Introduction

In the field of consumer electronics, organic light-emitting diodes (OLEDs) as solid-state lighting are progressively replacing the traditional display technology due to their superior lightweight, flexible and large viewing angle [1, 2]. Among diverse design strategies, the properties of three essential emitting elements (red, green and blue) are directly influence the performance of devices [3–5]. After decades of development, several excellent green and red emitting molecules have been designed and applied successfully to improve to the light emitting capability of OLEDs [6]. However, the overall performance and stability of blue (especially deep blue) fluorescence emitters and devices are still inferior to those red and green counterparts [7, 8]. The blue emitter can not only effectively reduce power consumption of the devices but also be utilized to generate light of other colors by energy cascade to lower energy fluorescent or phosphorescent dopants. To date, the marketable OLEDs mainly adopt the doped host-guest emitting layer. Nevertheless, the generally used heavy metals for phosphorescence are confined to iridium (Ir) and platinum (Pt) [9–11], which are rather expensive and dependent on limited global resources. Thus, organic molecule with free heavy-metal containing meets the eco-friendly theme [12]. At the same time, phosphorescent materials also suffer from the lack of deep blue emitters and their inherent short lifetimes when utilized to configure white organic light-emitting diodes (WOLEDs) [13]. Therefore, the development of high-performance deep-blue emitting OLEDs (CIE criterion: y < 0.15 and x + y < 0.30) and related materials is still a challenging issue [14, 15]. In order to realize the deep blue emission, the strategic molecular design should be focus on relatively short conjugation and weak intermolecular aggregation [16]. Also, the electron affinities of blue-emitting materials should be increased to realize balanced charge injection and transport, since the electron injection and transport ability in organic semiconductors is relatively low compared to that of hole [17, 18]. The n-type phenathroimidazole (PI) derivatives always adopt less conjugative configuration such as the twisted linkage or non-conjugative linkage [19–22]. The PI moiety as the core structure linked with a freely rotatable benzene ring and end-capsulated with donor units is the representative donor–π–acceptor dipolar molecule [23–27]. In our molecule PhImPOTD, electron-deficiency diphenylphosphine oxide is bonding with NH site at the imidazole ring, which could effectively inhibit molecular aggregation, π–π stacking and even fluorescence quenching in the solid state. It is noted that diphenylphosphine oxide unit, in contrast with the electron-rich group, also affords a hypsochromic shift function of emission to ensure the deep blue emitting OLEDs [28].

In this work, we report a triple twisted PI derivative, which is designed based on donor–π–acceptor approach and utilized phenyl bridge to concatenate two planar electron-transporting PI and hole-transporting dibenzothiophene (TD) units. In this way, such bipolar feature is beneficial to charge transport and balancing for obtaining high-efficiency devices in comparison with conventional unipolar materials. In addition, diphenylphosphine oxide is employed to construct multi-dimension framework by deprotonation on the site of imidazole. Therefore, the yielding sterically hindered and highly twisted molecular configuration is benefit to increase the photoluminescence quantum yield. Thermal, photophysical, and electroluminescent properties of the deep blue emitting PhImPOTD are comprehensively investigated. Hybrid WOLEDs, combining blue fluorophores and yellow phosphors, has been fabricated and investigated. It is approved that PhImPOTD possesses high luminescent efficiency, excellent luminous/thermo- stability and balanced charge carrier injection and transport, which has opened us new avenues for designing deep blue PI derivatives for high-performance OLEDs.

Experimental

Materials and Instruments

All the reagents and solvents used for the synthesis of the PhImPOTD were purchased from Aldrich, J&K and TCI companies and used as received. Dopant material PO-01 was purchased from Lumtec Corp. (taiwan). 1H magnetic resonance (NMR) spectra were recorded using a Bruker AVANCE III 500-MHz spectrometer, using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. High resolution mass spectra were recorded on a Bruker APEX IV fourier transform ion cyclotron resonance mass spectrometer. Elemental analysis for C, H, N and S were performed on a Elementar Analysensysteme GmbH. All manipulations involving air-sensitive reagents were performed in an atmosphere of dry Ar. Absorption and photoluminescence (PL) emission spectra of the target compound were measured using a Perkin Elmer Lambda-750 UV-Vis-NIR spectrophotometer and LS 55 fluorescence spectrometer, respectively. The luminescence quantum yield of compound was measured at room temperature and cited relative to a reference solution of 9,10-dipenylanthracene (Φ = 0.9 in cyclohexane) as a standard, and they were calculated according to the well-known equation:

$$ \frac{\varphi_{overall}}{\varphi_{ref}}={\left(\frac{n}{n_{ref}}\right)}^2\frac{A_{ref}}{A}\frac{I}{I_{ref}} $$
(1)

In Eq. (1), n, A, and I denote the refractive index of solvent, the area of the emission spectrum, and the absorbance at the excitation wavelength, respectively, and φ ref represents the quantum yield of the standard 9,10-dipenylanthracene solution. The subscript ref. denotes the reference, and the absence of a subscript implies an unknown sample. For the determination of the quantum yield, the excitation wavelength was chosen so that A < 0.05. For the solid samples, the quantum yield for the compound was determined at room temperature through an absolute method using an Edinburgh Instruments’ integrating sphere coupled to a modular Edinburgh FLS 920 fluorescence spectrophotometer. The values reported are the average of three independent determinations for each sample. The absolute quantum yield was calculated using the following expression:

$$ \varPhi =\frac{{\displaystyle \int {L}_{emission}}}{{\displaystyle \int {L}_{reference}}-{\displaystyle \int {L}_{sample}}} $$
(2)

In expression (2), L emission is the emission spectrum of the sample, collected using the sphere, E sample is the spectrum of the incident light used to excite the sample, collected using the sphere, and E reference is the spectrum of the light used for excitation with only the reference in the sphere. The method is accurate to within 10%. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on Perkin Elmer TGA 4000 and DSC 8000 thermal analyzers under nitrogen atmosphere at a heating rate of 10 °C min−1. Cyclic voltammetric (CV) measurements were carried out in a conventional three electrode cell using a Pt button working electrode of 2 mm in diameter, a platinum wire counter electrode, and a saturated calomel electrode (SCE) reference electrode on a computer-controlled CHI660d electrochemical workstation at room temperature. Reduction CV of the compound was performed in CH2Cl2 containing teterabutylammonium hexafluorophosphate (Bu4NPF6, 0.1 M) as the supporting electrolyte. Ferrocene was used as an external standard. Electrochemistry was done at a scan rate of 100 mV s−1.

Computational Details

The theoretical investigation of geometry optimization was performed with the Gaussian 09 program package [29]. Density functional theory (DFT) was calculated at Beck’s three-parameter hybrid exchange functional [30] and Lee, and Yang and Parr correlation functional [31] B3LYP/6-31G (d). The spin density distributions were visualized using Gaussview 5.0.8.

Device Fabrication and Measurement

Prior to the device fabrication, the patterned ITO-coated glass substrates were scrubbed and sonicated consecutively with detergent water, deionized water, and acetone, dried in drying cabinet, and then exposed to a UV-ozone environment for 30 min. After these processes, the substrates were transferred into a vacuum chamber for sequential deposition of all the organic layers by thermally evaporation with a base pressure (~4.0 × 10−4 Pa) at a rate of 0.1–0.2 nm s−1 monitored in situ with the quartz oscillator. LiF covered by Al is used as cathode without breaking the vacuum. All the samples were measured directly after fabrication without encapsulation at room temperature under ambient atmosphere. The current-voltage-luminance characteristics were carried out using a PR655 Spectrascan spectrometer and a Keithley 2400 programmable voltage-current source. The external quantum efficiency (EQE) and luminous efficiency (LE) were calculated assuming Lambertian distribution, and then calibrated to the efficiencies obtained at 1000 cd m−2 in the integrating sphere (Jm-3200). The configurations of Device A was ITO/MoO3 (10 nm)/NPB (80 nm)/PhImPOTD (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). The nominal hole-only and electron-only devices were fabricated with the configurations of ITO/MoO3 (0.2 nm)/NPB (40 nm)/PhImPOTD (30 nm)/NPB (40 nm)/MoO3 (0.2 nm)/Al (100 nm) (hole-only transporting Device) and ITO (100 nm)/LiF (0.1 nm)/TPBi (40 nm)/ PhImPOTD (30 nm)/TPBi (40 nm)/ LiF (0.1 nm)/Al (100 nm) (electron-only transporting Device). The doped WOLEDs device B was fabricated with the structure ITO/MoO3 (0.2 nm)/NPB (40 nm)/PhImPOTD (15 nm)/PO-01 (0.2 nm)/PhImPOTD (15 nm)/TPBi (40 nm)/LiF (0.1 nm)/Al.

Synthesis

Synthesis of Dibenzothiophene-4-Dioxaborolane (TD4B)

TD4B was synthesized according to the literature procedures [28]. Yield: 80%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 8.22 (d, J = 8.0 Hz, 1 H), 8.11 (t, J = 6.5 Hz, 2 H), 8.90 (d, J = 7.5 Hz, 1 H), 7.84 (t, J = 4.0 Hz, 1 H), 7.45–7.40 (m, 3 H), 1.35 (s, 12 H); HR-ESI-MS: [M + H]+ m/z calcd for C18H20BO2S: 311.12797, found: 311.12748; elemental analysis calcd (%) for C18H19BO2S: C 69.69, H 6.17, N 10.31, S 10.33; found: C 69.58, H 6.11, N 10.45, S 10.42.

Synthesis of 4-(Diphenylphosphinyl)Benzenamine (POBA)

To a 100 mL round bottom flask fitted with magnetic bar, was added NiCl2.6H2O (23.6 mg, 0.1 mmol), zinc (127.9 mg, 2.0 mmol), 2,2′-bipyridine (bpy) (31.2 mg, 0.2 mmol), 4-bromoaniline (206.4 mg, 1.2 mmol), diphenylphosphine oxide (202.1 mg, 1.0 mmol) and water (15 mL). The reaction mixture was then stirred at 70 °C for 24 h. After completion of the reaction, the mixture was allowed to cool to room temperature and added with CH2Cl2 and water. The organic layer was isolated and the remaining aqueous phase was further extracted with CH2Cl2 (10 mL × 3). Then the organic phases were combined and dried with anhydrous MgSO4, and purified by silica gel column chromatography using CH2Cl2/MeOH (15:1) as the eluent to afford a white power. Yield: 75%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 7.59–7.5 (m, 6 H), 7.52–7.48 (m, 4 H), 7.21–7.17 (m, 2 H), 6.63 (dd, J = 6.0, 2.5 Hz, 2 H), 5.79 (br s, 2 H); HR-ESI-MS: [M + H]+ m/z calcd for C18H17NOP: 294.10423, found: 294.10422; elemental analysis calcd (%) for C40H26N2O2: C 84.78, H 4.62, N 4.94; found: C 84.68, H 4.71, N 4.86.

Synthesis of 2-(4-Bromophenyl)-1-(4-Diphenylphosphinylphenyl)-1H–Phenanthro[9,10-d]Imidazole (PhImPOBr)

A mixture of 4-bromobenzaldehyde (92.0 mg, 0.5 mmol), phenanthrene-9,10-dione (104.0 mg, 0.5 mmol), 4-(diphenylphosphinyl)benzenamine (732.8 mg, 2.5 mmol), ammonium acetate (130.1 mg, 2.0 mmol), and acetic acid (30 mL) were refluxed under nitrogen in an oil bath. After 24 h, the mixture was cooled and concentrated under reduced pressure, the crude product was extracted by CH2Cl2/H2O. It was then purified by chromatography using CH2Cl2/MeOH (15:1) as an eluent to obtain the product as yellow powder. Yield: 62%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 7.21 (d, J = 8.0 Hz, 1H), 7.30 (t, J = 7.5 Hz, 1H), 7.39 (d, J = 8.5 Hz, 2H), 7.44 (d, J = 8.5 Hz, 2H), 7.53–7.55 (m, 5H), 7.60–7.68 (m, 5H), 7.70–7.75 (m, 5H), 7.91 (dd, J = 8.0 Hz, 2H), 8.70 (d, J = 8.5 Hz,1H), 8.78 (d, J = 8.0 Hz, 1H), 8.8 (d, J = 9.0 Hz, 1H); HR-ESI-MS: [M + H]+ m/z calcd for C39H27BrN2OP: 649.10389, found: 649.10200; elemental analysis calcd (%) for C39H26BrN2OP: C 72.12 H 4.03, N 4.31; found: C 72.18, H 4.13, N 4.43.

Synthesis of 2-(4-Dibenzothiophene Sulfone)-1-(4-Diphenylphosphinylphenyl)-1H–Phenanthro[9,10-d]Imidazole (PhImPOTD)

A mixture of PhImPOBr (974.3 mg, 1.5 mmol), TD4B (465.3 mg, 1.5 mmol), tetrakis(triphenylphosphine)palladium (173.3 mg, 0.15 mmol), tetrabutylammonium bromide (48.5 mg, 0.15 mmol), and aqueous solution of sodium hydroxide (2 mol L−1, 9 mmol) in THF (20 mL) was stirred under argon at 80 °C for 48 h. After quenched with aqueous NH4Cl solution, the mixture was extracted with CH2Cl2. The combined organic extracts were washed with brine and dried over anhydrous MgSO4. After removing the solvent, the residue was purified by column chromatography on silica gel using ethyl acetate as the eluent to give a white power. Yield: 64%. 1H NMR (TMS, CDCl3, 500 MHz): ppm δ = 7.35 (t, J = 8.5 Hz, 1H), 7.47–7.63 (m, 12H), 7.68–7.84 (m, 13H), 7.98 (t, J = 8.5 Hz, 2H), 8.23 (t, J = 7.0 Hz, 2H), 8.75 (d, J = 8.5 Hz,1H), 8.83 (d, J = 8.5 Hz, 1H), 8.92 (d, J = 8.0 Hz, 1H); HR-ESI-MS: [M + H]+ m/z calcd for C51H34N2OPS: 753.21240, found: 753.21298; elemental analysis calcd (%) for C51H34N2OPS: C 81.36, H 4.42, N 3.72, S 4.26; found: C 81.38, H 4.37, N 3.71, S 5.31.

Results and Discussion

Synthesis

The synthetic route for compound PhImPOTD is outlined in Scheme 1. The detailed procedures for the syntheses of the reaction intermediates and final products are depicted in synthesis part. PhImPOTD is composed of three main components: PI as the acceptor moiety, dibenzothiophene as the donor moiety and diphenylphosphine oxide as the branch of phenathroimidazole. Firstly, TD4B is successively achieved by bromination and borate acidification of dibenzothiophene at C-4 positions. Secondly, the diphenylphosphine oxide (PO) group is not directly treated with deprotonation for the NH group on imidazole. We chose the moderate synthetic route to construct 4-(diphenylphosphinyl)benzenamine by zinc and nickel co-catalysis. The key backbone PhImPOBr is prepared by one-pot cyclizing reaction with high yields. This synthesis method could conveniently construct PI derivatives with various structures by tuning aromatic aldehyde and primary amine [32]. The target molecule is obtained through the typical Suzuki-Miyaura cross-coupling reactions between the bromide intermediate PhImPOBr and the boronic ester TD4B catalyzed by Pd(PPh3)4–NaOH in 64% yields. In order to pursue the maximized yields, the selective aprotic solvents THF, toluene and 1,4-dioxane were utilized for this reaction. The results prove that the utilization of THF can be apt to the maximized yields. The identity of PhImPOTD is fully characterized by standard spectroscopic techniques, which gives satisfactory analysis data corresponding to chemical structure and demonstrates its high purity. PhImPOTD has good solubility in common organic solvents such as THF, dichloromethane, chloroform and toluene.

Scheme 1
scheme 1

Synthetic pathways toward bipolar molecule PhImPOTD

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Thermal Properties

The thermal properties of PhImPOTD are examined by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements, as shown in Fig. 1. PhImPOTD shows good thermal stability, which is indicated by high decomposition temperature (T d, corresponding to 5% weight loss) of 321 °C. Such value is ca. 60 °C higher than PI unit (T d = 262 °C). In addition, no phase transition, like the glass transition and the melting temperature, is observed in the tested temperature range, indicating the noncrystalline or amorphous characteristics of PhImPOTD [33]. The nonplanar PO and TD units in molecule hinder close packing, which is beneficial for the formation of amorphous thin film by vacuum deposition.

Fig. 1
figure 1

TGA thermogram of PhImPOTD (at 10 °C min−1 under notrogen atmosphere). Inset: Differential scanning calorimetry (DSC) spectrum of the first and second heating cyclings for PhImPOTD at a heating rate of 10 °C min−1 under nitrogen flushing

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Theoretical Calculations and Electrochemical Properties

To gain further insight into the structure-property relationship, we performed the geometry optimization of PhImPOTD using the DFT calculations with the Gaussian 09 series of programs using the B3LYP hybrid functional and 6–31G(d) basis set. Figure 2 presents the distributions of the frontier molecular orbitals (FMO), i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). The HOMOs are situated on the dibenzothiophene unit, while the LUMOs are mainly localized on the imidazole, diphenylphosphine oxide group and the linker phenyl segments due to the inductive effect of P = O. It is noteworthy that with a most twisted conformation due to TD at the 4-position of PI, it brings about the separated HOMO and LUMO level distribution localized on the D and A moieties, which indicates that HOMO–LUMO excitation would shift the electron density distribution from one side of the TD groups as the donor to the other side PI as the acceptor. These observations are in accord with the fact that TD is a hole transport unit and PI is an electron transport unit for PhImPOTD. It is more important, such separation of HOMO and LUMO can provide hole- and electron-transporting channel respectively. Moreover, the dihedral angels between the adjacent phenyl linker and PI are 31.7°. The dihedral angels between the imidazole and deprotonation group are 75.2°. Closer inspection reveals the conjugated phenyl linker and the adjacent TD plane intersect with the approximately perpendicular dihedral angels of 86.2°. The twisted conformation brings out the less overlapped spatial distributions of the HOMO and LUMO, which is beneficial to charge balance and exciton recombination in devices.

Fig. 2
figure 2

The optimized molecular geometries of PhImPOTD calculated with DFT on a B3LYP/6-31G(d) level

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The experimental LUMO and HOMO energy levels of PhImPOTD was estimated with cyclic voltammetry (CV), as shown in Fig. 3. PhImPOTD shows a single irreversible oxidation peak with the similar onset voltages around 1.20 eV, corresponding to the HOMO energy levels of −5.60 eV (assuming that the absolute energy level of the Fc/Fc+ redox couple was 4.40 eV below vacuum). The energy level of LUMO was calculated by using Eqs. (3) and (4):

$$ {E}_{\mathrm{g}}=1240/{\lambda}_{\mathrm{onset}} $$
(3)
$$ {E}_{\mathrm{LUMO}}={E}_{\mathrm{HOMO}}+{E}_{\mathrm{g}} $$
(4)
Fig. 3
figure 3

Cyclic voltammogram of PhImPOTD. In each case, the anodic scan was performed in CH2Cl2 at a scan rate of 100 mV s−1. The working electrode: platinum wire; the auxiliary electrode: platinum wire with a porous ceramic wick; the reference electrode: calomel electrode

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The corresponding LUMO energy level of PhImPOTD was calculated to be −2.41 eV. The molecular orbital data indicate that the HOMO/LUMO level of diphenylphosphine oxide substituted PhImPOTD are obviously lower than those of the electron-rich substituted one [28], which proved the electrophilic substituent group pulls the HOMO/LUMO levels down.

Photophysical Properties

The electronic absorption and steady-state photoluminescence spectra of PhImPOTD were measured at room temperature in dichloromethane and in thin film, as illustrated in Fig. 4 and Table 1. The compound features the intense absorption bands in the region of 230–390 nm in dichloromethane, which consist of three bands around 230, 260 and 339 nm. The strong absorption peak located at 230 nm originated from the π → π* transition of diphenylphosphinyl moiety [34]. The absorption bands at around 260 nm can be assigned to the isolated benzene ring connected with imidazole [35], the π → π* transition of dibenzothiophene (280–305 nm) partially merges with this absorption band, the broad absorption bands at 339 nm might be corresponds to the π–π* transition of the substituent on the 2-imidazole position to the PI unit [36]. Moreover, the optical gap in solution was 3.19 eV for PhImPOTD, estimated according to the onset of the absorption spectrum in CH2Cl2. In the photoluminescence spectra, PhImPOTD shows a structureless emission peak at 414 nm. As expected, the emission maxima of PhImPOTD in a thin film (at 438 nm) is bathochromic shifted by ca. 24 nm in comparison with that in solution, which is likely to be caused by π–π stacking and intermolecular aggregation in the solid state. The photoluminescence quantum yield (PLQY) is determined by 9,10-dipenylanthracene (Φ = 0.9 in cyclohexane) as a standard. PhImPOTD shows the PLQY is as high as 0.62 in CH2Cl2. However, PLQY is decreased to 0.34 in the solid state, which should be attributed by the aggregation quenching. Then, the solvatochromic shifts are investigated in different polarity solvents. These solvents are n-hexane, toluene, chloroform, ether, dichloromethane and acetonitrile, respectively. The orientation polarization of these solvents are 0.0012, 0.014, 0.15, 0.17, 0.22 and 0.31, respectively. The emission colors obtained from various solutions, and solvatochromic shifts of PL spectra for PhImPOTD are shown in Fig. 5. The wavelength of maximum PL intensity for PhImPOTD red-shifted from 398 (n-hexane) to 420 nm (acetonitrile), with increasing solvent polarity. Such solvatochromic behavior demonstrates the existence of CT moiety in the excited state [37].

Fig. 4
figure 4

Normalized absorption spectra and fluorescence spectra of PhImPOTD in CH2Cl2 at 10−6 M and in spin-coating film on a quartz plate at 298 K, respectively. Insert: photographs of PhImPOTD in CH2Cl2 and in the solid state under 365 nm hand-lamp irradiation; photographs of single crystal captured through optical microscope

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Table 1 The photophysical and thermal properties of PhImPOTD
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Fig. 5
figure 5

Solvatochromic shifts of PL spectra of PhImPOTD

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Electroluminescence Properties

To explore the EL properties of PhImPOTD, we construct the device A with a frequently used multilayered structure: indium tin oxide (ITO)/MoO3 (10 nm)/ N,N′-Bis-(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) (80 nm)/ PhImPOTD (30 nm)/ 1,3,5-Tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) (40 nm)/LiF (1 nm)/Al (100 nm), in which MoO3 and LiF are utilized as a hole and an electron-injecting layer, respectively. NPB and TPBi serve as a hole and an electron-transporting layer, respectively (Scheme 2). The key parameters of device are summarized in Table 2. From the EL spectra, shown in Fig. 6, we could see an obvious deep-blue emission with λmax at 444 nm and no vibronic feature under the applied voltages ranging from 6 to 10 V. Evidently, such the EL spectrum is very similar to the PL spectra observed from film. In detail, the full width at half maximum (FWHM) of EL and PL is 62 nm and 68 nm, replectively, Noticeably, no other emission from any other layers was observed for device A, suggesting that all excitons generated in the device are effectively confined in the emitting layer (EML). At the same time, no exmicer or exciplex emission was observed. Due to the substituent on the imidazole position and the polyaromatic hydrocarbon moieties, PhImPOTD exists the locally twisted structure, which results in large steric hindrance. Figure 7 and Fig. 8 show the current density-voltage-luminance (J–V–L) characteristics and plots of efficiency versus current density of the fabricated deep blue device, respectively. The device yields a maximum luminance of 1341 cd m−2, an external quantum yield of 1.00%, and a maximum luminous efficiency of 1.13 cd A−1. Despite the efficiencies of device with PhImPOTD are slightly lower than those of the reference devices [28, 36], it is definitely a worthwhile work that Commission Internationale de l’Eclairage (CIE) color coordinates of (0.15, 0.08) are attained. This value is very close to the NTSC (National Television Standards Committe) blue standard (CIE: 0.14, 0.08). Meanwhile, the device A shows low turn-on voltage of 3.6 V, which should be attributed to the well matched HOMO level of diphenylphosphine substituent PI emitters (5.6 eV) and the adjacent NPB (5.4 eV). To further understand the charge injection/transportation characteristics of PhImPOTD, we fabricated the single-carrier devices with the configurations of ITO/MoO3 (0.2 nm)/NPB (40 nm)/PhImPOTD (30 nm)/NPB (40 nm)/MoO3 (0.2 nm)/Al (100 nm) (hole-only transporting device) and ITO/LiF (0.1 nm)/ TPBi (40 nm)/PhImPOTD (30 nm)/TPBi (40 nm)/LiF (0.1 nm)/Al (100 nm) (electron-only transporting device). MoO3 and TPBi layers are used to prevent electron and hole injection from the cathode and anode, respectively. As depicted in Fig. 9, the current density–voltage (J–V) characteristics illustrate that the hole current density values of PhImPOTD approximate the same as the electron current density, which undoubtedly evidences the balanced bipolar charge transport capacity of PhImPOTD. All these results indicate that PhImPOTD is an excellent bipolar material, which can be served as non-doped deep blue OLEDs.

Scheme 2
scheme 2

Energy level of device A and chemical structures of the carrier transporting materials

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Table 2 Key performance parameters of non-doped deep blue devices
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Fig. 6
figure 6

Electroluminescence spectra for device A at different voltages. The inset shows a magnified view of the emission peaks

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Fig. 7
figure 7

Current density–voltage–luminance characteristics for devices A

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Fig. 8
figure 8

Efficiency versus luminance curves of non-doped blue device based on PhImPOTD

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Fig. 9
figure 9

Current density versus voltage characteristics of hole-only and electron-only devices for PhImPOTD

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WOLEDs have drawn tremendous intentions and been studied extensively because of their great promise for universal application in future solid-state lighting sources and backlights for full-color displays [38]. White-light emission can be acquired by complementary color (blue plus yellow or blue, green plus red) or broadband emission. To further explore the potential of deep blue emitter PhImPOTD [39, 40], we tried to fabricate device B through precisely controlling the ratio of PhImPOTD and the complementary yellow emitter PO-01 (acetylacetonatobis(4-phenylthieno[3,2-c]pyridinato-N,C2’) Iridium), aiming for achieving WOLEDs. The typical EL spectra of the WOLEDs at different voltages are shown in Fig. 10. The dual emissions EL spectra could be divided into their blue emission (444 nm) corresponding to the fluorophore PhImPOTD and yellow emission (556 nm) corresponding to the phosphor PO-01. The low driving voltage of 3.6 V for onset is realized owing to the well matching HOMO/LUMO level between EML and transport layers. (Fig. 11). PhImPOTD and PO-01 endow device B the high efficiencies with 4.83 cd A−1 for maximum current efficiency (C.E.), 2.15 lm W−1 for power efficiency (P.E.) and a maximum luminance of 8472 cd m−2 (Fig. 12). The white light emitting device is with the CIE coordinates of (0.32, 0.32) at the luminance of 1000 cd m2, which is very close to the standard white light point of (0.33, 0.33). In addition, a small offset of CIE coordinates of emitted light is observed under the various biases (at the luminance of 300–7500 cd A−1), which reflects good color stability for doped WOLEDs. The inset of Fig. 11 is a snapshot of the WOLED at 14.0 V; a suitable white light emission with a uniform emitting area is seen.

Fig. 10
figure 10

Electroluminescence spectra for device B at different voltages

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Fig. 11
figure 11

Current density–voltage–luminance characteristics for device B

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Fig. 12
figure 12

Efficiency versus luminance curves of doped white device B based on PhImPOTD

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Conclusion

In summary, a new phenanthroimidazole derivative PhImPOTD with ambipolar transport behavior has been synthesized and characterized. By combing the advantages of phenanthroimidazole backbone and the D–π–A structure, high efficient deep blue emitter was achieved. Further, the deprotonation of n-type imidazole moiety by electron-withdrawing diphenylphosphine oxide increased the unsymmetrical configuration, which could efficiently prevent the strong π–π stacking and molecular interactions. It is reasonable that PhImPOTD possesses high quantum efficiency and thermal stability. PhImPOTD has not only been used as emitter to fabricate deep-blue OLEDs, but also as host materials to construct highly efficient WOLEDs. Non-doped PhImPOTD multilayer device exhibits a deep-blue emission (CIE: 0.15, 0.08), together with a low driving voltage of 3.6 V, efficiencies of 1.13 cd A−1 for C.E., and 1.00% for E.Q.E. Doped WOLED with CIE coordinates of (0.32, 0.32) gave the performance of 4.83 cd A−1 for C.E., 2.15 lm W−1 for P.E. and 3.6 V for onset voltage. It also possesses good color stability under broad luminance. We believe that this work gives a novel clue for high performance deep blue and white emitting fluorescent OLEDs.