Iridium(III) Carbene Complexes Featuring Either Metal‐to‐Ligand Charge Transfer (MLCT) or Through‐Space Charge Transfer (TSCT) Blue Luminescence

Abstract

Through-space charge transfer (TSCT), rather than the commonly postulated metal-to-ligand charge transfer (MLCT) process, was proposed in getting the lowest lying excited state of newly designed Ir(III) blue phosphors. Accordingly, two benzo[d]imidazolylidene pro-chelates, L12H₂ ⁺ and L13H₂ ⁺, one with two cyano groups at the peri-benzo and N-aryl pendent and the other with its peri-cyano group being replaced with methyl substituent, were employed in syntheses of Ir(III) complexes f-ct12b,c and f-ct13b,c. Notably, complexes f-ct12b,c exhibited the traditional MLCT process, while f-ct13b,c were dominated by the TSCT transition, resulting in a smaller S₁-T₁ energy gap ΔE_ST. Next, it prompted us to explore whether their long-lived emission originated from phosphorescence or thermally activated delayed fluorescence (TADF). Although temperature-dependent emission studies favor TADF, the unresolved concerns are still discussed in depth. For application, OLED with the TSCT-based dopant f-ct13b delivered a maximum external quantum efficiency (EQE) of 22.2% and a max. luminance of 10 000 cd m^‒2, together with CIE_xy of (0.155, 0.120). Moreover, the hyper-OLED with f-ct13c sensitizer and v-DABNA terminal emitter exhibited a max. EQE of 28.2% and CIE_xy of (0.123, 0.129), demonstrating a new approach in developing efficient Ir(III) blue phosphors.