Fluorene–Triphenylamine-Based Bipolar Materials: Fluorescent Emitter and Host for Yellow Phosphorescent OLEDs

: In this study, two new bipolar materials were designed and synthesized: N 1 -(9,9-diphenyl-9 H -ﬂuoren-2-yl)- N 1 -(4,6-diphenylpyrimidin-2-yl)- N 4 , N 4 -diphenylbenzene-1,4-diam ine (FLU-TPA / PYR) and N 1 -(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)- N 1 -(9,9-diphenyl-9 H -ﬂuoren-2-yl)- N 4 , N 4 diphenylbenzene-1,4-diamine (FLU-TPA / TRZ). We fabricated two di ﬀ erent devices, namely a yellow phosphorescent organic light-emitting diode (PhOLED) and a non-doped ﬂuorescent OLED emitter with both FLU-TPA / PYR and FLU-TPA / TRZ. The FLU-TPA / PYR host-based yellow PhOLED device showed better maximum current, power and external quantum e ﬃ ciencies at 21.70 cd / A, 13.64 lm / W and 7.75%, respectively. The observed e ﬃ ciencies were better than those of the triazine-based FLU-TPA / TRZ. The non-doped ﬂuorescent device with the triazine-based FLU-TPA / TRZ material demonstrated current, power and external quantum e ﬃ ciencies of 10.30 cd / A, 6.47 lm / W and 3.57%, respectively.


Introduction
Organic light-emitting diodes (OLEDs) have become an interesting component in research and commercial markets. OLEDs show prominent advantages over traditional displays, namely their high contrast, high brightness, wide view angle, lack of back light requirements, light weight and thin films. OLEDs continue to be used for next-generation displays in mobile phones, televisions and other lighting resources, where they help to reduce the energy consumption [1][2][3][4][5][6].
There are three types of host materials that have been reported, namely hole transport (HT) type, electron transport (ET) type, and bipolar host materials.
Bipolar host materials have received much attention from research communities due to their bipolar nature. Bipolar host materials contain electron-donating and electron-accepting units in a single molecule.

Instrumentation
1 H and 13 C NMR analyses were performed with a JEON JNM-ECP FT-NMR spectrometer (Peabody, MA, USA) operating at 500 MHz. The UV-Vis absorbance property was analyzed with a Lambda 1050 ultraviolet visible (UV-VIS) spectrophotometer (Perkin Elmer, Waltham, MA, USA). The energy of the band gap (Eg) was obtained from the onset wavelength of the UV-Vis absorbance spectra. Photoluminescence (PL) spectra were measured with an HR800 spectrofluorometer (Horiba Jobin Yvon, Paris, France). Mass spectrometry analysis was carried out with a Xevo TQ-S spectrometer (Waters, Milford, MA, USA). An elemental analysis (EA) was performed with a ThermoFisher (Flash 2000) elemental analyzer (Loughborough, England). Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were recorded with a PerkinElmer DSC 4000 and TGA 8000 system (Melville, NY, USA) under a nitrogen atmosphere with a heating rate of 10 • C/min. The triplet energy level (ET) was estimated from the onset wavelength of the emission spectra at 77 K in toluene. The HOMO (highest occupied molecular orbital) value was determined by a AC-2 method with a photoelectron spectrometer (RIKEN, Saitama, Japan). The LUMO (lowest unoccupied molecular orbital) energy was calculated by adding the band gap energy to the obtained HOMO energy. OLED devices were constructed with a thermal evaporating system under a pressure of 5 × 10 −7 torr (Sunicel plus, Seoul, Korea). Current density-voltage-luminescence (J-V-L) performances were observed by an OLED I-V-L test system (Polarmix M6100, Suwon, Korea). The electroluminescence (EL) spectra were recorded with a spectroradiometer (Konica Minolta CS-2000, Tokyo, Japan).

Synthesis of 2-chloro-4,6-diphenylpyrimidine (6)
A mixture of 2,4,6-trichloropyrimidine (4, 0.63 mL, 5.45 mmol), phenylboronic acid (5, 1.39 g, 11.44 mmol), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh 3 P) 4 , 0.18 g, 0.16 mmol), 40 mL of a 2 M aqueous K 2 CO 3 solution, 40 mL of water, 25 mL of ethanol, and 80 mL of toluene was added into a two-neck round-bottom flask that was equipped with a condenser, and this mixture was refluxed at 110 • C for 10 h under a nitrogen atmosphere. After the completion of the reaction, the mixture was made up by using dichloromethane (100 mL) and water (80 mL). The organic layer was dried over anhydrous magnesium sulfate and then concentrated. The crude residues were separated by using a silica column and an n-hexane:dichloromethane (4:1) solvent system to obtain the intermediate (6)

OLED Device Fabrication
To construct the OLED devices, indium-tin-oxide (ITO)-coated glass substrates were washed in an ultrasonic bath with isopropyl alcohol and deionized water for 20 min. The cleaned substrates were subjected to a UV-ozone treatment for about 6 min. All the organic layers and the metal cathode were fabricated on the pre-washed ITO-coated glass substrate. The deposition rate of~5 × 10 −7 Torr pressure was applied with a Sunic organic evaporator (Suwon, Korea). All deposition processes were conducted inside a glove box under inert conditions. Each device size area was fabricated as 2 mm 2 .

Results and Discussions
The thermal properties of FLU-TPA/PYR and FLU-TPA/TRZ were studied with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. The glass transition temperatures of FLU-TPA/PYR and FLU-TPA/TRZ were 153 and 147 • C, respectively. The thermal decompositions of our FLU-TPA/PYR and FLU-TPA/TRZ were recorded at approximately 426 and 478 • C, respectively, with a 5% weight reduction. Both of our materials exhibited excellent thermal stabilities, which could improve the morphological stabilities during device operation. Thermal properties are shown in Figure 1 and summarized in Table 1.

OLED Device Fabrication
To construct the OLED devices, indium-tin-oxide (ITO)-coated glass substrates were washed in an ultrasonic bath with isopropyl alcohol and deionized water for 20 min. The cleaned substrates were subjected to a UV-ozone treatment for about 6 min. All the organic layers and the metal cathode were fabricated on the pre-washed ITO-coated glass substrate. The deposition rate of ~5 × 10 −7 Torr pressure was applied with a Sunic organic evaporator (Suwon, Korea). All deposition processes were conducted inside a glove box under inert conditions. Each device size area was fabricated as 2 mm 2 .

Results and Discussions
The thermal properties of FLU-TPA/PYR and FLU-TPA/TRZ were studied with thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under a nitrogen atmosphere. The glass transition temperatures of FLU-TPA/PYR and FLU-TPA/TRZ were 153 and 147 °C, respectively. The thermal decompositions of our FLU-TPA/PYR and FLU-TPA/TRZ were recorded at approximately 426 and 478 °C, respectively, with a 5% weight reduction. Both of our materials exhibited excellent thermal stabilities, which could improve the morphological stabilities during device operation. Thermal properties are shown in Figure 1 and summarized in Table 1.   The UV-Vis absorption and photoluminescent (PL) spectra of FLU-TPA/PYR and FLU-TPA/TRZ are depicted in Figure 2. FLU-TPA/PYR and FLU-TPA/TRZ showed similar absorption at 410 and 412 nm, respectively. The band gap energy values of FLU-TPA/PYR and FLU-TPA/TRZ were recorded at 2.41 and 2.49 eV, and they were associated with onset absorption wavelengths of 515 and 498 nm, respectively. FLU-TPA/PYR showed an extended absorption when compared to FLU-TPA/TRZ. The photoluminescent spectra's maxima were found at 548 and 606 nm, respectively, in the toluene solvent for FLU-TPA/PYR and FLU-TPA/TRZ. Triazine acceptor-based FLU-TPA/TRZ expressed a longer emission peak at room temperature. The triplet energies of FLU-TPA/PYR and FLU-TPA/TRZ were obtained as 2.57 and 2.77 eV, respectively, and this is highly important for a host material to prevent energy flow back from the dopant material. From this information, we believed that our materials would reveal good efficiency enhancement with yellow phosphorescent OLEDs. The HOMO (highest occupied molecular orbital) energy ere were calculated, and they were found to be −5.27 and 5.35 eV for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The LUMO (lowest unoccupied molecular orbital) energy values were obtained by adding the bang gap energy to the HOMO energy, and they were −2.86 eV for both FLU-TPA/PYR and FLU-TPA/TRZ. The frontier molecular orbital energies (FMOs) facilitated an effective charge transportation from both the electrodes. To understand the molecular orbital distribution, we used basic density functional theory (DFT) with a basic set of 6-31 G while using a Gaussian 09. The frontier molecular orbital distributions are displayed in Figure 3. The LUMO distribution was noticed over the electron-withdrawing triazine and pyrimidine moieties, while the HOMO distribution was noticed over the electron-donating triphenylamine unit, but the HOMO distribution showed considerable overlapping.   The HOMO (highest occupied molecular orbital) energy ere were calculated, and they were found to be −5.27 and 5.35 eV for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The LUMO (lowest unoccupied molecular orbital) energy values were obtained by adding the bang gap energy to the HOMO energy, and they were −2.86 eV for both FLU-TPA/PYR and FLU-TPA/TRZ. The frontier molecular orbital energies (FMOs) facilitated an effective charge transportation from both the electrodes. To understand the molecular orbital distribution, we used basic density functional theory (DFT) with a basic set of 6-31 G while using a Gaussian 09. The frontier molecular orbital distributions are displayed in Figure 3. The LUMO distribution was noticed over the electron-withdrawing triazine and pyrimidine moieties, while the HOMO distribution was noticed over the electron-donating triphenylamine unit, but the HOMO distribution showed considerable overlapping. The HOMO (highest occupied molecular orbital) energy ere were calculated, and they were found to be −5.27 and 5.35 eV for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The LUMO (lowest unoccupied molecular orbital) energy values were obtained by adding the bang gap energy to the HOMO energy, and they were −2.86 eV for both FLU-TPA/PYR and FLU-TPA/TRZ. The frontier molecular orbital energies (FMOs) facilitated an effective charge transportation from both the electrodes. To understand the molecular orbital distribution, we used basic density functional theory (DFT) with a basic set of 6-31 G while using a Gaussian 09. The frontier molecular orbital distributions are displayed in Figure 3. The LUMO distribution was noticed over the electron-withdrawing triazine and pyrimidine moieties, while the HOMO distribution was noticed over the electron-donating triphenylamine unit, but the HOMO distribution showed considerable overlapping.      The current density-voltage, current density-power, and current efficiencies are depicted in Figure 5 and summarized in Table 2. The turn-on voltage of the FLU-TPA/PYR and FLU-TPA/TRZ-based devices was 5 V. The current efficiencies of the FLU-TPA/PYR and FLU-TPA/TRZ-based devices were 21.70 and 18.72 cd/A, respectively. Consequently, the power efficiencies was observed as 13.64 and 11.76 lm/W for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The external quantum efficiency (7.75%) of the FLU-TPA/PYR-based device was higher than that of the triazine-based FLU-TPA/TRZ device (6.44%). Though both devices exhibited a similar turn-on voltage, the device efficiencies of the pyrimidine-based FLU-TPA/PYR were considerably higher than those of FLU-TPA/TRZ. Both the devices showed lower efficiency roll-off, which is one of the important factors for longer operation devices. The electro luminescent (EL) spectra of both devices are depicted in Figure 6. The maximum emission of both yellow devices was similar at 562 nm, which substantiated the yellow emission from the yellow dopant. and cathode, respectively, HATCN was used for the hole-injecting layer (HIL), Liq was used for the electron-injecting layer (EIL), TAPC was used as the hole-transporting layer (HTL), and TmPyPB was employed as the electron transport layer (ETL). The current density-voltage, current density-power, and current efficiencies are depicted in Figure 5 and summarized in Table 2. The turn-on voltage of the FLU-TPA/PYR and FLU-TPA/TRZbased devices was 5 V. The current efficiencies of the FLU-TPA/PYR and FLU-TPA/TRZ-based devices were 21.70 and 18.72 cd/A, respectively. Consequently, the power efficiencies was observed as 13.64 and 11.76 lm/W for FLU-TPA/PYR and FLU-TPA/TRZ, respectively. The external quantum efficiency (7.75%) of the FLU-TPA/PYR-based device was higher than that of the triazine-based FLU-TPA/TRZ device (6.44%). Though both devices exhibited a similar turn-on voltage, the device efficiencies of the pyrimidine-based FLU-TPA/PYR were considerably higher than those of FLU-TPA/TRZ. Both the devices showed lower efficiency roll-off, which is one of the important factors for longer operation devices. The electro luminescent (EL) spectra of both devices are depicted in Figure  6. The maximum emission of both yellow devices was similar at 562 nm, which substantiated the yellow emission from the yellow dopant.       Moreover, we fabricated non-doped fluorescence devices to understand the performances of our molecules as emitters (Figure 7). The non-doped device structure was as follows: indium-tin oxide (ITO) (150 nm)/1, 4,5,8,9,11-     The non-doped device performances of the triazine-based FLU-TPA/TRZ was much better than that of the pyridine-based FLU-TPA/PYR. The current, power and external quantum efficiencies of the FLU-TPA/TRZ-based device were 10.30 cd/A, 6.47 lm/W and 3.57%, respectively ( Figure 8 and Table 2). The maximum EL emission was observed at 549 nm, which was related to the CIE color coordinates (x, y) of 0.41, 0.55 with the emission of green color (Figure 9).
The non-doped device performances of the triazine-based FLU-TPA/TRZ was much better than that of the pyridine-based FLU-TPA/PYR. The current, power and external quantum efficiencies of the FLU-TPA/TRZ-based device were 10.30 cd/A, 6.47 lm/W and 3.57%, respectively ( Figure 8 and Table 2). The maximum EL emission was observed at 549 nm, which was related to the CIE color coordinates (x, y) of 0.41, 0.55 with the emission of green color (Figure 9).

Conclusions
Two bipolar materials were synthesized with an N 1 -(9,9-diphenyl-9H-fluoren-2-yl)-N 4 ,N 4diphenylbenzene-1,4-diamine donor and a triazine or pyrimidine acceptor. The bipolar materials FLU-TPA/PYR and FLU-TPA/TRZ were applied in yellow phosphorescent and non-doped fluorescent OLEDs. The yellow device with FLU-TPA/PYR showed good device characteristics when compared to the triazine-based FLU-TPA/TRZ. The current and power efficiencies of the FLU-TPA/PYR-based yellow device were measured at 21.70 cd/A and 13.64 lm/W, respectively. However, the FLU-TPA/TRZ-based non-doped fluorescent device revealed excellent device properties compared to the pyrimidine-based FLU-TPA/PYR. All devices exhibited a lower efficiency roll-off, and our new molecular design will be helpful to develop thermally stable host materials in the future.

Conclusions
Two bipolar materials were synthesized with an N 1 -(9,9-diphenyl-9H-fluoren-2-yl)-N 4 ,N 4 -dipheny lbenzene-1,4-diamine donor and a triazine or pyrimidine acceptor. The bipolar materials FLU-TPA/PYR and FLU-TPA/TRZ were applied in yellow phosphorescent and non-doped fluorescent OLEDs. The yellow device with FLU-TPA/PYR showed good device characteristics when compared to the triazine-based FLU-TPA/TRZ. The current and power efficiencies of the FLU-TPA/PYR-based yellow device were measured at 21.70 cd/A and 13.64 lm/W, respectively. However, the FLU-TPA/TRZ-based non-doped fluorescent device revealed excellent device properties compared to the pyrimidine-based FLU-TPA/PYR. All devices exhibited a lower efficiency roll-off, and our new molecular design will be helpful to develop thermally stable host materials in the future.