Efficient non-doped blue organic light-emitting diodes: donor–acceptor type host materials

The new blue light emitting materials, (E)-40-(1-(4-(2-(1-(naphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,10]-biphenyl-4-amine (NPI-PITPA), (E)40-(1-(4-(2-(1-(4-methylnaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro [9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,10]-biphenyl-4-amine (MeNPI-PITPA) and (E)-40-(1-(4-(2-(1-(4methoxynaphthalen-1-yl)-1H-phenanthro[9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazol2-yl)-N,N-diphenyl-[1,10]-biphenyl-4-amine (OMeNPI-PITPA) with dual charge transport properties have been synthesized and characterised. These compounds exhibit excellent thermal properties with very high glasstransition temperature and thus are favourable to form thin films under thermal evaporation for non-doped organic light-emitting diodes (OLEDs). The non-doped blue device based on OMeNPI-PITPA show maximum efficiencies (hex 4.90%; hc 5.90 cd A ; hp 5.10 lm W ) at low turn-on voltage and the device performances show that the phenanthroimidazole unit is a tunable building block for carrier injection properties and that they also can be used as hosts for green phosphorescent OLEDs.


Introduction
Organic light-emitting diodes (OLEDs) are of current interest from both scientic and practical points of view due to their potential utility in high-resolution at-panel displays. 1-5 The development of efficient green [6][7][8] and red devices 9 with pure color Commission International de l'Eclairage (CIE) coordination has been well reported. However, there is a lack of blue phosphorescent emission with long lifetime and pure color CIE due to wide band gaps. Though blue phosphorescent devices with high external quantum efficiencies have been reported, due to short device lifetimes they are not yet commercially used. [10][11][12][13] Therefore, to commercialise efficient blue OLEDs there is an urgent need to develop blue emissive materials.
High triplet energy blue emissive materials with balanced carrier transport properties may be used as hosts for green devices. [14][15][16][17] The high triplet energy enables green phosphors to harvest the triplet energy of the blue emitter; however, the doped blue electroluminescent materials are not suitable hosts for phosphorescent OLEDs due to their low triplet energy and poor carrier transport properties. 18 Thus, efficient hosts for green phosphors exhibit low efficiency when they are used as an emissive layer in blue OLEDs. [19][20][21] There is still a challenge to achieve full color OLEDs based on deep blue emitting materials.
Therefore, to develop the multi-functional organic materials that can be used as both emitter for blue OLEDs and hosts for green OLEDs, molecules with donor (D) and acceptor (A) or electron and hole transport moieties assembled together to form electron and hole transport channels are synthesised. The D-A molecule should have relatively weak charge transfer properties because of red shied emission and the small singlet-triplet splitting should have high triplet excited state energy which is used to excite the green phosphorescent dopant. Our continuous interest to design and synthesize the ntype imidazole derivatives as OLED emitters to improve the efficiencies of devices, [22][23][24][25][26][27][28][29][30][31] herein, we report the synthesis of a series of blue-emitting (E)-4 0 -(1-(4-(2-(1-(naphthalen-1-yl)-1Hphenanthro [9,10-d]imidazol-2-yl)vinyl)phenyl)-1H-phenanthro [9,10-d]imidazol-2-yl)-N,N-diphenyl-[1,1 0 ]-biphenyl]-4-amine derivatives and their application as the emitter in nondoped devices and host materials for green OLEDs. These derivatives show higher thermal stability, proper carries barriers and balanced charge injection property.
LCMS VL SD in electron ionization mode and electrochemical measurements were performed with CHI 630A potentiostat electrochemical analyzer with platinum electrode as the working electrode, platinum wire as the counter electrode and Ag/Ag + electrode as the reference electrode at a scan rate of 100 mV s À1 . About 0.1 M solution of tetrabutylammoniumperchlorate in CH 2 Cl 2 was used as supporting electrolyte. The UVvisible spectra was obtained with Perkin Elmer Lambda 35 UVvis spectrophotometer and corrected for background absorption due to solvent. Perkin Elmer Lambda 35 spectrophotometer with RSA-PE-20 integrating sphere attachment was used to record UV-vis diffuse reectance spectra. The emission spectra were recorded by a Perkin Elmer LS55 uorescence spectrometer. The absolute PL quantum yields were measured in dichloromethane using 0.5 M H 2 SO 4 solution of quinine (0.54) as reference. The solid-state quantum yield on the quartz plate using an integrating sphere where f unk is the radiative quantum yield of the sample, f std is the radiative quantum yield of the standard, 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 h unk and h std are the indexes of refraction of the sample and standard solutions. Thermogravimetric analyses (TGA) was performed with NETZSCH-Geratebau Gmbh thermal analysis STA 409 PCO. The differential scanning calorimetric (DSC) analyses were made under nitrogen atmosphere (100 mL min À1 ). The sensitivity of the instrument was set at 0.01 mg and the sample (10 mg) was heated from 30 to 700 C at the rate of 10 or 15 or 20 K min À1 .

Theoretical calculations
The ground state geometries were optimized using density functional theory method with the Becke three-parameter hybrid exchange and the Lee-Yang-Parr correlation functional (B3LYP) and 6-31G* as basis set using Gaussian 09 soware package. 32

Devices fabrication
The EL devices based on the of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA were fabricated by vacuum deposition of organic materials at 5 Â 10 À6 Torr on precleaned indium tin oxide glass substrate with resistance of 20 U per square. Organic layers were deposited onto the substrate at a rate of 0.1 nm s À1 . Aer organic lm deposition LiF and Al were thermally evaporated onto the surface of organic layer. The thickness of the organic materials and the cathode layers were controlled using a quartz crystal thickness monitor. A series of fabricated devices (I-V) with multilayer conguration is as follows: A mixture of phenanthrenequinone (2.08 g, 10 mmol), 4-nitrocinnamaldehyde (1.51 g, 10 mmol), substituted naphthylamine (4.65 g, 50 mmol) and ammonium acetate (3.08 g, 40 mmol) in ethanol (25 mL) was reuxed at 120 C for 12 h under nitrogen atmosphere. The reaction mixture was cooled and poured into methanol. The separated yellowish green crude product was puried by column chromatography using hexane : ethylacetate as the eluent (Scheme S1 †). Anal. calcd for C 33   2-(4-aminostyryl)-1-naphthyl-1H-phenanthro[9,10-d]imidazole (1.16 g, 3 mmol), ammonium acetate (1.54 g, 20 mmol) and glacial acetic acid (25 mL) was reuxed at 120 C for 12 h under nitrogen atmosphere. 33,34,39 The reaction mixture was poured into methanol and the separated white solid was ltered off, washed with water and puried by column chromatography using CH 2
Differential scanning calorimetric (DSC) scan performed at 10 C min À1 revealed a glass transition temperature (T g ) of 176, 189 and 191 C (Fig. 1). Owing to rigid molecular back bone and non-coplanarity geometry, the synthesized materials exhibit high T g and T d5 values which indicate that they could form morphologically stable amorphous lms upon vacuum thermal evaporation which is highly important for device fabrication since the high T m and T d5 could improve the life time of devices. 35 The lifetime decay curve is shown in Fig. 2 and the radiative lifetime of these compounds are 1.68 ns (NPI-PITPA) and 1.86 ns (MeNPI-PITPA) and 1.93 ns (OMeNPI-PITPA). Geometry optimization of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA has been performed by DFT/B3LYP/6-31G(d,p) level using Gaussian-03 and the optimized geometry is shown in Fig. 3 along with their corresponding molecular orbital distribution. The HOMO orbital of NPI-PITPA and OMeNPI-PITPA is localized at styryl phenanthrimidazole group, while the LUMO orbital distributes on PITPA. The HOMO orbital of MeNPI-PITPA is localized at methyl naphthyl fragment and styryl moiety, while the LUMO orbital distributes majority on phenanthrimidazole fragment. The HOMO and LUMO of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA display adequate separation in electron density features which benets the holeand electron-transport functions. 36 The calculated electron and   hole transfer integrals for OMeNPI-PITPA are 0.038 and 0.042 eV which reveal that OMeNPI-PITPA act as bipolar material. Moreover these compounds exhibit reduction and oxidation waves revealing that these derivatives should have good electron and hole transport abilities (Fig. 1).
The electronic spectral studies of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA have been measured in dichloromethane and the absorption and emission spectra have been displayed in Fig. 4. The absorption maxima around 256 nm may originate from naphthyl ring attached to nitrogen of phenanthrimidazole plane and the absorption band around 361 nm is assigned to p / p* electronic transition of the styryl phenanthrimidazole ring. This NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA derivatives show blue emission at 438, 441 and 443 nm, respectively, in CH 2 Cl 2 . The emission peak shi towards a longer wavelength as the polarity of the solvent increases (Fig. S1 †) and this variation is likely due to the polarization-induced spectral shi. 37 The lm state of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA show a red shi of 437, 445 and 449 nm, respectively but are still localized in the blue region with a small full-width at halfmaximum (FWHM) around 52 nm. This small FWHM implies the inconsiderable aggregation involved in its solid state and also due to elongated conjugation. 38 Fig. 5 shows the current density versus voltage characteristics of the hole-only and electron-only devices. The electron current density NPI-PITPA/MeNPI-PITPA/OMeNPI-PITPA based device is higher than the CBP-based device. This reveals that these materials have better electron injection and transport properties than CBP. The difference in current density between the hole-only and electron-only devices based on NPI-PITPA/MeNPI-PITPA/OMeNPI-PITPA is much smaller than that based on CBP at the same voltage suggesting these materials are potential bipolar material capable of transporting electrons and holes in the devices. [40][41][42] The observed intense blue emission and high T g for NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA suggest their suitability to serve as blue emitters in OLED applications. The device performances of the blue emitters are analysed by fabricating non-doped OLEDs with conguration of ITO/NPB (1,4-bis(1-naphthylphenylamino)-biphenyl) (50 nm)/NPI-PITPA (I)/MeNPI-PITPA (II)/OMeNPI-PITPA (III) (30 nm)/BCP (2,9dimethyl-4,7-diphenyl-1,10-phenanthroline) (15 nm)/Alq 3 (tris-(8-hydroxyquinoline)aluminum) (50 nm)/LiF (1 nm)/Al (100 nm) (Fig. 6). The device performances are detailed in Table 1. It is clear from Fig. 7 that the three new born NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA based devices exhibit high brightness at low voltage. The resulting blue EL spectra of the devices are very similar to the PL spectra of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA in the solid state (Fig. 4). The hole injection barriers between OMeNPI-PITPA and hole transport layer is very small and thus to combine the electron-hole radiative recombination in the emissive layer. Additionally the blue emitter OMeNPIP exhibit better thermal stability (T d -571 C & T g -191 C) and high quantum efficiency (f -0.94/0.97). These results reveal that OMeNPI-PITPA is a potential non-doped blue light emitting material. The small injection barrier 0.26, 0.38 and 0.49 (OMeNPI-PITPA; MeNPI-PITPA; NPI-PITPA) for charge carriers may account for the observed low turn-on voltages. The OMeNPI-PITPA lm fabricated by vacuum deposition exhibits a smooth surface morphology with a roughness of 0.28 nm. Aer annealing at 100 C for 7 h, the lm morphology is still unchanged (Fig. 6). The EQE of OLEDs can be calculated as follows: 43 where h out is the lightout-coupling efficiency (20%), h rc is the product of the charge recombination efficiency, 100% if holes and electrons are fully balanced and completely recombined to form excitons, h g is the efficiency of radiative exciton production (25%), F PL is the photoluminescence quantum yield of the emitters.
The maximum external quantum efficiency and current efficiencies of NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA based devices are 4.60, 4.70 and 4.90% and 4.8, 5.2 and 5.9 cd A À1 , respectively. This result could be attributed the more balanced charge-transporting properties within the emissive layer achieved by better charge injection provided by hole transport layer. As well as having high external quantum efficiency and current efficiency, the NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA  (Fig. 6). The device performances and EL spectra of these devices are shown in Fig. 6 and 4. The EL spectra are similar to the PL spectra of the doped thin lms (Fig. 4). Device based on OMeNPI-PITPA (30 nm):5 wt% Ir(ppy) 3 exhibits maximum luminance of 8215 cd m À2 , maximum current and power efficiencies are of 27.3 cd A À1 and 30.1 lm W À1 , respectively at low turn-on voltage of 2.7 V. The maximum external quantum efficiencies of the devices based on NPI-PITPA:Ir(ppy) 3 , MeNPI-PITPA:Ir(ppy) 3 and OMeNPI-PITPA:Ir(ppy) 3 are 17.0%, 18.3% and 19.0%, respectively. The device performances reveal that NPI-PITPA, MeNPI-PITPA and OMeNPI-PITPA are universal host materials for green phosphorescent emitters.

Conflicts of interest
There are no conicts to declare.