Temperature Dependence of Blue Phosphorescent

Temperature Dependence of Blue Phosphorescent - Temperature...

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Unformatted text preview: Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes Tissa Sajoto, Peter I. Djurovich,* , Arnold B. Tamayo, Jonas Oxgaard, William A. Goddard III, and Mark E. Thompson* , Department of Chemistry, Uni V ersity of Southern California, Los Angeles, California 90089, and Department of Chemistry, California Institute of Technology, Pasadena, California 91125 Received April 24, 2009; E-mail: djurovic@usc.edu; met@usc.edu Abstract: The photophysical properties for a series of facial ( fac ) cyclometalated Ir(III) complexes ( fac- Ir(C N) 3 (C N ) 2-phenylpyridyl (ppy), 2-(4,6-difluorophenyl)pyridyl (F2ppy), 1-phenylpyrazolyl (ppz), 1-(2,4- difluorophenyl)pyrazolyl) (F2ppz), and 1-(2-(9,9 -dimethylfluorenyl))pyrazolyl (flz)), fac-Ir(C N) 2 (C N ) (C N ) ppz or F2ppz and C N ) ppy or F2ppy), and fac-Ir(C C ) 3 (C C ) 1-phenyl-3-methylbenzimidazolyl (pmb)) have been studied in dilute 2-methyltetrahydrofuran (2-MeTHF) solution in a temperature range of 77- 378 K. Photoluminescent quantum yields ( ) for the 10 compounds at room temperature vary between near zero and unity, whereas all emit with high efficiency at low temperature (77 K). The quantum yield for fac-Ir(ppy) 3 ( ) 0.97) is temperature-independent. For the other complexes, the temperature-dependent data indicates that the luminescent efficiency is primarily determined by thermal deactivation to a nonradiative state. Activation energies and rate constants for both radiative and nonradiative processes were obtained using a Boltzmann analysis of the temperature-dependent luminescent decay data. Activation energies to the nonradiative state are found to range between 1600 and 4800 cm- 1 . The pre-exponential factors for deactivation are large for complexes with C N ligands (10 11- 10 13 s- 1 ) and significantly smaller for fac- Ir(pmb) 3 (10 9 s- 1 ). The kinetic parameters for decay and results from density functional theory (DFT) calculations of the triplet state are consistent with a nonradiative process involving Ir- N (Ir- C for fac- Ir(pmb) 3 ) bond rupture leading to a five-coordinate species that has triplet metal-centered ( 3 MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and 3 MC states. The energy level for the 3 MC state is estimated to lie between 21 700 and 24 000 cm- 1 for the fac-Ir(C N) 3 complexes and at 28 000 cm- 1 for fac-Ir(pmb) 3 . Introduction Cyclometalated iridium(III) complexes have been recently shown to have phosphorescence efficiencies approaching theo- retical limits ( ) 0.8- 1.0) and short radiative triplet lifetimes ( ) 1- 5 s)....
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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.

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Temperature Dependence of Blue Phosphorescent - Temperature...

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