Highly efficient non-doped blue electroluminescent materials for organic light-emitting devices

Abstract

We have synthesized the new blue-light-emitting materials 4,4′-bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl (NPIP), 4,4′-bis(4-methylnaphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl(MeNPIP) and 4,4′-bis(4-methoxynaphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl-1,1′-biphenyl(MeONPIP) through a two-step procedure using inexpensive catalysts. These compounds showed excellent thermal properties with a very high glass-transition temperature of 195–201 °C due to their rigid molecular backbones and they emit intense blue light in both solution and film. A device fabricated with MeONPIP shows maximum efficiencies (η_ex 6.92%; η_c −7.80 cd A⁻¹; η_p 6.10 lm W⁻¹) with a low turn-on voltage and the performances of the device suggest that the phenanthroimidazole unit is an excellent building block for tuning carrier injection properties as well as for blue emission.

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1. Introduction

The development of highly stable and efficient emitters of primary colors is a major task for making OLEDs into commercial products.^1–4 With a rapid improvement in the efficiencies of OLEDs, efficient fluorescence and phosphorescence emitters have already emerged. However, there are still some vital problems to be addressed, such as the lack of highly efficient and stable deep-blue-emitting materials with good color purity and the need for an economical production method that can be used in practical industrial applications.⁵ A doped blue-emitting device based on bis(4,6-difluorophenylpyridinato-N,C²)picolinatoiridium has an efficiency of 40 cd A⁻¹, but the fabrication of the device is complex.⁶ Though devices with a guest–host system as the emissive layer show improved efficiency, the fabrication of doped emissive layers is more expensive and complicated than that of non-doped layers.^7,8 Additionally, phase separation on heating could be an important cause of degraded performances in guest–host systems.⁹ To overcome these disadvantages, the fabrication of non-doped devices has attracted tremendous attention and these devices have exhibited an extremely high external quantum efficiency.^10–12

Non-doped devices with deep-blue dipyrenylbenzene and anthracene derivatives as emitters exhibit high efficiencies of 5.2 and 5.3%, with CIE coordinates of (0.15, 0.11) and (0.14, 0.12), respectively,¹² but their corresponding power efficiencies are relatively low. The wide band gap of the high-energy host emitters makes it difficult to inject charge carriers into the emissive layer from the adjacent layers and this is likely to be the reason for the low power efficiency. An n-type oligoquinoline as the emissive layer in a blue-emitting device exhibits high power and external quantum efficiencies of 4.3 lm W ⁻¹ and 6.6%, respectively, with CIE coordinates of (0.15, 0.16).¹¹

Recently, an efficient deep-blue emitter – namely, poly-aryl-substituted phenanthrimidazoles – has attracted great attention due to its easy synthetic process, excellent thermal properties, high quantum yields and potential bipolar properties.^13–20 As non-doped blue emitters, the external quantum efficiency of these derivative-based devices is 5% with CIE < 0.15 (ref. 15 and 17) and they were also shown to be highly efficient doped blue-fluorescent emitters by Ge and Wang et al.^18–20 We continue to be interested in designing and synthesizing n-type imidazole derivatives as OLED emitters to improve their efficiencies as devices, and herein we report a series of blue-emitting 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl derivatives and their applications as blue emitters in non-doped devices. These derivatives show high thermal stability, proper carrier barriers and balanced charge-injection properties.

2. Experimental

2.1. Materials and measurements

All the reagents and solvents used for the synthesis were purchased from Sigma-Aldrich. All reactions were performed under a nitrogen atmosphere. NMR spectra were recorded on a Bruker 400 MHz NMR spectrometer. The mass spectra were obtained using an Agilent LCMS VL SD in electron ionization mode, and cyclic voltammetry analyses were performed using a CHI 630A potentiostat electrochemical analyzer with a platinum electrode as the working electrode, platinum wire as the counter electrode and an Ag/Ag⁺ electrode as the reference electrode at a scan rate of 100 mV s⁻¹. About 0.1 M solution of tetrabutylammoniumperchlorate (TBAPF₆) in CH₂Cl₂ was used as the supporting electrolyte. The UV-visible spectra were obtained with a PerkinElmer Lambda 35 UV-vis spectrophotometer and corrected for background absorption due to the solvent. A PerkinElmer Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere attachment was used to record the UV-vis diffuse reflectance spectra. Photoluminescence spectra were recorded on a PerkinElmer LS55 fluorescence spectrometer. Lifetime measurements were carried out with a nanosecond time-correlated single photon counting (TCSPC) spectrometer Horiba Fluorocube-01-NL lifetime system. The PL quantum yields were measured in dichloromethane using a 0.5 M H₂SO₄ solution of quinine (0.54) as a reference/the solid-state quantum yield on the quartz plate using an integrating sphere:
where ϕ_unk is the radiative quantum yield of the sample, ϕ_std is the radiative quantum yield of the standard, and I_unk and I_std are the integrated emission intensities of the sample and standard, respectively. A_unk, and A_std are the absorbances of the sample and standard, respectively, and η_unk and η_std are the indexes of refraction of the sample and standard solutions. Thermal analysis of the phenanthrimidazoles was made with a NETZSCH-Geratebau Gmbh thermal analysis STA 409 PCO. The differential scanning calorimetric (DSC) and thermogravimetric analyses (TGA) were made under nitrogen atmosphere (100 mL min⁻¹). The sensitivity of the instrument was set at 0.01 μg and the sample (10 mg) was heated from 30 to 700 °C at a rate of 10 or 15 or 20 K min⁻¹. DFT calculations were performed with a Gaussian-03 package.²¹

2.2. Fabrication of devices

The EL devices based on the phenanthrimidazoles were fabricated by vacuum deposition of the materials at 5 × 10⁻⁶ torr onto a clean glass precoated with a layer of indium tin oxide as the substrate, with a sheet resistance of 20 Ω per square. The glass was cleaned before use by sonication successively in a detergent solution, acetone, methanol and deionised water. Organic layers were deposited onto the substrate at a rate of 0.1 nm s⁻¹. LiF and Alq₃ were thermally evaporated onto the surface of the organic layer. The thicknesses of the organic materials and the cathode layers were controlled using a quartz crystal thickness monitor. A series of devices (I–V) were fabricated with multilayer configurations as follows: (a) ITO/NPB (4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl) (hole-transporting layer) (50 nm)/4(I)/5(II)/6(III) (30 nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) (15 nm)/Alq₃ (tris(8-hydroxyquinolinato)aluminum) (50 nm)/LiF (1 nm)/Al (100 nm); (b) ITO/NPB (50 nm)/Alq₃ (50 nm)/LiF (1 nm)/Al (100 nm) (reference device IV): (c) NPB (50 nm)/6 (10 nm)/LiF (1 nm)/Al (100 nm) (reference device V). Measurements of current, voltage and light intensity were made simultaneously using a Keithley 2400 sourcemeter (Keithley, Cleveland, Ohio). The EL spectra of the devices were carried out in ambient atmosphere without further encapsulation.

2.3. Synthesis of 2-(4-bromophenyl)-1-aryl-1H-phenanthro[9,10-d]imidazole derivatives (1–3)

A mixture of 9,10-phenanthrenequinone (2.0 g, 9.6 mmol), 4-bromobenzaldehyde (1.78 g, 9.6 mmol), a substituted naphthylamine (11.5 mmol) and ammonium acetate (7.4 g, 96.1 mmol) in ethanol (25 mL) was refluxed at 90 °C for 12 h under nitrogen atmosphere. After cooling, the reaction mixture was poured into methanol and dried.²²

2.3.1 2-(4-bromophenyl)-1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole (1). Mp 257 °C. Anal. calcd for C₃₁H₁₉BrN₂: C, 74.56; H, 3.83; N, 5.61. Found: C, 74.51; H, 3.79; N, 5.52. ¹H NMR (400 MHz, CDCl₃): δ 7.17 (bs, 1H), 7.24–7.29 (m, 1H), 7.38–7.56 (m, 8H), 7.58–7.79 (m, 6H), 8.69 (d, J = 8.0 Hz, 1H), 8.75 (d, J = 8.4 Hz, 1H), 8.83 (d, J = 7.8 Hz, 1H). ¹³C NMR (400 MHz, CDCl₃): δ 120.7, 122.6, 122.8, 123.0, 123.3, 124.2, 125.0, 125.8, 126.2, 127.0, 127.4, 128.0, 129.1, 129.3, 129.6, 130.1, 130.9, 131.5, 137.4, 138.5, 149.6. MS: m/z. 499 [M⁺]. Calcd 498.07.

2.3.2 2-(4-bromophenyl)-1-(methylnaphthalen-4-yl)-1H-phenanthro[9,10-d]imidazole (2). Mp 245 °C. Anal. calcd for C₃₂H₂₁BrN₂: C, 74.86; H, 4.12; N, 5.46. Found: C, 74.79; H, 3.98; N, 5.41. ¹H NMR (400 MHz, CDCl₃): δ 2.54 (s, 3H), 7.18–7.28 (m, 2H), 7.31–7.54 (m, 10H), 7.59–7.68 (m, 2H), 7.73 (td, J = 7.9, J = 1.0 Hz, 1H), 8.69 (d, J = 8.1 Hz, 1H), 8.75 (d, J = 8.4 Hz, 1H) 8.84 (d, J = 7.5 Hz, 1H). ¹³C NMR (400 MHz, CDCl₃): δ 21.9, 120.7, 122.5, 122.8, 123.0, 123.3, 124.0, 124.2, 124.8, 125.6, 125.8, 126.1, 127.0, 127.2, 128.0, 128.4, 128.5, 129.2, 129.4, 130.6, 130.8, 131.3, 135.7, 137.2, 140.2, 149.6. MS: m/z. 513 [M⁺]. Calcd 512.09.

2.3.3 2-(4-bromophenyl)-1-(methoxynaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole (3). Mp 241 °C. Anal. calcd for C₃₂H₂₁BrN₂O: C, 72.60; H, 4.00; N, 5.29. Found: C, 72.57; H, 3.96; N 5.25. ¹H NMR (400 MHz, CDCl₃): δ 3.96 (s, 3H), 7.08 (d, J = 8.7 Hz, 2H), 7.21–7.33 (m, 1H), 7.38 (d, J = 8.7 Hz, 2H), 7.43–7.55 (m, 7H), 7.60–7.67 (m, 2H), 7.74 (td, J = 8.0 Hz, J = 1.1 Hz, 1H), 8.71 (d, J = 7.9 Hz, 1H), 8.75 (d, J = 8.4 Hz, 1H), 8.84 (d, J = 8.0 Hz, 1H). ¹³C NMR (400 MHz, CDCl₃): δ 55.9, 115.2, 120.7, 122.5, 123.0, 123.2, 123.4, 124.3, 125.1, 125.4, 125.7, 126.4, 127.0, 127.3, 127.5, 128.1, 128.3, 128.5, 129.3, 129.6, 130.1, 130.7, 130.9, 131.3, 137.5, 149.7, 160.4. MS: m/z. 529 [M⁺]. Calcd 528.31.

2.4. Synthesis of bis(naphthylphenanthroimidazolyl)biphenyl derivatives (4–6)

A mixture of 2-(4-bromophenyl)-1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazole (1) (2.0 g, 4.45 mmol), NiCl₂ (0.058 g, 0.45 mmol), Zn powder (0.29 g, 4.45 mmol), KI (1.11 g, 6.68 mmol) and PPh₃ (0.47 g, 1.78 mmol) in 20 mL of DMF was added to the flask and the reaction mixture was stirred at 90 °C for 24 h under nitrogen atmosphere. The Zn and inorganic salts were removed by filtration of the hot reaction mixture and the residue was washed with DMF. The crude product was purified by column chromatography using hexane: ethylacetate as the eluent. The synthetic route is shown in Scheme 1 and the NMR spectra of these compounds are displayed in Fig. S1.†

Scheme 1 Synthetic route of phenanthrimidazole derivatives.

2.4.1 4,4′-bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl (NPIP) (4). Mp 408 °C. Anal. calcd for C₆₂H₃₈N₄: C, 88.76; H, 4.57; N, 6.68. Found: C, 88.68; H, 4.51; N 6.57. ¹H NMR (400 MHz, CDCl₃): δ 6.84–7.77 (m, 32H), 8.72 (d, J = 8.0 Hz, 2H), 8.76 (d, J = 8.4 Hz, 2H), 8.89 (d, J = 7.8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 120.6, 122.7, 122.8, 123.2, 123.5, 124.2, 125.2, 125.8, 126.4, 127.2, 127.5, 128.3, 129.0, 129.5, 129.6, 130.0, 130.4, 130.9, 131.5, 137.7, 138.9, 149.8. MS: m/z. 838 [M⁺]. Calcd 838.3.

2.4.2 4,4′-bis(4-methylnaphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl-1,1′-biphenyl (MeNPIP) (5). MeNPIP was synthesized using a methodology similar to that for NPIP.

Mp 410 °C. Anal. calcd for C₆₄H₄₂N₄: C, 88.66; H, 4.88; N 6.46. Found: C, 88.57; H, 4.71; N, 6.39. ¹H NMR (400 MHz, CDCl₃): δ 2.59 (s, 6H), 7.17–7.77 (m, 30H), 8.72 (d, J = 7.9 Hz, 2H), 8.76 (d, J = 8.4 Hz, 2H), 8.89 (d, J = 7.8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 21.9, 120.7, 122.7, 122.8, 123.0, 123.3, 124.1, 124.8, 125.7, 126.3, 127.2, 127.4, 128.1, 128.3, 128.6, 129.4, 129.6, 130.5, 130.9, 131.3, 135.6, 137.3, 140.0, 149.8. MS: m/z. 867 [M⁺]. Calcd 867.3.

2.4.3 4,4′-bis(4-methoxynaphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl-1,1′-biphenyl (MeONPIP) (6). MeNPIP was synthesized using a methodology similar to that for NPIP.

Mp 415 °C. Anal. calcd for C₆₄H₄₂N₄O₂: C, 88.50; H, 4.71; N, 6.23. Found: C, 88.39; H, 4.61; N, 6.18. ¹H NMR (400 MHz, CDCl₃): δ 3.78 (s, 6H), 6.84–7.77 (m, 30H), 8.70 (d, J = 8.40 Hz, 2H), 8.74 (d, J = 8.81 Hz, 2H), 8.85 (d, J = 7.60 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ 55.7, 115.2, 120.8, 122.5, 123.0, 123.2, 123.3, 124.2, 125.6, 125.7, 126.5, 127.0, 127.3, 128.1, 128.3, 129.2, 129.6, 130.2, 130.8, 130.9, 131.5, 137.4, 149.8, 160.5. MS: m/z. 898 [M⁺]. Calcd 898.

3. Results and discussion

3.1. Photophysical properties of bis(naphthylphenanthroimidazolyl)biphenyl derivatives

The bis(phenanthroimidazolyl)biphenyl derivatives 4–6 were prepared from the homo-coupling reaction of the intermediates 2-(4-bromophenyl)-1-naphthyl-1H-phenanthro[9,10-d]imidazole derivatives 1–3 using an inexpensive NiCl₂/PPh₃ catalyst and Zn powder as the reducing agent at 90 °C for 24 h. The 2-(4-bromophenyl)-1-aryl-1H-phenanthro[9,10-d]imidazole derivatives 1–3 and bis(naphthylphenanthroimidazolyl)biphenyl derivatives 4–6 were characterized by ¹H and ¹³C NMR, high resolution mass spectroscopy and elemental analysis. Geometrical optimization of bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl derivatives 4–6 was performed at DFT/B3LYP/6-31G(d,p) level using the Gaussian-03 package and the optimized geometry is shown in Fig. 1. The central biphenyl moiety adopts a coplanar configuration and the inter-ring torsion angles between the biphenyl group and the two naphthyl phenanthrimidazolyl groups are ca. 36°. At this small angle, the extent of conjugation is reduced and thus the amount of charge transfer (CT) transition is increased and this is reflected in the solvatochromic studies. The naphthyl moieties are highly twisted about the phenanthrimidazolyl rings, with dihedral angles of about 81°. At this large angle, fluorescence quenching was suppressed due to aggregation in the film. Thermal properties were investigated by DSC and TGA under nitrogen atmosphere and the results are displayed in Fig. 2. All bis(naphthylphenanthrimidazolyl)biphenyl derivatives 4–6 exhibit good thermal stability and the decomposition temperatures with 5% weight loss (T_d5) have been measured as 421, 427 and 430 °C (Table 1). The bis(phenanthrimidazolyl)biphenyl derivatives 4–6 exhibited very high melting points of 408, 410 and 415 °C, respectively. On a second heating, no melting points were observed, even though the samples were given enough time to cool in air. Once they became amorphous solids, they did not revert to the crystalline state at all. After the samples had cooled to room temperature, second DSC scans performed at 10 °C min⁻¹ revealed glass transition temperatures (T_g) of 195, 200 and 201 °C. Owing to the rigid molecular backbones and non-coplanar geometries of the synthesized materials, they exhibit high T_g, T_m and T_d5 values, which indicate that they could form morphologically stable amorphous films upon vacuum thermal evaporation. This is highly important for device fabrication since the high T_g, T_m and T_d5 could improve the lifetimes of the devices.²³

Fig. 1 Optimized geometry of bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl derivatives 4–6 has been performed by DFT at B3LYP/6-31G(d, p) level using Gaussian-03.

Fig. 2 (a) The DSC and (b) TGA graphs of bis(1-(naphthalen-1-yl)-1H-phenanthro [9,10]imidazole-2-yl)-1,1′-biphenyl derivatives.

Table 1 Photophysical, thermal and electroluminescent properties^a

Parameters	NPIP (4)	MeNPIP (5)	MeONPIP (6)
a Reference devices IV/V: V1000 (V): 3.9/3.9; L (cd m^−2): 30 181/37 801; ηex (%): 4.81/4.97; ηc (cd A^−1): 5.78/5.89, ηp (lm W^−1): 4.75/4.83.b Fluorescence quantum yield measured in dichloromethane using 0.5 M H2SO solution of quinine (0.54) as reference/the solid-state quantum yield on the quartz plate using an integrating sphere.c The HOMO and LUMO energies were determined from cyclic voltammetry and absorption data.
Photophysical & thermal
λab (nm) (soln/film)	259, 364/260, 365	260, 367/262, 369	262, 369/264, 371
λem (nm) (soln/film)	425/432	427/449	449/452
Tg/Td (°C)	195/421	200/427	201/430
ϕ^b (soln/film)	0.69/0.50	0.71/0.62	0.78/0.64
HOMO^c/LUMO^c eV (optical/calculated)	−5.91/−2.29	−5.83/−2.34	−5.70/−2.89
kr × 10^9 (s^−1)	0.34	0.35	0.48
knr × 10^9 (s^−1)	0.15	0.13	00.2
Device	I	II	III
V1000 (V)	2.7	2.9	3.1
L (cd m^−2)	34 012	35 912	40 812
ηex (%)	5.20	6.10	6.92
ηc (cd A^−1)	6.81	6.92	7.80
ηp (lm W^−1)	5.19	5.86	6.10
EL (nm)	434	450	453

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The electronic spectra of the bis(naphthyl phenanthroimidazolyl)biphenyl derivatives were measured in dichloromethane and the absorption and emission spectra are displayed in Fig. 3a. The absorption maxima around 259 nm may originate from the naphthyl ring attached to the nitrogen of the phenanthrimidazole plane and the absorption band around 364 nm is assigned to a π → π* electronic transition of the phenanthrimidazole ring. The bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl derivatives 4–6 show emissions at 425, 427 and 449 nm, respectively, and the emission spectra vary with the solvent used. The emission peaks shift towards a longer wavelength as the polarity of the solvent increases (Fig. 3b) and this variation is likely to be due to the polarization-induced spectral shift.²⁴ In the film state, the molecules show red shifts of 432 nm (4), 449 nm (5) and 452 nm (6), but are still localized in the blue region, with a small FWHM around 58 nm; this small FWHM implies that there would be considerable aggregation involved in the solid state.¹² The quantum yields (ϕ) measured in CH₂Cl₂ solution are 0.69 (4), 0.71 (5) and 0.78 (6). The radiative (k_r) and non-radiative (k_nr) decay of the excited state of these compounds were obtained using the quantum yield and lifetime (τ) and are listed in Table 1. The lifetime decay curve is shown in Fig. 3c and the radiative lifetimes of these compounds fall in the range of 1.62–2.01 ns.

Fig. 3 (a) Normalized absorption and emission spectra of bis(1-(naphthalen-1-yl)-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl in CH₂Cl₂ (10⁻⁵ M) and in film; (b) PL spectra of 4, 4′ bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl (NPIP) in different solvents; (c) the lifetime decay curve of bis(naphthylphenanthrimidazolyl)biphenyl derivatives; and (d) the CV measurement of bis(naphthylphenanthrimidazolyl)biphenyl derivatives.

The electronic energies (HOMO and LUMO) of the non-doped blue emitters such as bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl derivatives were examined by cyclic voltammetry (CV) and the redox potentials were measured from the plot of potential versus current which is shown in Fig. 3d. CV analysis exhibits one quasi-reversible oxidation wave with an oxidative onset potential of 1.10 V (4), 1.00 V (5) and 0.90 (6) V, which gives HOMO energies of −5.91 eV (4), −5.83 eV (5), and −5.70 eV (6) by comparison to ferrocene (E_HOMO = E_ox + 4.8 eV).²⁵ The LUMO energies were deduced from the HOMO energies and the lowest-energy absorption edges of the UV-vis absorption spectra.²⁵ The LUMO energies, −2.29 eV (4), −2.34 eV (5) and −2.89 eV (6), are quite close to that of 1,3,5-tris(N-phenylimidazol-2-yl)benzene (TPBI), revealing that the electron-injection abilities of bis(naphthyl phenanthrimidazolyl)biphenyl derivatives are similar to TPBI. This reflects the more balanced carrier-injection properties existing in these materials. The electron density distribution of the HOMO for the bis(phenanthrimidazolyl)biphenyl derivatives is localized predominantly on the electron-rich phenanthro[9,10-d]imidazole planes, and the HOMO values range from −5.70 to −5.91 eV. The electron density of LUMO is distributed on the biphenyl center, with a small fraction on the phenanthrimidazole moiety (Fig. 4), but their values are increased from −2.29 to −2.89 eV. In accordance with our expectations, their corresponding band gaps are decreased (−3.62 eV (4); −2.49 eV (5); −1.81 eV (6)) due to the inductive effect of the substituent in the naphthyl phenanthrimidazoles 5 and 6 and some degree of space charge separation ability is found, which would be beneficial for the injection of carriers from the electrode.

Fig. 4 HOMO–LUMO contour maps for bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10] imidazole-2-yl)-1,1′-biphenyl derivatives.

3.2. Performances of devices using bis(1-(naphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl)-1,1′-biphenyl derivatives as blue emitters

The observed intense blue emission and high T_g for the bis(phenanthrimidazole)biphenyl derivatives suggest their suitability to serve as blue emitters in OLEDs. We have fabricated non-doped blue-emitting devices with a device configuration of (a) ITO/NPB (4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl) (hole-transporting layer) (50 nm)/4(I)/5(II)/6(III) (30 nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) (15 nm)/Alq₃ (tris(8-hydroxyquinolinato)aluminum) (50 nm)/LiF (1 nm)/Al (100 nm); (b) ITO/NPB(50 nm)/Alq₃ (50 nm)/LiF (1 nm)/Al (100 nm) (reference device IV); (c) ITO/NPB/6 (10 nm)/LiF (1 nm)/Al (100 nm) (reference device V). The performances of the devices are detailed in Table 1 and the energy levels and chemical structures of the materials used are illustrated in Fig. 5. It is clear from Fig. 6 that the three new devices based on bis(naphthylphenanthrimidazole)biphenyl derivatives (I–III) are turned on at a very low voltage of ≤3.1 V and exhibit maximum efficiencies. The resulting blue EL spectra of the three devices are very similar to the PL spectrum of bis(naphthylphenanthrimidazole)biphenyl derivatives in the solid state, indicating the same sources of EL and PL.

Fig. 5 Energy levels and the chemical structures of the materials used in the devices.

Fig. 6 (a) Luminescence-voltage; (b) external quantum efficiency-current density; (c) current efficiency-current density; (d) power efficiency-current density; and (e) normalized electroluminescent spectrum. Insets in (e): AFM images of MeONPIP based film (i) at room temperature and (ii) annealing at 100 °C.

Among the three new blue emitters, MeONPIP exhibits the best thermal stability (T_d – 430 °C; T_d – 201 °C) and highest quantum efficiency (ϕ – 0.78/0.64). Additionally, a MeONPIP film fabricated by vacuum deposition exhibits a fairly smooth surface morphology with a roughness of 0.26 nm. After annealing at 100 °C for 7 h, the film morphology is unchanged, which supports its suitability for device fabrication (Fig. 6). According to the energy level diagram shown in Fig. 5, electrons could smoothly travel into the bis(naphthyl phenanthrimidazole)biphenyl MeONPIP (6) layer by conquering small injection barriers of 0.1 eV from the BCP layers, which served as electron-transporting and hole-blocking layers.^10,12,26 On the other hand, the hole-injection barrier between MeONPIP and the hole-transport layer is also very small (≤0.3 eV) and thus the electron–hole radiative recombination effectively takes place in the emissive layer. These results reveal that MeONPIP is a potential non-doped blue-light-emitting material. The small injection barriers for charge carriers may account for the observed low turn-on voltages. The maximum external quantum efficiency and current efficiency achieved by these devices are 5.20, 6.19 and 6.92% and 6.81, 6.92 and 7.80 cd A⁻¹, respectively. These results could be attributed to the more balanced charge-transporting properties within the emissive layer achieved by better charge injection and confinement provided by the hole-transport layer. As well as having high external quantum efficiency and current efficiency, the devices based on bis(phenanthrimidazole)biphenyl derivatives 4–6 also preserve relatively high power efficiencies of 5.19, 5.86 and 6.10 lm W⁻¹ at low driving voltages. The electroluminescent performances reveal that MeONPIP is a potential non-doped blue-emitting material. A progressive step between ITO and the emissive layer facilitates the holes diffusing across the junction. The improved efficiency of the devices is due to the decreasing number of holes through the emissive layer, which results in a balance of the electrons and holes in the emitting layer and thus elimination of the nonproductive hole current. From the electrochemical properties of these blue-emitting materials used in the devices (Table 1), the HOMO energy level of these compounds lies between those of NPB (−5.4 eV) and Alq₃ (−6.0 eV) and could regulate hole injection by acting as a ladder between the energy levels of NPB and Alq₃. Therefore, the decreased number of holes should be attributed to the relatively low hole-transport ability of these materials, which is supported by comparing the current density–voltage characteristic of devices III and V with different thicknesses of the emissive layer based on compound MeONPIP. Device V with a 10 nm thickness of the MeONPIP layer showed a lower efficiency than that of device III with a 30 nm thickness. The dependence of the current density of the device on the thickness of the emissive layer indicates that MeONPIP and other phenanthroimidazole derivatives can prevent excessive numbers of holes entering the emissive layer when the thickness of the emissive layer is as thick as 10 nm. The higher efficiency was realized by improving the balance of holes and electrons. Synthesized blue emitters with almost identical molecular geometries exhibit similar thermal, electrochemical and photophysical properties. However, devices I–III displayed different electroluminescent performances (Table 1) i.e., there may be differences in molecular packing and nanostructures of NPIP, MeNPIP and MeONPIP in thin-film systems. The charge transport of organic semiconductors is strongly dependent upon molecular packing and morphology and it has been shown that organic molecules with similar molecular structures exhibit remarkably different mobilities. In EL devices, the carrier mobility and balance are dependent on the interfacial characteristics between the emissive layer and the NPB or Alq₃ layer, which have an effect on the EL performances.¹⁰ Although efficient non-doped OLEDs with extremely high external quantum efficiencies and excellent color purity have been reported, the corresponding power efficiencies are still relatively low (<4.5 lm W⁻¹).^{10–12,27,28} To the best of our knowledge, better electroluminescent efficiency is achieved by the devices based on the new non-doped blue-emitting bis(naphthylphenanthrimidazolyl)biphenyl derivatives.

4. Conclusion

We have reported the synthesis of efficient new blue-emitting bis(naphthylphenanthrimidazolyl)biphenyl derivatives with wide band gap by a two-step procedure using inexpensive catalysts. These compounds showed excellent thermal properties (T_d – 421/427/430 °C; T_g – 195/200/201 °C) and emit intense blue light. The non-doped devices with bis(naphthylphenanthrimidazolyl)biphenyl derivatives as the emissive layer show high electroluminescent efficiencies at low driving voltage. The device with 4,4′-bis(4-methoxynaphthalen-1-yl)-1H-phenanthro[9,10]imidazole-2-yl-1,1′-biphenyl exhibits a very high external quantum efficiency of 6.92%, current efficiency of 7.80 cd A⁻¹ and power efficiency of 6.10 lm W⁻¹.

Acknowledgements

One of the authors, Prof. J. Jayabharathi, is thankful to CSIR [No. 01/(2707)/13EMR-II], DST [No. EMR/2014/000094], DRDO (NRB-213/MAT/10-11) and UGC (F. No. 36-21/2008) for providing funds to this research study. Mr A. Prabhakaran is thankful to CSIR [No. 01/(2707)/13EMR-II] for providing a fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra11439c

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