σ–π and p–π conjugation induced NIR-emitting iridium(iii) complexes anchored by flexible side chains in a rigid dibenzo[a,c]phenazine moiety and their application in highly efficient solution-processable NIR-emitting devices†
To systematically investigate the σ–π and p–π conjugation effects on photoluminescence (PL) and electroluminescence (EL) properties, and to obtain high-efficiency near-infrared (NIR)-emitting iridium(III) complexes, in this work, 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 a significantly red-shifted NIR emission with high photoluminescence 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. Utilizing Ir-R as the phosphorescent dopant, the solution-processed organic light-emitting diodes (OLEDs) using CBP as the host and polymer light-emitting diodes (PLEDs) using PVK:OXD-7 as the cohost show outstanding EL performances. The highest external quantum efficiency (EQE) of 6.91% and a radiance of 26 100 mW sr−1 m−2 were obtained in the Ir-R based OLEDs with an emission peak at 730 nm, which are 4.7 and 2.2 times higher than those of the complex Ir-OR-based devices. This indicates that introducing σ–π conjugation effects induced by the anchored flexible n-octyl side chains in the rigid dibenzo[a,c]phenazine moiety is a better strategy for obtaining NIR-emitting iridium complexes for application in high-efficiency solution-processed devices.