Peripheral Ligand Effect on the Photophysical Property of Octahedral Iridium Complex: o-Aryl Substitution on the Phenyl Units of Homoleptic IrIII(C∧C)3 Complexes (C∧C = 1-Phenyl-3-methylimidazolin-2-ylidene-C,C2′) for Deep Blue Phosphorescence (2020)

To evaluate the efficacy of <i>ortho</i>-arylation in the second coordination sphere of octahedral iridium complex, a series of homoleptic <i>N</i>-heterocyclic carbene (NHC)-based <b>Ir(C<sup>∧</sup>C</b><sup><b>R</b></sup><b>)</b><sub><b>3</b></sub>-type complexes were designed and prepared by introducing various substituents (R = H, Me, Ph, MePh, and diMePh) at the <i>ortho</i>-position of the aryl unit of the orthometalated phenyl group. In solution, an unnoticeable increase of emission quantum yields was observed within the variation of the <i>ortho</i>-substituent of the sterically demanding side-branch, a diMePh- group, showing the radiative quantum yield of <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>diMePh</b></sup><b>)</b><sub><b>3</b></sub> (Φ<sub>PL</sub> = 1.9%), being higher than that of the unsubstituted carbene-based <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>H</b></sup>)<sub><b>3</b></sub> (Φ<sub>PL</sub> = 1.2%), due to a considerable difference in the nonradiative decay rate (<i>k</i><sub>nr</sub> = 65.40 × 10<sup>5</sup> s<sup>-1</sup> for <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>diMePh</b></sup><b>)</b><sub><b>3</b></sub> vs <i>k</i><sub>nr</sub> = 141.1 × 10<sup>5</sup> s<sup>-1</sup> for <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>H</b></sup>)<sub><b>3</b></sub>). Such a difference is attributed to the reduction of nonradiative pathway via the <sup>3</sup>MLCT → <sup>3</sup>MC transition by the widening gap between triplet emissive states and <sup>3</sup>MC state, and a rigidity increase in structure by steric hindrance of bulky aryl substituent. In contrast, significant increase of emission quantum yield was observed in the films cast by spin coating, and <i>fac</i><b>-</b>/<i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>diMePh</b></sup><b>)</b><sub><b>3</b></sub> (Φ<sub>PL</sub> = 60.1/49.1%) were the most efficient ones among NHC-Ir(III) complexes, compatible with the assumption that the secondary coordination effect, i.e. a peripheral constraint, was put into action. As the substituent R increases in size on going from H, Me, Ph, MePh, to diMePh, notable structural changes in the periphery are evident, while an increase of emission quantum yields is also seen. Such a peripheral difference was under scrutiny first with X-ray structural studies, and its manifestation in photophysics was investigated along with quantum calculations that finally addressed the peripheral effect being maximized at R = diMePh. In the application of PhOLED, the <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>diMePh</b></sup><b>)</b><sub><b>3</b></sub>-doped multilayer device showed highly enhanced efficiency with an external quantum efficiency (EQE) of up to 8.1%, compared to that of the <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>H</b></sup><b>)</b><sub><b>3</b></sub>-based device (1.2%), indicating the multiple positive effects of bulky aryl substitution of Ir(III) dopant. A deep-blue CIE chromaticity diagram (0.16, 0.09) was achieved from the device using <i>mer</i><b>-Ir(C<sup>∧</sup>C</b><sup><b>diMePh</b></sup><b>)</b><sub><b>3</b></sub> as a dopant.