Synthesis and electroluminescence property of new hexaphenylbenzene derivatives including amine group for blue emitters

Three new blue-emitting compounds of 5P-VA, 5P-VTPA, and 5P-DVTPA for organic light-emitting diode (OLED) based on hexaphenylbenzene moiety were demonstrated. Physical properties by the change of the substitution groups of the synthesized materials were systematically examined. Photoluminescence spectrum of the synthesized materials showed maximum emitting wavelengths of about 400 to 447 nm in solution state and 451 to 461 nm in film state, indicating deep blue emission color. OLED devices were fabricated by the synthesized compounds using vacuum deposit process as an emitting layer. The device structure was ITO/2-TNATA 60 nm/ NPB 15 nm/ EML 35 nm/ TPBi 20 nm/ LiF 1 nm/ Al 200 nm. External quantum efficiencies and CIE values of 5P-VA, 5P-VTPA, and 5P-DVTPA were 1.89%, 3.59%, 3.34%, and (0.154, 0.196), (0.150, 0.076), (0.148, 0.120), respectively. 5P-VTPA and 5P-DVTPA exhibited superior highly blue quality and thermal property such as high Td of 448°C and 449°C.


Background
Intensive studies have been conducted on organic lightemitting diodes (OLEDs) as they have a great potential to be applied to large full-color displays and mobile displays [1][2][3]. Most of the conjugated organic molecules have been reported as red, green, and blue electroluminescence (EL) [4]. It is required for those red, green, and blue emitters to show high EL efficiency, good thermal properties, long lifetime, and pure color coordinates (1931 Commission Internationale de l'Eclairage (CIE)) in order to be applied to large full-color displays. A red light-fluorescence emitter with CIE coordinates of (0.66, 0.34) and a long lifetime of more than 600,000 h at 24 cd/A has recently been developed. A green lightfluorescence emitter with CIE coordinates of (0.34, 0.62) and a lifetime of 400,000 h at 78 cd/A has also been achieved [5]. However, the best official results for a bluelight emitter are a short lifetime of only 10,000 h at 9.0 cd/A and CIE coordinates of (0.14, 0.12) with fluorescence materials [6]. Thus, the development of a blue emitter with high color purity, high efficiency, and a long lifetime is an extremely challenging research topic. Most existing studies of blue emitters use molecules with excellent fluorescence characteristics such as anthracene [7,8] and pyrene [9,10] as core or side moieties. Many studies have investigated the use of anthracene and pyrene as blue core moiety since they have high photoluminescence (PL) and EL efficiencies. However, these molecules can easily form excimers through packing because anthracene and pyrene have flat molecular structure that reduce EL efficiency and degrade color purity [11].
In this work, new blue-emitting compounds based on hexaphenylbenzene group are designed and synthesized as shown in Figure 1. Aromatic amine moiety as a side group was introduced into main core structure in order to prevent intermolecular interaction and improve hole mobility [12]. Also, the change of emission wavelength as well as device efficiency was evaluated according to the different side group.

Methods
Reagents and solvents were purchased as reagent grade and used without further purification. All reactions were performed using dry glassware under nitrogen atmosphere. Analytical TLC was carried out on Merck 60 F254 silica gel plate, and column chromatography was performed on Merck 60 silica gel (230 to 400 mesh) (Merck & Co., Inc., Whitehouse Station, NJ, USA). Melting points were determined on an Electrothemal IA 9000 series melting point apparatus (Bibby Scientific Limited, Stone, Staffordshire, UK) and are uncorrected. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Mercury-400 (400 MHz) spectrometer (Varian Medical Systems Inc., Palo Alto, CA) with TMS peak as reference. The optical absorption spectra were obtained by HP 8453 UVvis-NIR spectrometer (HP Company, Palo Alto, CA, USA). Thermal properties of the compounds were measured by thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) using a SDT2960 and DSC2910 (TA Instruments, New Castle, DE, USA). Voyager-DE-STR, elemental analysis was performed with a PerkinElmer 2400 analyzer (PerkinElmer, Waltham, MA, USA). PerkinElmer luminescence spectrometer LS50 (Xenon flash tube) was used for PL spectroscopy. Surface analyzer AC-2 (RIKEN KEIKI, Itabashi-ku, Tokyo, Japan) was used for work function measurement. EL devices were fabricated as the following structure: ITO/ 2-TNATA 60 nm/ NPB 15 nm/ EML 35 nm/ TPBi 20 nm/ LiF 1 nm/ Al 200 nm, where 4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine (2-TNATA) was used as a hole injection layer, N,N'-bis(naphthalene-1-ly)-N,N'-bis(phenyl) benzidine (NPB) as a hole transporting layer, the synthesized materials as emitting layer (EML), 1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl (TPBi) as an electron transporting layer and hole blocking layer, lithium fluoride (LiF) as an electron injection layer, ITO as anode, and Al as cathode. The organic layer was vacuum deposited by thermal evaporation at a vacuum base pressure of 10 −6 Torr and the rate of deposition being 1 Å/S to give an emitting area of 4 mm 2 , and the Al layer was continuously deposited under the same vacuum condition. The currentvoltage-luminance (I-V-L) characteristics of the fabricated EL devices were obtained using a Keithley 2400 electrometer (Keithley Instruments Inc, Solon, OH, USA), and light intensity was obtained using Minolta CS 1000A (Minolta Co., Ltd., Chuo-ku, Osaka, Japan).
Synthesis of hexaphenylbenzene-based compounds 1, 2, and 3 The most straight-forward preparation of compounds 1, 2, and 3 can be envisaged to proceed through a reaction sequence of the following steps, as depicted in Figure 2. Every step of the reaction sequence proceeded smoothly and efficiently to give a good or moderate yield of the product (see the experimental section for the synthetic details). Commercially available 4-iodotoluene (4) was reacted with phenylacetylene (5) through Sonogashira coupling [13][14][15] to give 6 in 92.5% yield, and then, the subsequent cyclization with tetraphenylcyclopentadie none through Diels-Alder reaction [16] was carried out to give compound 8 in 78.6%. Compound 8 was brominated and phosphonated to produce compound 10 in 74.0%. Typical Wittig-type reactions of aldehydes 12 and 13 with 10 and 11 gave 1 and 2 in 40.0% and 36.0% yield, respectively. It is noticeable that phosphonation yield of bromocompound 9 was 10% higher than the ylid  formation of 9, and the subsequent Wittig-Horner-Emmons reaction of aldehyde 12 with the phosphonate 10 gave a better yield than the reaction of aldehyde 12 with the ylid 11 (Wittig reaction). On the other hand, the reaction of aldehyde 13 with ylid 11 produced a better yield than the reaction of 13 with 10 for the synthesis of 2. Even although we have not optimized the above both reactions, we had to choose the Wittig-Horner-Emmons-type reaction for 1 and Wittig reaction for 2 after several trials. Accordingly, analogously prepared hexaphenylbenzene-based diphosphonate 18 reacted with aldehyde 12 to produce 3 in 32.0% yield.
Compounds 1, 2, and 3 and their precursor compounds are very soluble in aromatic solvents (i.e., toluene, odichlorobenzene, and benzonitrile) and other common organic solvents (i.e., acetone, CH 2 Cl 2 , CHCl 3 , and THF). The structure and purity of the newly synthesized compounds were confirmed mainly by 1 H NMR and elemental analysis. 1 H NMR spectra of 1, 2, and 3 are consistent with the proposed structures, showing the expected features with the correct integration ratios, respectively. The matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) mass spectra provided a direct evidence for the structures of 1, 2, and 3, showing a singly charged molecular ion peaks at m/z = 803.38 for 1, m/z = 679.35 for 2, and m/z = 1,073.24 for 3, respectively.

Results and discussion
The optical properties of synthesized compounds were summarized in Table 1 and Figures 3 and 4. Figure 3 shows ultraviolet-visible (UV-vis) absorption and PL spectrum data in solution state. Figure 4 exhibits spectrum data in film state. In the solution case, the solvent was used with chloroform (1 × 10 −5 M), and 50 nm in thickness was chosen for evaporated film on glass. In Figure 3 and Table 1 As shown in Figure 4 and Table 1, 5P-VA film showed maximum absorption value at 363 nm as well as PL max value at 460 nm. In the 5P-VTPA and 5P-DVTPA cases, two compounds showed similar absorption maximum values at 376 and 373 nm, but PL max values were slightly different as 451 and 461 nm.
In the case of 5P-VA, PL max value was large red-shifted from 400 to 460 nm according to film state. It means that intermolecular distance in film state was closed, and intermolecular π-  π* interaction was increased because of no bulky side group. However, 5P-VTPA and 5P-DVTPA having bulky side group of aromatic amine moiety had slightly red-shifted with 5 to 15 nm in film state.
5P-VTPA including diphenyl amine group in solution state showed large red shift of 46 nm in emission wavelength compared to 5P-VA having only alkyl amine and dimethyl amine (see Table 1).
DSC and TGA analyses to determine the thermal properties of the synthesized molecules were carried out (see Table 1). High T g and T d values indicate that the morphology of the material will not easily be changed by the high temperatures generated during the operation of OLED devices and are closely correlated with long OLED device life-times [17,18]. Two compounds showed high T g of 108°C and 110°C and high T d of 448°C and 449°C. Comparing on T m and T d of three compounds, two compounds having prevented molecular packing had the slightly decreased T m and the increased T g and T d . The increased T g and T d can be interpreted by the increased molecular weight.
Energy levels of three synthesized compounds such as HOMO, LUMO, and bandgap were estimated by ultraviolet photon spectroscopy of AC-2 and optical absorption spectroscopy (see Table 2). 5P-VA had HOMO and bandgap values of −5.50 and 2.99 eV, respectively. 5P-VTPA and 5P-DVTPA showed HOMO values of −5.65 and −5.60 eV and bandgap values of 2.95 and 2.89 eV, respectively. Bandgap was decreased and emission wavelength was red-shifted according to the change from alkyl amine side group to aromatic amine side group.
OLED devices of the three compounds as an EML were fabricated as ITO/2-TNATA 60 nm/NPB 15 nm/ EML 35 nm/TPBi 20 nm/LiF 1 nm/Al 200 nm. All organic films were prepared by evaporation under high vacuum of 10 −6 Torr. Figure 5 shows I-V-L characteristics of the three devices. It exhibits the current density and luminance according to the applied voltage. I-V-L curves of the three compounds showed typical diode characteristics, but 5P-VTPA and 5P-DVTPA devices had the relatively smaller operating voltage compared to that of 5P-VA. The related efficiency data were also summarized in Table 2. Comparing external quantum efficiency (EQE) of the three devices, two compounds of 5P-VTPA and 5P-DVTPA had relatively 1.7 to 1.9 times higher efficiency than 5P-VA. Two devices of 5P-VTPA and 5P-DVTPA had low operating voltage such as 7.31 and 7.87 V although 5P-VA had lower energy barrier of HOMO level between NPB and EML because of small value of −5.50 eV. Low operating voltage might be explained by faster mobility of 5P-VTPA and 5P-DVTPA compared to 5P-VA, and it caused the increased efficiency. EL maximum values were shifted to deep blue, and CIE values showed excellent pure blue color y values of 0.076 and 0.120. Thus, aromatic amine side group prevented the packing of molecular structure, and it caused the improved blue color and EQE value. TV application is asking less than 0.08 y value for cold white OLED device, but it is extremely difficult to achieve that value. The normalized EL spectra of the three compound devices were shown in Figure 6.

Conclusion
We demonstrated new blue fluorescence compounds based on hexaphenyl benzene derivatives. Those chemical structures can be varied by side groups of aliphatic and aromatic amine moiety. Three model compounds were designed and synthesized. Those were applied to OLED device as an EML, and the related properties were evaluated. Aromatic amine side groups can improve EL property such as color purity and operating voltage as well as EQE. 5P-VTPA, and 5P-DVTPA showed excellent CIE values of (0.150, 0.076), (0.148, 0.120) as a deep blue color. Especially, CIE value of 5P-VTPA can be applied to OLED TV application because of highly pure blue color. Also, 5P-VTPA and 5P-DVTPA exhibited superior thermal property such as high T d of 448°C and 449°C.