Pyrene Replace The Design, Synthesis And Optical And Electrical Properties Of Fluorene Derivatives

Research into blue-light-emitting materials remains one of the major challenges in the development of organic light-emitting devices because it is much more difficult to produce blue emission due to their intrinsic characteristic of having a wide bandgap irrespective of the type of materials. In this dissertation, after combining the high stability of C9 diaryl substituted fluorene derivatives with the high carrier mobility and hole-injection ability of pyrene, a series of novel blue-light-emitting materials were designed and synthesized. Then their physico-chemical and optoelectronic properties were investigated. The dissertation is divided into the following parts.1. Three novel conjugated molecules (PTF1, PTF2, and PTF3) (Figures 2.2 and 2.3), with pyrene substitution at the C9 of the fluorene moieties, were designed and synthesized by Suzuki coupling reaction. The chemical structures of the three compounds were confirmed by ¹HNMR, ¹³CNMR, MALDI-TOF-MS, and elemental analysis. Thermal analyses indicated that PTF1 and PTF2 were not stable enough in nitrogen because of steric hindrance, with onset decomposition temperature at ²00 °C, while PTF3 has highly thermal stability and high glass transition temperature (T_g). Although the absorption spectra indicated some additional peaks resulting from pyrene at C9 of fluorene, the photoluminescence (PL) spectra and PL efficiency of the materials were similar to that of ter(9,9-diphenylfluorene)(TDPF). Cyclic voltammetry (CV) illustrated that the three materials have higher HOMO energy levels than TDPF, implying that they have better hole-injection and hole-transporting properties. A typical semiempirical quantum chemical calculation methodology, Austin Modell (AM1), was employed to simulate the HOMO and LUMO of PTF3, which also showed that the pyrene at C9 of fluorene has significant contribution to the formation of molecular orbitals. The preliminary results of OLEDs which was evidenced by a three-layer OLED with the configuration of ITO/TCTA(8nm)/PTF3(30nm)/BCP(40nm)/Mg:Ag showed a pure blue emission with turn-on voltage of 5 V and maximum luminance of 2012 cd/m2.2. C9 pyrene substituted terfluorenes with alkyl groups were designed and synthesized by Suzuki coupling reaction (ATF1 and ATF2) (Figures 3.1 and 3.3). TGA showed that the decomposition temperature of ATF1 is only 200 °C, while ATF2 has highly thermal stability with T_g of 155 °G The electroluminescence (EL) characterization showed that the instability of ATF1 was originated from thedecomposition of methyl groups at C9 of fluorene, which reduced the performance of OLED and resulted in the instability of the EL spectra. Because of high thermal stability and the increasing viscosity resulting from the long-chain alkyl groups, EL devices of ATF2 can be prepared both by vacuum deposition and by solution spin-coating.3. Pyrene was incorporated onto the C2 and C7 of the fluorene moieties to obtain the monosubstituted derivatives of pyrene (MP1, MP2 and MP3) (Figures 4.1 and 4.2), and their chemical structures were fully characterized. MP1 and MP2 exhibited high thermal stability, while MP3 is not stable enough because of steric hindrance. The three materials exhibited stable bright blue emission in the solid state. The high HOMO (about -5.3 eV) indicated that they have improved hole-injection/transporting ability. Semiempirical AM1 was employed to simulate the HOMO and LUMO of MP1 and MP2. Both the simulation results and the absorption spectra indicated the important contribution of pyrene at C9 of fluorene moieties to molecular orbitals. The electroluminescent properties of MP1 and MP2 were preliminarily characterized. A three-layer blue OLED of ITO/TCTA (8 nm)/MP2 (30 nm) /BCP (40 nm)/Mg:Ag was fabricated with high efficiency (3.08 cd/A, 1.17 lm/W), low turn-on voltage (3.5 V), and high brightness (19885 cd/m²) in ambient air, which is comparable with the finest doped and nondoped blue OLEDs. These results revealed that these materials are promising as blue emitters for high efficiency OLEDs with a much simpler architecture. We thus exemplified that introducing 9-phenyl-9-pyrenyl fluorene onto large aromatic rings is a new methodology to employ fluorescent dyes with large planar aromatic rings as the blue emitters in nondoped OLEDs without hole-injection layers.4. The compounds EHOP1 and EHOP2 were designed and synthesized by incorporating the long chain alkyloxy groups onto the monosubstitution derivatives of pyrene, and their chemical structures were fully characterized. Thermal analyses exhibited that the two materials are with highly amorphous stability, and no phase transition was observed from room temperature to 300°C. The two materials decomposed at temperature higher than 430°C. They also exhibited stable solid blue emission and improved hole-injection/transporting ability. A blue OLED of ITO/TCTA (8 nm)/EHOP1 (30 nm)/BCP (40 nm)/Mg:Ag was fabricated without the hole-injection layer, with high brightness (8126 cd/m²), low turn-on voltage (4.5 V), and high efficiency (2.13 cd/A). Nevertheless, the device still demonstrated areasonably high efficiency of 1.21 cd/A when the luminescence reached its maximum.5. After the dibromination of pyrene, the 9-phenyl-9-pyrenylfluorene group was introduced onto the pyrene by Suzuki coupling reaction to obtain the disubstituted derivatives of pyrene (DP1) (Figure 6.1), and the 9,9-diphenylfluorene disubstituted pyrene (DP2) (Figure 6.1) was also synthesized to compare the special effect of pyrene groups at C9 of the fluorene moieties. The absorption spectra of the two compounds showed obvious difference. CV indicated that the HOMO of DP1 is higher than that of DP2, which is resulted from the special effects of the pyrenes at C9 of the fluorene moieties, and the PL spectra of the two materials showed bright blue emission. OLED with the same structure were fabricated under the same condition, and the EL results were discussed. Due to the special effect of the pyrenes at C9 of fluorene moieties, primary DC current-voltage analyses showed that the carrier mobility of DP1 is higher than that of DP2. A three-layer OLED with the configuration of ITO/TCTA(8 nm)/DP1(30 nm)/BCP(40 nm)/Mg:Ag exhibited a bright blue emission, with turn-on voltage of 5 V, maximum luminance of 6322 cd/m², and maximum current efficiency of 1.17 cd/A.6. 2,7-Dibromo-9-phenyl-9-pyrenylfluorene was polymerized to obtain polyfluorene derivatives (PDOPPF and PDAOPF-PPF) (Figure 7.2), and dibromopyrene was polymerized to obtain fluorene-pyrene copolymers (PDOFP and PDAOPF-P) (Figure 7.3). Thermal analysis showed that all of the four polymers have high thermal stability with T_g higher than 200 °C. The absorption spectra of PDOPPF and PDAOPF-PPF showed fine peaks resulting from pyrenes at C9 of the fluorene moiety and PDOFP and PDAOPF-P showed red-shift compared to polyflurenes. CV indicated that the HOMOs of PDOPPF and PDAOPF-PPF are higher than those of PDOFP and PDAOPF-P. The EL performance of the four polymers was tested and the device results were discussed. The performance of the EL devices using PDOFP and PDAOPF-P as emitters was better than the performance of devices using PDOPPF and PDAOPF-P as emitters. Polymer EL device of ITO/PEDOT:PSS(40 nm)/PDOFP (80 nm)/Ba(5 nm)/Al(130 nm) exhibited blue-green emission, with turn-on voltage of 5.5 V, and maximum luminance of 1081 cd/m² when the driving voltage reached 8 V.