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Unformatted text preview: pubs.acs.org/IC Published on Web 10/08/2010 r 2010 American Chemical Society Inorg. Chem. 2010 , 49 , 98919901 9891 DOI: 10.1021/ic1009253 Iridium Metal Complexes Containing N-Heterocyclic Carbene Ligands for Blue-Light-Emitting Electrochemical Cells Cheng-Han Yang, , Juan Beltran, Vincent Lemaur, J 9892 Inorganic Chemistry, Vol. 49, No. 21, 2010 Yang et al. and typically they emit in the red, orange, yellow, green, and sky-blueregions. Amongthesecomplexes, iridium complexes are interesting candidates in terms of color tuning because of the pronounced ligand-field-splitting effects.Recently, Wong et al. and Qiu et al. reported white LEECs based on cationic iridium complexes. 9 However, reports concerning the design and preparation of blue or even near-UV ionic phosphor- escent materials also based on other metal ions are relatively rare. 10 In particular, among the iridium(III) complexes, a great number of ancillary ligands (e.g., picolate-, pyrazolylborate-, and pyridylazolate-based ligands) have been explored in order to obtain blue emission for application as OLED materials. 11 Despite a lot of effort, experimental and theore- tical studies have pointed out the intrinsic difficulties in obtaining deep-blue emission because increasing the highest occupied molecular orbital (HOMO)- lowest unoccupied molecular orbital (LUMO) energy gap induces destabiliza- tionof the metal-to-ligandcharge-transfer(MLCT) state and hence a decrease in the MLCT character of the lowest luminescent triplet excited state. This implies that the excited state becomes more ligand-centered, with a stronger coupling to the intramolecular vibrational modes, and that the oscil- lator strength of the S- T 1 transition as well as the emission quantum efficiency is reduced, while the radiative lifetime is increased. It has been recently reported that the use of high- field-strength ligands such as carbenes results in a shift toward higher energy of the emission and an increase of the blue phosphorescent efficiency. 12 The number of papers exploiting carbene as high-field ligand, however, remains scarceeventhoughsuchligandsappeartobegoodcandidates to achieve high-performing OLED displays, and to the best of our knowledge, no reports on LEEC devices have been published. Herein, we report a new strategy to synthesize cationic blue-emitting, heteroleptic iridium(III)-based phosphors employing methyl- or n-butyl-substituted bisimidazolium salt N-heterocyclic carbene ligands as the ancillary chelate of a iridium(III) 4,6-difluorophenylpyridine complex. A full photophysical and electrochemical characterization has been performed and the experimental results compared and ratio- nalized with quantum-chemical calculations. These com- plexes display deep-blue emission and high luminescent quantum yields and, because of their excellent solubility in organic solvents, have been employed as blue phosphors to fabricate blue-emitting LEECs. The emission wavelengths of the devices obtained are still very blue, the bluest ever...
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This note was uploaded on 03/27/2011 for the course CHEM 2211L taught by Professor T.a. during the Spring '08 term at University of Georgia Athens.
- Spring '08