Design, Synthesis And Characterization Of Condensed Aromatic Compounds As Blue Electroluminescent Materials

Organic light emitting diodes (OLEDs) have potential use in applications of the next generation displays and lighting sources. Their advantage is seen clearly with the development of science and technology in electroluminescent materials. Compared with the liquid crystal displays (LCDs), virtues of a higher contrast, a sharper response, a lower driving voltage, smaller energy consumption, thinner thickness, lighter weight, a larger view angle, and etc. are bestowed upon OLEDs. Although some OLED products have been put into commerce, they are not mature enough to meet the requirement of the market and there is still much room for improvement. A lot of money and time are invested in the field of OLEDs to gain competitive advantage in the future display market.This paper mainly focuses on the design, synthesis and characterization of blue electroluminescent materials. Detailed contents are listed as follows.The first chapter is related to the basic knowledge of OLEDs. The history of OLEDs, emission mechanism, important parameters and some functional materials are included in this part.In the second chapter, the experimental methods and chemical reagents are written comprehensively.In the third chapter,7-hexyl-7H-phenanthro[3,4-c]carbazole (CPR) was synthesized. CPR was obtained using the photocylization as the main reaction step in a short time (ca 10 min) and high productivity (72%). The crystal structure, thermal property, electrochemical property, photoluminescent property, theoretical calculation and electroluminescent property were investigated thoroughly. CPR crystalized into space group P11/C The molecule possessed a helical structure. The non-planar molecule structure was beneficial for not only stable amorphous state in thin-film but also relieving concentration quenching in solid state. The thermal measurement showed the melting and decomposition points were 130.9 and 297 ℃, respectively. CPR underwent quasi-reversible process in the electrochemical measurement. The HOMO and LUMO energy level were -5.19 and -2.27 eV. In different solvents (toluene, dichloromethane, THF, trichloromethane, acetonitrile and methanol), the UV-Vis absorption peaks varied between 315 and 320 nm and the photoluminescence peaks varied between 425 and 428 nm. The fluorescence quantum yields in toluene and film state were 19% and 4%, respectively. Theoretical calculation showed the maximum absorption peak was attributed to the HOMOâ†'LUMO+1 transition. The molecule dipole moment in ground state was merely 2.57 Debye, which is responsible for stability of deep-blue OLEDs. The OLED device configuration:ITO/NPB (50 nm)/CPR (30 nm)/Bphen (20 nm)/Mg:Ag=10:1(150 nm)/Ag (50 nm), ITO/NPB (50 nm)/CPR:CBP=1:10 (30 nm)/Bphen (20 nm)/Mg:Ag=10:1 (150 nm)/Ag (50 nm). Device with CPR as the host emitter obtained the turn-on voltage of 5.4 V, maximum current efficiency of 0.42 cd A-1, maximum power efficiency of 0.11 lm W-1, maximum external quantum yield of 0.51%, EL peak of 444 nm and Commission International de l’Eclairage (CIE) coordinates of (0.15,0.09). In contrast, device with CPR as the guest emitter achieved the turn-on voltage of 5.1 V, maximum current efficiency of 0.42 cd A-1, maximum power efficiency of 0.17 lm W-1, maximum external quantum yield of 0.76%, EL peak of 462 nm, CIE coordinate of (0.15,0.10).In the fourth chapter,7-hexyl-7H-carbazolo[3,4-k]phenanthridine (CPD-1) and 14-hexyl-14H-carbazolo[3,2-k]phenanthridine (CPD-2) were synthesized. CPD-1 and CPD-2 were isomers. The helicene, CPD-1 crystalized into a space group of C2/c. CPD-2 together with water crystalized into orthorhombic system, Pna21space group. The molecule structure of CPD-2 was planar relative to that of CPD-1. The melting and decomposition points of CPD-1 were 110.6 and 304.1 ℃ respectively, while those of CPD-2 were 144.3 and 321.4 ℃. The HOMO and LUMO energy levels were-5.21 and -2.30 eV for CPD-1 and those of-5.19 and -2.43 eV for CPD-2. The UV-Vis absorption spectra of both compounds were almost same in different solvents. With the solvents polarity increasing from toluene to methanol. the photoluminescence (PL) peaks of CPD-1 changed from 437 to 462 nm, the PL peaks of CPD-2 from 446 to 473 nm. When CPD-1 as a guest emitter was doped in CBP with the weight concentration of 10%, the device performance attained the turn-on voltage of 6 V, maximum current efficiency of 0.28 cd A-1, maximum power efficiency of 0.10 lm W-1,EL peak of 442 nm and CIE coordinate of (0.17,0.10). When CPD-2 as a guest emitter was doped in the host emitter of CBP with weight concentration of 2%, the OLED presented the performance of the turn-on voltage of 5.9 V, maximum current efficiency of 0.33 cd A-1, maximum power efficiency of 0.08 lm W-1, PL peak at 450 nm, CIE coordinate (0.16,0.10).