Deep blue organic light-emitting devices enabled by bipolar phenanthro[9,10-d]imidazole derivatives

Abstract

Two blue fluorescent phenanthroimidazole derivatives (PhImFD and PhImTD) with a D–π–A structure are synthesized by attaching a hole-transporting dibenzofuran or dibenzothiophene and an electron-transporting phenanthroimidazole moiety and characterized. The nonplanar twisted structures reduce molecular aggregation, which endows both of the compounds with good thermal properties, and film-forming abilities as well as high quantum yields in CH₂Cl₂ and in the solid state. Non-doped organic light emitting diodes (OLEDs) are fabricated by employing the compounds PhImFD and PhImTD as emitters and exhibited promising performances. The devices show a deep blue emission with Commission Internationale de l’Eclairage (CIE) coordinates of (0.15, 0.11) for PhImFD and (0.15, 0.10) for PhImTD. PhImFD and PhImTD, with the desired bipolar-dominant characteristics, render devices with a low driving voltage of 3.6 V. The energy levels of the materials were found to be related to the donor units in the compounds with different substituents. Device B, using PhImTD as the emitting layer (EML), with well fitting energy levels and increased electron transport ability, possesses favorable efficiencies of 1.34 cd m⁻² for CE, 0.82 lm W⁻¹ for PE and 1.63% for EQE. PhImFD and PhImTD are utilized as blue emitters and the host for a yellow emitter, PO-01, to fabricate white organic light-emitting diodes (WOLEDs) that give a forward-viewing maximum CE of 8.12 cd m⁻² and CIE coordinates of (0.339, 0.330). The results demonstrated not only that the phenanthroimidazole unit is an excellent building block to construct deep blue emission materials, but also that chemical structure modification by the introduction of a suitable electron-donor substituent could influence the performance of devices.

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1. Introduction

Over recent decades, organic light-emitting diodes (OLEDs) have received increasing attention and are expected to be used as next-generation lighting.¹ Owing to the advantages of a planar lighting source, flexibility over a large area, being mercury-free, low-cost fabrication, and a high power-conversion efficiency, some tendencies toward replacing traditional light sources will gradually emerge.² In the commercial world, high efficiency and excellent stability are two constant goals. To achieve full-color electroluminescent displays, three color components, i.e., red, green, and blue, must be available.³ Although red and green light-emitting materials have acquired sufficient development to achieve the commercial application criteria of OLEDs,⁴ the design and optimization of blue OLEDs has remained a formidable challenge to date.⁵ A blue emitter can not only effectively reduce the power consumption of the devices but can also be utilized to generate light of other colors via an energy cascade to lower energy fluorescent or phosphorescent dopants.⁶ Forrest et al. proposed the concept of phosphorescent OLEDs (PHOLEDs),⁷ where the introduction of phosphors can boost the internal quantum efficiency up to the theoretical 100% through the use of triplet excitons for light emission.⁸ However, it is much more difficult to find a blue phosphorescent emission with a long lifetime and pure Commission International de l’Eclairage (CIE) color, due to the inherently wide bandgap.⁹ High CIE coordinates (y-coordinate > 0.25), short device lifetimes and environmental contaminants (heavy metals) are not suitable for commercial use.¹⁰ Therefore, in order to achieve marketable OLEDs, the hunt for highly efficient blue-fluorescent materials and devices still remains a subject of much current interest.

As is well known, the electron injection and transport ability in organic semiconductors is low compared to that of hole transport, therefore the basic design requirement for blue materials is to increase their electron affinities to realize balanced charge injection and transport. The n-type imidazole moiety has been widely employed as an electron-transporting material and as the electron-withdrawing group of bipolar host materials.¹¹ Meanwhile, phenanthroimidazole (PI) could readily construct a variety of sky-blue and deep blue electroluminescent materials as a near-ultraviolet fluorescent chromophore.¹² Therefore, PI, including an imidazole moiety, can act as an electron-acceptor when linked with an electron-donating group, which would exhibit ambipolar characteristics.¹³ Based on the inherent character of the imidazole group, a wide variety of substituents could be attached after deprotonation of the NH group on the imidazole, which would then be capable of inhibiting the strong π–π stacking and molecular interactions in the solid state.¹⁴ The formation of smooth and stable amorphous films could be realized using the twisted configuration.¹⁵ Simultaneously, the introduction of electron-deficient PI units effectively increases the electron injection and transport abilities and facilely adjusts the ionization potentials (IP) of the compounds,¹⁶ further reduces the injection barrier at the interface between the hole transporting layer (HTL) and the emissive layer (EML) and balances carrier recombination.¹⁷ Actually, energy and charge transfer processes in the EML should be more accurately controlled to achieve low driving voltages and high efficiencies.¹⁸ Therefore, developing high-efficiency deep blue OLEDs with a low driving voltage is important to promote their commercial applications in portable devices.¹⁹

Motivated by this research trend and our project on the synthesis of blue emitting compounds, non-doped blue-fluorescent devices using phenanthroimidazole derivatives are reported. In this paper, we describe the design and synthesis of two bipolar phenanthroimidazole derivatives by using the para-positions of a freely rotatable phenyl bridge between the C-2 position of the imidazole ring and C-4 position of the dibenzofuran or dibenzothiophene. PI acts as a π-acceptor and can be incorporated with an electron donor to form a bipolar molecule. We anticipate the rigid PI skeleton, as well as the freely rotatable aryl substituents on the C-2 positions of imidazole, are beneficial to the thermal and morphological stabilities without sacrificing the good electron-transport ability and high triplet energy imparted by the PI unit. In addition, the bulky and sterically hindered molecular configuration is advantageous for effectively enhancing the photoluminescence quantum yield.²⁰ The thermal, photophysical, and electroluminescent properties of the compounds are comprehensively investigated. The new PI derivatives PhImFD and PhImTD possess a high luminescent efficiency, excellent luminous/thermo-stability and balanced injection and transport of charge carriers, and have opened up new opportunities in the utilization of PI derivatives for applications and functions.

2. Results and discussion

2.1 Synthesis

The synthetic routes for PhImFD and PhImTD are shown in Scheme 1. The detailed procedures for the syntheses of the reaction intermediates and final products are depicted in the synthesis part. PhImFD and PhImTD are composed of two main components: phenanthroimidazole as the acceptor moiety and dibenzofuran or dibenzothiophene as the donor moiety. Firstly, FD4B and TD4B were achieved by bromination and borate acidification of dibenzofuran and dibenzothiophene at the C-4 positions, respectively. The important precursor PhImBr was synthesized via a one-pot cyclization reaction in good yields.²¹ This synthesis method could conveniently construct PI derivatives with various structures by tuning the aromatic aldehyde and primary amine. The target molecules are obtained through the typical Suzuki cross-coupling reactions of the bromide intermediate PhImBr and the precursors FD4B/TD4B, catalyzed by Pd(PPh₃)₄–NaOH, in 79–82% yields. In order to pursue the maximized yields, the selected aprotic solvents THF, toluene and 1,4-dioxane were utilized for this reaction. The results prove that the utilization of THF can contribute to the maximized yields. The identities of PhImFD and PhImTD were established using ¹H NMR, ¹³C NMR, high-resolution MS, and satisfactory elemental analysis data. Both of the molecules have good solubility in common organic solvents such as THF, dichloromethane, chloroform and toluene.

Scheme 1 Synthetic pathways toward bipolar molecules PhImFD and PhImTD.

2.2 Thermal properties

The compounds PhImFD and PhImTD with dibenzofuran and dibenzothiophene at the para-position of the C2-phenyl and an N1-phenyl on the phenanthroimidazole show high thermal stabilities. As illustrated in Fig. 1, the temperature of thermal-decomposition (T_d, at a weight loss of 5%) of PhImFD is 300 °C, while PhImTD possesses a higher T_d than PhImFD of 318 °C. The DSC results show that the glass-transition temperatures (T_g) of PhImFD and PhImTD are 110 °C and 115 °C, respectively, verifying that the small energetic disorder at the C2-phenyl of the phenanthroimidazole can efficiently enhance the morphological stability. The high T_d and T_g imply that they could form morphologically stable amorphous films upon thermal evaporation, which is a crucial parameter for application as OLEDs.

Fig. 1 TGA thermograms of PhImFD and PhImTD. Both measured at 10 °C min⁻¹ under a nitrogen flow. Inset: DSC spectra of the first and second heating cycles for PhImFD (a) and PhImTD (b) at a heating rate of 10 °C min⁻¹ under a nitrogen flow.

2.3 Photophysical properties

The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) emission spectra of PhImFD and PhImTD in a dilute solution (10⁻⁵ mol L⁻¹ in CH₂Cl₂) and in a film were measured to investigate their photophysical properties (Fig. 2). The absorption spectra exhibit no distinct differences and both of the molecules exhibit four bands at ca. 265 nm, 290 nm, 333 nm and 365 nm in CH₂Cl₂ solution. The strong absorption bands at around 265 nm can be attributed to the phenanthrene unit.²² The absorption peaks at around 290 nm could be assigned to the dibenzofuran and dibenzothiophene centered n–π* transitions.²³ The absorption bands at 333 nm might be due to the π–π* transition of the substituent on the 2-imidazole position to the PI unit.²⁴ The weak peaks at 365 nm originated from the π–π* transition of the PI unit. Moreover, the optical bandgaps in solution were 3.19 and 3.17 eV for PhImFD and PhImTD, estimated by the absorption edges. The photophysical data for the emissions are gathered in Table 1. The corresponding emission peaks of PhImFD and PhImTD in CH₂Cl₂ appeared at 421 and 423 nm, respectively. As expected, the emission maxima of PhImFD and PhImTD in a thin film (both at 440 nm) are bathochromically shifted by ca. 18 nm compared with those peaks in solution, which is likely to be caused by π–π stacking and the intermolecular aggregation of the bulk existing in the form of a solid.²⁵ PhImFD and PhImTD show high PL quantum yields of 0.58 and 0.62 in the CH₂Cl₂ solution, respectively. While the corresponding quantum yields for the powder are 0.39 and 0.44 for PhImFD and PhImTD, showing the potential application in OLEDs. In addition, in order to investigate the triplet energy level, cryogenic temperature (77 K) time-resolved phosphorescent spectra measurements were performed using 2-methyl-THF glass (Fig. S16†), and the energy levels were determined as 2.99 and 3.03 eV according to 0–0 transitions.

Fig. 2 Normalized UV-vis absorption and fluorescence spectra of PhImFD and PhImTD in CH₂Cl₂ at 10⁻⁵ M and in a spin-coated film at room temperature.

Table 1 The photophysical and thermal properties of PhImFD and PhImTD

	λmax,abs^a/nm	λmax,PL^a/nm	λmax,film^b/nm	λmax,ph^c/nm	ET^d/eV	Tg^e/°C	Td^f/°C	HOMO^g/eV	LUMO^h/eV	Eg^i/eV
a Measured in a CH2Cl2 solution at room temperature.b Measured in a spin-coated film at room temperature.c Measured in a 2-methyl-THF glass matrix at 77 K.d Estimated according to the 0–0 transitions of time-resolved phosphorescent spectra.e Tg: glass transition temperature, obtained from the DSC measurements.f Td: decomposition temperature at a weight loss of 5%, obtained from the TGA measurements.g HOMO was calculated from the onset value of the oxidation potential.h LUMO was calculated from the HOMO and the optical band gap, Eg.i Eg: the optical band gap was calculated from the absorption spectra.
PhImFD	263, 280, 333, 364	421	440	476	2.68	110	300	5.00	1.81	3.19
PhImTD	265, 292, 334, 374	423	440	477	2.70	115	318	5.49	2.32	3.17

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2.4 Theoretical calculations

Density Functional Theory (DFT) calculations (B3LYP/6-31G(d)) were carried out to investigate the structure–property relationships of the new compounds. Fig. 3 shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distributions of PhImFD and PhImTD. The electron clouds of the HOMOs of PhImFD(TD) were distributed on the FD(TD) unit because of the electron-rich dibenzofuran and dibenzothiophene units, and the electron clouds of the LUMOs of PhImFD(TD) were dispersed over the PI and extended phenyl unit owing to the electron-deficient imidazole unit. The HOMOs and LUMOs of PhImFD and PhImTD were completely separated, indicating that HOMO–LUMO excitation would shift the electron density distribution from one side of the dibenzofuran or dibenzothiophene group as the donor to the other side with the phenanthroimidazole as the acceptor. These observations are in accordance with the fact that FD(TD) is a hole transport unit and PI is an electron transport unit for PhImFD(TD). Moreover, the dihedral angles between the adjacent phenyl linker and phenanthroimidazole are 29.5° and 29.1° for PhImFD and PhImTD, respectively. Closer inspection reveals that the conjugated phenyl linker and the adjacent FD and TD planes intersect with the approximately perpendicular dihedral angles (84.5° and 87.1° for PhImFD and PhImTD, respectively).

Fig. 3 FMOs (HOMO and LUMO) of PhImFD and PhImTD calculated using DFT on a B3LYP/6-31G(d) level.

2.5 Electrochemical properties

Fig. 4 reveals the bipolar electrochemical character of PhImFD and PhImTD, as evidenced using cyclic voltammetry (CV) measurements. The electrochemical data are summarized in Table 1. The HOMO energy levels of the compounds were determined from the onset of the first oxidation potentials with regard to the energy level of the SCE (assuming that the absolute energy level of the Fc/Fc⁺ redox couple was 4.4 eV below vacuum). We observed one reversible oxidation potential at 0.6 V for PhImFD and 1.09 V for PhImTD. Through subtraction of the optical energy gap (E_g) from the HOMO energy level, we calculated the energy level of the LUMO to be −1.81 and −2.32 eV for PhImFD and PhImTD, respectively. The molecular orbital data indicate that the HOMO/LUMO level of the dibenzothiophene substituted derivative is obviously lower than that of the dibenzofuran substituted one. The differences in the energy level depend on the linkage units of the electronic donor moieties, because the dibenzofuran is a weaker electron donor than the dibenzothiophene group and molecules end-capped with dibenzothiophene are more coplanar, according to the DFT calculations. More importantly, no electropolymerization occurred during multiple cycles of CV scanning. Thus, the appending of the electron-accepting phenanthroimidazole units onto the dibenzofuran and dibenzothiophene renders PhImFD and PhImTD with promising electrochemical stability and bipolar characteristics, which may imply a greater separation of the HOMO and LUMO orbitals,²⁶ which is in good agreement with the result of the DFT calculations.

Fig. 4 Cyclic voltammograms of PhImFD and PhImTD. 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.

2.6 Electroluminescent properties

To evaluate the performance of PhImFD and PhImTD as emitters, multilayer OLED devices A and B were fabricated (see Fig. 5 for the device configurations). Devices A and B had compounds PhImFD and PhImTD, respectively, as the emitters. The EL spectra, luminance and current density vs. the applied voltage, and the current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) vs. the luminance of devices A and B are displayed in Fig. 6–8. The key performance parameters of the devices are summarized in Table 2. Devices A and B with the structure ITO/MoO_x (2 nm)/NPB (40 nm)/PhImFD or PhImTD (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (ITO is indium tin oxide, NPB is N,N′-bis-(1-naphthalenyl)-N,N′-bis-phenyl-(1,1′-biphenyl)-4,4′-diamine, and TPBi is 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl) are produced. MoO_x is utilized as a hole-injecting layer, and NPB and TPBi are selected as hole and electron-transporting layers, respectively. Both of the devices exhibit deep blue EL spectra with a peak wavelength at 445 nm, which show little or no vibronic features (Fig. 6). The EL spectra of the devices are almost the same as their corresponding photoluminescence spectra in the film state, indicating that the EL spectra are indeed from the emitting layers with no excimer or exciplex emission. Importantly, the saturated deep blue EL emissions are quite stable under the applied voltages ranging from 6 to 10 V. The full width at half maximum (FWHM) is around 70 nm and a CIE y coordinate value < 0.12, along with an (x + y) value < 0.28, is achieved, which effectively guarantees the saturated deep blue colors that remain almost unchanged over a range of luminance from 1000 cd m⁻² to 3000 cd m⁻². In addition, device B exhibits an obviously better performance when compared with device A. It is well known that efficient carrier injection at the interfaces between the different layers in OLEDs is essential for obtaining high performance devices. Thus, it is crucial for the EML to possess a shallow HOMO to facilitate hole-injection. However, compared with that of PhImFD, the HOMO/LUMO of PhImTD is even more remarkably lowered by 0.5 eV. On the one hand, the hole injection barriers between PhImTD and the HTL are very small (0.1 eV). On the other hand, the LUMO energy of PhImTD (−2.3 eV) is close to that of TPBi (−2.7 eV), and the electron injection barrier between PhImFD and TPBi is much too big (0.9 eV), which reveals that the electron injection ability of PhImTD is relatively easier than that of PhImFD. That is reasonable as in the dibenzothiophene substituted phenanthroimidazole the lower energy HOMO/LUMO fit the device HTL and ETL better when compared with the dibenzofuran substituted one. The results reflect that dibenzothiophene as the donor unit affords a more appropriate HOMO/LUMO level, which features the preference of balanced carrier injection and transportation. Meanwhile, it gives us a new method to introduce building blocks to tune the HOMO/LUMO energies. To further understand both the hole and electron injection/transportation characteristics of PhImFD and PhImTD, the single-carrier devices were fabricated (Fig. S10†). The current density–voltage (J–V) characteristics illustrate that the hole current density values of PhImFD and PhImTD are not much different but the electron current density values of PhImTD are higher than those of PhImFD, which is probably caused by the excellent electron transport of PhImTD. PhImTD has a bipolar charge transport capacity as evidenced by the considerable hole/electron current density, which means that the charge transport property of PhImTD is superior to that of PhImFD. As depicted in Fig. 8, device B gives a performance in terms of a maximum EQE of 1.63%, a maximum CE of 1.34 cd A⁻¹ and a maximum PE of 0.82 lm W⁻¹. Although the performance of the deep blue devices is lower than that of the PI-based device, it is improved in comparison with those of PI-based devices substituted with carbazole.²⁴ The CIE coordinates are (0.151, 0.098), which match well with the requirement of the CIE deep blue criterion to have a y coordinate value < 0.15 along with an (x + y) value < 0.30 (Fig. S11†).

Fig. 5 Device structures and an energy diagram for devices A and B.

Fig. 6 Electroluminescence spectra for devices A and B at different voltages.

Fig. 7 Current density–voltage–luminance characteristics for devices A and B.

Fig. 8 Efficiency versus luminance curves of the non-doped blue devices based on PhImFD and PhImTD.

Table 2 Key performance parameters of the non-doped deep blue devices

Material	Device	Von^a (V)	λmax (nm)	FWHM (nm)	CEmax^b (cd A^−1)	PEmax^b (lm W^−1)	EQEmax^b (%)	CIE (x, y)^c
a The voltage required for 1 cd m^−2.b Current efficiency (CEmax), power efficiency (PEmax), and external quantum yield (EQEmax).c The CIE coordinates were measured at 1000 cd m^−2.
PhImFD	A	3.6	445	73	1.31	0.88	1.21	(0.151, 0.115)
PhImTD	B	3.6	445	69	1.34	0.82	1.63	(0.151, 0.098)

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The field of white organic light emitting devices (WOLEDs) has inspired research activities on the basis of their great potential in lighting systems. The WOLEDs are generally realized by mixing red, green and blue emitters in a certain ratio, or by mixing two complementary colors (e.g. orange or yellow and blue) to provide the connecting line of coordinates across the white light region. To fabricate doped WOLEDs, PhImFD and PhImTD were utilized as blue emitters, and PO-01 (acetylacetonatobis(4-phenylthieno[3,2-c]pyridinato-N,C2′)iridium) with an emission peak at 560 nm was utilized as the complementary yellow emitter. The typical EL spectra of the WOLEDs at different voltages are shown in Fig. S13 and S14.† All of the EL spectra could be divided into their blue emission, corresponding to the fluorophore PhImFD and PhImTD, and yellow emission, corresponding to the phosphor PO-01. As a result, the device D that employed PhImTD gave the best performance with the maximum luminance of 3095 cd m⁻² and a maximum current efficiency of 8.12 cd A⁻¹ was achieved, as shown in Fig. 9 and S12 and Table S2.† Devices C and D exhibited a low turn-on voltage (<3.7 V). The white light CIE coordinates of (0.339, 0.330) for device D at the luminance of 1000 cd m⁻² are very close to the standard white light point of (0.33, 0.33). In addition, a small offset of the CIE coordinates of the emitted light is observed under the various biases (at the luminance of 2000–6500 cd A⁻¹), which reflects good color stability for the doped WOLEDs. The inset of Fig. S15† is a snapshot of the WOLED at 14.0 V; a suitable white light emission with a uniform emitting area is seen.

Fig. 9 Current density–voltage–luminance characteristics for devices C and D.

3. Experimental section

3.1 Materials and instruments

All the reagents and solvents used for the synthesis of the compounds were purchased from Aldrich, Acros, J&K and TCI companies and used without further purification. Dopant material PO-01 was purchased from Lumtec Corp. (Taiwan). ¹H and ¹³C-nuclear magnetic resonance (NMR) spectra were recorded using a Bruker AVANCE III 500 MHz spectrometer at 500 MHz and 125 MHz respectively, using DMSO-d₆ or CDCl₃ as the solvents 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, and N were performed on a Perkin-Elmer 2400 automatic analyzer. 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 PerkinElmer Lambda-750 UV-vis-NIR spectrophotometer and LS 55 fluorescence spectrometer, respectively. Phosphorescence spectra were measured in CH₂Cl₂ using an Edinburgh FLS 920 fluorescence spectrophotometer at 77 K cooling by liquid nitrogen with a delay of 300 μs using a Time-Correlated Single Photon Counting (TCSPC) method with a microsecond pulsed xenon light source for 10 μs to 10 s lifetime measurement. The luminescence quantum yields of compounds were 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:

In eqn (1), n, A, and I denote the refractive index of the 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 yields for the compounds were 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 (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 PerkinElmer TGA 4000 and DSC 8000 thermal analyzers under a 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 all compounds was performed in CH₂Cl₂ containing tetrabutylammonium hexafluorophosphate (Bu₄NPF₆, 0.1 M) as the supporting electrolyte. Ferrocene was used as an external standard. The electrochemistry experiments were done at a scan rate of 100 mV s⁻¹.

3.2 Computational details

The theoretical investigation of the geometrical properties was performed with the Gaussian 09 package.²⁷ Density functional theory (DFT) was calculated using Becke’s three-parameter hybrid exchange functional²⁸ and Lee, and Yang and Parr correlation functional²⁹ B3LYP/6-31G(d). The spin density distributions were visualized using Gaussview 5.0.8.

3.3 Device fabrication and measurement

Prior to the device fabrication, the patterned ITO-coated glass substrates were degreased with standard solvents, blow-dried using a N₂ gun, and exposed in a UV-ozone ambient environment for 30 min. All the organic layers were commercially purchased from Luminescence Technology Corp., and thermally deposited onto the ITO 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 with Al, was used as the cathode without breaking the vacuum. All of 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 Spectra scan 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 and B were ITO/MoO_x (2 nm)/NPB (40 nm)/PhImFD or PhImTD (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm), where MoO_x and the LiF anode/cathode buffer layer were for hole/electron injection. NPB and TPBi served as hole- and electron-transporting layers (HTL and ETL), respectively. The nominal hole-only and electron-only devices were fabricated with the configurations of ITO/MoO_x (2 nm)/NPB (40 nm)/PhImFD or PhImTD (30 nm)/NPB (40 nm)/MoO_x (2 nm)/Al (100 nm) (hole-only transporting device) and Al (100 nm)/LiF (1 nm)/TPBi (40 nm)/PhImFD or PhImTD (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (electron-only transporting device). The doped white OLEDs devices C and D were fabricated with the structure ITO/MoO_x (2 nm)/NPB (40 nm)/PhImFD or PhImTD (15 nm)/PO-01 (0.2 nm)/PhImFD or PhImTD (15 nm)/TPBi (40 nm)/LiF (1 nm)/Al.

3.4 Synthesis

Synthesis of dibenzofuran-4-dioxaborolane (FD4B). Dibenzofuran (11.76 g, 70 mmol) was dissolved in 100 mL of anhydrous THF and slowly lithiated using 1.6 M n-BuLi in hexane (44 mL, 70 mol) at −78 °C under an argon atmosphere. The solution was warmed to 0 °C and was stirred for 6 h. Afterward the reaction mixture was cooled to −78 °C and a solution of 1,2-dibromoethane (26.3 g, 140 mmol) in 20 mL anhydrous THF was added dropwise. The mixture was then stirred at room temperature for 10 h. After concentration under reduced pressure the crude brominated product was dissolved in CH₂Cl₂ and washed with water several times. The solution was concentrated again, and the resulting solid was recrystallized from an n-hexane/CH₂Cl₂ solution to give the 4-bromodibenzothiophene. Then, 4-bromodibenzothiophene (2.46 g, 10 mmol) was dissolved in 30 mL THF, and stirred in a dry ice/acetone bath for 30 min. 9.4 mL n-BuLi (15 mmol, 1.6 M) in 30 mL THF was added dropwise to this solution. The solution was stirred for 2 h, and then trimethyl borate (3.4 mL, 30 mmol) was added to the solution. The solution was stirred for 1 h at −78 °C and warmed to r.t., and then cooled in an ice bath, 2 M HCl (30 mL, 60 mmol) was added, and the solution was stirred at r.t. for 8 hours. The mixture was diluted with CH₂Cl₂ and extracted three times with water. After being dried with anhydrous Na₂SO₄, the organic phase was completely removed using a rotary evaporator to afford white dibenzofuran-4-boronic acid. A mixture of pinacol (0.8863 g, 7.5 mmol) and dibenzofuran-4-boronic acid (1.0603 g, 5 mmol) in toluene (50 mL) was stirred at 80 °C overnight. The solvent was evaporated under reduced pressure and the residue extracted with CH₂Cl₂/H₂O. The residue was purified by column chromatography using an ethyl acetate/hexane (1 : 10) mixture as an eluent to give a white powder. Yield: 86%. ¹H NMR (TMS, CDCl₃, 500 MHz): ppm δ = 8.05 (d, J = 7.5 Hz, 1H); 7.91 (dd, J = 8.0 Hz, 2H); 7.66 (d, J = 8.0 Hz, 2H); 7.43 (t, J = 8.5 Hz, 1H); 7.36–7.30 (m, 2H); 1.44 (s, 12H); HR-ESI-MS: [M + H]⁺ m/z calcd for C₁₈H₂₀BO₃: 295.15074, found: 295.15032.

Synthesis of dibenzothiophene-4-dioxaborolane (TD4B). The procedure for TD4B was similar to the preparation of FD4B starting from dibenzothiophene (12.86 g, 70 mmol) instead of dibenzofuran. Yield: 80%. ¹H NMR (TMS, CDCl₃, 500 MHz): ppm δ = 8.22 (d, J = 8.0 Hz, 1H); 8.11 (t, J = 6.5 Hz, 2H); 8.90 (d, J = 7.5 Hz, 1H); 7.84 (t, J = 4.0 Hz, 1H); 7.45–7.40 (m, 3H); 1.35 (s, 12H); HR-ESI-MS: [M + H]⁺ m/z calcd for C₁₈H₂₀BO₂S: 311.12797, found: 311.12748.

Synthesis of 2-(4-bromophenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole (PhImBr). A mixture of 4-bromobenzaldehyde (248.3 mg, 13.5 mmol), phenanthrene-9,10-dione (280.9 mg, 13.5 mmol), 4-methoxy-benzenamine (830.7 mg, 67.5 mmol), ammonium acetate (354.2 mg, 54.5 mmol), and acetic acid (60 mL) were refluxed under nitrogen in an oil bath. After 24 h, the mixture was cooled and filtered. The solid product was washed with an acetic acid/water mixture (1 : 1, 100 mL) and water. It was then purified using chromatography with CH₂Cl₂/petroleum ether (1 : 1) as an eluent to obtain the product as a green powder. Yield: 70%. ¹H NMR (TMS, CDCl₃, 500 MHz): ppm δ = 3.96 (s, 3H), 7.10 (d, J = 8.5 Hz, 2H), 7.29 (t, J = 7.0 Hz, 2H), 7.39–7.45 (m, 4H), 7.48–7.53 (m, 3H), 7.65 (t, J = 8.5 Hz, 1H), 7.73 (t, J = 7.0 Hz, 1H), 8.70 (d, J = 8.0 Hz, 1H), 8.76 (d, J = 8.5 Hz, 1H), 8.84 (d, J = 9.0 Hz, 1H); HR-ESI-MS: [M + H]⁺ m/z calcd for C₂₈H₂₀BrN₂O: 479.07535, found: 479.07449.

Synthesis of 2-(4-dibenzofuran-4-phenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole (PhImFD). A mixture of PhImBr (478.1 mg, 1 mmol), FD4B (294.1 mg, 1 mmol), tetrakis(triphenylphosphine)palladium (115.6 mg, 0.1 mmol), tetrabutylammonium bromide (32.2 g, 0.1 mmol), and an aqueous solution of sodium hydroxide (2 mol L⁻¹, 6 mmol) in THF (20 mL) was stirred under argon at 80 °C for 48 h. After quenching with an 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 using column chromatography on silica gel with CH₂Cl₂ as the eluent to give a white power. Yield: 82%. ¹H NMR (TMS, CDCl₃, 500 MHz): ppm δ = 3.96 (s, 3H), 7.14 (d, J = 8.5 Hz, 2H), 7.28–7.43 (m, 4H), 7.46–7.54 (m, 4H), 7.61 (d, J = 9.0 Hz, 2H), 7.66 (t, J = 7.5 Hz, 1H), 7.76 (t, J = 7.0 Hz, 1H), 7.81 (d, J = 8.0 Hz, 2H), 7.92 (dd, J = 7.0 Hz, 3H), 7.98 (d, J = 8.0 Hz, 1H), 8.71 (d, J = 8.5 Hz, 1H), 8.78 (d, J = 8.5 Hz, 1H), 8.93 (d, J = 8.0 Hz, 1H); ¹³C NMR (TMS, CDCl₃, 125 MHz): ppm δ = 160.43, 156.17, 153.30, 150.71, 136.73, 131.29, 130.10, 129.47, 128.57, 128.32, 127.30, 126.64, 126.32, 125.59, 125.07, 124.80, 124.12, 123.23, 123.11, 122.76, 120.86, 120.69, 119.98, 115.38, 111.86, 55.39; HR-ESI-MS: [M + H]⁺ m/z calcd for C₄₀H₂₇N₂O₂: 567.20670, found: 567.20704; 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-dibenzothiophene-4-phenyl)-1-(4-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole (PhImTD). The procedure for PhImTD was similar to the preparation of PhImFD starting from TD4B (310.2 mg, 1 mmol) instead of FD4B. Yield: 79%. ¹H NMR (TMS, CDCl₃, 500 MHz): ppm δ = 3.96 (s, 3H), 7.14 (d, J = 8.5 Hz, 2H), 7.28–7.43 (m, 4H), 7.46–7.54 (m, 4H), 7.61 (d, J = 9.0 Hz, 2H), 7.66 (t, J = 7.5 Hz, 1H), 7.76 (t, J = 7.0 Hz, 1H), 7.81 (d, J = 8.0 Hz, 2H), 7.92 (dd, J = 7.0 Hz, 3H), 7.98 (d, J = 8.0 Hz, 1H), 8.71 (d, J = 8.5 Hz, 1H), 8.78 (d, J = 8.5 Hz, 1H), 8.93 (d, J = 8.0 Hz, 1H); ¹³C NMR (TMS, CDCl₃, 125 MHz): ppm δ = 160.50, 156.09, 153.45, 150.67, 136.55, 131.32, 130.18, 129.51, 129.31, 128.57, 128.50, 128.32, 127.30, 126.68, 126.32, 125.61, 124.88, 124.12, 123.23, 123.11, 122.85, 120.90, 120.69, 119.98, 115.38, 111.64, 55.70; HR-ESI-MS: [M + H]⁺ m/z calcd for C₄₀H₂₇N₂OS: 583.18386, found: 583.18331; elemental analysis calcd (%) for C₄₀H₂₆N₂OS: C 82.45, H 4.50, N 4.81, S 5.50; found: C 82.33, H 4.47, N 4.75, S 5.62.

4. Conclusion

In conclusion, we have presented in this work the use of blue-fluorescent PhImFD and PhImTD, containing the n-type imidazole moiety and p-type dibenzofuran or dibenzothiophene moiety, for EL devices. Through DFT investigations, the bipolar units exist in twisting D–A molecules connected by the means of a freely rotatable para-linked benzene ring. In addition, the unsymmetrical configuration adopted at the conjunction of the dibenzofuran and dibenzothiophene units at the C-4 positions could suppress strong π–π stacking and molecular interactions, which is attributed to be the cause of the observed high quantum efficiency and thermal stability. While both have similar optical properties and FMO locations, PhImTD was chosen for the effective utilization as a non-doped emitter, where we demonstrated a deep blue (CIE coordinates: (0.15, 0.10)) electroluminescence device that exhibited an EQE of 1.63%, PE of 0.82 lm W⁻¹ and CE of 1.34 cd m⁻¹. An appropriate bandgap endows blue devices with a low driving voltage of 3.6 V. Meanwhile, WOLED devices using PhImFD or PhImTD as the host and yellow PO-01 as the dopant were realized with a maximum CE of 8.12 cd A⁻¹ and CIE coordinates of (0.339, 0.330). This work indicated that compared to dibenzofuran as the electron-donating unit, the rational modulation of the dibenzothiophene group more strongly influences the energy level match to achieve materials with the desired optoelectronic properties.

Acknowledgements

The authors greatly appreciate the financial support of the Chinese National Programs for Scientific Instruments Research and Development (No. 2012YQ03007502), National Science Foundation of China (No. 11090330) and China Postdoctoral Science Foundation (No. 2015T80018).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13814k

‡ These authors contributed equally.

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