Bulky, dendronized iridium complexes and their photoluminescence

Abstract

Solution-processed blue emitters are essential for low-cost organic light-emitting diodes (OLEDs) but still face challenges due to their poor color purity, low efficiency and limited operational stability. Herein, by extending the conjugation of ultraviolet-emissive, facial tris(diphenylbenzimidazolyl)iridium (Ir) (fac-(dpbic)₃Ir), we introduce two new types of solution-processed emitters, i.e. triisopropylsilylethynyl(TIPSE)-substituted fac-(dpbic)₃Ir (2) and fac-(dpbic)₃Ir-based polyphenylene dendrimers D1 and D2. The emissions of Ir-complex 2 and the dendrimers were successfully pushed toward a pure and sky blue color, respectively, due to the dominant ³π–π* nature of their emissive excited states. As a pleasant surprise, the troublesome aggregation-induced red shift of the emission of Ir-complex 2 could be totally suppressed by the bulky TIPSE moieties. Ir-complex 2 displays pure blue emission in a solution-processed, non-doped OLED (non-optimized) with moderate efficiency and without any observed aggregation effects, which paves a way for the future design of high-performance, non-doped phosphorescent emitters. The dendrimers exhibit strong sky-blue emission at 77 K but their emission is completely quenched at ambient temperature. This is demonstrated to result from the much elongated Ir–C_carbene bond by the strong steric hindrance of the bulky polyphenylene dendrons. The remarkably long Ir–C_carbene bonds of the dendrimers make their T_I states more easily accessible to the non-emissive ³MC state than those for fac-(dpbic)₃Ir and compound 2 as supported by the quantum chemical results. This finding also promises suggestions for designing better dendrimer-based blue emitters.