylisatin-2-one-3,3-diyl]} (P2). 290 mg, yield: 89%. ¹H NMR (300 MHz, CDCl₃, δ, ppm): 8.20 (d, 2H, J = 8.76 Hz, ArH), 8.12 (s, 2H, ArH), 7.74–7.87 (m, 6H, ArH), 7.66 (d, 2H, J = 8.43 Hz, ArH), 7.28–7.39 (m, 5H, ArH), 7.11 (t, 1H, J = 7.47 Hz, ArH), 6.99 (d, 1H, J = 7.8 Hz, ArH), 3.39 (s, 3H), 1.87–1.98 (m, 8H), 1.06–1.45 (m, 48H), 0.72–0.85 (m, 12H). ¹³C NMR (75 MHz, CDCl₃, δ, ppm): 14.10, 22.61, 22.62, 24.00, 26.76, 29.28, 29.35, 30.10, 31.83, 40.12, 55.24, 62.94, 118.41, 119.93, 121.32, 123.06, 126.05, 127.27, 128.23, 138.35, 139.83, 141.33, 141.69, 143.18, 151.41, 151.52, 157.14, 177.61.

3 Results and discussion

3.1 Synthesis of monomers and polymers

The M1/M2 was synthesized by the Suzuki-type aryl–aryl cross-coupling between the boronic ester (3 or 6) and the aryl halides. The polymers were prepared by a metal-free, superacid-catalyzed polyhydroxyalkylation (Scheme 1). N-Methylisatin was used as the hydroxymethylation component for coupling with the bifunctional aromatic monomers at the electron-rich positions (3,6 positions for carbazole (M1), 2,7 positions for fluorene (M2)), in the presence of TFSA, to give linear, high molecular weight polymers. The number average molecular weights (M_n) of P1 and P2 were estimated by gel permeation chromatography using tetrahydrofuran as the eluent (Table 1). The evidence of the polymerization includes the emergence of a new signal with δ = 3.40 ppm in the ¹H NMR spectrum of P1/P2, as can be ascribed to the methyl group in the N-methylisatin unit, indicating the connection of N-methylisatin unit with the M1/M2 unit (Fig. S3–S6†).

Scheme 1 Synthetic routes of the monomers and polymers.

Table 1 Physical and electrochemical properties of the polymers

Polymer	Mn (× 10^3)	PDI	Td^a (°C)	Tg (°C)	HOMO (eV)/Eox	LUMO^b (eV)	Eg^c (eV)	ET^c (eV)
a Corresponding to 5 wt% loss.b Estimated from the HOMO levels and the optical gaps.c Calculated from the onset of the spectra.
P1	49.9	1.65	414	185	−5.72 (0.92V)	−2.39	3.33	2.79
P2	76.5	1.86	405	110	−5.91 (1.11V)	−2.57	3.34	2.43

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3.2 Thermal properties

The thermal properties were investigated by thermogravimetric analyses (TGA) (Fig. 1) and differential scan calorimetry (DSC), and the data are shown in Table 1. The polymers exhibit good thermal stabilities, with comparatively high glass transition temperatures (T_g) (185 °C for P1, 110 °C for P2) and decomposition temperatures (T_d corresponding to 5 wt% loss) (414 °C for P1, 405 °C for P2). The high T_g and T_d are important for the practical PLED application of polymer materials, and could be expected to prevent device degradation during the long-term operation.

Fig. 1 TGA curves of P1 and P2.

3.3 Electrochemical properties

The electrochemical properties of the polymers were investigated by cyclic voltammetry (Fig. 2), and are summarized in Table 1. The onset of the oxidation potentials of P1 and P2 are 0.92 and 1.11 V versus ferrocene/ferrocenium (Fc/Fc⁺). The reduction processes of the polymers cannot be observed, and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are calculated according to the equation: E_HOMO = −e(E_ox,onset + 4.80) eV, E_LUMO = E_g − E_HOMO, with regard to the energy level of ferrocene 4.8 eV below the vacuum. The calculated HOMO/LUMO of P1 and P2 are −5.72/−2.39 eV and −5.91/−2.57 eV, respectively. The higher HOMO of P1 can be attributed to the electron-rich nature of the carbazole units, which increases the electron density of the polymer.

Fig. 2 Cyclic voltammograms of P1 and P2 in an acetonitrile solution.

3.4 Photophysical properties

The absorption spectra of P1 and P2 were measured in dilute CH₂Cl₂ solution and in the film state (Fig. 3). In solution, P1 shows absorption peaks at 252 nm and 299 nm, possibly corresponding to n–π* and π–π* transitions, respectively, with a weak absorption at ca. 330 nm, corresponding to the intramolecular charge transfer (ICT) with regard to the interaction between the electron-donor carbazole and the electron-withdrawing pyridine. In the case of P2, it shows a π–π* transition absorption band centered at 340 nm, and no obvious ICT transition can be observed, due to the weaker electron-donor nature of fluorene. Compared to solution absorption, P1 and P2 films show similar profiles, but broader absorption bands, ascribed to the molecular aggregation in the solid state. The optical gaps of P1 and P2 are 3.33 eV and 3.34 eV, respectively (Table 1), as estimated from the onset of the absorption in film.

Fig. 3 Absorption spectra of P1 and P2 in solution and in the film state (inset: absorption spectra of M1 and P1 in dilute solution).

To further study the effectiveness of the conjugation breaking in the polymer backbone, both the photoluminescence (PL) spectra and the absorption spectra of M1/P1 and M2/P2 were measured in CH₂Cl₂ solutions (10⁻⁵ M) (Fig. 4 and S2†). The PL spectra of P1/P2 in solution show a slight red-shift (only about 5 nm) compared with that of M1/M2, as well as in their absorption spectra (Fig. S2† and inset of Fig. 3), indicating that the conjugation lengths of the polymers are successfully confined into the repeating units because of the non-conjugated δ-C bond connection in the polymer backbone. However, the polymers in the film show a broader and more red-shifted emission than in solution, probably due to the severe molecular aggregation in the solid state.

Fig. 4 PL spectra of the monomers and polymers in film and in a diluted solution (10⁻⁵ M).

The phosphorescence spectra of P1/P2 were measured at 77 K in film, according to the literature method³⁰ (Fig. 5). The triplet energy levels (E_T) of P1 and P2 are located at 2.79 eV and 2.43 eV respectively, relative to the onset of the phosphorescence spectra. The lower E_T of P2 can be ascribed to the fluorene unit, which itself has lower triplet energy levels than the carbazole unit. In addition, the para linkages of fluorene with pyridine in P2 extend the π-electron delocalizations along the fluorene–pyridine ring, allowing a lower E_T. Compared to PICzFB (E_T = 2.73 eV), which we published earlier,²⁷ the slightly higher E_T of P1 (2.79 eV) could be ascribed to the all meta linkages between the carbazole and pyridine units, which possibly further interrupt the molecular conjugation along the repeating units and increase the triplet energy levels. It is worth mentioning that the E_T of P1 is also higher than for some copolymers published in literature, such as poly(9,9-dialkyl-3,6-dibenzosilole) (2.55 eV),^31,32 and poly(9,9-dioctylfluorene-2,7-diyl-co-2,8-dihexyldibenzo-thiophene-S,S-dioxide-3,7-diyl) (2.46 eV),³³ and also higher than that of the widely used green-emitting phosphorescent dopant tris(2-(4-tolyl)phenylpyridine)iridium (Ir(mppy)₃) (2.38 eV),³⁴ as well as the blue-emitting iridium(III) [bis(4,6-difluorophenyl)pyridinato-N,C2′]-picolinate (FIrpic) (2.65 eV).^35–37

Fig. 5 Phosphorescence spectra of P1 and P2 in 77 K in the film state.

When considering that a high E_T is an important prerequisite for host materials, to effectively prevent energy back transfer from the phosphorescent complex to the polymer host, it is thus expected that the present high-E_T copolymer will have great potential for application as a host for both blue and green phosphorescent emitters.

To evaluate the potential of the polymers as a host for green phosphorescent PLEDs, the absorption of Ir(mppy)₃ (tris(2-(4-tolyl)phenylpyridine)iridium) and the emission of the polymers were investigated to confirm the energy transfer between P1/P2 and Ir(mppy)₃ (Fig. 6). As seen from Fig. 6, Ir(mppy)₃ exhibits a strong absorption band centered at 370 nm, which overlaps well with the emission of polymers P1 and P2, implying a possibly effective energy transfer from the polymers to Ir(mppy)₃.

Fig. 6 PL spectra of P1 and P2 and absorption of Ir(mppy)₃.

The PL spectra of P1 and P2 doped with Ir(mppy)₃ were investigated (Fig. 7). The blended films exhibit a strong emission at 516 nm arising from Ir(mppy)₃, together with a minor contribution at 380 nm from the polymer. This indicates that an efficient energy transfer occurs from the polymers to Ir(mppy)₃ and that the back energy-transfers do not occur to a substantial degree. These results suggest that the polymers can be used as a host for green phosphorescent PLEDs.

Fig. 7 PL spectra of P1 and P2 films doped with Ir(mppy)₃ (5 wt%).

3.5 Electroluminescence properties

Single-layer PLEDs based on P1/P2 as the host and Ir(mppy)₃ as the guest with the devices configuration of ITO/PEDOT:PSS/LEL/CsF/Al were fabricated (LEL denotes P1/P2 films doped with 5 wt% Ir(mppy)₃). In contrast with the PL spectra, the EL (electroluminescence) spectra of both P1 and P2 exhibit only a green emission peaked at 515 nm derived from Ir(mppy)₃ and no emission can be observed from the polymers (Fig. 8), suggesting that both energy transfer and direct charge trapping/recombination at Ir(mppy)₃ were responsible for the EL mechanism.

Fig. 8 EL spectra of the device ITO/PEDOT/polymer: Ir(mppy)₃ (5 wt%)/CsF/Al.

Luminance–current density–voltage characteristics are shown in Fig. 9 and the results summarized in Table 2. The P1-based device shows best EL properties with maximum external quantum efficiency of 1.26%, luminance efficiency of 4.4 cd A⁻¹ and brightness of 4493 cd m⁻² in such a preliminary device configuration, compared to the P2-based device and our previously reported PICzFB-based device. The higher efficiency for P1-device could be ascribed to higher triplet energy as well as the balanced charge transportation/injection due to the excellent hole-transport carbazole and electron-transport pyridine units incorporated in the polymer backbone. According to the energy levels, P1 has a higher HOMO (−5.72 eV) than PICzFB (−5.82 eV) and P2 (−5.91 eV). The high HOMO facilitates the holes injection from PEDOT to the LEL and leads to lower turn on voltage (Table 2).

Fig. 9 Luminance–current density–voltage properties of the devices structure of ITO/PEDOT/polymer: Ir(mppy)₃ (5 wt%)/CsF/Al.

Table 2 Device performance with P1/P2 as the host and Ir(mppy)₃ as the guest (device structure: ITO/PEDOT/polymer:Ir(mppy)₃ (5 wt%)/CsF/Al)

Polymer^a	Von^b (V)	Lmax (cd m^−2)	LEmax (cd A^−1)	EQEmax (%)	Brightness = 100 cd m^−2
					V (V)	J (mA cm^−2)	LE (cd A^−1)	EQE (%)	CIE^c
a Doped with 5 wt% Ir(mppy)3.b Corresponding to a luminance of 1 cd m^−2.c Measured at 12 mA m^−2.
P1	10	4493	4.4	1.26	13	2.6	4.16	1.2	(0.31, 0.61)
P2	11	552	0.98	0.39	18	12	0.81	0.32	(0.34, 0.59)
PICzFB	15	1242	4.2	1.18	22	3.2	3.7	1.1	(0.32, 0.61)

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Although the efficiencies of the P1-based devices are lower than those of the multilayer devices reported in previous literature,^38,39 the results here were obtained in a very simple device structure; i.e., a single-layer device without blending additional electron/hole-transporting materials into the host, which would be advantageous in future markets. The presented results suggest that P1 actually has high triplet energy, as well as balanced carrier transportation, and could be a promising host polymer for green-emitting PLEDs. The relevant work into the P1-host for blue-emitting PLEDs is also in progress.

4 Conclusions

In summary, two novel bipolar host polymers based on carbazole–pyridine–carbazole and fluorene–pyridine–fluorene repeating units have been prepared, respectively, by metal-free superacid-catalyzed polyhydroxyalkylation. The polymers show good thermal stability, with high glass transition temperatures (185 °C for P1, and 110 °C for P2), a wide band gap about 3.33 eV, and considerably high triplet energy (2.79 eV for P1). Efficient single-layer green phosphorescent PLEDs have been realized using P1 as the host and Ir(mppy)₃ as the guest without blending any electron/hole-transporting materials. The superior results can be ascribed to the all meta linkages in the repeating units, as well as to the conjugation-confined main chain due to the δ-C bond interrupted polymer backbone.

Acknowledgements

The authors are grateful for financial support from the National Natural Science Foundation of China (Grant nos 21303256, 21221064 and 10974223).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06540a

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