Bulky, Dendronized Iridium Complexes and their Photoluminescence
Solution-processed blue emitters are essential for low-cost organic light-emitting diodes (OLEDs) but still define challenges due to their poor color purity, low efficiency and limited operational stability. Herein, by the extending the conjugation of an 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 emission of the 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 the 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 emissive at 77 K but their emission is completely quenched at ambient temperature. This is demonstrated to result from the much elongated Ir-Ccarbene bond by the strong steric hinderance of the bulky polyphenylene dendrons. The remarkably long Ir-Ccarbene bonds of the dendrimers make their TI states much easily accessible to the non-emissive 3MC state than those for fac-(dpbic)3Ir and compound 2 as supported by quantum chemical results. This finding also promises suggestions for designing better dendrimer-based blue emitters.