Four highly efficient cuprous complexes and their applications in solution-processed organic light-emitting diodes

Abstract

Four mononuclear cationic Cu(I) complexes featuring functional C-linked pyrazolyl pyridine diimine ligands have been synthesized and characterized. Under UV 365 nm at room temperature, these complexes emit similar orange light in solution and green light in thin film, which could be attributed to the limited charge and steric influence of these methyl or trifluoromethyl substituted phenyl appendages at the pyrazol ring. Moreover, multilayer organic light-emitting diodes (OLEDs) based on these Cu(I) complexes had different performance efficiencies. The device with complex 1 as the light emitting material had the highest current efficiency of 17.8 cd A⁻¹ and an external quantum efficiency (EQE) of 6.4%, while the device with complex 2 which shows the highest photoluminescence quantum yields gave the poorest device efficiency with a current efficiency of 13 cd A⁻¹ and an EQE of 4.7%.

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Introduction

Since the pioneering work of Tang et al.,¹ multilayered organic light-emitting diodes (OLEDs) have attracted considerable attention.^2–4 In the emitting layers of OLEDs, dopants are one of the most important components because they are keys to the electrochemical stability, decay time, color, efficiency, etc. of the devices.⁵ Up to the present, there are two kinds of dopants for highly efficient OLEDs which can surpass 25% of the theoretical intrinsic upper limit of conventional fluorescent emitters. One is the triplet harvesting phosphorescent materials, including third-row noble Os(II),^6–8 Ir(III)^9–13 and Pt(II)^14,15 complexes.¹⁶ Another is thermally activated delayed fluorescence^17–19 (TADF) materials containing pure organic skeleton compounds^20–22 and specific cuprous complexes^23–25 which are low-cost and environmentally benign.

In order to have highly efficient cuprous complexes, “small energy gap between T₁ and S₁” and “large energy gap between T₁ and S₀” are required. The former determines the level of thermally up-conversion from T₁ to S₁.^26,27 And the later probably hinder the thermal population radiationless deactivation.²⁸ For a Cu(I) complex with distorted tetrahedral coordination geometry, it undergoes a typical flattening distortion process to offer a lower T₁ state when it is excited.^29,30 Due to this flattening geometry, the Cu(I) ion becomes easier to be accessed by solvent and oxygen molecular, which, subsequently, accelerate the emission quenching.³¹ The introduction of the large steric ligand bis[2-(dipenylphosphino)phenyl]ether (POP) could effectively block the flattening process, and then increase the efficiency of the photoluminescence.^5,32–34 In these heteroleptical Cu(NN)(POP)⁺ complexes, the HOMOs have predominant contribution from the metal d orbital and the POP ligand, and the LUMOs are on the diimine ligands, indicating an emission from metal and ligand to ligand charge transfer ((M + L)LCT)) excited state.^35,36 The HOMOs of these complexes were lowered down by the electron deficient POP.³³ Meanwhile, their LUMOs and T₁ levels can be further elevated by the introduction of electron-rich and bulky steric hindrance diimine ligands which has already led to highly efficient photo- and electro-luminescence of Cu(I) complexes.^37,38

Although neutral Cu(I) complexes without small counterions achieve higher device performance than those with counterions,^39–44 these complexes containing N⁻-ligands and halogen counterions have much lower stability^45–47 or are more difficult to adjust the emission spanning the whole visible region.^48–52 Cationic emissive Cu(I) complexes are more stable and their emission would be easily adjusted.^27,53–55 Moreover, these complexes with diimine and POP ligands had been successfully fabricated into devices and excellent external quantum efficiency (EQE) up to 16% through the cost-effective solution spin-coating method had been realized.⁵⁶

In this study, four novel cationic cuprous complexes based on functional 3-C-linked pyrazolyl pyridine diimine ligands were synthesized and characterized. Significant different influences of functional groups for these complexes both on the photo- and electro-luminescence as compared to our previous reported blue-emitting cationic Cu(I) complexes³⁷ have been observed. For the complexes with functional N-linked 2-pyrazolyl pyridine diimine ligands, the substituent group largely influence their emission properties both on photo- and electro-luminescence, meanwhile, the trifluoromethyl substituted compounds showed the best device efficiency. The complexes in this report had similar photo-luminescence behaviours in PMMA and in dichloromethane, respectively, which suggest that the influences of the substituent groups are negligible. Moreover, complex 1 without trifluoromethyl and methyl group had shown the best device efficiency with the highest current efficiency of 17.8 cd A⁻¹ and EQE of 6.4% with the configuration: ITO/PEDOT:PSS(40 nm)/10 wt% of 1 (or 2, 3, 4):host(30 nm)/DPEPO(50 nm)/LiF(0.7 nm)/Al(100 nm). While, the device based on 2 gave the poorest device efficiency with current efficiency of 13 cd A⁻¹ and EQE of 4.7%.

Results and discussion

Synthesis and characterization

According to previous report,³⁷ cuprous complexes [Cu(phpzpy)(POP)]BF₄ (1), [Cu(2-mphpzpy)(POP)]BF₄ (2), [Cu(2-tfphpzpy)(POP)]BF₄ (3), [Cu(4-tfphpzpy)(POP)]BF₄ (4) (Fig. 1) were prepared from [Cu(CH₃CN)₄]BF₄ with one equivalent of POP and corresponding diimine ligand in dichloromethane. The as-made solids were purified and isolated through crystallization by layering diethyl ether on the surface of the dichloromethane solvent of these complexes. The obtained complexes were characterized with ¹H NMR, ³¹P NMR, thermal analysis, elemental analysis and Single-crystal X-ray diffraction analysis.

Fig. 1 Molecular structures of cuprous complexes.

The crystals for X-ray analysis were recrystallized in the dichloromethane/diethyl ether for 1 and in the ethanol for 2–4 as shown in Fig. 2. Selected bond distances and bond angles were listed in Table S2.† Similar highly distorted tetrahedral geometries were observed for these complexes with N–Cu–N and P–Cu–P angles within the range of 78.46–79.34° and 109.05–114.85°, respectively. The Cu–N2 bond length was shorter than that of Cu–N3, suggesting a stronger coordination between copper ion and pyrazol unit than that between copper ion and pyridine ring.

Fig. 2 ORTEP diagrams of 1–4 with thermal ellipsoids at 30% probability level. Solvent molecules, the anion, and H atoms were omitted for clarity.

Photophysical properties

Photophysical data of these complexes in the degassed CH₂Cl₂, PMMA and crystal powder were listed in Table 1. Their corresponding UV-vis absorption and emission spectra were shown in Fig. 3a. According to the previous studies,^38,57 the intense absorption bands below 300 nm belong to spin-allowed π–π* ligand-centered (LC) transitions of diimine ligand and POP ligand, and the tailed peak from 330 nm to 420 nm is attributed to the MLCT transition from the Cu(I) ion to diimine ligand. The onset wavelengths of absorption spectra which used to estimate the energy gap (ΔE_g) between HOMO and LUMO were consistent with the peak of excitation spectra (Fig. S3†). Similar ΔE_g values of these complexes were obtained for 1 (3.04 eV), 2 (3.08 eV), 3 (3.04 eV) and 4 (3.00 eV). Therefore, parallel emission spectra of these four complexes probably could be observed.

Table 1 Photophysical data of 1–4 in degassed CH₂Cl₂, PMMA films and ethanol processed crystalline powder

	Solution (in degassed CH2Cl2)			PMMA films			Powder
	λabs (nm) (ε × 10^−3 M^−1 cm^−1)	λem^a (nm)	τ^b (μs)	λem^a (nm)	τ^b (μs)	Φ^c (%)	λem^a (nm)	τ^b (μs)	Φ^c (%)
a Emission maximum.b Emission decay time (monoexponential fit, error ± 5%).c Photoluminescence quantum yield (error ± 10%) at 298 K.
1	253(31), 288(26), 357(3.4)	570	5.7	513	38.1	53	487	41	72
2	245(54), 284(43), 349(7.8)	556	4.2	506	38.2	47	494	61	91
3	245(35), 280(28), 354(4.1)	580	4.8	511	34.2	54	494	43	77
4	253(43), 287(35), 361(4.1)	576	5.2	515	38.8	46	498	49	70

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Fig. 3 (a) Absorption spectra of cuprous complexes 1, 2, 3 and 4 in CH₂Cl₂. (b) Emission spectra in degassed CH₂Cl₂. (c) and (d) Emission spectra in PMMA at 298 K and 77 K, respectively. (e) Emission spectra in ethanol processed crystalline state. (f) Emission spectra of 1 in ethanol (1a) and dichloromethane/diethyl ether (1b) processed crystalline powder.

In the CH₂Cl₂, all of these four complexes displayed bright yellow-orange emission under UV 365 nm (Fig. 3b). Quite similar emission wavelength and decay time of 3 and 4 suggest that the position of the steric trifluoromethyl group has limited influence on their emitting light. Additionally, the emission spectra of 3 and 4 had several nanometers of red shift, and 2 with methyl group had 14 nm of blue shift compared with that of 1. These results suggested that the emission spectra of these four complexes were mainly controlled by the charge character instead of the position of substituent group. Furthermore, 1 had the longest decay time among them, probably because it had not vibration of the appended methyl and trifluoromethyl groups as other complexes did. Certainly, complex 2 had the shortest emission life due to its additional C–H vibration of methyl unit.⁵⁸

In comparison with these complexes in the CH₂Cl₂, their emission in the PMMA films had blue shift of at least 50 nm at room temperature (Fig. 3c), their decay times were increased nearly by six times and their quantum yields were significantly improved. The changes for their emission behaviours are caused by the suppressed geometry distortion and non-radiative quenching process in the excited state with the increasing in rigidity of the matrix.^52,53 Meanwhile, the emission spectra for all of these four cuprous complexes at 77 K are red shift (Fig. 3d) which caused by the changes of the light emitting states (from the singlet to the triplet state). The changes suggest that the triplet state (T₁) could be thermally activated to the singlet state (S₁) for these complexes at room temperature. The hypothesis is consistent with the small energy gaps for these complexes estimated from the onsets of their emission peaks in PMMA, 0.16 eV for 1, 0.24 eV for 2, 0.16 eV for 3 and 0.17 eV for 4 which is small enough for thermally up-conversion.

Highly photoluminescence quantum yields (up to 90%) of these four cuprous complexes in the solid state prepared in the ethanol suggested that the effects of energy transferring between adjacent emitter molecules were not significant. Similar luminescence performances were also observed except slightly blue shift emission spectrum of 1 and increase in decay time and quantum yield of 2. However, because copper is internal heavy metal,⁵⁹ its MLCT exited state was largely influenced by the morphology even in the solid state.³⁶ As shown in Fig. 3f, complex 1 had 45 nm of emission spectral shift from blue-green of the crystal state prepared in ethanol to green of the solid prepared in CH₂Cl₂/Et₂O, and the emission peak of later state even had 20 nm of red shift compared with that of 1 in PMMA.

In order to understand the nature of the emitting state of these complexes, 1 was selected as a representative, its emission spectra in the solid state prepared in CH₂Cl₂/Et₂O was measured at 77 K and 298 K (Fig. 4a). The results showed that 1 had two inter-convertible exited states with 28 nm of emission spectral shift, which was confirmed by the decay time of 1 at varied temperatures from 77 K to 298 K as listed in Table S3.† These data were tested with the following TADF equations.^17–19,27

Fig. 4 (a) Emission spectra of 1 in dichloromethane/diethyl ether processed crystalline powder at 77 K and 298 K. (b) Temperature dependence of decay time for the dichloromethane/diethyl ether processed crystalline powder of 1.

Herein, k_B and T are the Boltzmann constant and the absolute temperature, respectively. τ(S₁) and τ(T₁) are the individual decay times of the lowest singlet and triplet excited state, and ΔE_ST is the energy gap between these two states. The fitted values of τ(S₁), τ(T₁) and ΔE_ST are 300 ns, 287 μs and 0.10 eV, respectively. At 77 K, the τ_obs is approximately assigned to pure T₁ state (Fig. 4b). The ΔE_ST value agrees very well with the estimated energy difference (0.12 eV) between onset of emission spectra at 77 K and 298 K. The small ΔE_ST is essential to facilitate thermal up-conversion from T₁ to S₁ at room temperature. The results showed that these data fitted very well with the TADF equation, suggesting these cuprous complexes can largely harvest the energy of T₁ state through thermally up-conversion and are promising alternatives for highly efficient OLEDs.

Electroluminescence performances

The tiled cuprous complexes were selected for solution processed OLED fabrication with the configuration: ITO/PEDOT:PSS (40 nm)/10 wt% of 1 (or 2, 3, 4):PYD2 (2,6-bis(N-carbazolyl)pyridine, 30 nm)/DPEPO (bis[2-(di-(phenyl)phosphino)-phenyl]ether oxide, 50 nm)/LiF (0.7 nm)/Al (100 nm) to evaluate their electroluminescence performances. In this configuration, PYD2 and DPEPO acted as hole-transporting and electron-transporting materials, respectively. The HOMO/LUMO levels of these complexes were determined by oxidation potential and their onsets of absorption spectra (5.64/2.6 eV for 1), (5.68/2.6 eV for 2), (5.6/2.56 eV for 3) and (5.56/2.56 eV for 4). The corresponding HOMO/LUMO levels of other materials used in the device were based on previous reports.³⁷ The structures of organic materials and the energy levels of device layers were shown in Fig. 5a. The functional groups on 3-C-linked pyrazolyl pyridine diimine ligands had different influence on electroluminescence compared with substituent groups of N-linked pyrazolyl pyridine diimine ligands. As shown in Fig. 5b–d and Table 2, these four cuprous complexes had similar green electroluminescent (EL) spectra with slightly blue shift emission spectra compared with their photoluminescence in PMMA, which might be due to the different rigidity of the matrix. Additionally, devices based on 2 and 3 exhibited relatively poor EL performances with turn-on voltage (V_on) of 6.3 eV and 5.9 eV, maximum current efficiency (CE_max) of 13.0 cd A⁻¹ and 14.5 cd A⁻¹, EQE of 4.71% and 5.54%, maximum brightness (B_max) of 1286 cd m⁻² and 1281 cd m⁻², respectively. Device with V_on of 6.9 eV, CE_max of 17.8 cd A⁻¹, EQE of 6.37%, B_max of 1378 cd m⁻² had been realized based on 1, and a slightly poorer efficiency was observed for 4. These results showed that the substituent groups on appendage phenyl are not in favour of the highly efficient device performance for this series of complexes, and the reasons for this are still needed to be verified.

Fig. 5 (a) Energy level diagrams and chemical structures of materials used in OLED. (b) EL spectra of device. (c) Current density–voltage–brightness (I–V–B) curves. (d) Plots of external quantum efficiency and current efficiency vs. brightness.

Table 2 EL characteristics of OLEDs

Dopant	CEmax (cd A^−1)	EQEmax (%)	Von (eV)	Bmax (cd m^−2)	λmax (nm)	CIE1931 (x, y)
1	17.8	6.4	6.9	1378	508	(0.22,0.45)
2	13.0	4.7	6.3	1286	498	(0.22,0.44)
3	14.5	5.5	5.9	1281	498	(0.21,0.41)
4	17.4	6.1	6.9	1366	506	(0.23,0.46)

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Conclusions

In this study, four novel highly emissive cationic cuprous complexes based on functional 3-C-linked pyrazolyl pyridine diimine ligands had been synthesized and characterized. It was found that the influence of substituent methyl and trifluoromethyl on photoluminescence was very limited at room temperature. These complexes all had similar photoluminescence behaviours both in the degassed CH₂Cl₂ and PMMA. However, different performance efficiencies of these complexes were observed in the solution processed OLEDs. Results of PL and EL had also demonstrated that cuprous complex 1 with substituent-free diimine ligand should be a more effective dopant material of OLEDs.

Experimental

General

All experiments were performed under N₂ atmosphere with standard Schlenk techniques unless specified. Solvents were freshly distilled over appropriate drying reagents. Other materials were purchased and used as received. [Cu(CH₃CN)₄]BF₄ was prepared according to the literature procedure.⁶⁰

Characterization

Samples for NMR, elemental analyses and thermogravimetric analysis (TGA) were dried at 90 °C for 30 min. ¹H NMR and ³¹P NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer. Elemental analyses (C, H, N) were determined with an Elementar Vario EL III elemental analyzer. TGA were done on a NETZSCH STA 449C Jupiter thermogravimetric analyzer with flowing nitrogen and Al₂O₃ crucible at a heating rate of 10 K min⁻¹ (Fig. S2†). Cyclic voltammetry (CV) was performed in a gastight single-compartment three-electrode cell with a BAS Epsilon electrochemical analyzer at room temperature. A glassy carbon disk and a platinum wire were selected as the working and auxiliary electrodes, respectively. The ferrocenium/ferrocene couple was used as an internal standard. The reference electrode was Ag/Ag⁺ (0.1 M of AgNO₃ in CH₂Cl₂). The CV measurements were carried out in anhydrous and nitrogen-saturated CH₂Cl₂ solution with 0.1 mol of n-tetrabutylammonium perchlorate (TBAP) and 1.0 mmol of analyte at a scan rate of 0.1 V s⁻¹. UV-vis absorption spectra were recorded with a Perkin-Elmer Lambda 45 UV/vis spectrophotometer. Photoluminescence spectra at both 298 K and 77 K were recorded on an Edinburgh Analytical instrument FLS980. The lifetimes at different temperatures were determined on an Edinburgh analytical instrument FLS980 with a picosecond laser diode. The PL quantum yields were defined as the number of photons emitted per photon absorbed by the systems and measured on Edinburgh analytical instrument FLS920 equipped with an integrating sphere. The film samples in photophysical studies were prepared through spin-coating of a mixture of Cu(I) complexes (10 wt%) and PMMA (90 wt%) at 1000 rpm.

Synthesis

3-Pyridine-2-yl-1H-pyrazolyl (A). According to previous report⁶¹ (Fig. S1†), a mixture of 2-acetylpyridine (10 g) and dimethylformamide dimethyl acetal (25 mL) was refluxed for 16 h into a dark brown solution. After removal of the excess solvent under vacuum, the crude brown oil was recrystallized from chloroform/petroleum ether to give 3-dimethylamino-1-(pyridine-2-yl)prop-2-en-1-one (B) as yellow crystalline solid (12 g). Then, B was added to a solution of hydrazine monohydrate (15 mL) in ethanol. The mixture was refluxed for 30 min. The solvent was removed under vacuum to get white solid, which further purified through crystallization in chloroform/petroleum ether to give colourless crystal.

2-(1-R-1H-pyrazol)pyridine. This preparation process is a revised method based on previous report.⁹ A (0.3 g, 2.07 mmol), 1,10-phenanthroline (0.149 g, 0.827 mmol), copper(I) iodide (0.079 g, 0.413 mmol), caesium carbonate (1.35 g, 4.13 mmol), 2.07 mmol iodobenzene for 2-(1-phenyl-1H-pyrazol)pyridine (phpzpy), 2-methyl iodobenzene for 2-(1-(o-tolyl)-phenyl-1H-pyrazol-3yl)pyridine (2-mphpzpy), 2-trifluoromethyl iodobenzene for 2-(1-(2-(trifluoromethanyl)phenyl-1H-pyrazol-3yl)pyridine (2-tfphpzpy) and 4-trifluoromethyl iodobenzene for 2-(1-(4-(trifluoromethanyl)phenyl-1H-pyrazol-3yl)pyridine (4-tfphpzpy) were dissolved in dry DMF (10 mL). The mixture was stirred vigorously for 1 h at room temperature and then heated to 100 °C under nitrogen for another 36 h. At room temperature, the residue was filtrated. The liquid phase was extracted with CH₂Cl₂ (3 × 30 mL) and washed with water, subsequently dried with anhydrous sodium sulphate. After the removal of solvent under vacuum, the crude product was purified through column chromatography on silica gel with ethyl acetate/petroleum ether.

Cuprous complexes. The four cuprous complexes were prepared according to the following general procedures: a mixture of [Cu(CH₃CN)₄]BF₄ (1 mmol) and phosphine ligand (POP, 1 mmol) in CH₂Cl₂ (10 mL) was stirred at room temperature for 0.5 h. Then, corresponding diimine ligands phpzpy (1 mmol) for 1, 2-mphpzpy for 2, 2-tfphpzpy for 3 and 4-tfphpzpy for 4 were added. The solvent was evaporated after the reaction mixture was stirred for another 1 h. Then, single crystal of 1 for X-ray diffraction was obtained through the diffusion of diethyl ether into the CH₂Cl₂ solutions of it, and the other crystals were obtained through slow evaporation the saturated ethanol solutions of them.
[Cu(phpzpy)(POP)]BF₄ (1). ¹H NMR (400 MHz, CDCl₃): δ 8.09 (d, 1H), 7.92 (m, 2H), 7.8 (d, 1H), 7.48 (t, 1H), 7.33–7.27 (m, 7H), 7.24–7.07 (m, 12H), 6.98–6.86 (m, 9H), 6.79 (dd 4H), 6.76–6.62 (m, 2H). ³¹P NMR: −12.78 (s). Elemental analysis: calc for C₅₀H₃₉BCuF₄N₃OP₂: C, 65.99; H, 4.3; N, 4.62. Found: C, 65.44; H, 4.28; N, 4.49%.
[Cu(2-mphpzpy) (POP)]BF₄ (2). ¹H NMR (400 MHz, CDCl₃): δ 8.19 (d, 1H), 7.96 (t, 1H), 7.83 (d, 1H), 7.68 (d, 1H), 7.52 (m, 2H), 7.39–7.26 (m, 8H), 7.20–7.12 (m, 8H), 7.06 (d, 2H), 6.96–6.90 (m, 4H), 6.73 (m, 7H), 6.52–6.49 (m, 2H), 6.18 (d, 1H), 1.69 (s, 3H). ³¹P NMR: −13.61 (s). Elemental analysis: calc for C₅₆H₄₂BCuF₄N₄OP₂: C, 66.22; H, 4.44; N, 4.54. Found: C, 66.47; H, 4.59; N, 4.46%.
[Cu(2-tfphpzpy) (POP)]BF₄ (3). H NMR (400 MHz, CDCl₃): δ 8.16 (d, 1H), 7.96 (t, 1H), 7.89 (d, 2H), 7.83 (d, 1H), 7.74 (d, 1H), 7.4–7.31 (m, 8H), 7.26–7.12 (m, 9H), 7.02–6.91 (m, 5H), 6.83–6.75 (m, 7H), 6.58–6.51 (m, 2H), 6.18 (d, 1H). ³¹P NMR: −13.22 (s). Elemental analysis: calc for C₅₆H₄₂BCuF₄N₄OP₂: C, 62.6; H, 3.91; N, 4.29. Found: C, 62.36; H, 4.28; N, 4.41%.
[Cu(4-tfphpzpy) (POP)]BF₄ (4). H NMR (400 MHz, CDCl₃): δ 8.21 (d, 1H), 8.10 (d, 1H), 7.99 (d, 1H), 7.94 (t, 1H), 7.45 (m, 4H), 7.39 (d, 1H), 7.36–7.30 (m, 5H), 7.26 (d, 1H), 7.20 (t, 4H), 7.10 (t, 4H), 7.05 (t, 1H), 7.00 (t, 4H), 6.91–6.83 (m, 8H), 6.75–6.71 (m, 2H). ³¹P NMR: −12.45 (s). Elemental analysis: calc for C₅₆H₄₂BCuF₄N₄OP₂: C, 62.6; H, 3.91; N, 4.29. Found: C, 62.3; H, 3.97; N, 4.17%.

X-ray crystallographic analysis

Diffraction data of complexes 1 and 3 were collected on a SuperNova, Dual, Cu at zero, Atlas diffractometer equipped with graphite-monochromated Cu Kα radiation (λ = 1.54184 Å). Diffraction data of complexes 2 and 4 were collected on a Rigaku Mercury diffractometer equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Structures were solved with direct methods and refined with full-matrix least-squares methods with SHELXL-97 program package. Hydrogen atoms were added in idealized positions. All nonhydrogen atoms were refined anisotropically. Details of crystal and structure refinement were listed in Table S2.† Selected bond length and bond angles were listed in Table S3.†

Device fabrication and characterization

The hole-blocking material bis[2-(di(phenyl)phosphino)-phenyl]ether oxide (DPEPO) was prepared according to literature, and purified through sublimation after recrystallization.⁶² Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) was purchased from Heraeus and filtered with 0.22 m filter before use. Other materials used in the device fabrication were purchased and used without further purification. A 40 nm thick PEDOT:PSS film was first spin-coated (at 3000 rpm) on a pre-cleaned ITO-glass substrate and then dried at 140 °C for 20 min. The emitting layer was then overlaid with spin-coating (at 1500 rpm) of CH₂Cl₂ solution with host and dopant (1 mg of 2 and 9 mg of PYD2 dissolved in 1.8 mL CH₂Cl₂). The film was then dried under vacuum for 1 h at room temperature. Subsequently, 50 nm of DPEPO, 0.8 nm of LiF and 100 nm of Al were deposited under pressure less than 4 × 10⁻⁴ Pa. The electroluminescence (EL) spectra were recorded on a HORIBA Jobin-Yvon FluoroMax-4 spectrometer. Current density–voltage–brightness (I–V–B) curves of these devices were recorded on a Keithley 2400/2000 source meter and a calibrated silicon photodiode at room temperature in air.

Acknowledgements

This work was supported by the 973 key program of the Chinese Ministry of Science and Technology (MOST) (2012CB821705), the Chinese Academy of Sciences (KJCX2-YW-319, KJCX2-EW-H01), the National Natural Science Foundation of China (21373221, 21221001, 91122027, 51172232) and the Natural Science Foundation of Fujian Province (2006L2005).

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Footnote

† Electronic supplementary information (ESI) available: Supplementary computational and experimental data. CCDC 1041001–1041004. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra04591f

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