Experimental And Theoretical Investigations Of Novel Blue-Light Materials For Organic Electroluminescent Devices

The existing organic electroluminescent (EL) materials of blue-emission face common problems in solid color, luminescence efficiency and service lifetme, etc. These problems stand in the way of the organic light-emitting diodes (OLEDs) to be applied to the RGB panchromatic display. This dissertation focuses on the developing high performance OLEDs with efficient and stable blue emission. Also, detailed theoretical investigation on the electronic structures of potential EL compounds have been made using quantum chemical calculation in order to reveal the structure-property relationship and to establish some general principles for the molecular design.Main results obtained in the experimental studies are: (1) Two novel blue light-emitting compounds, α-TMADN [2,3,6,7-tetramethyl-9,10-(1-dinaphthyl) -anthracene] and β-TMADN [2,3,6,7-tetramethyl-9,10-(2-dinaphthyl)- anthracene], were synthesized by introducing four methyl groups into the anthracene backbone of dinaphthyl anthracene (i.e. ADN). With a melting point higher than 320oC, both of them are of good thermostability. (2) Using pure β-TMADN and α-TMADN as emitting-layer material, two types of OLEDs with the same structure of ITO/NPB(50nm)/β-(or α-)TMADN(15nm)/Bphen(40nm)/Mg:Ag were prepared. Both diodes emitted stable and highly efficient blue light at 466nm. The luminous efficiencies reached 4.5cd/A(β- device) and 3.1(α- device), respectively, and were remarkably higher than that of the OLED with undoped ADN as the emitting-layer(1.9cd/A). The efficiency of the β-TMADN device was even higher than that of the device using dye doped ADN (3.5cd/A). (3) It was shown that a binary mixture of α- and β-TMADN of appropriate composition, instead of the pure emitting materials, may considerably improve the luminous efficiency of the OLED. A device sample with the optimal component, α:β=9:1 presented a maxium luminous efficiency up to 5.2cd/A.The instructive results of the theoretical investigations include: (1) The IR and Raman spectra as well as the equilibrium geometries of the two new compounds were predicted by using density functional theory (DFT) at the B3LYP/6-31G* level. The chemical shifts, ((1H-NMR), of them were calculated at the HF/6-31G*// B3LYP/6-31G* level. The results provided reliable evidence for the identification of the molecular structures. (2) The electronic structures of two metal chelates with multiple-dentate ligands, Al(azb-q) [(o,o′-dihydroxyazo- benzene)(8-quinolinoato) aluminum] and Al(Saph-q) [(salicylidene-o-aminophenolato), were calculated, respectively, at the B3LYP/6-31G* level. Then the characteristics of energy bands for both compounds, in an amorphous solid phase, were obtained by using the density of states (DOS) approach. The results demonstrated that the bridge atoms in the chelates play a primary role, dominating the forbidden zone width. The replacing atom, N, makes a great contribution to the lower edge of the empty band of Al(azb-q) and narrows its forbidden zone. (3) The electronic structure calculation of two-core-metal chelates, Al(Saph)-AA, Al(Saph)-DBM and Al(Saph)-DPM, was performed at the B3LYP/3-21G* level. The three chelates have the same Al cores and the first ligand, Saph (tridentate), but different second ligand (bidentate), AA, DBM and DPM. The obtained wavefunctions of frontier orbital pairs showed that the attribute of the chosen second ligand makes a great effect on the photoluminescence quantum yield of the two-core chelates. When the bidentate ligand is AA or DPM, groups with flexible chain, the components of HOMO and LUMO come from the same ligand, Saph, and the quantum yield of the material turns higher. When the ligand is DBM, in contrast, the components of HOMO and LUMO come from Saph and the bidentate ligand, respectively, and the quantum yield is much lower. The results coincide with the experimental results.