True Blue Blue-Emitting Aluminum(III) Quinolinolate Complexes

True Blue Blue-Emitting Aluminum(III) Quinolinolate Complexes

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True Blue: Blue-Emitting Aluminum(III) Quinolinolate Complexes Ce ´sar Pe ´rez-Bolı´var, Victor A. Montes, and Pavel Anzenbacher, Jr.* Department of Chemistry and Center for Photochemical Sciences, Bowling Green State Uni V ersity, Bowling Green, Ohio 43403 Received June 12, 2006 Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolino- late)phenolate complexes were synthesized. A tunable, blue-to- green emission is achieved by attaching electron-withdrawing modulators to the emisssive quinaldinate ligand. The electronic nature of modulator substituents attached to the position of the highest HOMO (highest occupied molecular orbital) density is used to modulate ligand HOMO levels to achieve effective emission tuning to obtain blue-emitting materials. Optical and electrochemical properties of the resulting complexes were investigated and compared to the results of density functional theory (DFT/B3LYP/ 6-31G*) studies. The resulting materials may find application as organic light-emitting device materials. Electroluminescent organometallic complexes are valued emitters for the fabrication of small-molecule-based organic light-emitting devices (SMOLEDs). 1 The unique electron- transport and emissive properties of tris(8-quinolinolate)alumin- um(III) (Alq 3 ) and its derivatives resulted in extensive use of Alq 3 in OLED fabrication (Figure 1). The advantage of aluminum(III) quinolinolate complexes is in their semi- conductor properties, namely, the ability to transport electrons and act as an electron-transport layer material and a host for various dopants. This, together with the ease of deposition by thermal vacuum evaporation, made aluminum(III) quino- linolate complexes widely used OLED materials. An obstacle in the fabrication of SMOLED-based full-color displays so far appears to be the limited availability of a pure blue color. 1 Additionally, in the case of blue-emitting complexes, the blue emission attests to the fact that the HOMO - LUMO gap (energy difference between the highest occupied molecular orbital and lowest unoccupied molecular orbital) is 3.2 - 3.3 eV regardless of the actual energy level of each molecular orbital. However, for successful device operation, the HOMO and LUMO energy levels of the electron-transporting mate- rial must also be aligned with the levels of the other materials in the OLED, namely, the dopant, both hole-blocking and buffer layers on the cathode side, and the hole-transport layer on the anode side. Few materials satisfy these conditions, which makes them highly desirable. 1 In the past, numerous research groups investigated alumin- um(III) quinolinolate complexes in studies focused on unrav- eling their photophysical and semiconductor properties. 2
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True Blue Blue-Emitting Aluminum(III) Quinolinolate Complexes

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