Temperature Dependence of Blue Phosphorescent Cyclometalated Ir(III) Complexes

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(CC′)_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 (Φ) 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 (Φ = 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 (1011−1013 s^−1) and significantly smaller for fac-Ir(pmb)_3 (109 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 (^3MC) character. Linear correlations are observed between the activation energy and the energy difference calculated for the emissive and ^3MC states. The energy level for the ^3MC state is estimated to lie between 21700 and 24000 cm^−1 for the fac-Ir(C^N)_3 complexes and at 28000 cm^−1 for fac-Ir(pmb)_3.

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