The σ−π and p−π Conjugation Induced NIR-Emitting Iridium(III) Complexes by Anchoring Flexible Side Chains in Rigid Dibenzo[a,c]phenazine Moiety and Their Application in High-Efficient Solution-Processable NIR-Emitting devices

To systematically investigate the σ−π and p−π conjugation effects on photoluminescent (PL) and electroluminescent (EL) properties, and get high-efficiency near-infrared (NIR)-emitting iridium(III) complexes, in this contribution, two rigid C^N ligands of dibenzo[a,c]phenazine (DBPz) derivatives respectively anchored by flexible side chains of n-octyl and n-octyloxy, and their corresponding iridium(III) complexes of Ir-R and Ir-OR were designed and synthesized. Their photophysical and EL properties were systematically investigated. It is found that the σ−π and p−π conjugation effects can make both iridium(III) complexes exhibit significantly red-shifted NIR emission with high photoluminescent quantum efficiency (PLQY) in comparison with their archetypal iridium complex without any side chains. More intense NIR emissions are achieved for Ir-R at 732 nm (PLQY: 24%) and Ir-OR at 775 nm (PLQY: 8%) in dichloromethane solution. Employing Ir-R as the phosphorescent dopant, the solution-processed organic light-emitting diodes (OLEDs) using CBP as host and polymer light-emitting diodes (PLEDs) using PVK: OXD-7 as cohost show outstanding EL performance. The highest external quantum efficiency (EQE) of 6.91% and a radiance of 26100 mW Sr-1 m-2 are obtained in the Ir-R based OLEDs with emission peak at 730 nm, which are 4.7 and 2.2 times high as those of the complex Ir-OR-based devices. It indicates that introducing σ−π conjugation effect induced by the anchored flexible n-octyl side chains in rigid dibenzo[a,c]phenazine moiety is a better strategy to obtain NIR-emitting iridium complexes for application in high-efficiency solution-processed devices.